Pabitra Chatterjee

Barthélemy Chollet
email: bart.chollet@grenoble-em.com

Olivier Trendel
email: olivier.trendel@grenoble-em.com

From conformity to reactance: Contingent role of network centrality in consumer-to-consumer influence

Keywords:

Central consumers in a group often are influential, because their social prominence commands conformity from other members. Yet, there can be another contradictory effect of centrality, such that other members regard it as a threat to their attitudinal freedom and express reactance instead of conformity. Whether a group member con-forms or reacts to the evaluation of a more central member might depend on the strength of their relationship, which determines the social cost of disagreeing. We provide evidence of such an interaction between centrality and relational strength with an experiment where participants with preexisting affective ties of varying strengths taste a snack in groups (Study 1) and a field study where participants connected by instrumental ties consume a complex service (Study 2). A scenario-based experiment manipulating centrality and strength of ties provides further evidence that reactance underlies the observed effects (Study 3).

Introduction

By sharing their opinions, information, and personal experiences with a product, consumers influence one another [START_REF] Cohen | Informational social influence and product evaluation[END_REF][START_REF] Zhu | To switch or not to switch: Understanding social influence in online choices[END_REF], some more than others [START_REF] Flynn | Opinion leaders and opinion seekers: Two new measurement scales[END_REF][START_REF] King | Overlap of opinion leadership across consumer product categories[END_REF]. A wellestablished goal in marketing research is to understand these disparities in influence, to be able to leverage the most influential consumers [START_REF] Godes | Commentary-invited comment on "opinion leadership and social contagion in new product diffusion[END_REF][START_REF] Libai | Customer-to-customer interactions: Broadening the scope of word of mouth research[END_REF]. One popular approach to this challenge is to consider any group of consumers as forming networks, whose structure determines the consumer-to-consumer influence processes [START_REF] Lee | The role of network centrality in the flow of consumer influence[END_REF]. Because social ties are unevenly distributed, some consumers occupy central positions, whereas others remain peripheral, and the differences in their influence are significant [START_REF] Friedkin | Structural bases of interpersonal influence in groups: A longitudinal case study[END_REF]. Marketing studies regularly confirm that network centrality (i.e., being connected to many other consumers) confers influence on a consumer, yet to the best of the authors' knowledge, these studies focus solely on products consumed individually (e.g., studying how social relationships influence a focal consumer in selecting a piano tuner, [START_REF] Reingen | Analysis of referral networks in marketing: Methods and illustration[END_REF]. Network positions likely also affect consumer-to-consumer influence processes in co-consumption contexts though, in which all parts of the network synchronously share the consumption experience (e.g. a group of colleagues eating at a restaurant). This article seeks to fill a gap by analyzing the influence that two consumers have on each other, as a function of their relative network centrality in a co-consumption group. With their co-presence, the social asymmetry associated with vastly different centralities becomes particularly salient and exerts situational pressure on peripheral consumers to conform with the evaluations of central consumers. But this pressure does not necessarily result in conformity. Because it conveys a potential threat to attitudinal freedom, it might spark reactance [START_REF] Fitzsimons | Reactance to recommendations: When unsolicited advice yields contrary responses[END_REF][START_REF] Mourali | The dual role of power in resisting social influence[END_REF].

By demonstrating the ambivalence of centrality, which may drive either conformity or reactance, this paper contributes to a network approach to consumer behavior. Prior research conceptualizes influence as either positive (peripheral consumer aligns attitudes or behaviors with those of a more central consumer) or non-existent (peripheral consumer ignores the opinion of the more central one), which is consistent with the practical aim of activating favorable influences. But this approach ignores the prediction of reactance theory that influence attempts might backfire and induce consumers to diverge radically from their source. To examine the conditions in which such "boomerang effect" may be likely, this research builds on prior findings that suggest reactance depends on the cost of resisting [START_REF] Miron | Reactance theory-40 years later[END_REF]. We argue that, for a focal consumer participating in a co-consumption episode, the cost of disagreeing with a highly central participant depends on the strength of their relationship, a network dimension that determines attachment, mutual binding, and constraints on action [START_REF] Granovetter | The strength of weak ties[END_REF]. With these considerations, the current research responds to a call from [START_REF] Lee | The role of consumers' network positions on information-seeking behavior of experts and novices: A power perspective[END_REF] to address the role of centrality in relation to tie strength, providing support to the notion that studies of consumer networks cannot separate structural and relational dimensions.

Accordingly, the current article presents two co-consumption studies, among groups of people who know one another with varying intensity. The studies both consist in analyzing all pairs of members (all dyads) within those groups, measuring the dyadic difference in their satisfaction with the consumable, the dyadic difference in their centrality scores and the strength of their tie. In Study 1, participants consumed a new snack together and then evaluated the product (shared condition). Another set of participants tasted the same product but had no possibility of communicating with or seeing any others (solitary condition). The results confirm the predicted interaction effect between centrality and tie strength on product evaluation only in the shared condition. Study 2 then confirms this interaction in a very different context, with a high involvement, utilitarian service consumed over several months (i.e., business education). Last, in study 3, a scenario-based experiment, centrality and tie strength are manipulated in order to provide further evidence that centrality generates threat to attitudinal freedom and that both variables interact to affect reactance behaviors.

Network centrality and consumer-to-consumer influence

Centrality is one of the most frequently considered network characteristics, because of its demonstrated effect on social power and structural influence [START_REF] Marsden | Egocentric and sociocentric measures of network centrality[END_REF]. High network centrality affects a variety of marketing outcomes, including new product adoption [START_REF] Katona | Network effects and personal influences: The diffusion of an online social network[END_REF][START_REF] Kim | Effects of social influence on consumers' voluntary adoption of innovations prompted by others[END_REF], product-related information-seeking behavior [START_REF] Lee | The role of consumers' network positions on information-seeking behavior of experts and novices: A power perspective[END_REF], and shopping behavior [START_REF] Gentina | Peer network position and shopping behavior among adolescents[END_REF]. In a similar vein, centrality appears correlated with opinion leadership [START_REF] Gentina | Unique but integrated: The role of individuation and assimilation processes in teen opinion leadership[END_REF][START_REF] Lee | The role of network centrality in the flow of consumer influence[END_REF][START_REF] Risselada | Indicators of opinion leadership in customer networks: Self-reports and degree centrality[END_REF]. Central consumers are influential, first, because the number of others they reach in their daily social interactions is greater than the number reached by more peripheral consumers. Centrality also confers a "social hub" position [START_REF] Goldenberg | The role of hubs in the adoption process[END_REF], such that the person serves as a passage point for information that flows throughout the network. With broader information sources, central consumers thus tend to be perceived as better informed, and their advice is more sought after by other consumers [START_REF] Lee | The role of network centrality in the flow of consumer influence[END_REF]. Finally, centrality provides preferred social status [START_REF] Ibarra | Power, social influence, and sense making: Effects of network centrality and proximity on employee perceptions[END_REF], such that central consumers enjoy more integration and acceptance in the network [START_REF] Gentina | Unique but integrated: The role of individuation and assimilation processes in teen opinion leadership[END_REF]. Their influence thus stems from the inclination of peripheral consumers to conform to their opinions, as part of an integration strategy (Van den Bulte, Wuyts, Dekimpe, Gijsbrechts, & Pieters, 2010).

Pressures to conform to central consumers when consumption is shared

Centrality in networks was found to influence consumer behavior in a variety of settings, from selecting a piano tuner [START_REF] Reingen | Analysis of referral networks in marketing: Methods and illustration[END_REF], to choosing to prescribe a new drug [START_REF] Iyengar | Social contagion in new product trial and repeat[END_REF], to deciding to affiliate with a social media platform [START_REF] Goldenberg | The role of hubs in the adoption process[END_REF][START_REF] Katona | Network effects and personal influences: The diffusion of an online social network[END_REF]. In these contexts, social interaction has the potential to intervene in consumers' thinking, either before (decision making) or after (evaluation) consumption. The consumption episode itself is not shared with others though; in prior studies, centrality does not operate during consumption.

Yet extensive literature on small group dynamics suggests that synchronous interactions with other consumers during consumption should affect the role of network centrality. Confronting other group members, especially peer group members [START_REF] Childers | The influence of familial and peer-based reference groups on consumer decisions[END_REF], results in significant attitude changes, due to social pressures to conform with what is perceived as the majority opinion [START_REF] Asch | Opinions and social pressure[END_REF][START_REF] Kaplan | The influencing process in group decision making. Group processes[END_REF]. Imagine a set of five consumers going out for dinner in a group. A reasonable prediction asserts that all of them refrain from sharing aspects of their judgments that they deem socially inappropriate when evaluating the meal [START_REF] Ramanathan | Consuming with others: Social influences on moment-to-moment and retrospective evaluations of an experience[END_REF]). Yet such a view is structure-blind, in that it ignores preexisting ties among the group, which may produce very different social positions for the individual members. As some in the group may already know each other well, and others may not, the shared experience generates a temporary social structure, assigning a specific centrality level to each participant, presumably with consequences for their perceived attitudinal freedom. Assume that in this example, a consumer B is a friend of all other participants, but consumer A is friendly only with B and barely knows the others. Given the unique group composition at that dinner, B has a much more central position than A. According to a network approach, the typical pressures for conformity in small groups will have a disproportionate effect on A. First, A lacks information to determine accurately what is socially appropriate for the group and perceives B as better equipped in this respect. Second, with her high centrality, during that dinner B will have a stronger influence in defining appropriateness, not just appraising it [START_REF] Friedkin | Structural bases of interpersonal influence in groups: A longitudinal case study[END_REF]. Using B″s opinion as a proxy for appropriateness and conforming to it thus will be an appealing strategy for A. Third, because she is more central, B is likely to fill more of the conversational space and can be more outspoken in sharing her opinion [START_REF] Lee | The role of network centrality in the flow of consumer influence[END_REF]. As research shows, central nodes in a network are more prone to assertiveness [START_REF] Brass | Potential power and power use: An investigation of structure and behavior[END_REF]. Overall, these arguments suggest that during shared consumption, peripheral consumers feel pressure to conform with the opinions of more central others.

Interaction between centrality and strength of ties

If centrality generates pressure to conform, then a reasonable prediction would be that less central consumers tend to respond to this pressure by simply conforming to more central ones. Following Asch's seminal work [START_REF] Asch | Opinions and social pressure[END_REF], the conformity literature has grown extremely strong, providing various explanations for that kind of response to normative pressure [START_REF] Chartrand | The chameleon effect: The perception-behavior link and social interaction[END_REF][START_REF] Cialdini | Social influence: Compliance and conformity[END_REF]. However, pressure to conform can also backfire and result in reactance, defined as a motivational state directed toward restoring freedom in response to perceptions that this freedom is under threat [START_REF] Miron | Reactance theory-40 years later[END_REF]. Following threats to attitudinal or behavioral freedom, people often react by asserting it "more forcefully than they would otherwise" (Kray, Thompson, & Galinsky, 2001: 948). For example, smokers might smoke more in response to pressures exerted by a spouse to quit the habit [START_REF] Miron | Reactance theory-40 years later[END_REF]. Marketing studies observe reactance among consumers opposing the norm or experts' recommendations [START_REF] Fitzsimons | Reactance to recommendations: When unsolicited advice yields contrary responses[END_REF][START_REF] Algesheimer | The social influence of brand community: Evidence from European car clubs[END_REF].

Because the situational pressure to conform and the urge to reassert freedom are countervailing forces, reactance theorists often focus on isolating the conditions in which one prevails over the other [START_REF] Miron | Reactance theory-40 years later[END_REF]. A key factor is the extent to which negative outcomes might result from reacting [START_REF] Crawford | Reactance, compliance, and anticipated regret[END_REF][START_REF] Heilman | Oppositional behavior as a function of influence attempt intensity and retaliation threat[END_REF]. Considering our case, negative outcomes lie in the social cost of disagreeing, which should depend, according to network theories, on the strength of the tie, a multifaceted notion that captures the "amount of time, the emotional intensity, the intimacy (mutual confiding), and the reciprocal services" between two persons [START_REF] Granovetter | The strength of weak ties[END_REF](Granovetter ( : p. 1361).

An intuitive take on the issue would be that disagreement is easier to express when the tie is strong, as intimacy might allow greater possibility to speak one's mind openly and thus freely express disagreement. Weak ties, on the contrary, supposedly command more restraining in the course of social interactions, less self-exposure and transparency in confiding, because reciprocity on the other end of the relationships isn't guaranteed [START_REF] Granovetter | The strength of weak ties[END_REF]. However, there is also considerable support for the reverse argument that social cost of disagreeing should increase with tie strength. With a strong tie, further interactions, beyond the focal consumption episode, are to be expected, which according to interdependence theory should make a difference [START_REF] Davis | Attitude alignment in close relationships[END_REF]. The episode is just one occurrence in a longer-term relationship-building process, thus negative outcomes are to be expected in the future (e.g., denial of social support), making the option of conforming more appealing than restoring attitudinal freedom through reactance. If the tie is weak though, the prospect of no or limited interactions in the future might leave more room for reactance [START_REF] Pallak | Interactive effects of commitment to future interaction and threat to attitudinal freedom[END_REF]. Other arguments rely on balance theory [START_REF] Davis | Attitude alignment in close relationships[END_REF]. Regardless of further interactions, disagreement with a strong tie carries a potential for discomfort that people tend to avoid; they rather search for opinion consistency [START_REF] Priester | Extending the bases of subjective attitudinal ambivalence: Interpersonal and intrapersonal antecedents of evaluative tension[END_REF]. Moreover, agreement avoids the painful suspicion that a difference in opinion might be symptomatic of decreased attachment, which reduces the risk of weakening the tie [START_REF] Heider | The psychology of interpersonal relations[END_REF]. If the tie is already weak though, it has limited value, so nothing is to be lost by disagreeing.

The above-mentioned mechanisms should be dramatically emphasized when tie strength is considered, not in the privacy of one-to-one interactions, but in the context of a larger group of other preexisting ties, with dyadic interactions taking place in the presence of other persons. Disagreement with a highly central interaction partner in that case should be even more difficult to express, as the salience of multiple other connections of the central member makes the focal tie appear less "exclusive" and more fragile. Also, there might be a fear that others in the group take disagreement as a sign of poor relationship with the more central member.

Taken together, these arguments suggest that a consumer's response to the pressure to conform to a more central consumer should be driven by relational strength in the dyad. When the tie is strong, difference in centrality levels should reduce disagreement (conformity effect); when the tie is weak, this difference should increase disagreement (reactance effect).

Study 1

Study 1 aims to test the predicted interaction between difference in centrality levels and tie strength on product evaluation in a controlled setting. In a taste test, participants first consumed as a group, then evaluated the food product individually. This study used food as the focal product because people commonly eat together, and influence of others on food evaluations already has been demonstrated [START_REF] Cruwys | Social modeling of eating: A review of when and why social influence affects food intake and choice[END_REF]. Participants tasted an Indian snack unavailable for purchase in the study setting (France). Thus, none of them was acquainted with this "exotic" foreign food, and the experience features ambiguity that typically favors social influence [START_REF] Deutsch | A study of normative and informational social influences upon individual judgment[END_REF].

According to our arguments, the interaction between difference in centralities and tie strength stems from a situational pressure to conform, which results from knowledge of others' opinions, gained through communication, and consciousness of the structure of preexisting social ties in the group. However, latent confounders such as shared traits also could prompt the predicted effects (Goldsmith-Pinkham & Imbens, 2013), resulting in agreement that is unrelated to social dynamics. Two participants may rate similarly (differently) the product just because they have similar (different) preferences and not because they influence each other by sharing their opinions. Therefore, in addition to the condition in which members of the groups could talk and share their opinions (thereafter referred to as the shared condition), another condition mandated that participants consume and evaluate the product individually (thereafter referred to as the solitary condition). The study was thus a one-factor (shared condition, solitary condition) between-subject experiment. In line with prior research that uses similar designs [START_REF] Falk | Clean evidence on peer effects[END_REF][START_REF] Ramanathan | Consuming with others: Social influences on moment-to-moment and retrospective evaluations of an experience[END_REF], the analysis of the data from the solitary condition functioned as if consumers were in groups, such that the analysis considered "pseudo-groups". If, as expected, the predicted effects arise in the shared condition but not in the solitary condition, they must be due to interaction dynamics, not latent confounders [START_REF] Iyengar | Social contagion in new product trial and repeat[END_REF].

Participants

129 students enrolled in a two-year undergraduate program at a French college of business voluntarily participated in the study (M age = 18.80 years, SD = 0.94 years; 64% female). This specifically targeted population offers adequate variability in the relational strength across members. All the courses of this program take place in the same location, a small building with 11 classrooms. Yet firstand second-year students never have classes together, and within each year, students have some classes in plenary sessions and most classes in small groups composed at the beginning of the academic year. Thus the relational strength of dyads of students varies, from knowing each other only by sight (e.g., two students from different program years) to knowing each other very well (e.g., two students in the same year and small group). This context is thus well suited for this experiment, which requires participants in variable network positions in a given co-consuming group.

Procedure and material

The data collection involved two steps. First, students completed a questionnaire and, on a roster of all students enrolled in the program, indicated their level of friendship with each student (to ease recall, names were listed by groups or cohorts, and then sorted alphabetically). The survey took place in a computer room during a course meeting, delivered by a teacher claiming to conduct research on creativity. This teacher did not appear as a stakeholder in the taste test phase.

Second, four weeks later, students were randomly assigned to the shared or solitary condition. Participants in the shared condition also were randomly assigned to co-consumption groups of four persons. To minimize the possibility of participants chatting about the study and jeopardizing its validity, the taste test data were collected in a single day. All participants received a preliminary briefing in an auditorium: They would take part in a product test for an Indian snack targeted at the French youth market. To improve the credibility of this cover story, an Indian researcher was introduced as the manager of the Indian company commissioning the survey, and two others were introduced as French professors providing consultancy services. All three were unknown to the participants.

The data collection for the shared condition occurred in a classroom (30 × 20 m 2 ). A table and chairs in the center of the room helped enable the shared consumption. Group members, seated around the table, received a reminder of the study purpose, as indicated during the preliminary briefing. Each participant received about 20 cl of the snack and 20 cl of water in plastic glasses. After asking them to consume as much or as little as they wanted, the experimenter left the room, saying he would give them each a questionnaire afterward so that they could offer their product opinions. The experimenter said nothing about whether participants could talk while consuming. After 8 min, the experimenter returned with pen-and-paper questionnaires, directed the participants to four chairs placed in the four corners of the room, and asked them to fill in their questionnaires on their own, without any discussion. While doing so, the participants could not see one another. This design reflects recommendations from prior research that suggests participants should be allowed to judge by themselves to give free expression to their reactance [START_REF] Clee | Consumer behavior and psychological reactance[END_REF]. Finally, the experimenter asked the participants not to discuss their experiences with those yet to take the taste test. To maintain equivalence between the two conditions, participants assigned to the solitary condition were also invited to participate in batch of four persons, following a random assignment. These participants underwent an identical procedure but were seated in isolated cubicles, and consumed and evaluated the product on their own.

Measures

In order to measure the dependent variable, participants rated their overall satisfaction with the snack on a semantic differential scale ("What is your level of satisfaction with the snack you just ate?"; 1= "I am not satisfied at all," 10 = "I am very satisfied"; [START_REF] Oliver | Customer satisfaction research[END_REF][START_REF] Kekre | Drivers of customer satisfaction for software products: Implications for design and service support[END_REF]. Because the key dependent variable is the level of disagreement between two group members, the measure relied on the dyadic difference in satisfaction. For the dyad {i,j}, the difference in satisfaction is | satisfaction isatisfaction j |, where satisfaction i is participant i's overall satisfaction with the snack. The dyadic values entered two square matrices, one for the shared condition and one for the solitary condition. Cell ij of a matrix contained the difference in satisfaction between i and j if i and j belonged to the same (real or pseudo) group and i ≠ j. Otherwise it was blank.

To assess the strength of ties, following [START_REF] Baldwin | The social fabric of a team-based MBA program: Network effects on student satisfaction and performance[END_REF], the sociometric questionnaire contained a measure of friendship that asked each respondent to indicate the amount of leisure time spent with each of the other students (0 = "do not spend any time together," to 5 = "hang out very often"). 1 Friendship ties were chosen because they resonate well with the hedonistic nature of the product consumed. That is, snacks often are consumed by people enjoying themselves with friends. In the resulting valued adjacency matrix, each cell contains a value (0-5) that indicates the level of friendship.

For each respondent, his or her degree centrality is the sum of preexisting ties that he or she has with other participants in the same consumption group. Formally, C i = ∑ K i≠ j t ij , where t ij represents the value of i's tie to j stored in the adjacency matrix, and K is the number of persons in the consumption group [START_REF] Knoke | Social network analysis[END_REF]. When network data is binary (two nodes either have a tie or have no tie), t takes either 0 or 1 and centrality is merely a count of ties. In our case, though, the network is valued: cells in the matrix vary with strength. Degree centrality is then a summation of strengths from the focal node to all others in the network [START_REF] Knoke | Social network analysis[END_REF][START_REF] Borgatti | Ucinet 6 for windows[END_REF]. For the dyad {i,j}, the difference in centrality is defined as |C i -C j |.

Finally, several covariates were measured. The dyadic difference in satisfaction with the new snack can be driven by similar (or dissimilar) ratings of specific product attributes [START_REF] Oliver | Customer satisfaction research[END_REF]. For example, two participants may love (hate) spiciness and therefore be similarly satisfied (unsatisfied) with the new snack for this reason. Therefore, participants rated their satisfaction with several attributes of the snack-sweetness, saltiness, spiciness, and fattiness-on 1-7 semantic differential scales ("How satisfied are you with each of the following characteristics of the snack?"; 1 = "not satisfied at all," 7 = "very satisfied"). We opted for a "one item per attribute" logic, consistent with prior demonstrations of its robustness [START_REF] Kekre | Drivers of customer satisfaction for software products: Implications for design and service support[END_REF][START_REF] Labarbera | A longitudinal assessment of consumer satisfaction/dissatisfaction: The dynamic aspect of the cognitive process[END_REF]). The differences in ratings then could be calculated for each dyad and each product attribute. For spiciness for example, the evaluation difference between participant i and participant j was |spiciness ispiciness j |.

Results

Data came from 88 2 participants in the shared condition (19 groups of 4 persons and 4 groups of 3 persons, forming 252 dyads) and 37 in the solitary condition (7 pseudo groups of 4 persons and 3 pseudo groups of 3 persons, forming 102 dyads). 3 It was organized into square matrices (88 × 88 in the shared condition, 37 × 37 in the solitary condition) that specified several types of relatedness among participants (tie strength, dyadic difference in centrality, dyadic difference in satisfaction). Standard hypothesis tests are impossible with such network data, because the observations are not independent. Thus, the analysis relies instead on double semi-partialling multiple regression quadratic assignment procedures (MRQAP), a non-parametric test [START_REF] Dekker | Sensitivity of MRQAP tests to collinearity and autocorrelation conditions[END_REF] available in the UCINET 6 package [START_REF] Borgatti | Ucinet 6 for windows[END_REF].

MRQAP starts with a regular ordinary least squares (OLS) estimation of regression coefficients across cells of the dependent, independent, and control matrices. The rows and columns of the dependent matrix are then randomly permuted, resulting in a new matrix used for another OLS regression, ultimately generating new coefficients and R-square values. This matrix contains a structure of dyadic ties that differ from the original on a cell-by-cell basis but are similar on a global level (i.e., each dyadic value appears the same number of times in the matrix), such that it preserves the level of autocorrelation on which further tests are conditioned [START_REF] Kilduff | Bringing the individual back in: A structural analysis of the internal market for reputation in organizations[END_REF]. The permutation and regression process repeats a high number of times (here, 1000 times), in order to generate a distribution that provides a reference for comparison with "real" observed values.

The key prediction for this study is that there is a significant negative interaction between difference in centrality and tie strength such that the dyadic difference in satisfaction increases with difference in centrality at low tie strength, due to reactance mechanisms, but decreases at high tie strength, due to conformity mechanisms. The results from the shared condition support this prediction (β tie-strength × difference-in-centrality = -0.25, p = 0.01; Table 1). The interaction of tie strength with difference in centrality is also negative and significant without any covariates in the model (β tie-strength x difference-in-centrality = -0.22, p = 0.02). An OLS regression is not adequate for hypothesis testing with network data, however in our case the coefficients from OLS and MRQAP are similar. Thus, to depict the interaction, we ran an OLS regression and plotted the interaction for the different levels of tie-strength [START_REF] Hayes | Computational procedures for probing interactions in OLS and logistic regression: SPSS and SAS implementations[END_REF]Fig. 1). Figure 1 shows that the difference in satisfaction increases with the difference in centrality in weak-tie conditions, as expected when reactance dominates. In other words, the level of disagreement in satisfaction increases with difference in centrality when there is no friendship to preserve and no or limited costs of reacting. However, the difference in satisfaction decreases with the difference in centrality in strong-tie conditions, as predicted for when conformity dominates.

The hypothesized interaction between tie strength and the difference in centrality also could be due to some latent confounds unrelated 1 The measurement of strength based on frequency sometimes relies on objective measures (e.g. "once a year", "once a month", etc.). We opted for a scale with only 2 anchors with rather subjective wording because asking to recall specific interactions might increase the recency bias which is typical in network studies [START_REF] Brewer | Forgetting in the recall-based elicitation of personal and social networks[END_REF]: a respondent urged to provide a specific number of interaction will usually base his/her response on the very last week of interaction much more than on the "typical week", even though the last week might be totally unrepresentative of usual interaction patterns [START_REF] Chang | Measuring the frequency of regular behaviors: Comparing the "typical week" to the "past week[END_REF].

2 One group of four had to be excluded from the sample because a member had not provided tie-related data.

3 As these numbers suggest, in a few groups one member did not show up on due time.

As a result, these groups are of size 3 instead of 4. Additional analyses excluding the dyads from those groups yield findings similar to those described thereafter. to the social dynamics at play during the co-consumption episode. The convergence or divergence observed in participants' evaluations could reflect similarity, rather than situational pressures to conform exerted on peripheral consumers. To rule out this possibility, a regression analysis was conducted on participants in the solitary condition (see above), who could not communicate, thus not gain knowledge of others' judgement. It yields non-significant coefficients for both tie strength and difference in centrality (β tie-strength = 0.10, p = 0.31; β difference-in-centrality = -0.11, p = 0.42) and for the interaction between tie strength and difference in centrality (β tie-strength × difference in centrality = -0.19, p = 0.14), contrary to the findings from the shared condition. Taken together, these findings confirm that communication is necessary for the predicted effects to occur; they do not result from latent confounds such as similar preferences or traits. That is not to say that similarity does not influence our findings at all though: during communication (in the shared condition), people might have unveiled their characteristics and in case of similarity (dissimilarity) agreed (disagreed) primarily with others whom they deem similar [START_REF] Allen | Categorization, belief similarity, and intergroup discrimination[END_REF]. Another concern with similarity is that it might affect not only agreement, but the nature of social relationships themselves, with similar people forming more easily strong ties [START_REF] Mcpherson | Birds of a feather: Homophily in social networks[END_REF]. These limitations are the reason why a study was designed to manipulate the variables of interest (study 3, Section 7).

Discussion

In this study, the predicted negative interaction between difference in centrality and tie strength arose from shared consumption. Social asymmetry in a dyad increases agreement only if the tie is strong, such that concern for preserving the tie demands conformity. It instead favors disagreement if the tie is weak, such that the costs of reactance are minimal.

Study 2

Study 1 offers evidence that the structure and strength of preexisting ties among consumers affect the patterns of social influence during the shared consumption of a low involvement, hedonic product (a snack), lasting a few minutes. The goal of Study 2 is to extend the scope of these findings by testing the hypothesized interaction in the case of a high involvement, utilitarian service consumed over several months: a graduate business education program. Previous research confirms that students think of themselves as consumers [START_REF] Ng | Education as service: The understanding of university experience through the service logic[END_REF]. A graduate business education program has great importance for students who anticipate positive career outcomes from completing it, and it also constitutes a shared experience that involves essentially group work and regular collective discussions [START_REF] Baldwin | The social fabric of a team-based MBA program: Network effects on student satisfaction and performance[END_REF]. Study 2 thus captures participants' evaluation several months after, instead of minutes after, making up their minds.

Participants

This study was conducted among 50 graduate students in a Master of Science in Marketing at a major French business school (different from the one in Study 1). The program was taught in English, and most students were foreigners living in France. They participated in the study at the end of the 10-month teaching part of the program. During that program portion, students attended all classes together daily; therefore, they all knew one another to some extent. All students shared the same consumption experience, which is a salient difference from Study 1, in which several smaller consumption groups were randomly assembled from the target population.

Procedure and material

Students received invitations to take a student satisfaction survey in the school's experimental lab. Participation was not compulsory, yet all students completed the survey and received a school t-shirt valued at €15 as a gift. The students came to the experimental laboratory in batches of 10, at scheduled hours. They were directed to cubicles, where an online questionnaire was open for them on lab computers.

After the students answered the questions about their program, they provided data about their ties with their classmates, supposedly for an unrelated study on relationships at work. As in Study 1, the tie-related data were obtained with the assistance of a roster, such that each student marked each classmate's importance as a source of advice and information. This instrumental, rather than affective, type of tie reflects the utilitarian nature of the consumable in this study. Finally, the respondents were thanked, given their gift, and requested not to discuss the survey with students who were yet to take it. The process took 25-30 min. Later, a debriefing email was sent to all the students.

Measures

The students rated the program (1 = "not satisfied at all," 7 = "very satisfied")4 in response to the query, "To what extent are you satisfied with the program" [START_REF] Oliver | Customer satisfaction research[END_REF]. As in Study 1, the dependent variable is the level of disagreement between students. Difference in satisfaction was operationalized exactly as in Study 1. To assess the strength of ties, following [START_REF] Baldwin | The social fabric of a team-based MBA program: Network effects on student satisfaction and performance[END_REF], participants answered, "To what extent do you consider this person a source of information and advice for school-related or course-related matters?" (1 = "never ask for advice," 5 = "ask for advice very often"), for all other students in the program. The difference in centrality was operationalized exactly as in Study 1. Finally, consistent with the notion that satisfaction scores can converge (diverge) as an outcome of similar (dissimilar) ratings of certain service attributes, participants had to provide satisfaction ratings on program-related attributes ("How satisfied are you with each of the following aspects of the program?"), using a 1-7 semantic differential scale (1 = "not satisfied at all," 7 = "very satisfied"). The attributes were administrative support, classroom facilities, course content, evaluation, help in job search, timely information, quality of professors, and workload (adapted from [START_REF] Gruber | Examining student satisfaction with higher education services: Using a new measurement tool[END_REF]. As in Study 1, differences in ratings then could be calculated for each dyad and each product attribute.

Results

The data, organized in square matrices (50 × 50), reflect 2540 unique dyads (49 dyads per respondent). The average tie strength was 2.44 (SD = 1.39). As in Study 1, the MRQAP confirmed the hypothesized negative interaction between difference in centrality and tie strength (β tie-strength × difference-in-centrality = -0.08, p = 0.049, Table 2). The interaction between tie strength and difference in centrality is also negative and significant when no covariates are included in the model (β tie-strength x difference-in-centrality = -0.15, p = 0.01).

We again used an OLS regression to illustrate the interaction for the different levels of tie strength. As Fig. 2 shows, the difference in satisfaction increases with the difference in centrality in weak tie conditions, as should be when reactance dominates. The difference in satisfaction instead decreases with the difference in centrality in strong tie conditions, as should be the case when conformity dominates.

Discussion

Study 2 replicates the interaction observed in Study 1, notwithstanding the dissimilarities between these studies. Therefore, the contingent role of centrality in driving either agreement or disagreement is also observed for high involvement consumables with great importance for their consumers.

Study 3

For both studies 1 and 2, the variables of interest (difference in centrality and tie strength) were not manipulated. Although the comparison of real groups in the shared condition with pseudo-groups in the solitary condition of study 1 provides some safeguards, the role of latent confounding factors cannot be ruled out completely. Our core theoretical argument is that centrality creates pressure to conform, resulting in some perceived threat to freedom, which translates either into conformity when the tie is strong or reactance when the tie is weak. Thus, the first objective of study 3 is to test that, when consumption takes place in group, centrality of the interaction partner increases the perceived threat to attitudinal freedom. Specifically, we tested that, compared to more central consumers, less central consumers will perceive a greater threat to their freedom. The second objective was to provide further evidence that centrality can backfire into reactance behaviors when the tie is weak more than when the tie is strong, a notion thatto our knowledgehas not been tested before. Specifically, we tested that less central consumers that consume with a weak tie, compared to a strong tie, will express more reactance behaviors. Last, because individuals differ in their urge to conform or dissent when facing pressure [START_REF] Santee | To agree or not to agree: Personal dissent amid social pressure to conform[END_REF], we tested the moderating effect of participants' need for uniqueness [START_REF] Tian | Consumers' need for uniqueness: Scale development and validation[END_REF].

Participants and procedure

Study 3 used a scenario-based experiment. 209 adults living in the United Kingdom (89 males, M age = 36.8 years) from the Prolific Academic website (www.prolific.ac) participated in exchange for 0.95 lb. The experiment used a 2 (centrality: low, high) × 2 (tie strength: weak, strong) between-subject design. Twenty-two participants' data were discarded because they failed to remember their centrality in the scenario and/or they reported that the name of the acquaintance (friend) listed was in fact a friend (mere acquaintance), leaving 187 participants for analyses.

All participants were first asked to provide the first name of a good friend and of a mere acquaintance living in the same city as them. They were then randomly assigned to one of 4 scenarios (Appendix A). Participants were asked to imagine that they were having lunch in a restaurant with either the acquaintance (weak tie) or the friend (strong tie) listed and two other persons that were either good friends of them, unknown to the acquaintance/friend (high centrality) or good friends of the acquaintance/friend, unknown to them (low centrality). For instance, participants in the low centrality/strong tie condition had to imagine having lunch with a friend and two other persons unknown to them but good friends of their own friend.

After reading the scenario, participants were asked to write down what they would think and feel in such a situation, they were then asked to rate their perceived threat to freedom with regard to the situation imagined and to rate their intentions to perform different behaviors, that ought to follow from reactance (e.g., leaving early), during the lunch [START_REF] Levav | Seeking freedom through variety[END_REF][START_REF] Miron | Reactance theory-40 years later[END_REF]. As markers of reactance, study 3 assessed reactance behaviors rather than disagreement in product/service evaluation following actual consumption (like in studies 1 and 2) because a scenario doesn't allow actual consumption.

Measures

Participants rated their perceived threat to freedom with regard to the situation imagined with three 10-point (not at all/extremely) scale items adapted from [START_REF] Jonas | Culture, self, and the emergence of reactance: Is there a "universal" freedom[END_REF] and [START_REF] Dillard | On the nature of reactance and its role in persuasive health communication[END_REF] ("How restricted would you feel in what you say"; "How restricted would you feel in what food you order"; "How restricted would you feel to speak your mind"; α = 0.77). Participants rated the likelihood that they would perform some reactance behaviors during the lunch 5 with five 9-point (very unlikely/very likely) scale ("Try to leave early"; "Spend a great deal of time checking my smartphone"; "Pay more attention than usual to things going on at other tables"; "Go to the restroom for no reason"; "Spend a great deal of time looking up at the TV screen in the restaurant"; α = 0.89). Participants need for uniqueness was 5 These behaviors were identified in a pretest performed on 171 other participants from the same website. assessed with the short form from [START_REF] Ruvio | Consumers' need for uniqueness: Shortform scale development and cross-cultural validation[END_REF], that included twelve 5-point scales (e.g., "I actively seek to develop my personal uniqueness by buying special products or brands"; α = 0.88).

Results

A 2 (centrality) × 2 (tie strength) ANOVA on perceived threat to freedom, indicated, as expected, that participants low in centrality perceived higher threat to their freedom (M Low Centrality = 5.27) than participants high in centrality (M High Centrality = 3.84, F(1, 183) = 24.74, p b 0.001). Tie strength did not have a main effect (F(1, 183) = 1.67, p = 0.20) nor did it interact with centrality (F b 1). This finding is consistent with our theorization that less central consumers feel a threat to freedom when confronted with more central members of a group.

The second step of our argumentation was that perceived loss of freedom stemming from difference in centrality triggers reactance if strength is low, i.e. if there is limited social cost to it. Thus we expected participants low in centrality consuming with a weak tie to express more reactance than participants consuming with a strong tie. For participants high in centrality, no such difference was to be expected. A 2 (centrality) × 2 (tie strength) ANOVA confirmed (marginally) the expected interaction between centrality and tie strength on reactance behaviors (F(1, 183) = 3.27, p = 0.07). For participants low in centrality, the ones consuming with a weak tie were more likely to perform reactance behaviors (M Weak Tie = 4.39) than the ones consuming with a strong tie (M Strong Tie = 3.25, t(92) = 2.79, p b 0.01). This difference was not significant for participants high in centrality (M Weak Tie = 2.33, M Strong Tie = 2.10, F b 1).

Concerning the moderating impact of need for uniqueness (NfU), we found a significant interaction between centrality and NfU on perceived threat to freedom (b = 1.39, t(182) = 3.60, p b 0.01). At low NfU (M-1SD = 1.89), we replicated the finding identified earlier that participants low in centrality perceive higher threat to their freedom than participants high in centrality (b = -2.43, t(182) = 6.14, p b 0.001). At high NfU (M + 1SD = 3.36), however, perceived threat to freedom didn't depend on centrality (F b 1). An interpretation of this finding is that, because they show higher private self-consciousness, autonomy and expressiveness in interpersonal behaviors [START_REF] Schlenker | Self-consciousness and self-presentation: Being autonomous versus appearing autonomous[END_REF], people high on NfU tend to speak their mind openly with less regard for situational pressure to conform than people low on NfU. We also tested for the impact of NfU on how centrality and strength of tie affect reactance behaviors, yet it was not significant (|t| b 1).

Discussion

Study 3 provides additional evidence in support to our view that centrality influences threat to attitudinal freedom and interacts with strength of ties to generate reactance. The manipulation of network variables, as well as the direct measurement of threat to freedom and reactance behaviors allow to better rule out alternative interpretations of the patterns observed in studies 1 and 2.

General discussion

Network centrality is often equated with power and influence in groups [START_REF] Friedkin | Norm formation in social influence networks[END_REF]; central consumers thus tend to be depicted as prominent influencers of more peripheral consumers [START_REF] Lee | The role of network centrality in the flow of consumer influence[END_REF][START_REF] Risselada | Indicators of opinion leadership in customer networks: Self-reports and degree centrality[END_REF]. Focusing on the context of shared consumption, the current findings suggest that this social asymmetry actually is a double-edged sword: The difference in centrality exerts some conformity pressure on the most peripheral consumer in the dyad but also may result in a boomerang effect, through reactance. Perceiving a threat to attitudinal freedom, a peripheral consumer might seek to restore it by evaluating the product in a way radically different from a central consumer's evaluation. Our research identifies strength in the dyad as driving which psychological force prevails, i.e. whether conformity prevails over reactance. When the tie is strong, the cost of dissenting is high, so the pressure stemming from social asymmetry results in conformity. When the tie is weak, the cost is low, and the urge to reassert attitudinal freedom prevails.

From a managerial standpoint, while prior work tends to invariably identify central consumers as valuable influencers (Van den [START_REF] Van Den Bulte | Opportunities and challenges in studying customer networks[END_REF], our findings invite practitioners to have a more nuanced approach. Centrality indeed confers influence, yet not necessarily positive influence. The qualitynot only the quantityof central consumers' relationships should be better taken into account and their leveraging should be activated primarily to reach their strong relationships. For this purpose, perhaps the most promising trend is the increasing availability of digital traces of social interactions between consumers, in particular within online brand communities. By either participating to or orchestrating such communities, managers can access information capturing relational strength such as the rate of message exchange between any two pairs of consumers or the volume of text in their message [START_REF] Howison | Validity issues in the use of social network analysis with digital trace data[END_REF]. Similarly, recent work provides ground for predicting strength of ties between social media friends from a variety of features such as their education or the type of words exchanged in posts made on each other's walls [START_REF] Gilbert | Computing and building around tie strength in social media[END_REF]. A more intrusive method would be to collect first-hand information on relational strength directly from consumers, a practice which would be particularly relevant in referral programs. Firms could survey customers willing to send a referral about the emotional closeness, the frequency of interaction or the duration of their relationship to the target (information which can eventually be cross-checked by surveying the target too), and activate rewards accordingly.

From a theoretical stand point, the findings contribute to the network approach to consumer-to-consumer influence, by studying structural (difference in centrality) and relational (strength of ties) features of consumer networks in conjunction [START_REF] Lee | The role of network centrality in the flow of consumer influence[END_REF]. Moreover, it extends this approach to shared consumption contexts, whereas most prior studies consider only individual consumption [START_REF] Goldenberg | The role of hubs in the adoption process[END_REF][START_REF] Iyengar | Social contagion in new product trial and repeat[END_REF], with the exception of [START_REF] Lee | The role of network centrality in the flow of consumer influence[END_REF] efforts to relate centrality to opinion leadership among participants of clubs that organize social events.

Second, because the study of co-consumption has ignored network concepts (Lee et al.'s work notwithstanding), the current article also has implications for continued research in this field. Prior studies usually rule out the possibility of a preexisting social structure that could affect social influence during co-consumption, by considering groups of strangers assembled solely for the purpose of the study [START_REF] Simpson | Bringing relationships into consumer decision-making[END_REF]. The goal is to ensure that any observed interpersonal influence stems from interaction dynamics, taking place during the co-consumption, such that no relational background can act as a confounding factor [START_REF] Cruwys | Social modeling of eating: A review of when and why social influence affects food intake and choice[END_REF]. As a result, they emulate situations in which consumers "have no past or future together and … little if any investment or commitment to their fleeting 'relationship'" (Simpson et al., 2012: 330). These studies cannot describe the many instances in which co-consumption takes place with known others rather than strangers. The current studies instead suggest that a different design-using groups of participants with preexisting relationships of varying intensity-can better reflect the reality in many shared consumption contexts.

The current research also contains some limitations. For example, it demonstrates that the structural and relational features of a dyad of consumers affect the chances that they agree in their evaluation right after consuming, but what happens later on? Does an evaluation formed under the pressure of social asymmetry endure, after the co-presence ceases? Research should determine whether agreements reached in these conditions lead to significant outcomes in the longer term, such as purchase, usage, or customer retention [START_REF] Haenlein | Social interactions in customer churn decisions: The impact of relationship directionality[END_REF].

The design of Study 1 prompted participants to share precisely for the purpose of evaluating (i.e., to test a hypothetical market launch), so it is not perfectly generalizable to shared experiences involving food in the real world. Groups usually snack in conjunction with another social activity, such as watching a movie, playing a game, or exchanging small talk, and the end purpose is not to form a specific evaluation of the product. Further research might adopt designs similar to that of Study 1 but make the product to be evaluated less central to the consumption episode.

Regardless of their network position, consumers possess different traits that might affect their urge to follow others' evaluations as well as their intentions to influence others [START_REF] Santee | To agree or not to agree: Personal dissent amid social pressure to conform[END_REF]. Accordingly, we found in study 3 that need for uniqueness moderated the impact of centrality on perceived threat to freedom. Further research could investigate the role of other traits. For example, Gentina and colleagues found that the relationship between centrality and opinion leadership was moderated by the susceptibility to normative influence [START_REF] Gentina | Unique but integrated: The role of individuation and assimilation processes in teen opinion leadership[END_REF].

Lastly, it would be interesting to study whether our results generalize to other (non-western) cultures. Prior work found that the expectations of individual freedom of choice are higher in individualistic compared with collectivistic cultures, thus resulting in higher propensity for reactance in individualistic cultures [START_REF] Miron | Reactance theory-40 years later[END_REF][START_REF] Steindl | Understanding psychological reactance. New developments and findings[END_REF]. Collectivists should thus manifest less reactance, hence less disagreement in our context. Another pattern related to culture worth investigating is that reactance depends on whether the threat affect the individual alone or the reference group at large [START_REF] Jonas | Culture, self, and the emergence of reactance: Is there a "universal" freedom[END_REF].

Acknowledgment

This work was supported by Agence Nationale de la Recherche [ANR-11-SOIN-0001].

Appendix A. Appendix

A.1. Low centrality scenario

It is a day off. You are about to go for lunch at a nearby restaurant and on your way you meet (name of friend/acquaintance provided) with two friends that are also looking for a place to eat. You decide to go for lunch together.

You have never met the two friends of (friend/acquaintance) before, so (friend/acquaintance) is the only one who knows everybody in the group.

One of the friends of (friend/acquaintance) suggests a new restaurant that just opened nearby and you all agree to try it out. The restaurant is quite busy but the waitress finds you a table that is well located. The dishes are very appetizing.

A.2. High centrality scenario

It is a day off. You are with two close friends and are about to go for lunch at a nearby restaurant and on your way you meet (name of friend/ acquaintance provided) who is also looking for a place to eat. You decide to go for lunch together.

(friend/acquaintance) has never met your two friends before, so you are the only one that knows everybody in the group.

One of your friends suggests a new restaurant that just opened nearby and you all agree to try it out. The restaurant is quite busy but the waitress finds you a table that is well located. The dishes are very appetizing.

Fig. 1 .

 1 Fig. 1. Relationship between difference in centrality and difference in satisfaction as a function of tie strength (study 1, shared condition, satisfaction was assessed on a 1 to 10 scale).

Fig. 2 .

 2 Fig. 2. Relationship between difference in centrality and difference in satisfaction as a function of tie strength (study 2, satisfaction was assessed on a 1 to 7 scale).

Table 1

 1 MRQAP regression results for dyadic difference in satisfaction (study 1, shared condition).

	Variable	Model 1			Model 2
		b	β	p	b	β	p
	Tie-strength	0.14	0.19	b0.01 0.27	0.38	b0.01
	Δ centrality	0.01	0.01	0.40	0.09	0.19	0.02
	Δ sweetness	-0.19 -0.18 b0.01 -0.18 -0.17 0.01
	Δ spiciness	0.17	0.13	0.02	0.18	0.13	0.02
	Δ saltiness	-0.05 -0.03 0.30	-0.06 -0.04 0.26
	Δ fattening	0.23	0.17	b0.01 0.22	0.16	0.01
	Tie-strength × Δ centrality				-0.04 -0.25 0.01
	R 2		0.11			0.13

Table 2

 2 MRQAP regression results for dyadic difference in satisfaction (study 2).

	Variable	Model 1			Model 2
		b	β	p	b	β	p
	Tie-strength	0.01	0.02	0.32	0.05	0.06	0.11
	Δ centrality	-0.01 -0.03 0.32	0.01	0.03	0.31
	Δ classroom facilities	0.06	0.08	0.05	0.06	0.08	0.04
	Δ course content	0.33	0.36	b0.01 0.33	0.36	b 0.01
	Δ evaluation	0.12	0.14	b0.01 0.12	0.14	b 0.01
	Δ help in job search	0.01	0.01	0.40	0.01	0.01	0.41
	Δ administrative support	0.05	0.07	0.05	0.05	0.07	0.06
	Δ information on time	0.01	0.01	0.46	-0.01 -0.01 0.52
	Δ quality of professors	0.10	0.12	0.02	0.10	0.12	0.02
	Δ workload	0.04	0.04	0.13	0.04	0.04	0.12
	Tie-strength × Δ centrality				-0.01 -0.08 0.049
	R 2		0.25			0.26

In this study we used a 1 to 7 satisfaction scale to respect the continuity with the satisfaction measures used by the business school to assess student satisfaction.


Laura Rampazzi
email: laura.rampazzi@uninsubria.it

M P Colombini

C Conti

C Corti

A Lluveras-Tenorio

A Sansonetti

M Zanaboni

Technology of medieval mortars: an investigation into the use of organic additives

Keywords: Amalfi, medieval mortars, organic additives, infrared spectroscopy, thermal analysis, microscopic analyses, gas chromatography -mass spectrometry, mortar technology

This work proposes a multi-analytical approach to determine the additives in historical mortars, the use of which is widely described in bibliographical sources, but has rarely been reported in the literature. A protocol to thoroughly analyse mortars was created (optical microscopy, X-ray diffraction, infrared spectroscopy, thermal analyses and gas chromatography -mass spectrometry). These techniques had already been carried out on samples from various sites from the Roman to the modern era, finding that additives had only been used in the mortars from the internal masonry at our sampling site: the medieval military shipyard of Amalfi (Italy). The investigations yielded information on the production technology, and FT-IR and GC-MS revealed a saccharide material-based additive in the mortars, of plant origin. The FT-IR spectra suggested the presence of a natural gum, which has been used since ancient times to strengthen the cohesion properties of mortars and their resistance to tensile stress.

INTRODUCTION

The most common components of historical mortars are known through the bibliographical sources on ancient technologies. A wide number of scientific papers have confirmed calcium carbonate as the most common binder component, together with a hydraulic binder, and usually with sand being the main aggregate phase. The use of additives in mortars has rarely been reported in papers on conservation science or the history of technology. Most of the literature focuses on the addition of organic compounds to the mortar paste for various purposes [START_REF] Sickels | Organics vs. synthetics: their use as additives in mortars, in Mortars, cements and grouts used in the conservation of historic buildings[END_REF][START_REF] Arcolao | Le ricette del restauro: malte, intonaci, stucchi dal XV al XIX secolo[END_REF][START_REF] Carbonara | Trattato di restauro architettonico, vols I and II[END_REF]). In the past few years, we have analysed a large number of historical mortars (architectural mortars and stuccoes) from various sites, dating from Roman times to the modern era [START_REF] Bugini | Classification of multi-layered plasters from St. Abbondio Cloister in Como (Italy): an analytical tool for building archaeology[END_REF][START_REF] Rampazzi | Integrated approach to the characterisation of historical mortars: the case study of St. Lorenzo Basil in Milan[END_REF][START_REF] Corti | Thermal analysis and archaeological chronology: the ancient mortars of the site of Baradello (Como, Italy)[END_REF][START_REF] Bugini | Features of a Roman clay plaster (Brixia, Lombardy, Italy)[END_REF]. The aim of our multi-analytical approach [START_REF] Rampazzi | Development of an analytical protocol for the characterisation of historical mortars[END_REF]) is to shed light on the main features of mortars; that is, the composition of the binder and aggregate fractions, the morphology and structure, the binder/aggregate ratio and the mineralogical phases. Using this approach, we have previously studied the organic phases, and in the case of Baroque stuccoes, an egg-based additive was successfully characterized by combining spectroscopic and chromatographic techniques [START_REF] Rampazzi | The stuccoes of St. Lorenzo in Laino (Como, Italy): the materials and the techniques employed by the 'Magistri Comacini[END_REF](Rampazzi et al. , 2012)). In the current study, we found organic additives in mortars from the brick bedding in the masonry at our archaeological site in Amalfi (Italy)-a medieval shipyard/ building, known in Italian as Arsenale (which is not to be confused with the English word 'arsenal').

The Amalfi Arsenale is the only surviving example of a medieval shipyard in southern Italy [START_REF] Gargano | Fortificazioni e marineria in Amalfi angioina[END_REF]. The ships and galleys were constructed in a building dating back to the ninth century, the ruins of which still remain. The grandiose Gothic construction was located on the waterfront, to the left of the 'Porta della Marina', and extended as far as the quay. It was renovated in 1240 and 1272, and was partially destroyed during a devastating sea-storm in 1343. The need to build using a mortar that was resistant to high humidity levels and sea salt spray had to be considered by the ancient builders. The need to use mortars with organic additives may have arisen due to this issue. Ten pillars divide the Arsenale into two stone brick passages (Fig. 1), with only 12 pointedarch arcades still standing out of the 22 that had originally opened out towards the sea [START_REF] Gargano | Fortificazioni e marineria in Amalfi angioina[END_REF]). The building is currently 44.60 m long and 6.65 m wide. The blocks of stone of the interior walls are joined together by a bedding mortar, which was sampled and analysed. The bedding mortars are whitish in colour and irregular in shape and thickness, the latter varying between 2 and 4 cm; the texture of the mortar is rough and the presence of organic additives cannot be discerned from its appearance.

Figure 1 The Arsenale of Amalfi: details of the stone brick passages.

This was an opportunity to shed light on the building techniques and medieval engineering, and to verify the recipe for the mortar mix of the historical masonry. Also, attempts to highlight the presence of organic substances were planned in the analytical survey.

The literature [START_REF] Sickels | Organics vs. synthetics: their use as additives in mortars, in Mortars, cements and grouts used in the conservation of historic buildings[END_REF][START_REF] Carbonara | Trattato di restauro architettonico, vols I and II[END_REF]) reports the use of additives with varying aims:

• improving the durability and hardness;

• reducing the drying shrinkage;

• increasing the resistance to traction;

• delaying/accelerating the setting time;

• facilitating adhesion on to the surface;

• stabilizing any emulsion, if present;

• increasing resistance to decay caused by freeze/thaw cycles; and

• increasing the plasticity and workability.

The use of organic compounds dates back to ancient times and became common in Europe in the medieval epoch under Byzantine influence. Pliny the Elder in his Naturalis historia and Vitruvius in his De architectura mention that the Romans commonly used additives in order to enhance the resistance of mortars [START_REF] Pliny | Natural history[END_REF][START_REF] Vitruvius | Ten books on architecture[END_REF]. The most common additives were animal glue, casein and other dairy products, beer, animal fat, linseed oil, albumen, blood and natural resins [START_REF] Sickels | Organics vs. synthetics: their use as additives in mortars, in Mortars, cements and grouts used in the conservation of historic buildings[END_REF][START_REF] Pecchioni | Le malte antiche e moderne tra tradizione e innovazione[END_REF]. Pliny mentions a special mortar called 'maltha', composed of lime slaked in wine, hogs' lard and fig juice, and designed for wet rooms, such as thermal baths [START_REF] Arcolao | Le ricette del restauro: malte, intonaci, stucchi dal XV al XIX secolo[END_REF]. Cennino Cennini, in his Il libro dell'arte, describes a mortar produced with lime putty and cheese, which adhered extremely well to stone blocks [START_REF] Cennini | Il libro dell'arte[END_REF]). In the 18th century, Francesco Milizia reports a similar mixture, with the addition of oil in order to enhance the impermeability of mortars used in aqueducts and cisterns [START_REF] Arcolao | Le ricette del restauro: malte, intonaci, stucchi dal XV al XIX secolo[END_REF]. The organic products mentioned above were used without any specific knowledge of their properties in the recipe for the mix and the performance they were able to induce. Most probably, they were simply added, making use of local resources, after which the resulting performance was observed empirically. Carbohydrate-based materials such as malt, sugar, molasses and gum arabic were also often used. Lime and sugar derivates react quickly to form calcium saccharate, which strengthens the cohesion properties of the mortars and the traction resistance, especially in a warm and dry climate and in the case of large masonry, where the lime sets slowly [START_REF] Carbonara | Trattato di restauro architettonico, vols I and II[END_REF]). Cardenas and co-workers investigated the role of polysaccharides in lime mortars also mixed with nopal [START_REF] Cardenas | On the possible role of Opuntia ficus-indica mucilage in lime mortar performance in the protection of historical buildings[END_REF]. Cactus juice was commonly added to mortars in historical Mexican buildings, to improve the performance against water penetration and cracking.

Cardenas demonstrated that the mechanical properties increased due to the formation of an interpenetrated network of cactus polysaccharide mucilage and Ca(OH)2. In the 17th century, fir pine cones were added to mortars: the tannin and sugar content of the cones ensured water resistance and the plasticity of the paste [START_REF] Arcolao | Le ricette del restauro: malte, intonaci, stucchi dal XV al XIX secolo[END_REF]. As far as modern mortars are concerned, additives are again used both to slow down and accelerate the setting time, and to increase the workability [START_REF] Pecchioni | Le malte antiche e moderne tra tradizione e innovazione[END_REF]. When added to repair mortars, cactus gum increased the mechanical performance [START_REF] Alonso | Preparation and characterisation of ancientlike masonry mortars[END_REF] and the carbonation front [START_REF] Ventola | Traditional organic additives improve lime mortars: new old materials for restoration and building natural stone fabrics[END_REF]. The plasticity and resistance of Portland cement are also increased when it is mixed with cactus extract, due to the formation of complexes with Ca(OH)2 [START_REF] Chandra | Use of cactus in mortars and concrete[END_REF]).

Despite the above-mentioned investigations, to the best of our knowledge the determination of additives in historical mortars has rarely been documented. By extraction treatment with chloroform-ethanol and infrared analysis, [START_REF] Luxan | Ancient gypsum mortars from St. Engracia (Zaragoza, Spain)characterization-identification of additives and treatments[END_REF] identified the presence of wax, resin and drying oil in ancient gypsum mortars from the St Engracia Basilica (16th century, Zaragoza, Spain). Elsewhere, HPLC analyses have pointed to the use of egg white in Byzantine lime mortars [START_REF] Kurugol | Physico-chemical, petrographic, and mechanical characteristics of lime mortars in historic Yoros Castle (Turkey)[END_REF]. Kuckova identified proteinaceous compounds in Romanesque mortars by peptide mass mapping and mass spectrometry analyses [START_REF] Kuckova | Application of peptide mass mapping on proteins in historical mortars[END_REF]. The same authors also set up two efficient spectrometric methods to analyse protein additives in mortars [START_REF] Krizkova | Peptide mass mapping as an effective tool for historical mortar analysis[END_REF]. Sticky rice has been determined in 18th-century mortars [START_REF] Zeng | A case study and mechanism investigation of typical mortars used on ancient architecture in China[END_REF], while [START_REF] Duran | Study by thermal analysis of mortars belonging to wall paintings corresponding to some historical buildings of Sevillian art[END_REF] have highlighted the presence of cellulosic materials from fibres in mortars dating back to the 16th-17th centuries. Plant gums have been identified by ion chromatography in Macedonian tombs (Greece) wall paintings of the fourth to third centuries BC [START_REF] Colombini | Ion chromatography characterization of polysaccharides in ancient wall paintings[END_REF].

The composite nature of bedding mortars is normally investigated by an integrated protocol of analyses, since a complementary approach is strongly recommended for reliable and complete results on the binder and aggregate fractions. X-ray diffraction (XRD) analysis and optical microscopy (OM) highlight the mineralogical phases. Fourier transform infrared spectroscopy (FT-IR) spectra determine the nature of the inorganic compounds and of the organic additive. Thermogravimetric analysis and differential scanning calorimetry (TG/DSC) determine organic compounds and quantify the binder used in the preparation of the mortar, indirectly determining the binder/aggregate ratio, while gas chromatography -mass spectrometry (GC-MS) identifies the organic phases. The analyses also help to verify the state of conservation of the mortars, determining decay markers such as efflorescences and gypsum. In this survey, the abovementioned techniques constituted a multi-analytical protocol for the characterization of the mortars from the Arsenale in Amalfi.

MATERIALS AND METHODS

The samples were collected by using a scalpel, according to the indications of UNI-Normal 3/80 (CNR-ICR 1980).

Figure 2 shows the location of the samples. Small fragments of each one were analysed as polished cross-sections and thin sections.

Optical microscopy

Preliminary morphological observations were carried out on micro-fragments using a LeitzWild M420 stereomicroscope. Thin cross-sections (30 μm thick) of the samples were observed in reflected light using a Leitz Ortholux microscope with an Ultropack illuminator, and in polarized light using a Nikon Eclipse E400Pol microscope.

All microscopes were equipped with a digitalimage capture system.

X-ray diffraction

Fine powders of the samples were analysed using a Bruker AXS D8 ADVANCE diffractometer with the following conditions: generator settings 40 kV and 40 mA; fenditure divergence slit 1.0; RS 0.2 mm; R = 217.5 mm; radiation Cu-Kα, λ = 1.5418 Å; pyrolytic graphite monochromator with diffracted ray; geometry θ:θ; soller slits 2.3°; continuous scan 1°/min; Δ2θ = 0.02°; scan range 5-55°; and deep quarter sampler 200 μm.

Fourier transform infrared spectroscopy

The samples were analysed as KBr (Sigma-Aldrich FT-IR Grade) pellets using a BioRad Excalibur Series FTS 3000 FT-IR spectrophotometer and a DTGS detector, in the transmission mode (400-4000 cm-1, 4 cm-1 resolution, 16 scans).

Regarding the extraction protocol, around 50 mg of the fine powders of the samples were put in a glass test tube, completely dipped into organic solvents (around 0.5 mL) and sonicated for 15 min. The extractions were conducted using hexane (Sigma-Aldrich 95% anhydrous), toluene (Sigma-Aldrich 99.8% anhydrous) and ethyl acetate (Sigma-Aldrich 99% anhydrous), in order to extract non-polar and polar components, respectively [START_REF] Doménech Carbó | Fourier transform infrared spectroscopy and the analytical study of works of art for purposes of diagnosis and conservation[END_REF][START_REF] Derrick | Infrared spectroscopy in conservation science[END_REF][START_REF] Colombini | The characterization of paints and waterproofing materials from the shipwrecks found at the archaeological site of the Etruscan and Roman harbour of Pisa (Italy)[END_REF]. The test tubes were centrifuged for 3 min. The soluble fraction was placed on a NaCl plate and analysed in the transmission mode after gentle evaporation of the solvent. Solvent blanks were prepared to check for contaminations. Extractions with deionized water (MilliQ Millipore grade) were also carried out. The soluble fraction was placed on a watch glass and left to evaporate. The residue was analysed as KBr pellets.

Thermal analysis

Determinations were performed using a NETZSCH STA 409 PC. Samples were placed in alumina crucibles, and the temperature programme ranged from room temperature to 900°C, at a heating rate of 10°C min-1 under a pure nitrogen atmosphere. Raw data from thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) were corrected on the basis of a background curve.

Gas chromatography -mass spectrometry analysis

Various selected samples were analysed according to a complex analytical procedure that characterizes all the organic compounds from the same microsample, avoiding interferences from inorganic media [START_REF] Lluveras | GC/MS analytical procedure for the characterization of glycerolipids, natural waxes, terpenoid resins, proteinaceous and polysaccharide materials in the same paint microsample avoiding interferences from inorganic media[END_REF]). The system used was a 6890N GC System Gas Chromatograph from Agilent Technologies, coupled with a 5975 Mass Selective Detector (also Agilent Technologies) with a single quadrupole mass spectrometer equipped with a PTV injector. The mass spectrometer operated in the electron impact (EI) positive mode (70 eV). The MS transfer line temperature was 280°C, the MS ion source temperature was kept at 230°C and the MS quadrupole temperature was 180°C. The samples were digested in a Milestone MLS-1200 MEGA microwave oven. The acidic hydrolysis of proteins was performed using powers of 250 W for 10 min and 500 W for 30 minutes, with 30 mL of 6N HCl at 160°C for 40 min. The acidic hydrolysis of polysaccharides was performed using a power of 500 W for 20 min, at 120°C, with 500 mL of TFA 2M. Saponification was performed using a power of 200Wfor 60 min at 80°C, with 300 mL of KOH in EtOH 10 wt%. The detailed analytical procedure and operating conditions are described elsewhere [START_REF] Lluveras | GC/MS analytical procedure for the characterization of glycerolipids, natural waxes, terpenoid resins, proteinaceous and polysaccharide materials in the same paint microsample avoiding interferences from inorganic media[END_REF]). The detection (LOD) and the quantification (LOQ) limits of amino acids, aldoses, uronic acids and fatty and dicarboxylic acids were calculated at a statistical significance level of 0.05 and used for the quantitative evaluation of each fraction. The LODs of the proteinaceous, lipid and saccharide materials were 0.35, 1.22 and 0.64 μg, respectively.

The LOQs of the proteinaceous, lipid and saccharide materials were 0.75, 1.47 and 1.48 μg, respectively.

RESULTS AND DISCUSSION

Mortar investigation

Although our focus was on organic additives, we also made a complete characterization of the mortar. In fact, combining the analysis of the inorganic fraction with the organic added substances enabled us to better understand the design criteria for the mortar mix. The mineralogical characterization of the mortars was performed by comparing the results achieved with X-ray diffractometry (Table 1) and polarized microscopy observations (Fig. 3). On the basis of the XRD results, samples were selected for the preparation of thin sections. Each mortar showed a high binder/aggregate ratio, with the aerial binder mainly consisting of calcite [CaCO3], even though there was frequently a small quantity of magnesium carbonates as nesquehonite [Mg(HCO3)(OH)•2(H2O)], magnesite [MgCO3] and dolomite [MgCa(CO3)2].

The wide variety of carbonates could be ascribed to the geological features of the area, which are characterized by Liassic limestones and dolomites. The aggregate was exclusively silicatic, consisting mainly of feldspathoids as analcime [NaAlSi2O6•H2O] and leucite [K[AlSi2O6]]. These minerals are commonly found in volcanic rocks enriched in alkaline elements; in fact, they occur in the most abundant Monte Somma-Vesuvio lavas, together with augite [(Ca,Mg,Fe2+,Fe3+,Ti,Al) The occurrence of forsterite [Mg2SiO4], the magnesium-rich end-member of the olivine solid solution series, verified by examining thin sections, confirmed the use of a volcanic aggregate belonging to the Monte Somma-Vesuvio lavas for the preparation of the mortars. Although to a lesser extent, the aggregate was also characterized by more common minerals such as K-feldspars [KAlSi3O8], muscovite [KAl2(AlSi3O10)(F,OH)2] and quartz [SiO2].

A10 +++ + - + ++ - - - A11 +++ +/- - - - - ++ + A12 +++ ++ - + - - - + A13 ++ +++ - + - - - + A14 +++ - - +++ - - + + A15 +++ + - ++ - - - + A16 +++ - - - - - - +/- A17 +++ ++ - +++ + - - +++ A18 +++ ++ - + - - - +++ A19 +++ + - ++ - +++ - - A20 +++ ++ - - - + - +++ A22 +++ ++ + + - + - ++ A23 +++ +/- + ++ - + - ++ A24 +++ + - ++ + - - ++ A25 +++ + - ++ - - - ++ A26 +++ + - ++ - + - ++ A27 +++ ++ - + + - - ++ A28 +++ + - ++ - - - - A29 +++ - + + - ++ - - A30 +++ + - ++ - - - -
Two granulometric fractions of aggregate were measured: quartz and feldspars have a finer granulometry, ranging from 0.1 to 0.3 mm; feldspathoids and forsterites show a coarser size, from 1 to 4 mm. Neither natural pozzolan nor other hydraulic additives have been detected. Some decay products were determined, such as gypsum [CaSO4•2H2O] and halite [NaCl]. The first is probably caused by the sulphation process of the lime binder fraction; the latter is clearly due to the close proximity to the seashore. The pattern of XRD and the observations of the thin sections did not provide any evidence of organic substances in the mix.

The detection of quartz in the pillar mortars proves that in these samples. another kind of aggregate (sand) was used together with the volcanic fragments. In fact, it is well known that the co-presence in rocks of quartz and feldspathoids is not possible. The use of a sand aggregate was probably due to the need to enhance the mechanical resistance of the mortar. Infrared spectra confirmed the XRD results as far as calcite and silicates are concerned. Analyses by medium infrared spectroscopy can rarely determine the silicate family; thus it is not possible to discriminate between the different phases highlighted with the mineralogical investigations. The presence of calcite was suggested by the asymmetric C-O stretching band between 1430 and 1440 cm -1 and by the absorbances at 874 (out-of-plane bending vibration), 713 (in-plane bending vibration), 1798 and 2514 cm -1 (combination modes). Silicates present bands in the regions of 1200-900 and 500-400 cm -1 , due to their SiO4 tetrahedra. Gypsum was determined in low amounts only in a few samples, which

showed characteristic peaks at 671 and 600 cm -1 (S-O bending bands). Some organic matter was found in all the samples, in most cases just as traces, with peaks in the regions around 2800-2900 cm -1 -that is, C-H signals-and around 1650 cm -1 . This suggested the presence of the carbonyl function, which is normally in the range 1680-1820 cm -1 , but which could have been lowered by conjugation effects or by the presence of a calcium salt, probably calcium saccharate. We thus made further analyses in order to identify the organic compound. A preliminary FT-IR survey on organic solvent-soluble portions of the most meaningful samples was designed, so as to focus on the organic family for GC-MS analyses.

Investigation of organic fraction

Infrared spectroscopy (FT-IR) All the samples were extracted using various organic solvents, as described in the 'Materials and Methods' section, and the residues were analysed using a FT-IR spectrophotometer. Analyses of residues after extraction with hexane and toluene presented no absorbance peaks, except for not yet evaporated solvent.

In contrast, spectra recorded after ethyl acetate extraction showed peaks typical of organic compounds. Generally, the C-H bond is suggested by stretching and bending absorbance signals, respectively, at 2921 and 2854 cm -1 . The stretching absorbances of C=O and C-O bonds were observed around 1635 cm-1 and 1230 cm -1 , respectively. Figure 4 shows the spectrum of sample A27 after extraction with ethyl acetate. Some peaks are due to residues of the inorganic fraction; that is, calcite and silicates. Most samples left no residues after extraction with hot water, except for samples A7, A9, A15 and A23, the absorbance peaks of which are summarized in Table 2. The FT-IR spectra confirmed aliphatic C-H stretches around 3000-2800 cm -1 and group C=O stretching absorption around 1650 cm -1 . Figure 5 presents the spectra recorded after extraction of sample A23 with water and ethyl acetate. (1421, 874 and 712 cm -1 and organic compounds (2926, 2854, 1638, 1366, 1234, 1075 and 920 cm -1 ). The comparison with reference standards of aged organic binders and the spectra found in the literature [START_REF] Derrick | Fourier transform infrared spectral analysis of natural resins used in furniture finishes[END_REF][START_REF] Cui | Characterisation and properties of Acacia senegal (L.) Willd. var. senegal with enhanced properties (Acacia (sen) SUPERGUM™): Part 4. Spectroscopic characterisation of Acacia senegal var. senegal and Acacia (sen) SUPERGUM™ arabic[END_REF] Due to natural and anthropic decay, only very low amounts of organic matter are generally recovered in ancient works of art. Thus, only the samples (A7, A8, A11, A15, A23 and A27) that after extraction with ethyl acetate showed the strongest diagnostic peaks belonging to organic compounds-for example, signals around 2900, 2800, 1730, 1630, 1230 and 1070 cm -1 -were selected for GC-MS analyses, in order to have the best chance of determining their nature.

Gas chromatography -mass spectrometry analysis (GC-MS) There was no proteinaceous or lipid material in any of the samples, which showed a content below the limit of detection (LOD) of the procedure; however, the saccharide content was above the LOD of the procedure. The chromatogram of the saccharide fraction of sample A15 is presented in Figure 6, as an example of the chromatographic profiles obtained from the samples. Table 3 shows the relative percentage sugar composition, as well as the saccharide content, calculated as the sum of the nine sugars quantified, for all samples. The glycoside profiles reveal several sugars, suggesting that a polysaccharide material could be present in the samples. The absence of glucose and of peaks corresponding to ketose sugars enables materials such as starch to be ruled out as the additive added to the mortars. However, neither the quantitative nor the qualitative profiles are in agreement with those in the literature for reference materials (Lluveras-Tenorio et al. 2012a). Given the high content of inorganic material, responsible for the modification of sugar profiles (Lluveras-Tenorio et al. 2012b), the low percentage of organic material in each of the samples in terms of the total amount of sample analysed (around 0.01%) and the absence of other organic materials (e.g., proteinaceous) that could contribute to the saccharide profile (Lluveras-Tenorio et al. 2012b), it is not possible to identify the source of the saccharide material present in the samples.

However, as both aldoses and uronic acids were present, the glycoside profiles seem to point to the presence of a polysaccharide material of plant origin. Thermogravimetric analysis (TGA) Thermal analysis may also reveal the presence of natural gums, by the weight losses around 90°C and 320°C, respectively due to water loss and polysaccharide depolymerization [START_REF] Zohuriaan | Thermal studies on natural and modified gums[END_REF]. The investigation was carried out on the same samples as analysed by GC-MS, confirming the presence of gum.

In particular, a weight loss was observed around 340°C. The signal was weak, as the organic fraction was expected to be a minor component of the mortar sample.

Figure 2

 2 Figure 2 The location of the mortar samples .

Figure 3 A

 3 Figure 3 A thin section micrograph of the mortars observed in crossed polarised light: (a) sample A29; (b) sample A14. The main components of the mortar aggregates are feldspathoids, the coarse grains clearly visible in (a); olivine, the crystal with high interference colours in the upper part of (b); quartz and feldspars, with their low order interference colours, visible both in (a) and (b) (bar = 1 mm).

Figure 4

 4 Figure 4 An FT-IR (infrared spectroscopy) transmission spectrum of sample A27 after extraction with ethyl acetate, showing the signals of calcite(1421, 874 and 712 cm -1 and organic compounds (2926, 2854, 1638, 1366, 1234, 1075 and 920 cm -1 ).

  suggested that the samples obtained after extraction with water and ethyl acetate might contain polysaccharides. The strong bands around 3300 cm -1 , ascribed to O-H group stretching, and around 1080 cm -1 , due to the C-O fingerprint, are particular features of polysaccharides and usually have the same intensity. Other clues are the weak signals around 2800-3000 cm -1 (methyl and methylene symmetric and asymmetric stretching modes) and the moderately strong carbonyl signal at 1620 cm -1 , assigned to the carboxylic group, and in particular the asymmetric stretching mode. Although some bands might be hidden by the calcite bands, most of the typical signals are clearly evident. A study of the literature enabled us to recognize a plant gum, possibly gum arabic. The peaks around 1070 cm -1 , often present in the samples, could be assigned to β-(1→6) and β-(1→3)-linked-galactan; that is, polymerized galactose, the basic unit of gum arabic[START_REF] Renard | Acacia senegal gum: continuum of molecular species differing by their protein to sugar ratio, molecular weight, and charges[END_REF]. Also, the peaks around 1420-1450 cm -1 , present in some cases, probably indicate COO-asymmetric stretching of the compound. The aging of gum arabic initiates a new, weak absorbance at 1730 cm -1 , due to oxidation reactions, which was sometimes observed in the samples[START_REF] Caruso | Caratterizzazione ed invecchiamento di leganti pittorici a base di gomme vegetali[END_REF].

Figure 5

 5 Figure 5 An FT-IR (infrared spectroscopy) transmission spectrum of sample A23 after extraction with ethyl acetate (above) and water (below), showing the signals of organic compounds around 2920, 2852, 1634, 1385 and 918 cm -1 .

Figure 6 A

 6 Figure 6 A chromatogram in single ion monitoring (SIM) of the saccharide fraction of sample A15. IS, internal standard mannitol.

  Figure 7 shows the TG/DSC curve of sample A23. The differential scanning calorimetry (DSC) thermogram shows a peak around 300°C, which may be associated with the exothermic transition typical of the depolymerization of natural gums (Zohuriaan and Shokrolahi 2004).

Figure 7 A

 7 Figure 7 A thermogravimetric analysis and differential scanning calorimetry (TG/DSC) thermogram of sample A23, showing the DSC peak of depolymerization of natural gums at 340°C. Weight losses at 67, 410 and 712°C are due to water loss, organic compounds and CaCO3 decomposition, respectively. Continuous curve, DSC; long-dashed curve, TG; dotted line, first derivative of TG curve.

  2(Si,Al)2O6] and olivine [(Mg,Fe)2SiO4].

	Sample label Calcite Mg-Carbonates Quartz Feldspathoids K-feldspars Muscovite Gypsum Halite
	A1	+++	+	-	++	-	+	-	+++
	A2	+++	-	-	+	++	++	-	+++
	A3	+++	++	-	+	-	-	-	++
	A4	+++	-	-	+	+	-	+	-
	A5	+++	+	-	+	+	-	-	+
	A7	+++	+	-	+	-	+	-	+
	A8	+++	+/-	+	+	-	+	-	+
	A9	+++	+	-	++	-	-	-	-

Table 1

 1

X-ray diffraction semiquantitative data. [Legend: +++ very abundant (>40%); ++ abundant (15-40%) : + present (3-15%); +/-scarce (<3%)].

Table 3

 3 Relative percentage sugar composition of samples quantified.

	Sample label	Xylose	Arabinose	Rhamnose	Fucose	Galacturonic acid	Glucuronic acid	Glucose	Mannose	Galactose	Saccharide content	(μg)
	A23	33.1	15.9	4.9	2.7	-	-	-	30.5	20.6	1.3
	A27	23.4	20.7	2.6	2.6	-	3.7	-	20.2	26.8	1.1
	A7	16.4	24.6	10.1	4.3	-	3.0	-	21.1	20.4	0.6
	A8	35.1	34.3	3.9	2.3	-	-	-	17.6	13.0	1.2
	A11	19.8	14.1	8.8	6.0	-	-	-	20.0	19.7	1.9
	A15	14.6	6.1	3.9	2.3	-	-	-	28.7	21.0	1.1

ACKNOWLEDGEMENTS

The authors would like to thank Professor Giuseppe Gargano for valuable discussions and Dr Roberto Mauri for the sketch of the Arsenale.

CONCLUSIONS

The mortars taken from the Arsenale were investigated using both mineralogical and chemical techniques. The mineralogical techniques revealed the minerals were present both in the aggregate and in the binder fractions. The results identified an aerial mortar, where the aggregate phase was made up of volcanic rocks. The presence of halite was due to the proximity to the seashore. The FT-IR analyses were important in determining the organic components and suggesting the presence of a saccharide material, possibly a natural gum. FT-IR confirmed its role as a rapid determination method for the preliminary screening, since the results influenced the choice of confirmatory sensitive analytical techniques, such as GC-MS and TGA. The GC-MS results enabled us to rule out the presence of lipid and proteinaceous materials and to confirm the presence of a saccharide material in all the analysed samples: the glycoside profiles were in agreement with the presence of a polysaccharide material of plant origin. The TGA investigations also revealed the presence of a natural gum, thus indicating the use of organic additives to strengthen the mixture and confirming the mortar recipes for the historical masonry. This study demonstrates that the presence of organic additives can be detected even if the mortar is rough and no clues are perceivable in its texture, due to its rough appearance. In fact, very often there are organic additives when a smooth and partially glossy surface is visible. This is often the case for stuccowork or mortars applied as a final render. Thus, evidence of organic additives in a mortar bedding is considered quite unique.