Joosje Van Bennekom
email: j.van.bennekom@rijksmuseum.nle.van.bork@rijksmuseum.nlflorian.tereygeol@cea.fr

Ellen Van Bork

Florian Thereygeol

The Tricks of the Trade: Research into Adam Van Vianen's Silver Alloy

When examining the alloy of several works of Adam van Vianen, who was one of the founders of the Dutch Kwabstijl, with X-ray fluorescence, a much higher silver and lower bismuth percentage and was found in the silver than was commonly used by his contemporaries. It is the hypothesis that Van Vianen used a high percentage of silver, as it is more malleable, and he could therefore obtain extreme deformation of the silver without the formation of cracks. Recipes on silver production and refinement in the silversmith workshop that would have been commonly used at the time were selected to serve as a basis for making reconstructions in silver. These books were -the Probierbuchlein by Calbus of Freiberg and the Description of Leading Ore Processing and Mining Methods by Lazarus Ercker. Reconstructions were made, and the 'test' procedure, leading to 'blick' silver, generated highly refined silver, the silver being very malleable. It is therefore concluded that this recipe could have been used on purpose by Adam van Vianen to make his elaborate objects. On the other hand, there exists the possibility that this low bismuth content might be generated by coincidence, as a side effect to the multiple annealing and pickling cycles the objects went through.

1 This project is run by Joosje van Bennekom, head of the metals conservation department at the Rijksmuseum.

2 Olympus handheld Spectra; data were acquired with the next settings: 10mm collimator, 30 sec measurement tie, Ashmolean standards calibrated. PyMCA calibration file for silver was made by A. Pappot (PhD student Rijksmuseum) and A. Heginbotham (Getty Institute). 3 See for example Louise E. van den Bergh-Hoogterp on silver from Adam van Vianens "Marcus Curtius" ewer, now in the Metropolitan Museum, New York (Stavelij, 1999, p.4-9). Artdealers Jacob Roosjens en Emiel en Esther Aardewerk also mentioned the different alloy composition to Joosje van Bennekom on different occasions. 4 Data collected by Rijksmuseum, circa 120 objects form known provenance from the Netherlands.

Introduction

XRF measurements on Dutch silver

Currently the metals conservation studio at the Rijksmuseum is working on a database on the alloy of (Dutch) silver. 1 X-ray fluorescence, calibrated with a silver standard set with many different ranges in impurities and recalculated by PyMCA 2 is used the gain insight in the composition of silver from the middle ages onwards. The information gained could give information on when or where an object was made, and into workshop practice throughout history.

The silversmith Adam van Vianen and his brother Paulus are the founders of the Auricular style, known as the Kwabstijl in Dutch, which was developed in in the early 17 th century in the Netherlands. This unique style breaks with all former conventions in silver works of art, with organic contours in the shape of the silver, contrary to the classical symmetrical style of the Renaissance. It is said in literature and that Adam van Vianen used a different alloy than silversmiths that lived at the same time. 3 The Kwab exhibition that was organized at the Rijksmuseum in 2018 gave the perfect opportunity to determine the alloy of several works made by Adam van Vianen, as many of his objects were on show (table 1). The results of the measurements showed that the silver percentage in these objects was much higher (97-98%), and the bismuth content level lower (circa 0.02%), than what was the standard of assay office (resp. silver 93,5%, bismuth roughly between 0.025-0.035%) and is thus commonly found in silver 17 th The following research questions arose from these findings:

1. What recipes on making the silver well workable and malleable could have been used by Dutch 17 th century silversmiths?

2. What kind of alloy composition do these recipes give, and is this composition similar to the one that Adam van Vianen used?

3. Would Adam van Vianen have used this alloy on purpose?

2 Recipes in treatises

Introduction

Two contemporary sources were consulted for recipes on silver processing and refinement; the Probierbuchlein by Calbus of Freiberg written in 15245 , and the Description of Leading Ore Processing and Mining Methods, from 1574 by Lazarus Ercker. 6 It is likely that these books were known at the beginning of the 17 th century in Holland as they were written in German, a language that was understood by most of the Dutch. Also, the Probierbuchlein has a chapter on "How Assay Weights Are Made in the Netherlands", showing that there was interaction between Germany and Holland on silversmithing as early as the 16 th century. In both books recipes are given that describe the procedure for preparing silver in larger quantities for the workshop in detail.

Three recipes from the Probierbuchlein were selected; 'How a test should be made', 'How to Purify and Refine Silver for Graduation' and 'How to make silver malleable again'7 . The titles of the recipes make them interesting even though they are not very detailed. The first and most elaborate recipe also appears in the book of Ercker, and gives a recipe how to prepare a large quantity of silver in the workshop. This recipe is the most important because it gives many practical details. The second recipe, with in its title the word 'graduation', should be interpreted as 'how to make silver pure so it can be used for alloying with copper to the required standards', which would give a very pure silver.

The third recipe seems useful as Van Vianen used very pure and thus malleable silver, for his heavily worked objects. It indicates that there was a remedy for silver that was not very malleable, or when the silversmith wanted to deform the silver to a great extent. Although the recipe mentions 'making the silver malleable again', insinuating that it is intended for brittle or unworkable silver, which occurs when there is too much lead or other impurities in the silver, the recipe might have also been used to enhance the properties of already workable silver.

The main recipe in Ercker is, as said, similar to the Probierbuchlein's 'How a test should be made': 'How to refine silver, and how to make correctly the tests used in the process'. 8 The recipe has many similarities with the one from the Probierbuchlein, although there are some variations, and Erckers recipes are more detailed. The scorifier for example, that is also used in Erckers recipe to refine the silver, is filled with woodash and topped with bone ash, instead of bone ash alone.

The fact that both books have a recipe for making this so called 'test' shows that this procedure was a common method used in silversmiths' workshops to refine larger quantities of silver. The method is similar to cupellation, the difference being that the cupellation method was used for small quantities, mainly for testing the silver by syndics of the guild. The 'test' is this same process but on a larger scale, a recipe which a silversmiths would have used very often, to refine their scrap metal that would contain solder, or impurities of other metals-, or to refine silver coins to make an object from. As a silversmith, the purity of the silver is very important. First, to avoid problems of other metals in your alloy, that could cause cracking and other unwanted reactions; secondly, to have almost a hundred percent pure silver to start with to easily alloy it by adding copper to the silver to obtain the grade that was requested by the guild (commonly 92.5% silver, 7.5 % copper). How to refine silver, and how to make correctly the tests used in the process "To refine silver in the fire means to purify and soften cupelled silver (which is still impure and not quite malleable) by firing it in a test. This is done in two ways: either under wood before the bellows, or under a muffle with nothing but charcoal.

First I shall describe how the tests are made…" take wood ashed from which lye has been made and which are no longer biting…"Get an earthenware pan, unglazed…rinse it with water…then put in the pan a two finger thickness of the {moistened} ashes, gently stamp them down with a wooden tamper which should have eight prongs, then add more ashes, tamp them down too, and repeat this till the pan is full…The ashes next to the upper rim should be pressed down smoothly in the test with a turned wooden ball….hollow out the inside...When the test is hollowed out, take a hair sieve fill it with ground bone ash and dust the test therewith until it is very white"….{then it is tested that the heat generated by the bellows is directly tunneled into the middle of the test} "Break the cupelled silver into small pieces; put a little straw in the test and the silver on top. Add burning coals and then charcoal until the silver and the test are completely covered, then work with the bellows and the silver will melt easily and start to drive."… "Then the charcoal is removed, and the pieces are covered with fir, spruce, pine or alderwood, the blast should hit the silver underneath and thus refine in the blast. What lead has been left after the first driving will go into the test".

Experimental

Silver reconstructions/Reconstructions

Reconstructions of the selected recipes were carried out in the open-air museum on the Carolingian silver mines of Melle as part of the Projet Collectif de Recherche called 'L'expérimentation paléométallurgique: de l'analogie à la modélisation'. 10 The Rijksmuseum metals conservation studio has developed a so called 'historical alloy' which is a silver-copper alloy containing traces of bismuth, lead and zinc, which are commonly found in silver roughly from before 1875. As X-ray fluorescence analyses showed that with the levels of these 'impurities' are a bit above what is found in 17 th century silver, this alloy was used for the experiment, as the bismuth level could be lowered by refining. Both recipes in the Probierbuchlein and Ercker do not explicitly mention the addition of lead for refinement. However, after a discussion with Dr. Florian Tereygeol, the project leader of the 'L'expérimentation paléométallurgique: de l'analogie à la modélisation', it became clear that the silver alloy could only be refined by adding lead, 11 as this is essential in the refining process. It is unclear why this is not mentioned in the recipes. Possible the authors thought that it was common knowledge that lead is necessary for the refinement of silver, and it was therefore didn't mention it in their recipes. The amount of lead that needed to be added was calculated according to a scheme that was given in the 'Manuel d'Essayeur' 12 . Even though it is a later source, it seemed useful as the process of refinement didn't change much until the second half of the 19 th century. For silver that contained 0.05% copper it was calculated that four times more lead than silver was necessary to remove the copper from the silver13 . The following experiments were derived from the recipes. Four experiments were based on the main recipe with the 'test'. The test consists of an earthenware dish (scorifier), filled with bone ash, which is placed in the furnace, and heated until the silver melts (with or without lead) until it 'blicks'. This moment is well known among metallurgist or silversmiths and it is described in many metallurgical recipes. It describes an important visual moment in the oven, when the silver is molten, and is so hot it starts to roll around with lead oxides (litharge) floating on top of the molten silver . The blick moment occurs when the litharge -or impurities-is completely absorbed by the bone ash and the silver looks completely 'clear', like an orange dome. It is said that the word 'blick' derives from the Dutch 'bliksem' or German 'blitsen' (lightning), a sudden bright moment.

The four experiments varied in adding no lead or (extra) lead and heating time. The first two experiments were done in duplo. The recipe with a surplus of lead was added to overcome the possibility that the calculation in the 17th century might not have been so exact as the calculations we made based on a table from the 19 th century. The two last recipes were based on the red ochre and Venetian glass entries, and were only executed once. The red ochre experiment was set up in a crucible instead of a scorifier, to be able to mix the silver with the powder well without spoiling. The Venetian glass and soap were added in a scorifier with bone ash. The temperature was raised in this case, because soda-lime glass will melt only at high temperatures (1000 0 C). During the time in the oven, we noted that the soap seemed to lower the melting point of the glass, because it melted very well. The potassium in the soap might have helped in lowering the melting point.

The tests were prepared by wetting sieved bone ash which was pressed into an earthenware scorifier. In the centre, some bone ash was removed to make a depression. Finally, the bone ash was compacted by hand, polished with an agate ball and left to dry overnight. The tests were carried out in a cupellation furnace big enough for the larger earthenware dish. An earthenware muffle was inserted, and charcoal of high quality was used to heat the furnace. The bullions that were produced during the experiments were separated from the tests and analysed with the XRF (see paragraph 4). They were sawn in half with a jeweller's saw, so one half could be used for further processing and the other half was kept as a reference. The halves were subsequently hammered, annealed and pickled 16,17 six times, and again measured with the XRF, to determine difference in bismuth content after a normal working procedure for manufacturing object, and thus making them better comparable to the data from the Van Vianen objects. This procedure also gave insight into the malleability of the silver.

Recipe

Results

Silver melting experiments

Experiments 1, 2 and 4 delivered a clean bullion, without any deposits left on the surface. Experiment 3, without lead, resulted in an irregular bullion with a black surface. Experiment 5 gave a very interesting result. It seemed as if the glass had absorbed all the impurities from the melt: green, yellow and glossy grey spots appeared at the surface of the glass. These areas where measured with the ARTAX XRF 18 , and they appeared to be copper and zinc, but no lead was found. Experiment 6 gave a bullion with an irregular shape and holes through and through, with a greyish surface.

14 Red ochre was prepared on the research site by filing a piece of red ochre stone near Rodez, France, from the collection of Dr. Florian Tereygeol. 15 Venetian glass is a term that is not easily translated in a chemical composition. According to the glass conservator of the Rijksmuseum, Venetian glass was known to be of very pure quality (silica, chalk and sodium). We decided to use pure sodium chalk glass balls (https://www.laboratorium.shop/product/bohemia-glasparels-o-2-mm-massief-natronkalkglas-1kgzak/ ). The Venetian soap probably was probably just of form of the common soft or lye soap still used nowadays, based on fatty acids cooked with lye. Table 5: XRF results of the bullion composition, average Experiment 1, 2 and 4, have the highest silver content. Experiment 1 (until blick), has the lowest copper content and a very low bismuth and lead content. Experiment 2, (beyond blick), resulted in less pure silver, and thus proportionally more copper. Experiment 4, (excess of lead), resulted in pure silver, but with proportionally more lead and copper present in the alloy. The results from experiment 3 and 5 are not representative, because with experiment 3 a thick copper rich surface layer had formed on the silver, and with experiment 5 a black surface layer had formed, together with many holes in the bullion, which made the measuring difficult.

Processed silver

The bullions could be placed in three categories in terms of their working properties: good, medium or bad malleability (table 6). The until 'blick' recipes (exp. 1) and the Venetian glass recipe (exp. 6) gave the best results, as no large cracks appeared during hammering, only some minor cracks along the edges. The beyond blick (exp. 2) and the red ochre (exp.5) recipes produced silver that started cracking after a few rounds of hammering, but was still relatively malleable. Exp. 3 and 4 gave silver that started cracking after one round of hammering.

Good malleability Medium malleability Bad malleability

Exp The results of the XRF measurements after the bullions were heated, annealed and pickled (after h.a.p.) can be seen in table 7, and were compared with the initial results, directly after casting.

Tabel 7: In white the bullion composition after hammering, annealing and pickling 6 x, average mean, For nickel and lead there is no consistency in increase or decrease after the h.a.p. treatment in the silver alloys. Copper diminishes largely, what was expected, as the pickling is a step in silversmithing necessary to remove the copper oxides from the surface, prohibiting these oxides from being incorporated in the surface of the silver during hammering. The silver, zinc and bismuth content diminished less drastically, but are leached out of the surface. The only element that decreases in almost all cases is gold, which is as expected, it will not react with the sulfuric acid.

Discussion

Malleability of the silver

Experiments 3 (without lead) and 4 (surplus of lead) have bad malleability and already started cracking during the first round of hammering. Their composition is quite different: experiment 3 has 4,2 % Cu and 95,6% Ag where experiment 4 has 0,7% Cu and 98,9% Ag after HAP. The Pb en Bi levels, however, are very similar.

The 'test' procedures and the Venetian glass recipe all gave malleable silver. These recipes contain a low copper percentage and a high silver percentage. The lead percentage falls in between 0.2 and 0.3 %. Lead is known to embrittle silver. 19 However, the silver measured for the Dutch silver database in the 17 th century usually contains in between 0.25 and 0.35 % lead, which could therefore be considered as a workable percentage, not causing immediate cracks in the silver. The cracking in our tests hence cannot be explained by of the presence of lead, as this is in the same range or even lower.

The blick 'without lead', red ochre and Venetians glass experiment contain a very low lead percentage and show medium to severe cracking, or, in the case of Venetian glass no cracking. This could indicate that a high or low percentage of lead does not influence the malleability of the silver. 20 Considering the accurateness of the recipes, whether they deliver workable silver, one can say that the 'test' blick recipe delivers good material, and well cleaned from all impurities, therefore a good starting material for alloying. Both the red ochre and Venetian glass experiment, the last one promising, did not deliver a pure silver alloy, although the trace elements copper, lead and bismuth did diminish. It is known from other research that glass was used as a refinement method for gold; it would be interesting to try this recipe again with different settings. 21 A remarkable fact to note here is that both recipes do not mention the adding of lead to purify the silver, but the experiments showed that our silver needed lead to get purified. Why then, do the recipes do not explicitly mention lead? The Probierbuchlein does not mention it at all, and the recipe in Ercker mentions lead almost in passing at the end of its recipe: "What lead has been left after the first driving will go into the test", which makes this an important sentence. Could it be that the 'test' procedure was the step after the first -larger scale -refinement (that took place near the mines)? The Saigerhütten process was one of the most used methods to produce silver: copper ore (containing small amount of silver) was roasted with lead on fires, and the molten lead would extract the silver from the copper cakes. The copper could be worked further, and the lead rich silver needed to be purified from the lead on a large scale in a hearth. 22 Because of this large scale, it is well possible that a lot of lead was still inside the silver after this first purification. It seems likely that the 'test' procedure is used as the successive step. It also seems plausible that silversmiths knew of these differences in use, and knew that with contaminated silver without extra lead they had to add extra lead themselves. Some other recipes in Ercker, to be investigated at a later stage, do mention the addition of lead, for example when melting coins for reuse.

Silver recipes compared to Adam van Vianen's silver

The main question was whether the recipes to prepare silver or to make it more malleable that were known in the time of Adam van Vianen, could result in an alloy that was similar to the alloy 17 th century used for his extremely raised and worked objects.

From the recipes that were tested, two of them refine the silver in such a way that the alloying elements are almost in the same ratio as the Adam van Vianen silver, as can be seen in table. 8. The bismuth content in Vianen's silver is very similar to the 'blick' and 'beyond blick' experiments. The lead content is a bit higher in Adam van Vianen's silver. The copper content in Vianen's silver though is clearly higher. An explanation could be that the silver was otherwise too soft and therefore vulnerable, hence Van Vianen added a small amount of copper to make the blick silver sturdier and more resilient. However, there exists another possibility that would deliver an alloy with very low trace elements in the silversmiths' workshop, by coincidence. Earlier experiments that were done where a silver samples were heated and pickled 20 times without hammering, showed no decrease in de bismuth level. 23 This current research does, however, show a slight decrease of bismuth after HAP. It is therefore possible that the hammering of the silver has an influence on the trace elements that are dissolved by the acid treatment. By hammering the silver, the surface enlarges and therefor trace elements that were hidden in the bulk, are exposed and subsequently dissolved in the pickle. It should be taken into account that for the earlier experiments vitriol was used and for the current research tartaric acid was used. As the pieces of Van Vianen were most likely heated and pickled numerous times, due to the extreme deformation and decoration of the silver, it is plausible that the repeated hammering and pickling did have an influence on the alloy composition and the amount of trace elements. When the silversmith starts with a silver alloy alloyed in the common way with 7.5% copper (as requested by the guild) it could be possible, when the object is very elaborate and has gone through many HAP cycles, that the trace elements dissolve, and thus diminish. The typical alloy from Adam van Vianen would then not be an conscious choice, but a coincidence, caused by the multiple cycles of hammering annealing, and pickling of the objects.

Conclusion

The questions that were posed in the introduction can only be partly answered.

1. What recipes on making the silver workable and malleable could have been used by Dutch 17 th century silversmiths?

The two books that were probably wide spread and well known in the Netherlands, which contain several recipes to deliver workable silver, were used for this research. The 'test' procedures 'until blick' delivered a silver alloywith a high silver content and a good malleability.

The Venetian glass and red ochre experiments though, did not result in an alloy with a high silver content, which did not seem to affect the malleability of silver from the Venetian glass experiment.

On the contrary, it did deliver a workable piece of silver. The glass had extracted impurities and incorporate these elements in its matrix.

2. What kind of alloy composition do these recipes give, and is this composition similar to the one that Adam van Vianen used?

The 'until blick' recipesilver alloys with a high silver content and a good malleability, and could therefore be considered similar to that of Adam van Vianen. The Venetian glass experiment though, did not result in an alloy with a high silver content, but this did not seem to matter for the malleability of this silver. The silver from the other experiments differed from Van Vianen's alloy either in terms of alloy or malleability.

3. Would Adam van Vianen have used this alloy deliberately?

Until now Adam van Vianen's elaborate objects are the only objects in the Dutch silver database that show such a low percentage of bismuth. It is likely that Adam van Vianen was aware that the silver of the 'test' procedure ('blick silver') was of the highest purity possible at that time, and that this silver was very malleable. Because Adam van Vianens was one of the most appraised silversmiths of his time, it is also possible that he could afford it to invest in high quality silver.

However, the fact that de bismuth levels also dropped slightly after HAP could be another explanation for the low bismuth levels Van Vianen's work. In future, when other very heavily worked objects that can be compared to the works of van Vianen are added to the database, the bismuth percentage should be compared to the content of Adam Van Vianen's, to see whether they show the same effect.

Future research

We hope this project will be followed up by more research into the workshop receipts from the Probierbuchlein and Lazarus Erckers book. Besides that, there are some questions after the research presented here should be addressed:

1) Where does the bismuth come from: from the original silver ores (copper or lead ores), or is it added later when the silver is alloyed with copper? The objects from Adam's contemporaries show a higher bismuth content. But these silversmiths would also have worked with refined -and hence bismuth free-silver (970 and higher), graduating it afterwards with copper to 935% silver. In what phase of the making of an object does the bismuth introduced. 2) Another question is: when the unrefined silver contains different amounts of bismuth per batch, do they end up with all the same low bismuth content after the 'test' procedure or does this vary? 3) Finally, new coupons with a high bismuth content should be treated according to the HAP procedure, to determine how much the bismuth percentage diminishes.

Table 1 :

 1 century Dutch silver. 4 composition of the main components of six objects by Adam van Vianen, average

		Ni	Cu	Zn	Ag	Au	Pb	Bi
	Adam van Vianen	0.016+/-0.009	1.776+/-1.194	0	97.523+/-1.386	0.187+/-0.113	0.248+/-0.118	0.015+/-0.001
	(6 objects, n=40)

Table 2 :

 2 Historical developed silver alloy, average

		Ni	Cu	Zn	Ag	Au	Pb	Bi
	Silver before (n=15)	0.011±0.007	6.035±0.257	0.018±0.007	93.569±0.298	0.104±0.003	0.233±0.025	0.053±0.002

Table 3 :

 3 Experiments done according to recipes from Probierbuchlein and Ercker 3.2. Processing the silver

		Experiment	Material added	Time heating	0 C
	"Test"procedure	1. Melting until blick	16 gr. Ag -73 gr Pb	+/-60 min.	900-960
	from Probier-	2.Melting beyond	16 gr. Ag -73 gr Pb	+/-60 min.	900-960
	buchlein and	blick
	Ercker	3.Melting until blick,	16 gr. Ag -0 gr Pb	+/-60 min.	900-960
		no lead added
		4.Melting until blick,	16 gr. Ag -93 gr Pb	+/-60 min.	900-960
		surplus lead added
	Probierbuchlein 5.Red ochre 14 (in	16 gr. Ag-Powdered	5-10 min. , 3x	900-960
		crucible)	Fe2O3, enough to cover
			the silver
		6.Venetian glass and	16 gr. Ag -16 gr. glass	30 min.	1000
		soap 15 (in scorifier)	balls -3 table spoons
			soap

Table 4 :

 4

		Ni	Cu	Zn	Ag	Au	Pb	Bi
	Silver before (n=15)	0.011±0.007	6.035±0.257	0.018±0.007	93.569±0.298	0.104±0.003	0.233±0.025	0.053±0.002
	1) Blick (n=4)	0.034+/-0.003	0.427+/-0.008	0	99.439+/-0.045	0.132+/-0.003	0.01+/-0.004	0.017+/-0.001
	2) Beyond blick (n=4)	0.019+/-0.005	1.207+/-0.245	0	98.396+/-0.321	0.123+/-0.015	0.266+/-0.105	0.023+/-0.006
	3) Without lead (n=1)	0	77.566	0.723	21.082	0.006	0.606	0.11
	4) Surplus lead (n=2)	0.026+/-0.004	0.72+/-0.064	0	99.019+/-0.162	0.127+/-0.002	0.075+/-0.015	0.016+/-0.001
	5) Red ochre (n=2)	0.012+/-0.011	8.843+/-5.264	0.002+/-0.008	90.961+/-5.203	0.109+/-0.006	0.01+/-0.001	0.021+/-0.002
	6) Venetian glass	n.m.	n.m.	n.m.	n.m.	n.m.	n.m.	n.m.

casting results, bullion after cooling down in test or scorifier

The separated bullions were measured four times with the calibrated handheld XRF Olympus Delta. It is standard to analyse 16 elements in the alloy, however, only the most important elements, that show large differences, are presented below (table

5

). The composition of the Rijksmuseum alloy, used for the experiments, is mentioned in the first row. The results of the experiments are stated in row 2-5. The Venetian glass experiment could not be measured at this stage, as it was still completely encapsulated by the glass.

Table 6 :

 6 Malleability results after hammering, annealing and pickling

Table 8 :

 8 'Blick' and 'beyond blick refinement compared to Adam van Vianen silver.

		Ni	Cu	Zn	Ag	Au	Pb	Bi
	Adam van Vianen	0.016+/-0.009	1.776+/-1.194	0	97.523+/-1.386	0.187+/-0.113	0.248+/-0.118	0.015+/-0.001
	(6 objects, n=40)
	Beyond blick and	0.021+/-0.005	0.514+/-0.157	0	99.211+/0.224	0.129+/-0.009	0.122+/-0.123	0.016+/-0.002
	h.a.p. (n=8)
	Blick and h.a.p. (n=8) 0.025+/-0.007	0.357+/-0.082	0	99.333+/0.189	0.133+/-0.008	0.109+/-0.131	0.015+/-0.001

A.G. Sisco & C.S. Smith "Calbus of Freiberg (and unknown authors), Bergbuchlein und Probierbuchlein, 1524, Lepizich" Yale University Press, New York, 1948.

A.G. Sisco & C.S. Smith "Lazarus Ercker, Description of Leading Ore Processing and Mining Methods, 1574, Prague" Chicago Press, Chicago, 1951.

A.G. Sisco & C.S. Smith, 1948, p.112 ev

A.G. Sisco & C.S. Smith, 1951. p.71

1 mark in Dutch weigh system of Trois pounds: circa 250 grams (see https://www.goudenzilverweging.nl/gsd_tg, seen on 16/3/2020)

We were granted research time by Dr Florian Tereygeol, research director of the archeological department, French National Centre for Scientific Research (CNRS).

Some silver that had to be refined would already be lead rich, because the product would come or 1) from lead mines where it would not have been necessary to add extra lead or from 2) the Saigerhutten process as by product from copper ores as leached out silver rich lead cakes. The recipes do not mention the addition of extra lead to the silver. Also for the refinement of silver of coins or scrap metal it would have been necessary to add extra lead. Maybe it was thought that the addition of lead is common knowledge in purification and cupellation of silver and it therefore wasn't explicitly mentioned in these recipes?

M. Vauquelin, 'Manuel d'Essayeur', Paris, Klostermann et fils, 1812, p.29.Table of d'Arcet, who calculated exactly how much lead was necessary to refine silver (per different amount of copper inside).

If the silver was more contaminated (20% of impurities), it would have needed eleven times the weight in lead.

Possibly, a combination of trace elements causes cracking, for example a too high sum of bismuth, lead and nickel. When these elements are summed up for the samples that gave malleability and the ones that started cracking, no significant differences are found between the malleable and cracking silver. It should also be taken into account that the shape of a halve bullion is odd, as the thickness varies a lot from the center to the edges, which can cause additional stress to the material when hammering, and thus cause cracking. A silversmith would have cast the silver into a barre or sheet of an even thickness, which would diminish the stress in de material during hammering. However, the bullions can be intercompared as they roughly have the same shape.

21 Rehren, Th., Nixon, S. Refining gold with glass-an early Islamic technology at Tadmekka in Journal of Archaeological Science 49(2014) 33-

41.22 See M.P. Guirado, F. Tereygeol & F. Peyrat 'Initial experiments on silver refining: How did a cupellation furnace work in the 16 th century" in HistoricalMetallurgy, no 44, part 2, 2010 (126-135), were the refinement of silver on a larger scale in a furnace, based on descriptions by Agricola is researched.

Experiments were done at the Rijksmuseum metals conservation studio in 2019. The samples were heated with a torch until they were cherry red. They were subsequently cooled in water and pickled in 10% sulfuric acid in water solution for ca. 2 minutes


E Ferrari

F Mercier

E Foy

F Téreygeol

New insights on the interpretation of alum-based fake silver recipes from 3 rd century AD by an experimental archaeology approach in the laboratory

Keywords: Ancient metallurgy, alum, doubling silver, archaeometry, experimental archaeology

New insights on the interpretation of alum-based fake silver recipes from 3rd century CE by an experimental archaeology approach in the laboratory

Introduction 1

Alum rarely appears in the field of metallurgy (Giumla-Mair, 2005). The collection of texts that goes under the name of papyri of Leiden and Stockholm [START_REF] Halleux | Les alchimistes grecs, Papyrus de Leyde[END_REF] represent an important exception. In the documents, dating back to the 3 rd century AD, the term 'alum' appears 15 times, which makes it the most common additive in the metallurgical recipes described. As archaeological finds are concerned, it remains challenging to prove whether alum is used as finishing or during the alloying process. The question arises as to how it is possible to link historical sources with objects from archaeological excavations concerning the use of such an elusive material. One possible way is the experimental approach. In this paper, we aim at reproducing ancient recipes in the laboratory. This approach might be relatively simple when dealing with technical texts from the Renaissance period while several problems arise in the case of older metallurgical and alchemical recipes. First, it is unclear whether the person who writes is himself an artisan.

Secondly, the document we read is a translation into a modern language, meaning the translator already did an interpretation of the text. Indeed the actual limitation when applying an experimental approach to this type of documents is our interpretation. The researcher might decide to exclude certain actions or materials for which one does not understand the purpose: this may reflect negatively on the outcomes of the study. At the same time, we should be able to identify what is confusing and, in our modern view, pointless.

Aim of the study

In this study, we aim at replicating the processes to obtain 'edulcorated' silver when mixing copper oxides (in form of copper scale) with silver as described in the papyri of Leiden and Stockholm.

In a copper-rich silver alloy, up to 85% in mass of copper can be present without a considerable change in the aesthetical properties of the final product compared to pure silver. 2 The alloying process can highly reduce the cost of the final artefact but it can be also interpreted as a fraud.

In the ancient texts considered here, the use of a by-product of copper production, the copper scale is reported. The copper scale corresponds to the oxide particles, often in form of platelets, that are detached from the metal as hammering is applied during the annealing process. In the described recipes, such particles are never used as such, but, before being added to silver for melting, they undergo a pre-treatment in an alum-containing bath. This is possibly related to the intention of producing a silvery-looking material that would be added to silver during the casting.

Corpus of the study

The present study is based on two recipes reported in the papyri of Leiden and Stockholm. In both described processes a copper scale undergoes a 'pre-treatment' using a solution containing alum and, afterwards, it is melted with silver. These texts were identified in 1830 and a first description can be found in [START_REF] Berthelot | Introduction à l'étude de la chimie des anciens et du moyen âge[END_REF]. In his edition, Robert Halleux provides all the specific information for these documents (Halleux, 1981, 5-78).

We report the text #6 as found in the papyri of Leiden and Stockholm (English translation from Halleux, 1981, page 111):

Holm 6 « Other » [doubling silver]
Immerse the copper scale in a brine and soak for 6 days in alum and shiny earth dissolved in fresh water. Then melt by adding silver.

The recipe is very short. According to the interpretation of Robert Halleux (Halleux, 1981, page 111, note 9), the process is carried out in two stages, followed by a melt. The first step consists in dipping the copper scale in an unspecified brine. In the second step, the scale is plunged into a second bath of alum and "shiny earth". Robert Halleux proposes a parallel with recipe #12 in the papyrus of Leiden3

Leid 12 « Production of Asem » (i.e. silver alloy, see [START_REF] Halleux | Le sens d'Asemos dans le papyrus chimique de Leyde et dans l'alchimie grécoégyptienne[END_REF] Take the copper scale, soak in vinegar and lamellar white alum and soak for 7 days, then melt with a quarter of copper, 8 of earth of Chio, 8 of Samos earth drachmas, 1 of Cappadocian salt, 1 of lamellar alum. Mix and melt by adding Adramyttion silver. (Halleux, 1981, p. 87).

In this latter recipe, the binary rhythm of the work is clearly stated. The second step which consists in including the earthy elements and which is described here as a cast, can only be a chemical treatment in solution: this might be an indication of the fact that the writer of the recipe is not fully acquainted with the metallurgical work. In addition, unlike the first, this second recipe promotes the use of vinegar as a reactive element.

From reading both recipes, the interpretation of them leads to several questions:

1. Are we dealing with a forgery recipe? 2. What is the role of the alum solution bath? 3. Which is the most efficient way to obtain the desired effect (whitening of the scale surface)? 4. What is the meaning of working with copper scale to produce a copper-silver alloy?

In order to answer these questions, we set up an open experimental plan following on one side the process described in the texts stricto sensu (apart from the supposed cast in Leid 12) and, on the other side, the interpretation suggested by Robert Halleux for the Holm 6 text. Our approach focuses on the 'pre-treatment' of copper scale. Finally, a comparative test is carried out by melting the treated scale with silver. Both the raw materials and the products obtained from the different experiments are characterized using a multi-analytical approach.

Materials and methods

Choice of raw materials

In order to follow the processes described above in the most accurate way, it is necessary to employ materials, which we do not usually find in a chemistry lab.

Sea salt, vinegar and alum are employed without any further purification. Drinking water (pH=7.5 at 20°C) is used instead of the deionized one. The role of the different earths cited in the ancient texts is not investigated here as we suppose it not to be of primary importance for the whole process. Sea salt: unrefined sea salt is employed. Main component is NaCl.

Vinegar: red wine (6°) vinegar is employed. Measured pH at 20°C is 2.9.

Copper scale: copper scale is generally obtained by delamination of the oxide layer during hot-working of copper. The one employed in this study is produced by heating at 900°C a copper foil (99%, Alfa Aesar) of ca. 25 µm thickness. The atmosphere in the oven is not controlled and a natural ventilation is provided. After 2 hours, the foil is completely transformed: it appears black and brittle. Diffraction and Raman analysis are performed on the scale in order to determine its composition and are presented in the Results section. Synthetic cuprite (97%, Sigma-Aldrich) is employed in one of the experiments.

Alum

A natural crystal of alum retrieved from a drugstore in Yemen is employed (Figure 1). The exact crystallographic identification is presented in the Results section. Some considerations about this material are necessary. As Maurice Picon pointed out in the occasion of the conference on alum in the Mediterranean basin (Borgard et al, 2005), various natural, artificial and synthetic materials can be ascribed to the term 'alum'. Among the artificial alum, meaning the one produced by transforming the mineral, we can find potassium or ammonium alum

((NH4)Al(SO4)2•12H2O).
Natural alums are hydrated double salts of aluminium sulphate and another cation; the most common being halotrichite (FeAl2(SO4)4•22H2O), pickeringite (MgAl2(SO4)4•22H2O) and alunite (KAl3(SO4)2(OH)6). In order to be employed, they undergo a simple handpicking, while more complex production process is required to obtain artificial alum from alunite or, more recently, from pyritic schist.

Experimental plan

Pre-treatment of the copper scale: choice of the procedure

The interpretation of the texts mentioned above lead to the set-up of several recipes selected for the experimentation.

The 'pre-treatment' solutions are reported here:

 Solution 1 ("sol.1") is prepared by dissolving alum in 10 mL water until saturation is reached. 10 mL of vinegar are added to the solution. Final pH is 2.5 at 20°C.  Solution 2 ("sol.2") is prepared by dissolving alum in 20 mL water until saturation is reached. Final pH is 3.2 at 20°C.  Solution 3 ("sol.3") is prepared by dissolving sea salt in 20 mL water until saturation is reached. Final pH is 7.3 at 20°C.  Solution 4 ("sol.4") was prepared by mixing 10 mL alum-saturated solution in 10 mL sea salt-saturated solution. Final pH is 2.9 at 20°C.

Leid 12 : 'Two-steps procedure'

Step 1: 1 g of copper scale is immersed for 7 days4 in sol.1 (alum-vinegar). After 7 days, a first sampling is made on the scale.

Step 2: sol.1 is eliminated and the scale is immersed in sol.4 (salt-alum) for other 7 days. The scale is air dried and analysed.

Leid 12 : 'One-step procedure' 1 g of copper scale is immersed for 7 days in sol.2 (alum). No rinsing and no drying are employed.

Holm 6 : 'Two-steps procedure'

Step 1: 1 g of copper scale is immersed for 7 days in sol.3 (salt). After 7 days, one part of the scale (sample 3a) is air dried without rinsing. The other part (sample 3b) is rinsed with water and air dried.

Step 2: Sample 3a and 3b are immersed separately in sol.2 (alum) for 7 days. The scale is air dried and analysed.

Holm 6 : 'One-step procedure' 1 g of copper scale is immersed for 7 days in sol.4 (salt-alum). In this case, different processes are applied after immersion:

 No rinsing and no oven drying: sample 4a  3 times water rinsing followed by air drying: sample 4b  1 time water rinsing followed by air drying: sample 4c Holm 6 : 'synthetic cuprite' 1 g of synthetic Cu2O is immersed for 14 days in "sol. 4": sample 5.

Melting operations

After the first phase, two melting experiments were realised. Each was performed in an electric oven at T=1000°C for 15 minutes in a ceramic crucible. 1 g of silver and 0.2 g of copper scale are employed.

In one case, copper scale without any pre-treatment is added and in the second case, the pretreated copper scale 'sample 4b' was employed for the melting. We base our choice on the fact that the sample undergoes a change in colour comparable to the one described in the reference texts.

Characterization techniques

Different analytical techniques are employed for the characterization of both the raw materials and the materials obtained with the experimental study.

X-Ray Diffraction (XRD) analysis is carried out using a Rigaku RU200 diffractometer equipped with a Mo X-Ray tube. An acceleration voltage of 55 kV and a current of 21 mA are used. The monochromatic and focalized beam has an energy corresponding to 17.5 keV and a surface of 100x100µm 2 . The sample is irradiated by a flux in the order of 20•10 6 photons/s. Diffraction images are collected on photosensitive screens and diffraction patterns elaborated using the software Fit2D (ESFR). The identification of the phases is made using the software DiffracEVA (Bruker AXS) with reference to the X-Ray diffraction database of the International Centre for Diffraction Data (ICDD).

Using the same apparatus, X-Ray Fluorescence (XRF) spectra are obtained through a SDD detector with an active area of 10 mm² and then analyzed thanks to the PyMca software [START_REF] Solé | A multiplatform code for the analysis of energy-dispersive X-ray fluorescence spectra[END_REF].

For the Raman spectrosocpy, a Renishaw ® Invia Reflex µ-spectrometer is employed. It is equipped with a 532nm Nd:YAG laser and an optical microscope. The surface analyzed is of about 1µm 2 . Calibration is made via a Silicon wafer with a Raman band at 520.5 cm -1 .

The spectra are collected using a 50X objective; the energy used was 5 to 1% of laser power corresponding to 1 to 0.1 mW. Raman spectra have a resolution of 2 cm -1 and are processed using the software Wire3.4. They are presented here without baseline correction.

Results

Characterization of raw materials: copper scale and natural alum

Figure 2 shows the X-ray diffraction pattern of the copper scale. Two copper oxide are identified cuprite (Cu2O) and tenorite (CuO) are equally present. The presence of both copper oxides derives from the lack of control on the atmosphere during heating. Optical microscope observations point out the two different appearances of the copper scale: one side appears shiny while the other is matte. The difference is highlighted when performing Raman spectroscopy on both sides of the copper scale sheet (Figure 3). On one side (recto), the signature of tenorite is identified by the bands at 299, 349 and 635 cm - 1 [START_REF] Deng | In situ Raman spectroscopy of copper and copper oxide surfaces during electrochemical oxygen evolution reaction: identification of Cu II oxides as catalytically active species[END_REF]; on the other side (verso), the characteristic shifts of cuprite are identified (218, 198 cm -1 ) [START_REF] Bouchard | Catalogue of 45 reference Raman spectra of minerals concerning research in art history and archaeology, especially in corroded metals and coloured glass[END_REF].

The identification of both cuprous and cupric oxides in the scale leads to the conclusion that the material obtained is similar to the one obtained by the smith during annealing and hammering operations. It consists of small sheets of copper oxide whose composition is subjected to the variability of ventilation and temperature that characterize the traditional fireplace.

The alum used for this experimentation is analyzed. XRF analysis coupled with XRD analysis allow to classify it as a potassium-based alum (Figure 4); other trace elements such as of Zn, Pb and Cu are detected. By performing XRD analysis, the presence of quartz is highlighted: both quartz and the trace elements are a clear indication that the sample is of natural origin. Figure 5 shows the Raman spectrum of the alum crystal. The signature is characteristic of potassium alum [START_REF] Kishimura | Thermal dehydration of potash alum studied by Raman spectroscopy and X-ray diffraction analysis[END_REF]. Main shifts are: an intense band at 989 cm -1 , a broad band as doublet at 450 and 618 cm -1 . The doublet at 1096 and 1133 cm -1 is also observed, as well as the weak band at 326 cm -1 .

Results of the different processes before melting

With the exception of sample 5, all the experiments produce a change in color of the copper scale. The surface structural characterization by Raman analysis shows that immersion in the different solutions induces the deposition of a whitish layer of alum on the surface of the otherwise black-red scale.

In Figure 6, the Raman spectra corresponding to the different samples are reported. In samples 1, 2, 4a, 4b and 4c, potassium alum is detected as major component. In sample 4a, cuprite is also detected (by the 219 cm -1 pick) as minor component. In samples 3a and 3b, tenorite is detected as the major component, although some alum seems to be present on the surface. Sample 5 was not analyzed because, after 7 days of immersion in the chosen solution, the cuprite powder did not wet at all. We proceed with filtering of the solution and the powder shows the same red color of the original synthetic cuprite. Thus, this experiment is considered as a failure.

Results after experimental melting

is not optimal considering that the stirring is not possible considering the type of furnace and the low amount of matter involved (1.2 g). 5 A comparison of the XRF spectra for the melting experiments with and without pre-treated copper scale is shown in Figure 7. In both cases, the copper scale, with and without pre-treatment, forms an alloy with silver. In the case of the copper scale pre-treated with the salt-alum solution (sample 4b), we observe that the alloying process is more efficient as a larger amount of copper is found in the alloy compared to the not pre-treated scale. The efficiency of the alloying process drastically increases.

Discussion

The central aspect of all recipes detailed in the introduction is the color. Reference to the color is found in both steps: first, in the preparation of the copper scale that will be added to silver and, secondly, in the description of the final alloy obtained by the mix of copper scale and silver.

Following the interpretation of R. Halleux, we can assume that the purpose of treating the copper scale with alum is to deceive the client who is assisting to the melting. The goldsmith needs to add the copper scale, which has, without any treatment, a black-to-red color, to the white silver metal. The pre-treatment of the copper scale, as it is described in the texts, leads to a silvery-looking material. This is what is obtained by pre-treating the copper scale with solutions 1, 2, 4b, 4c and, to a lesser extent, 4a.

Moreover, it must be kept in mind that the alloy itself will remain silvery-white as long as the mass of copper does not exceed 85%. The title of the recipe ("doubling of silver") can be interpreted as an increase in mass. Therefore, the fraud is possible until 85% in mass of copper is added. By adding 85 to 92% in mass of copper, the color of the alloy will shift to a red hue that may arise suspicion, and beyond 92%, the alloy obtained can no longer be mistaken with silver.

Even without taking the term "doubling" literally, adding the right amount of copper scale, will easily allow to deceive the customer.

Moreover, the meaning of the recipes seems not to be related merely to a fraud by a change in color of the raw materials. In fact, as demonstrated by the XRF analysis on the alloy, the amount of copper that can be introduced into the solid solution with silver increases when a pretreatment of the copper scale based on alum is performed. This results in a more efficient process. Therefore, the smith could consider adopting the pre-treatment operations involving the use of alum.

We did not proceed further with the experiment, but it is likely that the alloy can be obtained even in the case of different coloration of the copper scale. These differences highlight two aspects of the treatments that are not mentioned in the text: the importance of rinsing after the immersion in solution of the scale and the change in color as a function of the time after immersion. In the case of sol. 4, only drying and rinsing operations allowed to obtain a whitish scale. In parallel, the recipe based on 'Leid 12', the copper scale turns white after the second and last immersion. The intermediate test carried out using only 'sol.2' containing a saturated solution of alum, also leads to white scale but only after several days of exposure to air and without any further operation such as rinsing or drying. Such protocol does not appear in any of the recipes.

A last interesting remark concerns the experiment carried out using synthetic cuprite powder.

The composition of the raw material is the same as expected for the copper scale but its reactivity with the solution is different. Such difference could be due to the nanometric grain size of the synthetic powder; the presence of pollutants adsorbed on the surface of the grains derived from the industrial synthesis could also influence the reactivity of the mineral. The failure of such experiment stresses the importance of using materials as similar as possible to those that could be found in ancient times.

Conclusion

The experimentation carried out leads us to the conclusion that the procedures described in the two recipes have a double meaning. On one side, the operation described turns the black copper scale into a whitish material, more suitable to trick a client who is assisting to the melting of an artefact. In this sense, this family of recipes seems to be part of a workshop practice aiming at deceiving "visually" a customer. The process involving the pre-treatment of the scale using alum can be related to an act of forgery.

On the other side, we demonstrate that it seems possible to melt silver and copper in a more efficient way by adding a copper scale pre-treated with the alum solution. We observe here a case in which recycling of a waste material has a positive effect on the production process. The goldsmith could easily get cheap copper scale from a coppersmith, considering that the scale can be defined as a production waste of the latter.

At the present state, it is not possible to assess whether or how common the process described in the papyri was for the production of silver objects; a regular production of pre-treated scale seems to be difficult to realize on a large-scale production. Nevertheless, the experimentation shows that alum, if employed following a precise procedure, can have a positive effect on the alloying process of silver.

Figure 1 .

 1 Figure 1. Raw materials, from left to right: copper scale, alum and sea salt.

Figure 2 :

 2 Figure 2: Diffraction pattern of the copper scale obtained after heating in the muffle furnace. The scale as it appears after heating (green) and the scale grinded (blue). In red, the characteristic signature of cuprite (Cu2O), and in black that of tenorite (CuO).

Figure 3 :

 3 Figure 3: Raman spectra of the copper scale.

Figure 4 :

 4 Figure 4: XRD analysis and X-ray fluorescence spectra of the alum crystal (the peaks of Cu, Zn and Pb are highlighted in the upright panel.

Figure 5 :

 5 Figure 5: Raman spectra of the alum crystal (red) and of a specimen of alum-K (RRUFF ID: R040134.2).

Figure 6 :

 6 Figure 6: Raman spectra of the copper scale after immersion in the different solutions. The presence of alum was determined thanks to the peak between 970 and 985cm -1 .

Figure 7 :

 7 Figure 7: XRF spectra of, in red, the alloy obtained by melting the silver with copper scale (no pre-treatment), and, in black, by melting silver and copper scale pre-treated with alum.

This paper is part of a Special Issue on the Contributions of Experimental Archaeology to Excavation and Material Studies, guest edited by Frederik W. Rademakers, Georges Verly, Florian Téreygeol and Johannes Auenmüller.

Mechanical properties of the alloy improve compared to pure silver.

This observation seems consistent considering the fact that the two recipes were originally part of the same collection of texts and they were separated after their discovery[START_REF] Halleux | Les alchimistes grecs, Papyrus de Leyde[END_REF].

7 days is the immersion time adopted for all the recipes based on Leid 12.

We did not investigate further the quality of the alloy formed in this way since the conditions are too far from those of a melting carried out in a historical furnace.

Acknowledgments

This work was funded by grants from the the Programme Collectif de Recherche "Paléométallurgie et expérimentations -Recherches sur les chaines de production des métaux aux périodes anciennes" conducted at the Experimental platform at Melle (France).

List of figures

The scale as it appears after heating (green) and the scale grinded (blue). In red the characteristic signature of cuprite (Cu2O), and in black that of tenorite (CuO).


Frederik W Rademakers

Georges Verly

Florian Téreygeol

Johannes Auenmüller

Claire Somaglino

Adeline Bats

Roeland Paardekooper

Jean-Philippe Marnais

Jean-Charles Méaudre

Contributions of Experimental Archaeology to Excavation and Material Studies

come

The formal conference was held on 25, 26 and 27 September at two locations in Paris. The first day, hosted by the Lettres Sorbonne Université, focused specifically on research taking place in the Nile Valley. The second and third days at MSH Paris-Saclay covered sessions dedicated to individual material categories, such as technical ceramics, precious metals and imaging techniques. These more conventional conference and discussion sessions were followed by experimental sessions held at the Plateforme expérimentale de Melle from 18 September to 1 October (Figs. 1234). There, the conference participants gathered to set up, participate in, or witness a range of experiments, many of which were prepared in the months preceding the conference. These included the casting of bronze plates in a furnace reconstructed after a recently re-excavated feature in Kerma, Sudan [START_REF] Bonnet | Un atelier de bronziers à Kerma[END_REF][START_REF] Bonnet | Le Temple Principal de la Ville de Kerma et son Quartier Religieux[END_REF][START_REF] Verly | Reprise de la fouille du four de bronziers de Kerma[END_REF][START_REF] Verly | PCR. Paléométallurgies et expérimentations. Rapport intermédiaire[END_REF], crucible-based copper smelting based on late prehistoric finds from Laos [START_REF] Cadet | Late prehistoric copper smelting in the Lao PDR: Experimental reconstruction based on the Vilabouly Complex evidence[END_REF], exploring recipes of silver refining from the 16 th century CE [START_REF] Van Bennekom | Explorative studies in 16th century silver refining recipes[END_REF], identifying ancient counterfeiting through 3 rd century CE coinage moulds (Méaudre et al. 2021), and interpreting 3 rd century CE alum based fake silver recipes [START_REF] Ferrari | New insights on the interpretation of alum-based fake silver recipes from 3rd century CE by and experimental archaeology approach in the laboratory[END_REF].

Following the conference, participants as well as other scholars who were unable to attend were invited to submit their papers for the Special Issue presented here in the Journal of Archaeological Science: Reports. As for the conference, the use of experimental archaeology is central to all contributions. This current Special Issue is preceded by a first volume resulting from the First International Conference on Archaeometallurgy (ICA I), held in Brussels in 2015 (Verly et al. 2019). The 2015 conference focused more specifically on non-ferrous metallurgies and some of the theoretical principles underlying the methodology of experimental archaeology (see Téreygeol 2019, Verly and[START_REF] Verly | Méthodologie en archéologie expérimentale -Définition et protocole[END_REF]references therein). This editorial does not aim to provide an overview of the long and interesting history of experimental archaeology, with its rich and varying traditions across the academic and non-academic world. Rather, it briefly explains the framework underlying this Special Issue, and provides an overview of the included papers.

Contributions of Experimental Archaeology to Excavation and Material Studies

Experimental archeology is primarily a research tool developed and used within interdisciplinary teams, often in relation to excavation sites. The first contribution of experimental reconstructions is to allow a better a posteriori interpretation of archaeological findings, structures and artefacts encountered in excavation. These attest to particular techniques employed in the past, which can be more comprehensively understood in themselves through experimental archaeology.

In addition to the interpretation of structural/architectural archaeological features, experimentation is used as a tool for the interpretation of man-made materials, such as metals, glass, ceramics, pigments and others, and the specific remains associated with their production. Experimental archaeology offers a tool by which hypotheses surrounding production remains can be tested. This a posteriori approach has been applied to the study of ancient production systems for many decades already and is indeed foundational for disciplines such as archaeometallurgy (e.g., [START_REF] Coghlan | Some experiments on the origin of early copper[END_REF][START_REF] Crew | Firesetting experiment at Rhiw Goch[END_REF], Dungworth and Doonan 2013[START_REF] Rothenberg | The Ancient Metallurgy of Copper: Archaeology-Experiment-Theory[END_REF][START_REF] Tylecote | Partitioning of trace elements between the ores, fluxes, slags and metal during the smelting of copper[END_REF][START_REF] Tylecote | Experimental smelting techniques: Achievements and future[END_REF] and lithics research (e.g., [START_REF] Audouze | La Chaîne Opératoire a 70 Ans : Qu'en Ont Fait Les Préhistoriens Français[END_REF][START_REF] Johnson | A history of flint-knapping experimentation[END_REF][START_REF] Keeley | Experimental Determination of Stone Tool Uses[END_REF][START_REF] Spear | A brief discussion of obsidian use-wear experiments[END_REF][START_REF] Walker | An experimental study of the morphological characteristics of tool marks[END_REF][START_REF] Warren | The experimental investigation of flint fracture and its application to problems of human implements[END_REF]). Gaining experimental experience and thus a fuller understanding of any kind of prehistoric and ancient material cultureis also part of several university programmes in the context of dedicated classes at, e.g., the universities of Leiden, Exeter, York and Manchester, to name but a few. Furthermore, and next to several smaller undertakings, the EXARC network has to be mentioned for its organisation and promotion of experimental archaeology across an even broader horizon beyond the academic world.

A specific aim of the ICA II conference reflected in this Special Issue was to explore the possibility to go beyond the classical approach of experimentation as a test of hypotheses formed at the end of the excavation. Firstly, experimental experience can equally be positioned upstream of the fieldwork and it can be of great help to better organise excavation strategies (e.g., [START_REF] Timberlake | Predictive experimental archaeology as a tool in the study of ancient mining and metallurgy[END_REF]. Secondly, similar structures can be excavated later by the team that carried out the experiments. This undoubtedly allows these exceptional witnesses to be investigated more thoroughly and more data to be recorded. This a priori approach is, however, not yet fully integrated in experimental archaeology. As such, this Special Issue not only illustrates the utility of experimental archaeology in understanding a range of archaeological materials, but equally highlights the important contributions experimental archaeology can offer to the practice of archaeological excavation.

In this regard, some key questions can be formulated:

 How does experimentation improve the 'reading' and understanding of complex structures, e.g., those related to material production activities?  What can the individual acts and processes of experimentation contribute to the various acts and processes of excavation (and vice versa)?  How can archaeological structures be interpreted in terms of the particular requirements of a reconstructed or reconstructible craft?  Can the use of historical texts contribute to the (re-)construction of experience and enable our sensory development for a better reading of the field?  How can particular production chains be experimentally approached for which little or no archeological evidence exists?  How can we expand from craft structures to the micro-economy of a whole site?  What are the human means put into place at the time to meet production requirements?  How does experimental taphonomy influence our approach to sites and structures? Each question constitutes an avenue for reflection and this list is of course not exhaustive.

An important point to stress here is that the extent to which experimental archaeology is elaborated can vary significantly, but even at different scales it can provide a useful contribution if evaluated carefully. As outlined by [START_REF] Verly | Méthodologie en archéologie expérimentale -Définition et protocole[END_REF], there are several phases through which a fully developed experimental protocol should go: gathering and evaluating the source materials, defining an interdisciplinary field, defining the teleologies for each individual element of the experiments, proposing a strict work protocol, defining the observation and registration methods, running the experiments and gathering data, testing and refining hypotheses, analysing results and publishing.

An experimental protocol matures over time, with the interdisciplinary team gaining experience and improving their protocol accordingly. As such, it may take many years before an experimental setup is adequate to fully address the questions initially set out by the team. However, the results of the learning phases from the preceding years are equally interesting, and worth publishing along with the guidelines set out above. Indeed, experimental work will rarely offer any final answers, but rather it will refine the hypotheses to be tested. The papers collected in this issue reflect this inevitably continuous state of research that characterises experimental archaeology: some papers present the first results of an experimental approach, while others offer the digests of many years of experimental replication. As editors, we are very happy to present a variety of contributions reflecting the nature of experimental archaeology in this way.

Overview of contributions

This Special Issue includes the following papers, in order of appearance: [START_REF] Hodgkinson | Working with fire: Making glass beads at Amarna using methods from metallurgical scenes[END_REF] present an experimental approach to the production of glass beads at Tell el-Amarna, Egypt, during the New Kingdom (c. 1550-1070 BCE). They draw on (indirect) iconographic evidence as well as excavation data to test whether such beads could be made in small-scale, non-specialised settings, which are expected to leave only ephemeral evidence in the archaeological record. They discuss their results in light of broader technological traditions attested in Egypt during this period, and within Amarna more specifically, and outline further work required to better understand this hitherto little studied technology. [START_REF] Téreygeol | The reverberatory furnace for ore smelting: An experiment on a South American innovation[END_REF] publish an experimental series on the use of a reverberatory furnace for silver ore smelting during the colonial period in the Andes, based on data obtained while prospecting 17 th century settlements in the Bolivian Lipez. This series, which ran from 2010 to 2015, allows to observe the mode of operation of this type of furnace for the first time. The repetitions of the experiment make it possible to appreciate the performance of the apparatus. These experiments also enabled the researchers to produce an analogical structure leading to a better interpretation of the remains in the field. [START_REF] Leroy | First experimental reconstruction of an Angkorian iron furnace (13th-14th centuries CE): Archaeological and archaeometric implications[END_REF] introduce the first experimental reconstruction of an Angkorian iron furnace dating to the 13 th -14 th century CE. On the one hand, the study aims at a better understanding of the smelting technology evidenced by the furnace itself, for which structural elements are often missing in the archaeological record. On the other hand, the experimental products provide a guide to the interpretation of the archaeological finds not only from a technological perspective, but equally towards provenancing and dating those finds. Finally, the experiments offer a platform for communicating research to the community and exchanging knowledge with local archaeologists. [START_REF] Bats | The production of bread in conical moulds at the beginning of the Egyptian Middle Kingdom: The contribution of experimental archaeology[END_REF] has developed a new chaîne opératoire for Egyptian Middle Kingdom (c. 2050-1900 BCE) bread production based on an integrated study of iconographical sources, archaeological evidence and a series of experiments in which earlier ideas and new protocols were tested and/or (in-)validated. Bats not only shows how the shape of the Middle Kingdom bread moulds informs about their horizontal position during firing and baking, but also how and when the unbaked bread dough was filled into the repeatedly used moulds. The use of "old wheat" and low gluten flour made from emmer and of a sourdough starter greatly facilitated the successful baking of bread according to the Egyptian evidence. The project has received public attention as it was prominently featured in the Spanish National Geographic online magazine in February 2021. [START_REF] Disser | Transformations of the chemical signature of slag inclusions throughout experimental refining and first shaping of bloomery iron: new methodological developments[END_REF] focus on changes in the chemical signature of slag inclusions introduced during smithing. Their experiments are designed to evaluate the influence of a smithing flux on the inclusions encountered in bloomery iron, while equally testing the efficiency of different flux types. Furthermore, an elaborate analysis programme is integrated within the experimental protocol, and the authors offer a detailed overview of the workflow for statistical treatment of such provenance data. As such, they provide a data processing method for the efficient identification and analysis of slag inclusions. [START_REF] Chantran | Boiled, fried, or roasted? Determining culinary practices in Medieval France through multidisciplinary experimental approaches[END_REF] have established a reference collection for the interpretation of cooking traces through experimental archaeology. Cooking vessels were used for boiling, frying and roasting experiments and subsequently subjected to use-wear analysis. Starch grain extraction for optical microscopy analysis acted as a complementary method to establish criteria towards distinguishing different cooking methods. The diet study of early Medieval Couzan provides an example of this multidisciplinary methodology, highlighting important advantages as well as possible limitations.

Van [START_REF] Van Bennekom | Explorative studies in 16th century silver refining recipes[END_REF] conducted an experimental series on the question of silver refining in the Netherlands (17 th century CE), based on the work of the silversmith Adam van Vianen. The research question arose following the analyses carried out on these silverware pieces and the experimental protocol was set up with the help of a set of three recipes reported in the Probierbüchlein of Calbus of Freiberg and the treatises of Lazarus Ercker. The mastery of test furnaces and refining techniques acquired over a long period of time [START_REF] Téreygeol | La coupellation des alliages cuivre -argent : Approches expérimentales de l'essai d'argent par voie sèche[END_REF] made it possible to carry out a series of experiments showing the relevance of the proposed solutions to obtain a fine silver. Questions remain open, notably on the bismuth content. [START_REF] Ferrari | New insights on the interpretation of alum-based fake silver recipes from 3rd century CE by and experimental archaeology approach in the laboratory[END_REF] present another way of experimentation that is directly integrated in laboratory practice. Here, we find the experimental dimension well known to chemists. The Leiden papyrus recipe collection from the 3 rd century CE provides the starting point for this work. This time, the texts are questioned in their operational dimension. The experimental series has shown that far from being a fanciful recipe, the use of alum with copper scraps makes it possible to produce an alloy with silver and thus confirms the title of the recipe which aims at increasing the mass of the metal. [START_REF] Workman | In search of the invisible hearth: An experimental perspective on early Levantine iron production[END_REF] have conducted iron smelting experiments on the basis of early Iron Age evidence from Tell es-Safi/Gath, Israel. Their contribution exemplifies an approach in which structural remains for smelting (furnace) and smithing (hearth) activities are missing from the archaeological record. Drawing on a detailed characterisation of production waste from the ancient workshops, they have developed experiments to evaluate mineralogical and microstructural changes occurring in technical ceramics during bloomery smelting in a simple 'bowl-style' furnace. These serve as a reference guide for the detection of invisible hearths and furnaces, represented by the inner vitrified wall fragments of such structures.

Hunkeler (2021) reports on the cartonnage fragments found in tomb KV40 in the Valley of the Kings in Western Thebes, Egypt, and provides insights into the 22nd Dynasty (c. 945-720 BCE) dating and typology of the respective coffins. A thorough review of earlier hypotheses on the manufacture of cartonnage and an in-depth study of the fragments from KV40 and of a further data set from close-by Dra Abu el-Naga enable the author to propose fresh ideas about materials as well as manufacturing and decoration procedures of this particular type of Egyptian coffin. In the following, an innovative experimental study is presented that not only aims at gaining a better understanding of the materiality of cartonnage itself, but also at comprehending the highly fragmented state in which the fragments in KV40 were encountered due to their exposition to fire and moisture. [START_REF] Verly | The chaine opératoire of Middle Kingdom smelting batteries and the problem of fuel: Excavation, experimental and analytical studies of ancient Egyptian metallurgy[END_REF] present the digested results of many years of experimentation on copper smelting furnaces from Middle Kingdom Ayn Soukhna, an Egyptian logistical hub and harbor on the Red Sea shore. The paper focuses on the identification of the fuel used in smelting, which is rarely targeted specifically in archaeometallurgical studies. Through an iterative protocol of experimentation, excavation and archaeometric analysis, the authors identify the combined use of fresh wood and donkey dung as the fuel of choice at Ayn Soukhna. Their work offers a detailed example of experimental archaeology informing excavation practices and vice versa, and the utility of integrating such an iterative approach in the long-term agenda for archaeological fieldwork. [START_REF] Cadet | Late prehistoric copper smelting in the Lao PDR: Experimental reconstruction based on the Vilabouly Complex evidence[END_REF] have organized a first ambitious approach on copper smelting techniques in Laos for the late Prehistoric Period. It a rare attempt in more than one respect. Firstly, the ores chosen include chalcocite in addition to the classic chalcopyrite and malachite. Secondly, tests on the ore charge assembly led successfully to the production of matte and metallic copper. Finally, the crucibles were reproduced to show by analogy that the use of unfired ceramics seems to have been favoured. [START_REF] Loepp | Some observations on the replication of precious metal artefacts in experimental archaeology[END_REF] presents a critical and detailed assessment of two copper salt bonding recipes reported by Dioscorides and Pliny and the surface treatment of gold objects based on the Leiden papyrus. Through a detailed documentation of the experimental replication, this paper offers a practical reading of ancient recipes from a goldsmith's perspective. The experimental protocol further involves (micro-)analysis of the experimental products in the evaluation of working hypotheses and sets out important guidelines for the identification of gold working techniques on ancient artefacts. Ars et al. (2021) present one of the few approaches to what can be called virtual experimentation. Using virtual modelling, the authors clearly demonstrate the constraints related to ventilation in a 16 th century CE French mine at Sainte-Marie-aux-Mines. Importantly, these tests strongly reinforce the hypotheses established during excavation, highlighting the existence of an artificial ventilation system that had never before been characterized during excavation but of which Agricola (1556) reported in his De Re Metallica. Méaudre et al. (2021) discuss the discovery of a 3 rd century CE brass workshop associated with the production of counterfeit money in the north of France, which provides the necessary material for thorough considerations on the production processes of counterfeit money. This reflection focuses mainly on how clay moulds were produced allowing to copy the coinage. The success of coin casting also opens up future thoughts on monetary quantification. Finally, the implemented casting processes leads to a reconsideration of certain artifacts from the excavation that could be similar to fragments of the clay envelope holding the stacks of moulds.

Concluding remarks

The first two colloquia made it possible to define a methodology and to demonstrate the importance of experimental archaeology not only as a toolkit to test and understand materials and production chains, but also as a developed instrument prior to excavatingand thus for the most part destroyingany archaeological remains. The better excavators understand the remains and findings in their individual function, development and taphonomy, the better their excavation and documentation strategy. A third colloquium is planned to address specifically the interaction between the relevant disciplines, such as History, Archaeology, Numismatics, Chemistry, Physics and Natural Sciences. The relationship of these disciplines to experimental archaeology is already apparent in several of the papers presented here. The third conference will highlight the opportunities for multidisciplinary research supported by archaeological experimentation. Some finishing thoughts are to be taken into consideration for future research. In particular, we need to:  Develop dedicated places like experimental archaeology platforms to allow the scientific experimental approach to develop and mature over time  Multiply and repeat experiences  Use materials that meet the requirements of archaeological teleologies  Reproduce the experimental protocols close to the excavation sites as well as in experimental structures linked to centres for archaeometry research  Create multidisciplinary teams with access to laboratories to develop protocols, excavate structures and analyse samples during all research phases  Understand the iterative character of experimental work in order to propose a specific operating chain and not an absolute operating mode  Exclude from the objectives of experimental archaeology: the assertion of a precise production figure, production according to profitability (a modern notion), the assertion of a single action, etc.  Understand the vitality and potential of the experimental approach to answer questions concerning history, archaeology and archaeometry

Figure 1 :

 1 Figure 1: The central structure at Melle, within which a large forge and closed ovens are housed (left) and visitors to the experimental platform (right).

Figure 2 :

 2 Figure 2: A reconstruction of Late Prehistoric smelting in Laos by Mélissa Cadet and colleagues.

Figure 3 :

 3 Figure 3: A gold experiment in a portable furnace by David Loepp (left) and a silver refining experiment by van Bennekom and colleagues beneath the central structure at Melle (right).

Figure 4 :

 4 Figure 4: A reconstruction of the Kerma bronze casting furnace by Georges Verly, Frederik Rademakers and colleagues.

Acknowledgements

We thank here again the Lettres Sorbonne Université (ED 1, FIR), CNRS UMR 8167 Orient & Méditerranée (team Mondes pharaoniques) and the MSH Paris-Saclay, as well as the French Ministry of Culture. Furthermore, we are very grateful for the support towards realizing this conference and its publication received from the Egyptian Antiquities section at the Museum of Art and History, Brussels, the Laboratoire Archéomatériaux et Prévision de l'Altération (LMC IRAMAT UMR5060 CNRS et NIMBE UMR3685 CEA/CNRS), the KU Leuven -Department of Earth and Environmental Sciences, the Institut für Ägyptologie und Koptologie Westfälische Wilhelms-Universität Münster, Archéosite d'Aubechies, the Mines d'argent des Rois Francs de Melle and the Belgian Science Policy. Finally, we want to thank all participants of the conference and the individuals working before and behind the scenes, including the Brussels Master's students NAME, NAME, NAME and NAME. … who all made this conference and the experiments such a great success.

was made possible thanks to the support of the Lettres reconstruction of the Kerma casting furnace.