CLAUSULA 24ª. MESA DE CONTRATACIÓN

As shown above, with very few exceptions, the collector universally recommended in fire assay for gold and silver is pure lead metal, because of its strong affinity with noble metals, and probably its availability and low cost. This lead needs to be free of any other metal to avoid the contamination of the assay, especially when assaying for silver, as lead and silver often occur together. If silver- free lead is not available, the lead has to be assayed separately to determine its silver content, before assaying the ore, metal or alloy to be tested. This control of the purity of the lead by the assayer is strongly advised by all three sixteenth-century authors (Hoover and Hoover 1950: 239; Sisco and Smith 1951: 45-46; Smith and Gnudi 1990: 139).

The description of the fluxes to add to the charge, when necessary, can be more ambiguous. For instance, Biringuccio unclearly refers to his fluxes for assaying ores as “whatever you have found goes with it best” (Smith and Gnudi 1990: 140). The criteria for choosing such specific fluxes apparently lie in experiments to be carried out by the assayer himself (Smith and Gnudi 1990: 143), as no precise guidelines or typical examples are provided in Biringuccio’s text, rather an apparently random list of potential materials which could be used as fluxes.

Similarly, when Agricola refers to fluxes, he uses the Latin word “additamenta” which is translated as ‘flux’ by Hoover and Hoover. Although a flux is nowadays defined as a substance which lowers the melting temperature of the charge, Agricola’s ‘flux’ includes the above definition as well as chemical reagents which can reduce, oxidise, sulphurise, desulphurise and collect materials of the charge (Hoover and Hoover 1950: 232). He also calls fluxes “those things which can liquefy it [the ore] and purge it of its dross” (Hoover and Hoover 1950: 241), therefore remaining vague on what compounds to add when one looks at the many recipes he gives, and leaving the choice to the assayer while conducting his experiment. He lists many compounds which can be used as fluxes, and several of which are reported below, and classifies them into four distinct categories (Hoover and Hoover 1950: 232-234). The selection of one or more fluxes for one particular assay seems to depend on the material(s) contained in the ore and on the fumes exhaled by the ore when heated (Hoover and Hoover 1950: 235-236). In the case of gold ores, Agricola also advises in various situations – even if not detailing which ones – the use of sal

artificiosus or sal torrefactus, which are not readily understandable by the modern

scholar; of “some powder compound which melts ore”; of glass-gall, which can be defined as a “neutral salt skimmed off the surface of melted glass” (Smith and Forbes 1969: 62; Tanimoto and Rehren 2008); or of roasted argol, a crude potassium hydrogen tartrate (Hoover and Hoover 1950: 242-243).

On the contrary, Ercker gives very precise recipes, which he uses to assay gold and silver ores. For gold ores, he produces his flux by mixing and melting litharge with antimonysulphide. Metallic iron in the form of filings is then to be added if the ore does not contain any iron, so that the sulphides combine with iron while gold and silver form an alloy with the antimony and the lead (Sisco and Smith 1951: 114),

layer due to the affinity of silver for sulphur. In the case of rich and clean gold concentrates, he also recommends a flux made of burnt argol and saltpetre (potassium nitrate) (Sisco and Smith 1951: 108-110). For silver ores, and particularly the refractory ones, he uses a “glass of lead”, which is mostly composed of lead silicates (Sisco and Smith 1951: 33-34). More details about these fluxes are given below in the context of smelting.

Other fluxes are given for the crucible fusion of particular ores or specific situations in assaying, such as the presence/absence of an element in the ore or the refractoriness of the ore, among the main parameters influencing the selection of appropriate fluxes. Some examples are salt, vitriol (iron sulphate, copper sulphate), sal ammoniac (ammonium chloride), alkali carbonates (potassium and sodium carbonates), borax, crushed glass, pebbles, slag from previous smelting, metals (iron, copper), sulphides (pyrite, galena), etc.

A last important point to be made here is that, despite the relatively confusing depiction of the various fluxes and recipes, the proportion of reagents to be added are accurately quantified for each ‘recipe’. The justifications for these particular ratios are not explained, and may simply be a reflection of the author’s own experience – in the case of Biringuccio or Ercker, or observation – for Agricola.

The sixteenth-century treatises explain how to prepare the equipment needed for fire assay, how and when to combine the various collectors and fluxes and in which quantity, and finally how to conduct this operation. These written recipes are of higher interest if they find their equivalent in practice and this was where the scientific study of the assemblage from the Oberstockstall laboratory became relevant. As will be seen in chapter 5 of this thesis, this material has helped understand the practical side of fire assay in a particular context.

The metallurgical large-scale counterpart to fire assay being smelting, the following section will focus on industrial scale processes as described in the three written sources introduced above.