The most informative crucible remains concerning the identification of the raw materials that were processed in these vessels are the matte residues and metallic phases included in the crucible slag. The results of their analyses will therefore be discussed first. These considerations will be followed by the interpretation of the
data regarding the bulk of the slag, which allows the determination of the potential ore gangue and/or fluxes added to the crucible charge.
Matte and metallic inclusions of the crucible slag residues
The matte phases in the slag suggest the processing of a sulphidic ore, part of this sulphur not having been oxidised during the reducing fusion in the crucible, and forming phases separate from both the silica-rich and the metallic systems. The recurrence of antimony, copper, lead and silver implies the processing of a fahlore, a complex sulphidic ore ((Cu, Fe, Pb, Ag)12(Sb, As, Bi)4S13), probably for its content
in precious metals. The identification of zinc in various slag matrices, even in low concentrations (Table 5.1, p. 121), fits this idea, as sphalerite (ZnS) is a very common sulphide and frequently occurs with fahlores and silver ores. More generally, fahlores usually grow in association with base metal sulphides such as sphalerite, pyrite, galena, etc. The matte inclusions in the slag confirm that three layers probably formed in the crucibles: the lead-rich metallic bullion at the bottom, a sulphide-based system in between (the matte), and an oxide-based system on top (the slag). These three melts would separate by density in the crucible due to their non-miscibility in the liquid state. However depending on the overall viscosity of the melt, possible mixing during pouring, and the cooling rate, some sulphides and metal prills would be trapped in the slag, forming these discrete globules.
The main part of the lead detected in the matte and metallic residues most likely originates from the addition of metallic lead used as collector for the precious metals, although part of it may come from the ore itself, in the form of galena. The silver from the ore is mostly collected by the lead, while a small portion remains trapped in the matte. While the metal would be further processed to extract the silver, it appears that the slag was discarded; therefore any silver trapped in the matte cakes and in matte or metal globules within the slag would be lost. Thus, if the results of the assay were to be quantified, the amount of silver in the ore would be slightly underestimated. It will be shown later in this chapter, however, that matte was probably also occasionally scorified with some additional silver-free lead, so the amount of silver present in this phase would be quantified as well. Matte may have also been refined together with the lead bullion in a scorifier if the overall purpose of the operation was to quantify the noble metal content of the ore, without a specific
The metallic prills give further information about the metals which were extracted from the ore(s), containing antimony, copper and silver. As stated above, despite the other metals, the occurrence of silver, in the broader context of other refining equipment such as cupels, strongly supports the idea of the assay of ores for their silver content.
Crucible slag remains
There are several possible origins for the crucible slag, which are not mutually exclusive. Some of the crucible ceramic could have melted during use and contributed to the slag formation. However, not all the elements present can be explained by this scenario. The slag is relatively rich in iron when compared to the ceramic matrices of these crucibles (Martinón-Torres and Rehren 2005b): the SiO2/FeO ratio is ca. 11 in the ceramic while it only varies from ca. 2 to 9, with an
average of ca. 4, in the slag, meaning there is almost three times more iron than expected, if the slag were simply fused ceramic (Fig. 5.23). A similar situation is visible for the calcium oxide concentration, which is about eight times higher than it would be if originating from the ceramic only: the SiO2/CaO ratio is ca. 82 in the
ceramic while it ranges from ca. 2 to 13 in the slag, with an average of ca. 6 (Fig. 5.24). Furthermore, the SiO2/Al2O3 ratio is higher in the slag (ca. 6) than in the
ceramic (< 2) (Fig. 5.23). This could result from either a selective melting of the ceramic fabric and/or, more likely, an additional source for silica, especially for the crucibles showing a clean-cut interface between ceramic and slag and therefore no significant chemical interaction between the two. Thus, the contribution from the ceramic to the slag is very limited. Finally, we know that unused crucibles are free of zinc, antimony and lead (Martinón-Torres and Rehren 2005b), so these are clearly elements that formed part of the charge and, in turn, contaminated the crucible fabric.
0 2 4 6 8 10 12 14 ceramic average OB 307/S2 OB 461/S1 OB 479/S1 OB 494/S1- S2 OB 498/S1 OB 509/S1 OB N001/S1 OB N001/S2 OB N003/S1
Fig. 5.23. Diagram comparing the SiO2/FeO (blue) and SiO2/Al2O3 (purple) ratios
between the ceramic fabric (wt% of oxides averaged after Martinón-Torres and Rehren 2005b) and the glass matrix of the crucible slag specimens.
0 10 20 30 40 50 60 70 80 90 ceramic average OB 307/S2 OB 461/S1 OB 479/S1 OB 494/S1-S2 OB 498/S1 OB 509/S1 OB N001/S1 OB N001/S2 OB N003/S1
Fig. 5.24. Diagram comparing the SiO2/CaO ratio between the ceramic fabric (wt%
of oxides averaged after Martinón-Torres and Rehren 2005b) and the glass matrix of the crucible slag specimens.
On this basis, it can be argued that the gangue of the ore tested could have been rich in quartz and/or calcite, enriching the slag in silica and calcium oxide. Quartz is a common gangue mineral for many ores – in particular gold ores and several sulphidic ores –; and both quartz and calcite are the gangue of many copper ores and complex sulphidic ores such as fahlores. Biringuccio mentions limestone as a gangue mineral of copper ores and silver- or gold-bearing lead ores (Smith and Gnudi 1990: 53). Furthermore, although Agricola explains that adding lime – among other reagents – could be of help in smelting poor ores (Hoover and Hoover 1950: 388- 390), evidence seems to show that, in the present case, glass was more plausibly added instead of limestone or calcite (see below). The high levels of calcium oxide in the slag may thus be the result of an accidental or intentional introduction of gangue fragments attached to the sulphidic minerals to the charge and/or the addition of glass, in which calcium oxide is used as stabiliser, as flux.
Besides the partial fusion of the crucible fabric and the presence of siliceous gangue, other potential sources for the silica in the slag may be considered. Another source could be a silica-rich flux added to lower the melting temperature of an iron- containing ore, by forming more easily melted iron silicates. Agricola recommends adding “stones which melt easily in the fire”, among which he counts quartz, to smelt ores that “heat and fuse slowly” (Hoover and Hoover 1950: 380). This would be the case for a refractory zinc- and iron-containing ore with a calcite gangue, such as the one tentatively identified in these crucibles. Lastly, following some of the contemporary writers (Hoover and Hoover 1950: 238; Smith and Gnudi 1990: 143, 333), the chymist working in Oberstockstall may have used crushed glass as a flux or part of a mixture of fluxes. This could explain the combined high concentrations of silica, potash and lime in the slag (Table 5.1, p. 121). Although these element concentrations show no positive correlation, this is probably due to the tendency of potash to evaporate at high temperature, and to the fact that, as mentioned above, gangue minerals could also have contributed to the lime and silica levels in the slag. It should also be noted that the amount of potash in some of the slag cannot be explained as a result of ash contamination alone, since it has previously been shown that the crucibles were heated from the outside and the quantity of charcoal ash that would be required to produce such enrichment is not conceivable in crucibles of this size range. Lime could partly come from the gangue as well as from the glass. Early
Wedepohl 1998), and was readily available at the laboratory of Oberstockstall. Thus, the raised silica levels could originate from the addition of glass to the charge. There may have been further additions of glass or quartz in the course of the reaction, but the complexity of the resulting slag system does not allow to clarify this.
Turning to the iron content in the slag, this probably originates from pyritic ores, the most common sulphides. Gold often occurs disseminated in veins of pyrite or arsenopyrite, and a crystal of arsenopyrite, a minor ore for gold, has been identified by Soukup and Mayer (1997) in the assemblage of Oberstockstall. It may be worth noting that, due to its apparent similarity with gold, pyrite is widely known as the ‘fool’s gold’ or the ‘gold of the alchemists’. This element also fits the structure of fahlores, potentially the main ore processed at this site. However, iron could also have been added as a flux to drive the ore to melt. As already mentioned, Ercker advises the addition of iron filings, that is metal, if the ore does not contain enough of it (Sisco and Smith 1951: 114). At estimated furnace temperatures (1100-1200 °C), metallic iron would react with silica, alumina and other gangue materials, to form a relatively fluid slag, which would allow an easier and finer separation of the different phases. When smelting gold-rich pyrites on a small-scale in a crucible, Agricola recommends iron filings or hammer scales to be used in addition to antimony sulphide, copper filings and lead as flux; he also uses iron scales as a flux combined with lead oxide-rich additives when smelting poor gold ores on a large scale in the blast furnace (Hoover and Hoover 1950: 397-398). The latter method described by Agricola is probably not what is documented in the Oberstockstall crucibles analysed here, given that if iron oxide from the slag was to react with a sulphidic ore at high temperature, the sulphur would drive off and the formation of sulphur-rich matte, as documented in the archaeological material, could not have taken place. Besides, these recipes are given in the context of smelting, though on both scales, but are not particularly duplicated in the chapter dedicated to assaying. Nonetheless, although this procedure is relatively unlikely to have been regularly performed, it may have been experimented with. Starting with such oxide-rich additives would make the formation of a thick matte layer possible only if the sulphur included in the ore was contained within the crucible, e.g. by a quickly formed slag layer or the addition of glass or salt, which would act as a cover for the melt and retain most of the sulphur. This phenomenon has been observed by
during which copper minerals were reduced to metal, and matte formed when some slag was added as a flux on top of the charge. However, it remains very difficult to further establish which chemical reactions would control the system, since these would depend on their mechanisms and kinetics at the estimated temperatures. Overall, all one can say from the slag analyses is that iron could be part of the ore, the flux or both.
The bulk soda content in the glass matrices of the crucible slag varies between 3 and 7 wt%, which is much higher than the concentration in the ceramic matrix. This, together with the fact that the few feldspar inclusions of the crucible fabric are mostly potassium-based, clearly demonstrates that this soda enrichment in the slag does not find its origin in the ceramic. Besides, the Na2O/Al2O3 molar ratio in the
slag glass ranges from 0.2 to 4.0, with an average of ca. 1.3 (Table 5.1, p. 121). This ratio being generally higher than 1 seems to indicate that sodium has not been introduced by accident, e.g. from a feldspar (NaAlSi3O8, Na2O/Al2O3 molar ratio =
1), but intentionally added (I. Freestone, pers. comm.). This sodium-rich raw material could have been common salt, some sodium carbonate or nitrate, as recommended by the main Renaissance authors. These salts would act either as a cover for the charge, which would help prevent projections and therefore losses at the beginning of the reaction, or as a flux, because they would immediately melt in the hot crucible and drive the rest of the charge to melt. The hypothesis of the addition of rock salt as a flux seems to be supported by the apparent correlation between sodium and chlorine levels in the contaminated fabric of several used crucibles (Martinón-Torres 2005: 111) The concentration of chlorine in the various glass matrices of the slag, which is mostly around 1 wt%, could also be an indication of this use of rock salt. The amount of chlorine ions soluble in a glass is however limited by the structure of the glass itself to ca. 1 wt%, additional chlorides then forming a separate melt (Tanimoto and Rehren 2008). The glass matrix of the slag thus appears saturated in chlorides, which suggests that chlorine was not introduced in the glass from the burial environment but results from an intentional addition of a chlorine compound, which would have most likely decomposed at high temperature and individual ions would have then entered the glass matrix of the forming slag. This observation may, however, still not be a good enough confirmation of the addition of rock salt, since there may be another or additional source for sodium in
potash-lime-silica glass was the main glass type used during the Renaissance in northern Europe, soda-lime-silica glass was regularly imported from Italy, particularly Venice (Henderson 2000: 90-100). The high amount of sodium oxide in the slag could therefore also be due to the addition of such type of glass. This would be in conformity with the occurrence of Venetian glass objects in the assemblage from Oberstockstall (Von Osten 1998: 70). As mentioned above, the use of glass as flux seems to be suggested further by the high levels of calcium oxide in the slag.
In summary, based on the analytical results, it appears that a relatively large number, and possibly the majority, of the triangular crucibles in the laboratory assemblage from Oberstockstall was used for the reducing fusion of sulphidic minerals, such as tetrahedrite, pyrite or galena, having a gangue composed of calcite and/or quartz. Most likely, several fluxes were added to the charge and contributed to the simultaneous formation of metal, matte and slag. Lead was probably introduced as collector for the noble metals, but it may have also been part of the ore, since galena is a common mineral occurring in sulphidic argentiferous ores. Similarly, iron may have been used as a flux: adding iron metal to a fahlore, such as tetrahedrite, would produce pyrrhotite as well as metallic copper and antimony, since iron would preferentially combine with sulphur and facilitate the reduction of the base metals in the ore. Iron can also originate from a second source, as it could have been a potential component of the ore to be tested. Potash and soda present in the slag are probably not elements from the ore, and were almost certainly part of additional reagents. Fluxes such as glass, iron filings and some salts were probably added to the ore to perform the first stage of the metallurgical sequence under reducing conditions.
Altogether, this reconstruction is consistent with written sources; Biringuccio, Agricola and Ercker all recommend the use of various fluxes for the reducing fusion and testing of ores (cf. chapter 4). Of these, some glass and salt – probably a combination of potash-rich glass with rock salt or sodium carbonate, or some soda- lime-silica glass with saltpetre or potassium carbonate or a mixture of all these –, together with iron filings seem the most likely possibilities here, helping the melting of a fahlore.
texts and laboratory practice as documented in Oberstockstall. One of the most remarkable differences appears to be the roasting stage, recommended by the former when processing sulphidic ores, and apparently absent in the latter, which led to the formation of crucible matte. These similarities and differences will be discussed in more detail later (cf. chapter 7).
Oxidised crucible bullion and matte phases
Oxidised bullion and matte remains were identified within the triangular crucibles, as presented in the analysis above. The differentiation between oxidised matte and bullion residues was possible based on the thorough characterisation of the matte cakes and fragments (Fig. 5.7, p.116), which is described in more detail below. As will be seen, these isolated pieces of matte usually show a similar microstructure to each other, i.e. a complex mixture of sulphur-based phases, mainly composed of iron, copper, antimony, and lead, with many magnetite crystals and entrapped metallic prills of antimony and/or copper, and slag particles.
The low bulk sulphur content (3 wt% SO3) of specimen OB 307/S1 seems to
indicate that the metallurgical remains of this crucible are from an oxidised bullion. This is reinforced by the oxidised phases of lead and copper, which do not show any residual sulphidic core, contrary to the clear matte phases of OB 307/S2, which are now present as sulphates. Thus, the existence of both metallic and matte residues in the same vessel– as well as a slag layer (Table 5.1, p. 121) – verifies the hypothesis of the formation of three layers in the triangular crucibles.
Based on its lead-rich matrix (87 wt% PbO), the residue removed from the inside of crucible OB 464 seems to be a lead bullion, now partly oxidised. The separate phases of pure oxidised lead and pure copper confirm the metallic origin of this layer and the most likely use of lead as metal collector. It is clear that this formed under a matte layer, due to its numerous inclusions containing sulphur in high concentration. Contrary to most of the metallic and sulphidic remains, this sample has no antimony, but a high arsenic level instead (4 wt% As2O3 in the matrix). This again supports the
idea of an ore as the raw material to have been processed in these crucibles, since fahlores are either dominated by antimony (tetrahedrite) or arsenic (tennantite). According to this, the sample tested here was probably of the tennantite family, therefore slightly different from the bulk of the raw materials identified in this
concentrations of silver (4 wt% Ag) in the lead prills indicate that this ore was also processed for its content in noble silver, similarly to the antimony-rich ores. In addition, this particular sample shows that in a bullion produced from an ore devoid of antimony and dominated by another element, such as arsenic, antimony is not detected at all, which further indicates that in the samples where antimony has been detected, it most likely originates from the ore. This seems to rule out the possibility of antimony sulphide being routinely used as a flux or an additive, such as precious metals collector, as advised by Agricola and Ercker (cf. chapter 4 and see above). If