Johanna Salvant
email: johanna.salvant@culture.gouv.fr

Etienne Barthel

Michel Menu

Nanoindentation and the micromechanics of Van Gogh oil paints

Understanding the mechanical properties of ancient paintings is a major issue for conservation and restoration. One strategy is to measure the mechanical properties of reconstructed paints: however the aging process is poorly known, so it is also desirable to measure mechanical properties directly on ancient paint samples. Using nanoindentation, we have characterized submillimetric samples recovered from restoration of two Van Gogh paintings and compared the results with reconstructed paint samples. We demonstrate that the reduced modulus and hardness of historical paints can be measured at a very local scale, even differentiating between each paint layer. Our reconstructed paint samples exhibit elastic moduli comparable to literature values, but the values measured on the two 19 th century paint samples are found to be significantly larger. Similarly, the compositional dependence of the elastic modulus is consistent with literature results for our reconstructed samples while our preliminary results for ancient samples do not readily fall into the same pattern. These results all point out to a significant impact of long term aging, in a manner which is difficult to predict in our present state of understanding. They demonstrate that nanoindentation is a very adequate tool to improve our knowledge of art paint mechanics and aging.

Introduction

In the field of conservation and restoration, a significant concern is to master the mechanical response of paintings in order to reduce mechanical damage and even correct deformation. However an oil painting is a system of great complexity: it consists of an organic support (canvas, wood) on which a thick coating has been applied. This coating typically comprises several layers, such as size, ground, colored oil paint and varnish, while the colored layer itself is often a multilayer. Understanding the mechanical behavior of such oil paintings is a daunting task. Here we focus our study on the paint multilayer itself. In the life of a painting, a primary source of stress is the tensile residual stress resulting from paint cross-linking and drying [1,2] while additional stresses arise from variation of environmental factors such as relative humidity or temperature [3][4][5][START_REF] Mecklenburg | Proceedings of the third international conference Colour and conservation[END_REF][START_REF] Mecklenburg | Museum Microclimates[END_REF]. Stresses also result from the deformation of the support, whether deliberately applied during restoration processes (restretching, lining) or accidentally (handling). Although mechanical stresses may result in undesirable macroscopic deformation of paintings, the main stress-related problem affecting oil paintings is the cracking of the paint layer [2].

Cracking naturally occurs to relieve the tensile stresses accumulated in the comparatively rigid paint layers. If you stretch a canvas, for a given deformation, the elastic energy stored in the paint layer is proportional to its Young modulus. This stored elastic energy may result in cracks. However, if the paint material exhibits non elastic behavior, such as viscoelasticity or viscoplasticity, the stored elastic energy may also be released gradually by stress relaxation, in a damage-less process. As a result, it is necessary to measure the mechanical properties of paint layers, focusing both on the instantaneous (elastic) response and on the rheology.

Interest in the investigation of mechanical behavior of paintings has been growing since the late 1960s. Bearing in mind the complexity of the system we are dealing with, it is hardly a surprise that a great diversity of techniques has been used. In many studies, macroscopic mechanical tests have been performed on strips of reconstructed paint films. Newly-made paint was applied on supports (usually polyester films or glass plates) and left drying for a few months to a few years. Centimetric strips of free paint films were then removed from the support and characterized by tensile testing. From such studies, valuable information was gained on the influence of the type of pigments, aging of paint, temperature, relative humidity and solvents on the mechanical properties of paint films [5,[START_REF] Mecklenburg | Proceedings of the third international conference Colour and conservation[END_REF][START_REF] Whitmore | [END_REF][9][START_REF] Young | Modern Paints Uncovered[END_REF][START_REF] Fuesers | 9 th International Conference on NDT of Art[END_REF]. Effects of solvents and of relative humidity on paint properties were also investigated by thermomechanical measurements on small samples of reconstructed paint, providing information indirectly related to mechanical properties, such as the compressibility as a function of temperature or viscoelastic behavior [START_REF] Hedley | [END_REF]13]. All these results emphasize the fact that an oil paint is a slowly evolving system. Paint drying and cross-linking are indeed longtime processes and extrapolation of the results to centuries old paints is uneasy. This notion suggests that for a full understanding of the mechanics of ancient paintings [2,[START_REF] Boon | Die Malkunst -Spurensicherung an einem Meisterwerk: Ausstellungskatalog des Kunsthistorischen Museums Wien[END_REF], it is necessary to measure the mechanical properties not only of reconstructed paints, but also of ancient paints.

A few studies report the use of strips from ancient paintings or primed canvas as test samples to investigate the mechanical behavior of paintings [3,[START_REF] Mecklenburg | Some aspects of the mechanical behavior of fabric supported paintings[END_REF][START_REF] Young | [END_REF]. A more general application of these methods to ancient paintings is problematic. Since one would normally refrain from lacerating a valuable historic painting into stripes for mechanical testing, it is necessary to find a method to investigate the mechanical response of paintings on a less destructive basis. It would also be desirable to test the mechanical response of paintings on a very local basis. Indeed, for an elaborate system such as a painting, the overall mechanical response depends upon the individual responses of the substrate and the many layers in a complex way. Even for the paint multilayer by itself, complex rupture processes may arise due to elastic modulus mismatch within the multilayer, with strong impact on the crack path selection [17]. Therefore, when measuring the elastic properties of oil paints, an average effective modulus measured for the whole painted canvas or even just the paint multilayer is not enough: ideally we would need to identify the elastic modulus of each single layer.

Nanoindentation is a local method of mechanical investigation which can partly fulfill these two requirements. Nanoindentation has been developed in the 1980s and rapidly became a standard tool to investigate the mechanical properties of small size structures such as thin films and coatings, grains, small second-phase precipitates or materials involved in microscale and nanoscale devices. Reduced elastic modulus and hardness are commonly derived from nanoindentation tests [START_REF] Fischer-Cripps | Nanoindentation[END_REF]. In this respect nanoindentation has already been widely used to study modern organic composite coatings. For example, it has been used in the field of automotive paint to characterize reduced modulus and hardness of each paint layer separately [START_REF] Bertrand-Lambotte | [END_REF]20]. Mar resistance of automotive coatings could also be investigated by performing nanoscratch technique on the topcoat layer separately from the other layers [21,22].

In this paper, we have applied nanoindentation to 19 th century paint fragments recovered after restoration. Submillimetric samples from two paintings by Van Gogh were embedded in resin and polished into cross sections. We demonstrate that for the relatively thick paint layers typical for Van Gogh the reduced elastic modulus, hardness and creep properties of each individual layer can be measured with reasonable confidence. We can clearly discriminate between the mechanical responses of various layers, and establish a relation between mechanical response and layer composition. Our results also show that typical values of reduced modulus obtained on these ancient materials are considerably larger than the reduced moduli measured on modern, reconstructed samples. Our results demonstrate that nanoindentation is a suitable tool for local investigation of historic paintings since very small fragments such as recovered from restoration can be tested. From nanoindentation of cross sections we also gain the benefit that each individual paint layer can be probed. Our results suggest that longtime aging (over periods of the order of a century) has to be taken into account for the understanding of the mechanical response of paintings.

Experimental

Materials

Oil paint is primarily a dispersion of pigments in a drying oil. Pigments are very fine insoluble particles with wavelength-selective absorption resulting in their ability to color materials, usually metal oxides or metal salts. Drying oil consists of glycerol triesters of fatty acids, with a high proportion of polyunsaturated fatty acids. Most common drying oils used in paints are linseed, poppy, walnut or safflower oil. As a result of the presence of both organic and mineral components, oil paint is a complex material, bearing strong similarities with technical composites made of a polymer matrix filled with particles [23,24].

Several samples from ground and paint layers of Van Gogh paintings have been investigated. Here we report representative results from two samples. Samples VG1 (Fig. 1) and VG2 (Fig. 2) have been retrieved respectively from Portrait du Docteur Paul Gachet (La Faille number F754, June 1890, Musée d'Orsay, Paris) and La Salle de danse à Arles (F547, December 1888, Musée d'Orsay, Paris). Sample sizes (length by thickness) are 700 µm by 240 µm (VG1) and 370 µm by 65 µm (VG2) (Table 1). The samples were embedded in resin and polished, resulting in cross sections exhibiting the different layers. The cross sections have been investigated by SEM-EDX to characterize the layer thickness and the main mineral compounds present. For these two samples, Table 1 indicates the weight percent of the major chemical elements detected as a percentage of the total mineral fraction. Sample VG1 presents two off-white ground layers (layer 1 and 2) and one blue painting layer (layer 3) (Fig. 1a). The thicknesses of the individual layers range between 40 and 140 µm (Table 1). Ground layers are made of lead white (2PbCO 3 •Pb(OH) 2 ) (layer 2) or of a mixture of calcium carbonate (CaCO 3 ) and lead white (layer 1) (Table 1). The paint layer is a mixture of zinc white (ZnO), cobalt blue (CoO • Al 2 O 3 ) and lead white (layer 3). Sample VG2 is a fragment of one single layer (Fig. 2). The yellow pigment is chrome yellow (PbCrO 4 ).

Samples from modern reconstructed oil paints were also investigated. Reconstructed lead white and zinc white oil paint have been made by grinding pigment powders in commercial purified linseed oil (Laverdure, huile de lin Catégorie A) with a glass muller on a frosted glass plate. The minimum amount of linseed oil possible for each pigment to obtain a workable paste was used to grind pigments. The respective weight of pigment powder and of linseed oil used to make 100g of each paint are indicated in Table 1: lead white paint (LW) contains 91 weight percent (% wt) of pigment and zinc white paint (ZW) 84 wt% of pigment, corresponding respectively to 58 % and 47 % volume concentration of pigments in the oil/pigment mixtures (Table 1). The paste has been applied on commercially primed canvas. After few months of drying, the reconstructed paint samples have been embedded in resin and polished. Samples LW and ZW are similar in thickness (respectively 350 µm and 185 µm) to the ancient Van Gogh paint samples, but are larger in length (respectively 1200 µm and 1960 µm).

Characterization of the mechanical properties

Nanoindentation testing was performed with an instrumented nanoindenter XP (MTS Nano Instruments, TN, USA), using a diamond Berkovich tip. A continuous measurement of the force, penetration and contact stiffness at 45 Hz was performed while loading at a rate of 0.05 s -1 . To probe creep, the load was held constant at its maximum value for 60 seconds before unloading (Fig. 3). Reference measurements were performed on silica as standard. On each location, two to four series of 5 to 10 indents were carried out. The recorded measurements were converted into reduced elastic modulus E*=E/(1ν 2 ) and hardness using the Oliver and Pharr (1992) method [25]. Here E is the Young modulus and ν the Poisson ratio. The tip shape was modeled as a sphere terminated cone, following Loubet et al. (1993), with an estimated sphere radius of 20 nm [26]. Note that the exact value of the tip radius does not impact the following results which are taken at larger penetrations. As far as individual positioning of the indents is concerned, position calibration of the translation stage was carried out before measurement. For such minute samples, optimum positioning is needed and due to long distance drifts, calibration of the position should be implemented close to each individual sample, and not only for the positioning stage as a whole. After the measurements, the samples were observed by optical microscopy in order to assess the precise location of the indents in the multilayers.

Indentation methodology

The characteristics of the samples (at the scale of the layer) and of the materials (at the scale of the pigment) further determined our choice of indentation parameters. Figure 1b shows the cross-section of sample VG1: the grain size of the pigments is typically in the range of 0.5 to 30 µm. As a result we are faced with two conflicting constraints: 1) the total area available is strictly limited, due to the small size of the samples and some fraction of this area should be avoided, for instance close to a large inclusion; 2) the paint layers exhibit strong heterogeneity. This material heterogeneity and the limits of the polishing of the surfaces make it impossible to measure at very low depths. In fact, typical indentation curves exhibit significant "overshoots" at low penetrations (Fig. 4), which signal a modified surface, possibly due to polishing. Beyond a few hundred nanometers, fortunately, the indentation curves usually converge to a plateau (Fig. 4). Therefore the targeted penetration depth should be as large as possible, and a minimum of 2 µm seems recommended. However large penetrations result in wide indents, occupying more of the specimen area. In order to be able to pack a significant number of indents in the limited space available, we decided to relax the usual rule that indents should be spaced typically three times their width, and chose a spacing of 15 µm for our 2 µm deep indents. We therefore expect some degree of overlap between the deformation fields, which may affect the accuracy of our results in a manner which is difficult to quantify, but also we expect this effect to be limited with regard to the error incurred due to heterogeneity of the samples. Indeed no significant deviation was observed when using reduced spacings for the reconstructed samples.

Results and discussion

Results: reproducibility, resolution and distribution

After identification of each indent by optical microscopy (Fig. 2), the values of reduced modulus and hardness obtained for those indents which were found to lie well inside a given layer were used to compute the average mechanical response of each layer (Table 2). Indents found to lie on or within one indent width from a layer boundary have been discarded. We notice that a significant mechanical contrast between the various samples is observed. For the ancient samples investigated in this study, the average reduced moduli range between 4.0 and 13.6 GPa, depending on the nature of the layer investigated. The hardness is comparatively low with mean values ranging between 0.1 and 0.44 GPa. Despite the heterogeneity of the layers, the reproducibility of the results appears satisfactory in view of the computed standard deviations (standard deviation for the reduced modulus: 0.6 -1.6 GPa; standard deviation for the hardness: 0.03 -0.09 GPa). We also observe that strong differences in reduced modulus between different layers can be identified with confidence: in sample VG1, the reduced moduli of the first two layers (layer 1 and 2) are around 12-13 GPa, while the reduced modulus of layer 3 is half lower, around 6 GPa. In addition, a sharp contrast is observed between the reconstructed paint samples and the 120 years old samples from Van Gogh paintings. The reduced moduli of the former range between 0.5 and 2.1 GPa, to be compared to the 4.0 to 13.6 GPa observed in the latter.

Beyond the direct measurement of reduced modulus and hardness, some information can be gathered on time-dependent mechanical processes. Indeed, creep has been observed on all the paint samples tested. On figure 5, the displacement into the surface is plotted against time during the hold period for the reference (silica) sample, for layer 2 of sample VG1 and for reconstructed sample LW. While silica exhibits negligible creep as expected, the modern sample is characterized by a significant time dependent response. It is interesting to note that the ancient samples also experience creep of the same order of magnitude. We have found that such a time dependent response is typical of all the ancient paint layers studied here. These results demonstrate that despite their significantly larger reduced modulus, these paints still flow, even after drying for over a century.

Discussion

The differences in reduced modulus and hardness observed for the various samples from ancient or reconstituted paints result from a complex combination of factors, pertaining to the matrix, the pigment and to their interaction. However two major parameters can be stated. On the one hand, the filler content will obviously affect the final mechanical properties after drying. Unfortunately the filler content is often difficult to assess especially for small samples of ancient paint. Also we may expect that during paint formulation, the pigment content has been chosen based on both optical and rheological requirements for the application of the paint layer, and not on the basis of final mechanical properties. Indeed we found that the useable ranges are rather narrow in practice, and limited variations of pigment content should be expected. On the other hand, the mechanical properties of the composite will also depend upon the "dried" cross-linked matrix. This oil paint "drying" process is the result of autoxidation and subsequent polymerization of the unsaturated fatty acids groups in the triglycerides [27]. The process of autoxidation, the insertion of dioxygen within the unsaturated fatty acids, results in the formation of reactive hydroperoxydes, which can be involved in crosslinking reactions. This sensitive process can obviously be affected by a number of factors, and this is likely the most sensitive parameter to be considered in the understanding of the mechanical properties of paints.

The results we obtained for reconstituted samples are comparable in magnitude to literature results obtained by tensile measurements on free paint films. By both techniques, moduli are found to range between 0.5 and 2 GPa. We also note that our reconstructed zinc white sample ZW exhibits significantly higher reduced modulus than reconstructed lead white LW (2.1 vs. 0.5 Gpa). Zinc white paint is usually described as a very brittle paint compared to paints based on other pigments. Indeed, Mecklenburg et al (2005) previously reported that zinc white oil paint is a stiff and brittle paint, with a very high Young modulus (1.7 GPa), significantly higher than lead white paint (Young modulus of 0.3 GPa) [5]. Our nanoindentation results are therefore fully consistent with the tensile test results. Mecklenburg et al (2005) and Fuesers and Zumbühl (2008) studied the impact of up to 14 year aging on the mechanical properties of free paint films [5,[START_REF] Fuesers | 9 th International Conference on NDT of Art[END_REF]. No saturation of the mechanical response is observed over this long aging time so that extrapolation to centuries is difficult. Direct comparison with the mechanical response of the present ancient samples is made difficult because the pigment contents were not specified in the earlier studies. It is unlikely however that the significant difference (for example from ca 0.5 GPa to ca 11-14 GPa for lead white) could be due to the difference in pigment content alone, all the more so as we were careful to investigate the highest possible pigment content in our own reconstituted samples. We conclude that aging over 120 years does impact the mechanical response significantly compared to a mere 15 year aging, and that this aging is primarily driven by the oil matrix cross-linking.

Within the limited range of ancient paint samples investigated here by nanoindentation, we can provide a few suggestions as to the impact of the nature of the pigments. From sample VG1, it appears that the layers which exhibit a higher reduced modulus (between 11 and 14 GPa) are characterized by a high relative content of lead white (around 55 and 90 weight percent of lead (sample VG1, respectively layers 1 and 2)). Mecklenburg et al (2005) indicate that the presence of zinc white, even as secondary pigment in lead white paint or in light color paint, often induces brittleness of the oil paint film within as early as three years [5]. It is therefore especially surprising that layer 3 of VG1, which contains a majority of zinc white and cobalt blue, together with some lead white (Zn 33 % atomic, Al 37% atomic and Pb 13% atomic of the mineral fraction) exhibits a quite low reduced modulus (6.1 GPa) in comparison to the layers from which zinc white is absent. In this respect, we note that the chrome yellow paint layer (VG2) also exhibits a low reduced modulus (4 GPa). Again, since the relative organic content is unknown for the ancient paint fragments, it impossible to draw definite conclusions but we can at least say that in terms of composition, the results we obtained on the ancient samples are counter-intuitive. Once again, we are led to conclude that the aging process, which operates over very long periods of time, is difficult to infer directly from the results obtained on reconstituted paint samples and that more measurements on ancient paint samples are needed.

Finally we would like to emphasize the significance of the creep behavior evidenced in figure 3. Creep, which is further penetration at fixed load due to rearrangement in the material, implies the possibility of stress relaxation. If the material is subjected to constant deformation, the same material rearrangement process will result in the decay of the tensile stresses. In practice, in the context of paintings, stress relaxation is therefore a process by which cracking may be avoided after intentional or accidental support deformation. It is therefore useful to understand not only the instantaneous response such as elastic modulus, but also the time dependence of the mechanical response as a function of material parameters such as --especially --pigment content and the aging of the material. In its own limited way, nanoindentation is also capable of providing such information.

Conclusions

In conclusion, the potential of nanoindentation as a local method to characterize the mechanical properties of paint layers has been explored. Small sub-millimeter samples can be used, which opens up for the investigation of material retrieved from valuable ancient paintings. We find that our nanoindentation results for reconstituted samples are fully consistent with literature data obtained by tensile testing. However, our preliminary results on ancient paintings evidence significantly larger reduced moduli, which points out to the impact of long time aging. Nanoindentation also enables the investigation of reduced modulus and hardness layer by layer within a paint multilayer, which provides for a better understanding of the micromechanics of the system. In our example, we find that the strong contrast between the reduced modulus of the two lead white based layers and the predominantly zinc white layer from sample VG1 is at variance with accepted ideas on the respective impacts of lead and zinc whites. These results further point out to the difficulty of extrapolating aging behaviors to the timescales relevant for ancient paintings. Significant creep has also been evidenced for these ancient samples, which suggests that these materials are capable of stress relaxation, a useful insight for restoration purposes.

Micrometric samples are often taken off paintings for conservation treatments and/or research (structure of paint layers, chemical composition, soap formation, etc). In this context, nanoindentation could give the opportunity to investigate the influence of various parameters on a large range of naturally aged samples for better general understanding of the mechanical behavior of oil paints and their aging over historic periods of time. More generally we can imagine a number of applications for this method. The rather complete information provided by nanoindentation could also give the opportunity to identify materials similar in reduced modulus and hardness to the original paint material if paint loss should be restored. It could also help to define the required properties of consolidating adhesives to be used for restoration. Potentially, before performing conservation treatment with solvents, its impact on the mechanics of the paint could be tested directly on a submillimetric sample with nanoindentation.

Table and figure captions

Table 1: Characteristics of the samples investigated (size of the sample, layer thickness and chemical composition).

Table 2: Reduced modulus and hardness (range and mean) obtained for the different samples investigated.

Tables and figures

Figure 1 :

 1 Figure 1: (a) Optical microscope image of sample VG1, showing the off-white ground layers (layer 1 and layer 2, applied on top of the canvas) and the blue painting layer (layer 3, applied on top of the ground layers) (b) SEM backscattering image of sample VG1 showing the three layers and their heterogeneity.

Figure 2 :

 2 Figure 2: Optical microscope image of sample VG2 after nanoindentation measurements.

Figure 3 :

 3 Figure 3: Loading -unloading measurement with a hold segment (at constant load) of 60 seconds (sample VG1).

Figure 4 :

 4 Figure 4: Reduced modulus as a function of penetration for ten measurements on sample VG2.

Figure 5 :

 5 Figure 5: Creep: displacement into the surface as a function of time during the hold period (constant load) in case of silica standard, sample from Van Gogh painting (VG1) and sample from modern reconstructed paint (LW).

Figure 2 Figure 3

 23 Figure 2

Figure 4

 4 Figure 4

Figure 5

 5 Figure 5

Table 1

 1

		Sample size :
	Van Gogh paint samples	largest length by largest thickness		Layer(s) -color, thickness (µm)		Main compound(s)	Weight percent of the main elements in the mineral fraction (SEM-EDX)
		(µm)
				Layer 1 -off white, 60	Calcium carbonate, lead white	56% Pb 22% Ca
	VG1	700 x 240	Layer 2 -off white, 40	Lead white	92% Pb
				Layer 3 -blue, 140	Zinc white, cobalt blue, lead white	40%Pb 32%Zn 15%Al 5%Co
	VG2	370 x 65	Layer 1 -yellow, 65	Chrome yellow	74% Pb 17% Cr
	Reconstructed paint samples	Sample size : largest length by largest thickness (µm)		Layer(s) -color, thickness (µm)	Pigment ground in linseed oil (manufacturer)	Weight percent of oil	Weight percent of pigment	Volume concentration of pigment (%) in paint
							Lead white
	LW	1200 x 350	Layer 1 -white, 350	(Natural Pigment, Dutch process	9	91	58
							manufacturing)
	ZW	1960 x 185	Layer 1 -white, 185	Zinc white (Kremer)	16	84	47
	Table 2
			Reduced modulus E (Gpa)		Hardness H (Gpa)	Number of
	Sample	Profile	Range		Mean	Standard deviation	Range	Mean	Standard deviation	tests in the layer
	VG1	layer 1 11.3 -17.5	13.6	1.6		0.29 -0.63	0.41	0.09	18
		layer 2 10.1 -13.6	11.6	1.0		0.34 -0.64	0.44	0.08	10
		layer 3	4.6 -9.0	6.1	1.2		0.14 -0.46	0.24	0.08	15
	VG2	layer 1	3.0 -4.7	4.0	0.6		0.04 -0.14	0.10	0.03	10
	LW	layer 1	0.3 -1.1	0.5	0.2		0.001 -0.01	0.006	0.003	33
	ZW	layer 1	1.1 -2.8	2.1	0.5		0.02 -0.08	0.04	0.015	22

Acknowledgements

We thank Y. Adda for the original suggestion, P. Lehuédé for giving us the opportunity to work together on this project, A. Distel and J.-P. Rioux for providing us with the microsamples from Van Gogh paintings, and J. Teisseire and S. Poissonnet for some initial measurements.


Keywords: secondary school, attitudes, science education, scientific literacy, quantitative research attitude towards biotechnology, quasi-experimental design

This article evaluated the impact of a four-lesson science module on the attitudes of secondary school students. This science module (on cancer and modern biotechnology) utilises several design principles, related to a social constructivist perspective on learning. The expectation was that the module would help students become more articulate in this particular field. In a quasi-experimental design (experimental-, control groups and pre-and post-tests) secondary school students' attitudes (N= 365) towards modern biotechnology were measured by a questionnaire. Data were analyzed using chi-square tests. Significant differences were obtained between the control and experimental conditions. Results showed that the science module had a significant effect on attitudes, although predominantly towards a more supportive and not towards a more critical stance. It is discussed that offering a science module of this kind can indeed encourage students to become more aware of modern biotechnology, although promoting a more critical attitude towards modern biotechnology should receive more attention.

Introduction

Background

As a scientific discipline, modern biotechnology goes hand in hand with cultural, social, and public policy controversies. The development of theories and techniques enables scientists to alter the genetic code of practically all-living organisms. Genes and gene-combinations, that control a wide variety of traits, are described. Several genetic anomalies causing disorders such as cystic fibrosis, Huntington's disease, and several types of cancer have been identified.

Biotechnological applications of all kinds are in the making and already evident in a growing range of genetically modified foods in supermarkets. Discoveries from the field of biology can fundamentally change society and human self-perception in the 21 st century.

This scientific revolution requires a scientifically literate population, meaning that people should be able to make informed and balanced decisions about scientific issues concerning their careers, their daily lives, and society as a whole (National Academy of Sciences, 1996).

Promoting scientific literacy is widely recognized as a major goal of school science education [START_REF] Millar | Twenty First Century Science: Insights from the Design and Implementation of a Scientific Literacy Approach in School Science[END_REF]. Although there is considerable agreement about the fact that science education should provide understanding, skills and values for young people to learn to cope with science in their lives, there is much uncertainty on how to achieve or improve this [START_REF] Deboer | Scientific literacy: Another look at its historical and contemporary meanings and its relationship to science education reform[END_REF][START_REF] Hodson | Some thoughts on scientific literacy: Motives, meanings and curriculum implementations[END_REF][START_REF] Jenkins | Scientific literacy and school science education[END_REF][START_REF] Kolstø | Scientific literacy for citizenship: tools for dealing with the science dimension of controversial socioscientific issues[END_REF][START_REF] Laugksch | Scientific literacy: A conceptual overview[END_REF]. Consequently, there are varying interpretations of how and what kind of abilities should be incorporated into school science curricula in order to help students become scientific literate. The question is what is important for students to know, value, and be able to do in situations involving science and technology? Current thinking about the desired outcomes of science education emphasises scientific knowledge and an appreciation of science's contribution to society. These outcomes require an understanding of important concepts and explanations of science, and the strength and limitations of science in the world [START_REF] Oecd | Scientific Literacy[END_REF]. Conceptualisations of scientific [START_REF] Millar | Beyond 2000: Science education for the future (The report of a seminar series funded by the Nuffield Foundation)[END_REF], an appreciation of the nature, aims and general limitations of science [START_REF] Jenkins | School science education: Towards a reconstruction[END_REF], to the abilities of a semi-professional scientist [START_REF] Hazen | Science matters. Achieving scientific literacy[END_REF][START_REF] Thomas | Why should we promote the public understanding of science[END_REF]. This paper follows [START_REF] Millar | Twenty First Century Science: Insights from the Design and Implementation of a Scientific Literacy Approach in School Science[END_REF] starting point in that science education should be the aspiration to include scientific literate competences that students need, to be able to live and participate with reasonable comfort, confidence, and responsibility in a society that is deeply influenced and shaped by the applications, ideas and values of science [START_REF] Millar | Twenty First Century Science: Insights from the Design and Implementation of a Scientific Literacy Approach in School Science[END_REF]. These competencies require students to demonstrate, on one hand, cognitive abilities, and on the other hand, values, motivations as they meet, and respond to socioscientific issues [START_REF] Bybee | Achieving Scientific Literacy: From Purposes to Practices[END_REF][START_REF] Holbrook | The Nature of Science Education for Enhancing Scientific Literacy[END_REF][START_REF] Kolstø | Scientific literacy for citizenship: tools for dealing with the science dimension of controversial socioscientific issues[END_REF][START_REF] Shamos | The myth of scientific literacy[END_REF][START_REF] Zeidler | Tangled up in views: Beliefs in the nature of science and responses to socioscientific dilemmas[END_REF].

Attitudes towards modern biotechnology

The purpose of science education should be helping students to be able to participate in discussions about science, to be sceptical and questioning of claims made by others about scientific matters, and to make informed decisions about the environment, their own health and well-being (in accordance with [START_REF] Driver | Establishing the norms of argumentation in classrooms[END_REF][START_REF] Goodrum | The status and quality of teaching and learning of science in Australian schools. A research report prepared for the department of education, training and youth affairs[END_REF][START_REF] Kolstø | Scientific literacy for citizenship: tools for dealing with the science dimension of controversial socioscientific issues[END_REF]National Science Council, 1996). According to [START_REF] Osborne | Science for citizenship[END_REF], this broad focus will help students to tackle everyday decisions with a science or technology dimension, such as whether to buy a tube of genetically modified tomato paste.

In this study, we examine the effects of science education on the development of stable, informed, or critical attitudes of students towards modern biotechnology, which are needed to cope with this field of research in every day life. Therefore, it is important to construct a measure that will be sufficiently sensitive to capture changes in the structure of its composition [START_REF] Millar | Twenty First Century Science: Insights from the Design and Implementation of a Scientific Literacy Approach in School Science[END_REF]. The tripartite theory of attitude [START_REF] Breckler | Empirical validation of affect, behaviour and cognition as distinct components of attitude[END_REF] [START_REF] Katz | A preliminary statement to a theory of attitude structure and change[END_REF][START_REF] Rosenberg | Cognitive, affective and behavioural components of attitudes[END_REF] provides a helpful framework in the construction of this measure of changes. In general, an attitude can be described as 'a summary of evaluations, representing favourable or unfavourable feelings towards a specific or psychological object' [START_REF] Ajzen | Attitudes and the attitude-behavior relation: Reasoned and automatic processes[END_REF][START_REF] Eagly | The psychology of attitudes[END_REF][START_REF] Weinburgh | Gender, prior academic performance and beliefs as predictors of attitudes toward biology laboratory experiences[END_REF][START_REF] Zacharia | Beliefs, attitudes, and intentions of science teachers regarding the educational use of computer simulations and inquiry-based experiments in physics[END_REF]. In this case the object is modern biotechnology.

According to the tripartite theory of attitudes, attitudinal responses can be classified into three key components; an affective, a cognitive, and a behavioural component [START_REF] Breckler | Empirical validation of affect, behaviour and cognition as distinct components of attitude[END_REF][START_REF] Eagly | The psychology of attitudes[END_REF][START_REF] Katz | A preliminary statement to a theory of attitude structure and change[END_REF][START_REF] Rosenberg | Cognitive, affective and behavioural components of attitudes[END_REF]. The cognitive as well as the affective component influence evaluations, which in turn affect behavioural intentions [START_REF] Ajzen | Nature and operation of attitudes[END_REF][START_REF] Heijs | Biotechnologie; houdingen en achtergronden [Biotechnology; attitudes and influencing factors[END_REF][START_REF] Tesser | Attitudes and attitude change[END_REF]). In the case of attitudes towards modern biotechnology, in the cognitive component, the evaluation of modern biotechnology follows from beliefs, thoughts, and knowledge of the object. The affective component of attitudes reflects how students feel about genomics, for instance anxieties and fears about this contemporary technology. Furthermore, attitude is one of the important determinants of intentions and behaviour, for example consumption or protest (theory of planned behaviour) [START_REF] Ajzen | Attitudes and the attitude-behavior relation: Reasoned and automatic processes[END_REF][START_REF] Armitage | Efficacy of the theory of planned behaviour: A metaanalytic review[END_REF][START_REF] Zacharia | Beliefs, attitudes, and intentions of science teachers regarding the educational use of computer simulations and inquiry-based experiments in physics[END_REF]. Our line of argument is that when students have a solid knowledge base on basic biological and genetic concepts, when they display an affective reaction of concern or comfort towards biotechnology issues (as opposed to an indifferent reaction), and they have comprehensible ideas on how to behave or make decisions when confronted with modern biotechnology, i.e. when students have profound attitudes, they can be considered scientific literate ('genomic literate'). In a previous study, an attitude instrument (questionnaire) was developed and a sample of 574 Dutch secondary school students were asked to answer this questionnaire in order to determine their attitudes [START_REF] Klop | An exploration of attitudes towards modern biotechnology; A study among Dutch secondary school students[END_REF]. Based on principal component analyses, a set of several independent underlying factors within the affective, cognitive, and behavioural components were found (see table 1 for descriptions). In a subsequent cluster analysis, four interpretable attitude-clusters based on that set of factors could be described, representing four different groups of students (attitude clusters).

Previous study on attitudes towards modern biotechnology

The four emerging patterns were labelled 'confident supporter' (22 % of the students), 'concerned sceptic' (18 %), 'not for me' (17 %) and 'not sure' (42 %) (See Figure 1 for a graphic representation). The 'confident-supporters' were a positive, pro-biotechnology and well-informed group of students, who seemed to welcome biotechnology in their daily lives. This group can be labelled as 'more scientifically literate', for they seemed to be well aware of scientific concepts and processes, and were able to take a clear position regarding environmental, health and personal issues. The 'concerned sceptics' were also a wellinformed group of students, and also labelled as more scientifically literate. Not only did they show a solid knowledge base on basic biological and genetic concepts, they demonstrated a sceptical, concerned, and questioning stance towards claims made about modern biotechnology as well. The smallest group, the 'not for me'-students, was very negative about biotechnology. Their beliefs and affective reactions were very negative, and unfortunately, they displayed poor knowledge and understanding of the subjects. The last cluster, the so- In other words, more than half of the 16-year-old students holds a relatively unprofound attitude towards modern biotechnology. These students had a limited knowledge base of the key concepts and principles of modern biotechnology (especially the 'not for me'-group), and unclear or poorly developed views or opinions on important social and ethical issues. They

were not sure about their intentions towards possible biotechnological applications, and were not sure what to expect of genomics in general. Even students with somewhat more knowledge on the subject (the 'not sure' group) seemed to have little awareness and showed little care about the possible impact modern biotechnology could have on society and thereby their own (future) lives. In other words, they did not use their 'scientific knowledge and ways of thinking for personal and social purposes'.

[Insert table 1 about here ]

The question is how 'scientific literacy' can be promoted in science classes; in what ways can science education encourage students to learn about (bio-) technological issues concerning society, their careers, and their daily lives, so-called socioscientific issues [START_REF] Sadler | Socioscientific issues and the affective domain: scientific literacy's missing link[END_REF][START_REF] Zeidler | The role of moral reasoning and the status of socioscientific issues in science education: Philosophical, psychological and pedagogical considerations[END_REF][START_REF] Zeidler | Tangled up in views: Beliefs in the nature of science and responses to socioscientific dilemmas[END_REF], and develop a critical opinion? In order to help young people engage in the social practice of scientists, learning contexts must be chosen so that students can make sense of it, and give them a feeling of responsibility to participate critically. However, at the level of educational practice, inspiring examples are relatively sparse. Moreover, empirical research into the effectiveness of such educational practices appears to be lacking [START_REF] Hodson | Time for action: Science education for an alternative future[END_REF]. Therefore, we decided to examine the effects of a new and innovative Dutch science module, on genomics and cancer, on students' attitudes towards genomics. By analyzing the design of the science module and the effects of the module on the different attitudinal components of the secondary school students, we hope to contribute to a greater understanding of how to support young learners in developing their need to cope with science in their lives. We will first present the structure of the science module and then make the underlying design principles explicit.

Features of a new science module

The genomics research centre of excellence (CGC)1 of the University Medical Centre Utrecht developed a new science module for the upper levels of secondary education. The socioscientific topic of the science module is genomics and cancer-research; titled 'Read the language of the tumour' ('Lees de taal van de tumor'). A so-called 'travelling DNA-lab' gives students the opportunity to meet with new and sophisticated research techniques. By giving a realistic picture of genomic-research, the module aims at students' acquisition of knowledge on the subject of genomics. Moreover, it is intended to stimulate the opinion forming and critical reflection of students towards genomics and the implications of the applications on society [START_REF] Waarlo | Biology education in a genomic world. Empowering citizens, patients and consumers[END_REF].

The science module consists of four lessons; an introductory lesson, two practical/hands on lessons (in succession), and a reflection lesson. During the introductory and reflection lesson instruction and guidance was given by the teacher him-/herself. The practical lessons, a 'DNA-lab setting' at school, was supervised by two trained students of the university.

Teachers that signed up for the science module received a detailed teacher manual and workbooks for their students. The introduction lesson included a brainstorm-session and opportunity to raise questions on the topic of cancer and cancer research. The lesson was designed to connect with students' prior knowledge of the subject matter, since students were already presumed to have at least some background knowledge and ideas about social or ethical matters relating to cancer and/or biotechnological research. After activating prior knowledge and clarifying ideas or difficulties, students were invited to discuss their questions about and experiences with cancer and cancer research in small groups first and then in the whole class.

During the second and third lessons, students had to perform an assignment in a genomics lab setting. They worked in small groups (two or three students), under the supervision of two university students. In this genomics-laboratory setting, using a hands-on approach, the students were invited to use actual 'genomic techniques'. This gave them an opportunity to visualize abstract biological concepts: observing (and in some cases, touching) preserved cancer tumours, extracting DNA from a thymus gland (calf), and demonstrating pathogenic defects in genes by carrying out a polymerase chain reaction (PCR) and gel-electrophoresis.

Combined with exploration and discussion of the relevance and complications of cancer research for patients, their relatives, and society, genomics was placed in a social and moral context.

A week after 'the lab-lessons', during the fourth lesson, the students were asked to reflect on their hands-on experiences. They had to draw conclusions from the experiments and to complete a fictional counsel form that laboratory researchers use to write down their findings and conclusions. The students were given the role of a researcher by having to give treatment recommendations to a doctor. They had to read 'non-specialist' articles on socioscientific issues (breast cancer) in class and to reflect on their own questions formulated at the introductory lesson. There was room for ethical discussions, so the experiments could be The science module utilized several design principles, which can be derived from a social constructivist perspective on learning. The metaphor of participation is often used to characterise this concept of 'learning' [START_REF] Salomon | Individual and social aspects of learning[END_REF][START_REF] Sfard | On two metaphors for learning and the dangers of choosing just one[END_REF]. In essence, social constructivist educational theories interpret learning as increasingly competent participation in the discourse, norms, and practices associated with particular communities of practice [START_REF] Lave | Situated learning: Legitimate peripheral participation[END_REF][START_REF] Wenger | Communities of practice: Learning, meaning and identity[END_REF]. Becoming a more central participant in society is not just a matter of acquiring knowledge and skills. It also implies becoming a member of a community of practice. For this to happen, learning contexts must be chosen, so that students can make sense of the subject matter and hence give them a feeling of responsibility to participate critically in the practice in question.

Over the last decade, elements of social constructivist conceptions of learning have been used in science education [START_REF] Frijters | Effects of dialogic learning on value-loaded critical thinking[END_REF][START_REF] Ogborn | Constructivist metaphors of learning science[END_REF]. In particular, the interest in how students learn to think critically about social issues increases (e.g. [START_REF] Driver | Establishing the norms of argumentation in classrooms[END_REF][START_REF] Kolstø | Scientific literacy for citizenship: tools for dealing with the science dimension of controversial socioscientific issues[END_REF]Sadler & Zeidler, 2005). Improving science education is interpreted as helping young people engaging with the social practice of scientists. Against the background of this social constructivist perspective on learning, we can describe the module 'Read the language of the tumour' in terms of five design principles: 1. Simulating active learning. Generally speaking, active learning is a process where students engage in higher-order thinking tasks such as analysis, synthesis, and evaluation. From a social constructivist point of view, the active role of learners is explicitly linked to the processes of making sense. Students are not seen as 'passive receivers' of information, but as active interpreters of social meanings. [START_REF] Ogborn | Constructivist metaphors of learning science[END_REF] advocated learning arrangements in science education in which the learner is actively involved in the integration of new experiences and information into what he or she already knows. In the module, the active contribution of students was facilitated in several ways. Throughout the module, students were encouraged to formulate and ask their own questions about cancer and cancer research.

In the brainstorm session (first lesson) they had to write down their own opinions and questions, discuss them in a small group, and afterwards within the context of a class discussion. Furthermore, active learning was stimulated by making use of authentic learning tasks (see ad 4.). [START_REF] Wells | Dialogic Inquiry. Towards a sociocultural practice and theory of education[END_REF] a class should function as a community of inquiry in which each student makes her or his own contribution. This social constructivist element is also present in science education research. A large number of studies have shown that inquiry-based science activities have positive effects on students' cognitive development, self-confidence, science achievement, attitude improvement towards both science and school, and conceptual understanding of science as a whole compared to a more conventional approach to science education [START_REF] Butts | Does participating in an undergraduate elementary science methods course make a difference[END_REF][START_REF] Gibson | Longitudinal impact of an inquiry-based science program on middle school students' attitudes toward science[END_REF][START_REF] Jarrett | Science interest and confidence among preservice elementary teachers[END_REF][START_REF] Zacharia | Beliefs, attitudes, and intentions of science teachers regarding the educational use of computer simulations and inquiry-based experiments in physics[END_REF]. [START_REF] Rutherford | Hands-on: A means to an end[END_REF] stated that 'hands-on and learning by inquiry are powerful ideas, and we know that engaging students actively (…) pays off in better learning'. One of the building blocks of the module is the assumption that the actual performance of (genomics) techniques, combined with an exploration of the social and moral implications of cancer, can positively influence scientific literacy. The students were invited 3. Using authentic tasks. Authentic tasks resemble tasks performed in a non-educational setting (real-life tasks or activities) and require students to apply a broad range of knowledge and skills [START_REF] Newmann | Five standards of authentic instruction[END_REF][START_REF] Roth | Authentic school science: Knowing and learning in open-inquiry science laboratories[END_REF]. The tasks refer to complex situations, contain open-ended, ill-defined problems and often require a multidisciplinary approach as well as collaborative work [START_REF] Ten Berge | Analyzing the design of an authentic task in higher education: teaching entrepreneurship for software products in a fictive company[END_REF]. Authentic tasks are believed to help students to become aware of the relevance and meaningfulness of what they are learning, because the tasks mirror real-life experiences and provoke active and constructive learning [START_REF] Lowyck | Constructivisme: ontwikkeling van een concept[END_REF]. Thus, besides developing knowledge, skills and attitudes, it is assumed that authentic tasks increases motivation [START_REF] Herrington | An instructional design framework for authentic learning environments[END_REF]. This makes authentic tasks particularly suitable for helping young people to engage with the social practice of scientists and stimulate scientific literacy. According to [START_REF] Grabinger | Rich environments for active learning[END_REF] science and technology components should be looked upon from students' perspectives. In the module, authentic tasks were developed around the scientific concept of genomics using issues that are meaningful in students' lives (cf. [START_REF] Goodrum | The status and quality of teaching and learning of science in Australian schools. A research report prepared for the department of education, training and youth affairs[END_REF]. The module was about cancer and cancer-research, which provides a realistic and authentic context, as almost everyone has a relative who has dealt or is dealing with cancer.

Stimulating inquiry-based learning. According to

Stimulating reflection.

From a social constructivist perspective education should aim at learning to participate in society in a critical and aware manner. Performing authentic tasks in itself does not necessarily result in such an outcome. Issues to be dealt with should be made explicit, for example through dialogue in the classroom. Dialogue is generally considered a powerful instrument for reflection [START_REF] Wells | Dialogic inquiry in education[END_REF]. Several researchers have noted the important role of reflection as a learning activity in developing scientific literacy ( [START_REF] Sadler | The morality of socioscientific issues: Construal and resolution of genetic engineering dilemmas[END_REF][START_REF] Zeidler | Beyond STS: A research-based framework for socioscientific issues education[END_REF][START_REF] Zeidler | Tangled up in views: Beliefs in the nature of science and responses to socioscientific dilemmas[END_REF]. By reflecting on thoughts, feelings and actions, students create a meaningful picture of their experience of the world, for which they will take responsibility. Empirical studies on effectiveness of science education state that science education should not only focus on knowledge and understanding, but also by reflecting on the affective and ethical side of biotechnology (for example [START_REF] Chiappetta | A quantitative analysis of high school chemistry textbooks from scientific literacy themes and expository learning aids[END_REF][START_REF] Lee | Development of an instrument for measuring cognitive conflict in secondary-level science classes[END_REF][START_REF] Wilkinson | A quantitative analysis of physics textbooks for science literacy themes[END_REF]. In this science module, in the final lesson, the students reflected on the hands-on experience by writing down their findings and conclusions. Moreover, they read articles in class and reflected on their own questions formulated during the introductory lesson. Throughout the module, the students were encouraged to engage in (ethical) discussions with their peers in order to reflect on their own experiences, feelings, and thoughts.

5. Using socioscientific subject. Finally, cancer and cancer-research encompass socioscientific issues. Issues, such as cloning, stem cell research, genetic testing, and genetically modified foods will play a significant role in 'everyday live' in the (near) future. These issues are not only of great importance to scientists; they will have great impact on the whole society and are therefore termed socioscientific issues (SSI) [START_REF] Kolstø | Scientific literacy for citizenship: tools for dealing with the science dimension of controversial socioscientific issues[END_REF][START_REF] Zeidler | Tangled up in views: Beliefs in the nature of science and responses to socioscientific dilemmas[END_REF]. An important factor of scientific literacy is the ability to negotiate these socioscientific issues and make informed decisions regarding these issues [START_REF] Sadler | Socioscientific issues and the affective domain: scientific literacy's missing link[END_REF][START_REF] Sadler | Moral and ethical dimensions of socioscientific decision-making as integral components of scientific literacy[END_REF]. In examining previous research on how these issues can be incorporated into science curricula and classroom practice, we found that most research has been done on students' reasoning about these complex issues with inherent social implications (see Sadler & Zeidler, 2005;[START_REF] Zeidler | The role of moral reasoning and the status of socioscientific issues in science education: Philosophical, psychological and pedagogical considerations[END_REF][START_REF] Zeidler | Beyond STS: A research-based framework for socioscientific issues education[END_REF]. It has been suggested that SSI are taught most effectively through argumentation in the classroom [START_REF] Conner | The significance of an approach to the teaching of societal issues related to biotechnology[END_REF][START_REF] Steele | The challenge in teaching biotechnology[END_REF]. This requires subject matter that provides a meaningful, rich source of dilemmas for students to consider, such as cancer [START_REF] Conner | The significance of an approach to the teaching of societal issues related to biotechnology[END_REF]. The science module focused on several dilemma's of 13 biotechnology relevant to the students' lives, such as family, lifestyle choices, preventive treatments, which were linked to knowledge of genetics in general as well as to biotechnology. The nature of the topic therefore provided students the opportunity to think about and discuss this socioscientific issue.

The five design principles described are derived from science education literature. The empirical basis, however, is rather weak. The research area is dominated by small-scale studies and there is a lack of experimental research in this area with regard to the effectiveness of the proposed design principles in classroom settings. The nature of most of the studies allows for limited conclusions regarding the possible effects of such a learning arrangement on attitudes. It remains unclear whether, for example, more critical attitudes towards biotechnology have been elicited, and whether they are based on a broader understanding. The combination of the design principles described here seems to promote scientific literacy, but more evidence is needed. The present study attempts to answer some of the questions left unanswered by performing a quasi-experimental study using the new Dutch science module 'Read the language of the tumour'.

Research question and hypotheses

The main purpose of this study was to investigate the effects of the science module on the development of the attitudinal aspects of students' scientific literacy towards modern biotechnology. As described before, the majority of students could be labelled as less scientifically literate on this particular field; a poor cognitive base combined with unclear The research question can be phrased as follows: What is the effect of a science module, utilising several design principles, related to a social constructivist perspective on learning, on attitudes of secondary school students towards modern biotechnology?

The following central hypothesis guided this study: The science module has a more positive effect on the development of students' attitudes than the regular science classes.

If the module was successful, the low scientific literate group has enhanced their knowledge base, as well as their awareness of genomics. Consequently, they will either move to the group of 'confident supporters' or become more critical in their opinion and move to the 'concerned sceptics group. More specifically, we expected to observe the following changes in the attitude post-test compared to the pre-test and the control group: a) a smaller percentage of students in the 'not sure' group b) a smaller percentage of students in the 'not for me' group c) a larger percentage in the 'confident supporter' group d) a larger percentage in the 'concerned sceptic' group Apart from possible changes in group-membership, we will also examine what the effects of the science module were on the different factors in each of the three attitude components. For instance, can changes be detected in scores on biotechnology knowledge (in the cognitive component (see Table 1)? We implemented a pretest -posttest experimental design to examine these hypotheses. The experimental condition consisted of students who besides their regular biology classes on genetics and biotechnology, participated in the science module.

The control condition included students who did not partake in the science module, but only followed the regular biology curriculum on genetics and biotechnology. A total of 386 students (51.5% male) from 17 classrooms (year 11-12) from ten secondary schools in the Netherlands participated in the study. Twenty-one respondents were excluded from further analysis because of incomplete pre-or post-test data, or outlier scores. Therefore, the total dataset included 365 respondents. The average age of the participating students was 16. Schools in the experimental condition were randomly selected from all schools participating in the DNA-Lab project. Schools in the control condition were randomly selected from a general list of all Dutch secondary schools. In order to correct for possible effects of background variables, we selected schools that were comparable in terms of (a) the percentage of students with immigration and religious backgrounds, (b) students' socioeconomic background characteristics and (c) the period in which the regular biology lessons on the subject of genetics was taught.

Method

Research design

Pre-and post-tests were administered to students in the experimental and the control condition. Table 2 illustrates the design of the study. Students in the experimental condition received 'practical workbooks' with explanations, instructions, and assignments. Teachers received instruction manuals, including practical instructions and teaching guidance. Students in the control condition completed the pre-/or post-test, but did not participate in the science module. These students attended regular biology lessons on the subject of genetics, which includes lessons on modern biotechnology.

For reliability reasons (see the requirements) we made a distinction between three experimental groups and two control groups. Experimental group 1 (case study) differs from experimental group 2 in the sense that in this particular group of students, in addition to the

[Insert table 2 about here ]

To determine the effects of the science module, the following requirements had to be met 3 :

1. The different groups of students needed to have the same starting point, as measured by the attitudes-pre-test.

The results of the chi-square test showed that there was no statistically significant difference between the pre-test scores for all experimental and control.

2. The possible impact of the attitudes-pre-test-experience on learning during the module, and consequently on the attitudes-post-test needed to be ruled out.

Therefore we compared the post-test scores of the experimental 2 group (pre-test, treatment, and post-test) and the experimental 3 group (no pre-test, treatment and posttest).The results showed there was no statistically significant difference between these two groups.

3. The possible intervention effect due to the researcher's presence in the case-studyclasses should be accounted for.

To exclude this possibility, we performed a chi-square test comparing the post-tests of the case study group (experimental 1) and the post-test of the experimental 2 group. The results showed no significant differences between these two different groups.

4. External incidents that affect the post-test should also be considered. For example, if geneticists found a cure for cancer by genetically modifying cells, during the time of 2 The interview and observations are described in a subsequent article of a qualitative nature.

3 Results of Chi-Square test for comparison between (scores of) experimental and control groups are available from the authors if needed. For that reason, we analysed the results of the pre-test of control group 1 with the post-test of control group 2. No statistically significant difference could be established between these two control groups.

Analyses showed that all requirements were met. Therefore, we conclude that differences between conditions, and between pre-and post-test, cannot be ascribed to design effects.

Instrument

To measure students' attitudes towards biotechnology, we used a previously developed questionnaire, based on the general tripartite theory of attitudes (see [START_REF] Klop | An exploration of attitudes towards modern biotechnology; A study among Dutch secondary school students[END_REF].

The first section of the instrument was designed to obtain (socio-) demographic information about the students (only in pre-test). The second and third parts of the instrument included four categories of items: knowledge items, cognitive evaluation items (beliefs), affective evaluation items, and behavioural intention items (see Table 1, and we refer to [START_REF] Klop | An exploration of attitudes towards modern biotechnology; A study among Dutch secondary school students[END_REF] for a detailed description of the development of the instrument). Based on principal component analyses, several distinct and independent cognitive, affective, and behavioural factors were found, as described in Table 1. Cluster analysis resulted in the four different attitudes as described previously; 'confident supporter', 'concerned sceptic', 'not sure', and 'not for me' (see figure 1).

[Insert figure 1 about here ]

Analyses

To check the central hypothesis of the study, cluster-membership of students in the pre-test were compared to cluster-membership in the post-test, and experimental groups were 18 compared to control groups. Because of the nominal measurement level of the dependent variable (cluster membership), the comparison is done using chi-square tests. This test compares the distribution of students before the module to the distribution after the module, as well as possible significant differences between the experimental and control condition.

Results

The results of the comparison of the experimental groups with the control groups are presented first. Secondly, the results regarding the post-test compared with the pre-test within the experimental groups are described. We conclude this section with an analysis of the changes concerning the attitude components.

Comparison experimental groups and control groups

Using a chi-square test, the post-tests of the experimental groups (1, 2, and 3) and the post-test of the control groups (which received no treatment) were compared. A significant difference of distribution of students in the four attitude-clusters was found between the experimental and control groups in the post-test-scores χ 2 (3, N = 348) = 9.53, p < .05 (see Table 3). The largest differences could be found in the percentage of 'confident-students' in the experimental group versus those in the control group (43.9% vs. 30.3%). and between the 'not sure-students' in the experimental group and the 'not sure's' in the control group (40.3% vs. 3). The first three hypotheses can be confirmed; 1) At the end of the science module, there were significantly more students in the 'confident' group and 2) less in the 'not for me' group, compared to the control group. 3) The percentage of students in the 'not sure' group was somewhat smaller in the experimental groups (40.3% versus 46.1%). The fourth hypothesis, that there would be more students in the 'sceptic' group, could not be confirmed. There were even somewhat more sceptics in the control condition (14.3% versus 18.4%).

46.1%) (Table

[Insert table 3 about here ]

Comparison of pre-and post-tests within experimental condition

A comparison was made between 'attitude cluster-membership' before and after the science module within experimental groups. This comparison shows the possible changes in distribution of students over the four attitude-clusters. Table 4 presents the results of the chisquare analyses, showing whether shifts in the distribution are statistically significant.

We hypothesized a decrease of students in the 'not sure' group. In the pre-test, 35.1% of students belonged to the 'not sure' group. In the post-test, this group has grown slightly to 37.1%. Therefore the first hypothesis must be rejected. The majority of this 37.1% belonged to the same cluster at the pre-test (41.1%, see the column percentages in Table 4), but a considerable percentage originated from the 'concerned sceptic' cluster (26.8%). Another part of the post-test 'not sure'-cluster consisted of students who initially belonged to the 'confident supporter'-(21.4%) and 'not for me' groups (10.7%).

The second hypothesis, a smaller percentage of students in the 'not for me' group, can be confirmed. There was a decline of 6.0% in the pre-test to 2.0% in the post-test. Of the three students in the 'not for me' group, two started out as a 'not for me'-student, and one came from the 'not sure' group (see Table 4).

According to hypothesis 3, the percentage of students in the 'confident supporter' group should increase. The group of 'confident supporters' increased from 39.1% in the pre-test to 3 % in the post-test. Hypothesis 3 can therefore be confirmed. Sixty-three percent already belonged to this cluster at the start of the module and 31.5% initially belonged to the 'not sure' cluster, 4.1% were 'concerned sceptics' and 1.4% 'not for me's' (see Table 4).

Finally, hypothesis 4 must be rejected. A higher percentage of students in the 'concerned sceptic'-group was not observed. The percentage of students in this group even decreased from 19.9% to 12.6%. More than half of them remained sceptics (63.3%). The other 36.7% consisted mostly out of students who initially belonged to the 'not sure' group (31.6%) and a small part of 'confidents' (5.3%) (see Table 4).

[Insert table 4 about here ]

Effect of science module on attitude components

A remarkable result from the analyses comparing pre-and post-tests, concerns the increase of the 'not sure' cluster. Contrary to our expectations, a reasonable number of 'sceptics' as well as 'confidents' ended up not being sure what to think of modern biotechnology anymore.

Does this result indicate a decrease in scientific literacy?

We examined what the effects of the science module were on the different attitude factors, by conducting pairwise t-tests on each of the attitude factors (see Table 2 for a description of all factors). First, we examined the attitudinal changes of the entire experimental group, and subsequently of the post-not sure group. With this, we examined in more detail why students changed from being confident or sceptical to being unsure. The results are shown in the Table 5 and6 unavoidable, t(150) = -3.01, p < .001 and worries, t(150) = 3.00, p < .001 (reversely coded, see Table 5). These results suggest that the students showed a significant improvement in scientific literacy in terms of their knowledge base and positive awareness of genomics.

However, no significant movement towards a more critical stance could be established, explaining the rejection of the fourth hypothesis (a larger percentage in the 'concerned sceptics' group).

[Insert table 5 about here ]

Secondly, t-tests were used to detect the mean differences between pre-and post-test scores of the final 'not sure'-students, coming from the other three attitude-clusters.

For the 'confident supporters' turning into 'not sures' there was a significant effect for the behavioural factors. The students showed less intentions of consuming when there is a personal benefit to gain (own intentions), t(11) = 2.39, p < .05. The intentions of using medical applications, such as genetic tests also declined, t(11) = 2.22, p < .05, and consuming intention under critical or environmental conditions (e.g., environmentally friendlier) also declined, t(11) = 2.28, p < .05. Apparently, a more reserved position towards behavioural intentions made these students change into 'not sure'.

A clear shift in affection was observed in the 'concerned sceptic' group. The expressed worries towards biotechnology reduced, t(14) = 4.04, p < .001 (reversely coded), and feelings of biotechnology as an unavoidable process became stronger, t(14) = -3.51, p < .001. The presceptics also showed a more positive stance towards behavioural intentions, except for medical intentions (own intention, t(14) = -2.16, p ≤ .05; critical intentions t(14) = -2.43, p < .05). Apparently, with a more positive affective and intentional standpoint, these students lost a little of their concern and scepticism, and consequently moved to the 'not sure' group. As far as the 'not-for-me's' are concerned, a significant improvement on the scales measuring the cognitive component was observed. There was a significant progress on content knowledge of biotechnology and its applications, t(5) = -4.45, p < .05, and a more positive beliefs towards modern biotechnology, t(5) = -2.80, p < .05. By changing into 'not sure's, this group was still not able to make up their mind completely, but did show a more solid cognitive base.

Discussion

Being scientific literate means understanding the world we live in and being interested in it, taking part in discussions of and about science, and being sceptical and questioning claims made by others about scientific matters so that we can make informed decisions about the environment and personal health and well-being [START_REF] Goodrum | The status and quality of teaching and learning of science in Australian schools. A research report prepared for the department of education, training and youth affairs[END_REF]. In our view, and as far as modern biotechnology is concerned, scientifically literate people have an accurate knowledge base on basic biological and genetic concepts, display an affective reaction of concern or comfort towards biotechnology issues, and have clear ideas on how to behave or make decisions when confronted with modern biotechnology (in accordance with [START_REF] Millar | Twenty First Century Science: Insights from the Design and Implementation of a Scientific Literacy Approach in School Science[END_REF]. In other words, having a well-considered confident or sceptical attitude toward modern biotechnology [START_REF] Klop | An exploration of attitudes towards modern biotechnology; A study among Dutch secondary school students[END_REF]. The question is how can students' attitudes towards modern biotechnology become more articulate through education? In what ways can science modules encourage students to learn about so-called socioscientific issues and develop their own soundly based attitudes?

This study examined the effects of an innovative science module on the attitudes of secondary school students towards modern biotechnology. We made use of a new Dutch science module for the upper levels of secondary education. The socioscientific topic of the science module was genomics and cancer, the underlying design principles, inspired by a social constructivist 23 perspective on learning. We hypothesised that if the module was successful in developing attitudes, more students would move to the group of 'confident supporters' or become more critical in their opinion and shift to the 'concerned sceptics' group, and consequently, fewer students would be found in the 'not sure' or 'not for me' clusters.

Based on the combination of design principles and the socioscientific and relatively new subject matter [START_REF] Conner | The significance of an approach to the teaching of societal issues related to biotechnology[END_REF][START_REF] Sadler | Socioscientific issues and the affective domain: scientific literacy's missing link[END_REF][START_REF] Zeidler | Beyond STS: A research-based framework for socioscientific issues education[END_REF] we had reason to believe that even a small module could bring about some changes in attitudes.

Changes were indeed observed, and our hypotheses were partly confirmed. The module did result in a larger group of confident supporters, also in comparison with the control condition.

The expected increase in the numbers of 'concerned sceptics' was, however, not observed.

The 'sceptic' group even decreased in size. We offer three explanations for this finding. The first explanation concerns the number of lessons in the module: the changes were brought about in only four lessons. Students might have been overwhelmed by the (in particular 'progenomics', see next paragraph) module and as a consequence adopted ways of thinking about modern biotechnology without having time to think critically about its construct.

Elaborating on this first explanation, we give a second reason for the growth in the 'confident supporter' group, and the reduction in the 'concerned sceptic' group. There may have been a possible overexposure of the positive sides of modern biotechnology during the lessons.

Although some critical references on societal issues were offered in the workbook of the students, the emphasis of the module was on the benefits of cancer research using biotechnology. For that reason, the likelihood of students changing into 'a confident supporter' is greater than the likelihood of them turning into 'concerned sceptics'. From the perspective of biotechnological research-institutions or universities, this might be seen as a positive side effect, but it is certainly not the purpose of teaching for scientific literacy. 24 Therefore, we would like to argue that in the interest of fostering scientific literacy among students, science education modules such as the one described in the present study should focus on all aspects of genomics, the advantages as well as the disadvantages, the technical as well as the ethical.

A third explanation for the decrease in the 'concerned sceptic' group might be the quality of the fourth lesson of the module. Observation data gathered during the science module, and other research on this science module suggested that many teachers omitted (most of the) reflection activities (see [START_REF] Knippels | Mobile DNA labs; an evaluation of five innovative educational modules[END_REF], for a general evaluation of the science module; [START_REF] Waarlo | Biology education in a genomic world. Empowering citizens, patients and consumers[END_REF]. This means that a relatively large group of students was not invited to think critically about their newly acquired knowledge and feelings and the discussions they had had with their peers on the subject. These are, however, important factors in developing scientific literacy [START_REF] Sadler | The morality of socioscientific issues: Construal and resolution of genetic engineering dilemmas[END_REF][START_REF] Zeidler | Beyond STS: A research-based framework for socioscientific issues education[END_REF][START_REF] Zeidler | Tangled up in views: Beliefs in the nature of science and responses to socioscientific dilemmas[END_REF]. There is relatively little attention devoted to reflection on the learning content (deep understanding and insight) and reflection on students' own thinking and learning processes (meta-cognition) in most subjects in secondary education [START_REF] Volman | Qualities of instructional-learning episodes in different domains[END_REF]. These explanations lead to a recommendation for improving the science module: if there is more time spent, and a greater emphasis placed on reflection activities, it may help students to move from the 'not sure' group to the 'sceptics' group.

An unexpected finding in the present study concerned the substantial group of the students that moved from the 'confident supporter' group, or the 'concerned sceptic' group, to the 'not-sure' group. Our previous study has demonstrated that this particular group of students has a rather undefined attitude towards modern biotechnology; they are not sure what to think, feel, or do with it and their overall knowledge of the subject is rather poor. This may be a perfectly understandable position of 'the average teenager', and we expected that the science module would give them a more solid foundation to base their attitudes on, and that this partially occurred. All students in the experimental condition showed a significant improvement on the cognitive and affective component, as far as their knowledge base and positive awareness of genomics goes. This also implies that the meaning of 'being not-sure' after the module has changed. Especially since several 'confident supporters' and 'concerned sceptics' made a transition towards 'not sure'. During the science module, students acquired new knowledge, learned about new dilemmas, discussed these dilemmas with peers, and did hands-on work that was supervised by interesting students from a university, etc. In hindsight, it is understandable that due to all these experiences, and increase in their knowledge level, some of these students have started questioning their own views and behavioural intentions.

In that sense, these students have become 'less sure' about what to think. In our instrument, we made no (quantitative) distinction between ambivalent or questioning responses from indifferent responses [START_REF] Gardner | Measuring ambivalence to science[END_REF]. Future research should therefore include a measure of ambivalence.

Another suggestion for future research would be the design of a long-term effect study. In this study the time in between pre-and post-attitude test to follow students' attitudinal changes was approximately one to one and a half month. What is the persistence of the effects? What happened with the changes in attitudes in for instance six months time, have the effects vanished or maybe intensified? This will provide not only valuable information about the effectiveness of science education, but also about the durability of attitude changes.

In summary, we have suggested that the science module could help secondary school students become more articulate in their attitudes towards modern biotechnology. The expectation was that the module would help secondary school students develop a more pronounced attitude towards modern biotechnology. The science module indeed helped students to become somewhat more scientific literate by the improvement of their knowledge base and display of affective reactions towards biotechnology issues. Nevertheless, students were insufficiently invited to think critically about their newly acquired knowledge, feelings, and the discussions on the subjects that went on in the classrooms. This resulted in an under-representation of critical and sceptic students at the end. Besides, when socioscientific issues are discussed only one-sidedly, for example by leaving out the ethical dilemmas, again students are not invited to take a critical stance.

All students must be aware of the complexity of this expanding scientific discipline, so they will be able to participate, to be sceptical and questioning about scientific matters, and to make informed decisions for personal, social, and global benefit. 1 Numbers of respondents can vary between pre-and post-test, as some students did not complete both questionnaires.

F

2 As seen in the "requirements" section, control group 2 is not significantly different from control group 1. For this reason, both control groups can be considered as one group.

  understanding lay articles in newspapers and popular magazines

  line of argument, a profound attitude requires (1) a solid knowledge base of basic scientific constructs (cognitive component), (2) a clear stand on one's own feelings and emotions on important (social and ethical) issues (affective component) and (3) the ability to make informed decisions about the environment, ones own health and well-being (behavioural component).

  sure'-group, formed the largest group. Their views tended to be rather indistinct and more difficult to interpret; they showed neither anti-biotechnology nor pro-biotechnology affections, and their overall understanding of the subjects was rather diffuse.

  broader, societal context and students could reflect on experiences, feelings, and thoughts.

  an inquiry-based and hands-on approach. Students learned about concepts of cancer, cancer-research, and genomics by examining a real world, open-ended scenario and worked towards providing solutions that made sense to them.

  opinions. The question was to what extent the science module could bring about more balanced and decisive attitudes.

  -and post-attitudes tests, interviews were held with selected students and classroom practice was observed 2 .

  , it may affect students' attitudes towards genomics and override the effect of the science module.

.

  The results comparing the mean pre-test score to the mean post-test score of the students in the experimental condition revealed an overall significant improvement on two of the three factors measuring the cognitive component; knowledge of biotechnological applications, t(150) = -2.90, p < .001, and beliefs, t(150) = -3.01, p < .001. There was also an increase in two of the three factors that measured the affective component;

  more certainty about their own opinion. T-test analyses showed that

Figure 1 .

 1 Figure 1. K-means cluster analysis of the attitude-pretest-scores of 327 secondary school students, combined with the dataset of the previous attitude-test scores i . Confident supporters (n = 113), concerned sceptics (n = 66), not sure's (n = 123) and not for me's (n = 25). Scores are standardised values. *Negative score on 'worries-factor' indicates fewer worries about modern biotechnology

Table 1 .

 1 Attitude factors with scale name, description, typical items, reliability and descriptive values, based on principal component analyses

	o r
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Table 2 :

 2 Design of the study

			Attitudes	Experimental
			pre-test	science module
						38 (2 groups)
	Control	Control group 1	√ √ √ √	-	-	88 (4 groups)
	groups	Control group 2 2	-	-	√ √ √ √	64 (3 groups)

Table 3 .

 3 Result of Chi-Square test for comparison between post-test scores of experimental groups and post-test scores of control groups

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				Clusters post-test		Total
			Confident	Sceptic (n) Not sure (n) Not for me	(N)
			(n)			(n)
	Experimental	Treatment	43.9% (86)	14.3%	40.3% (79)	1.5% (3)	100%
	condition			(28)			(196)
	Control condition	No	30.3% (46)	18.4%	46.1% (70)	5.3% (8)	100%
		treatment		(28)			(152)
	Chi-Square= 9.53; df = 3; Asymp. Sig. (2-sided) p< .05

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Table 4 .

 4 Result of Chi-Square test for comparison of cluster distribution of the students based on pre and post-test scores of experimental groups

	Cluster post-test	Total
	confident sceptic	not	not for
		sure	me

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Table 5 .

 5 Mean attitude component scores for all participants on the experimental condition; obtained t-and significance of differences following paired sample analysis

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		Paired Differences
	Attitude factors Mean difference SD	t	df	Sig. (2-tailed)
	Pair 1 bio&gen pre -bio&gen post	-.00	. 3	-0.33 150	.75
	Pair 2 biotech pre -biotech post	-.03	.11 -2.90 150	.00
	Pair 3 beliefs pre -beliefs post	-.13	.55 -3.01 150	.00
	Pair 4 basic emotion pre -basic	.05	.47	1.26	150	.21
	emotion post
	Pair 5 Unavoid. pre -unavoid. post	-.13	.51 -3.01 150	.00
	Pair 6 worries pre -worries post	.17	.68	3.00	150	.00
	Pair 7 own intention pre -own	-.07	.66 -1.21 150	.23
	intention post
	Pair 8 med.intention pre -	-.01	.72 -0.14 150	.89
	med.intention post
	Pair 9 crit.intention pre -crit.intention	-.06	.67 -1.05 150	.30
	post

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The Cancer Genomics Centre (CGC) is a strategic collaboration of research groups from the Netherlands Cancer Institute, the Erasmus Medical Center, the Hubrecht Laboratory and the University Medical Centre Utrecht.