Crucibles and moulds

Crucibles and moulds are fairly easy can be affected, losing its normal enough copper to produce this to recognise and their identification structure and, at times, developing a colour. Lead and tin have low should alert the fieldworker to the vesicular, spongy texture. Some melting points and so were not presence of metalworking on site. crucibles have an added outer layer usually melted in crucibles; any This provides an early opportunity of less refractory clay which is domestic pot would do instead. to link the finds to associated usually heavily vitrified. materials (like scrap and waste Crucibles come in a great varicty of metal, furnace lining and slags) and Small quantities of the metal being sizes and shapes, though at any one structural features, such as hearths. melted may have become chemically period a more limited range was in bound in the vitrified surface of the use. From the Roman period Some of the vessels that may be crucible, or physically trapped there onwards, some crucibles arc wheel- identified as crucibles in the course as droplets. Chemical analysis of thrown but hand-made crucibles of excavations were actually used the slag layer or the metal droplets continue and are still found in for processes other than metal can indicate, with more or less medieval times. Their volume runs melting. Glass working is one of certainty, the composition of the from thimble-sized to more than pint these processes (the vitreous metal being melted. In some cases beer-mug size. The larger sizes deposits are relatively homogeneous analysis may be the only way of sometimes date to the Roman period and are mainly inside the crucible). identifying processes like the but most are later medieval or post- Precious metals were refined using manufacture of brass by medieval. Some forms are relatively vessels that can be confused with cementation. Visual examination, well-dated but simple hand-made crucibles; these processes and the with the naked eye or under low thumb pots are virtually undatable. finds associated with them are magni~cation (xl0-x30), can often Most crucibles were open-topped described in Datasheet 2. give some idea of the metal being though a few types had lids or the melted. Copper and its alloys can be rims were pinched together to Metal melting crucibles Crucibles seen as green corrosion products, produce an enclosed form. A few are ceramic containers used to hold like on metal objects, or as bright crucibles had knob-like handles on metals being melted. Their fabrics red patches where they are the side or lid. must be refractory (able to chemically bound in the vitrified withstand high temperatures) and surface of the crucible. Silver is On sites where large numbers of are almost invariably reduced fired visible as dark brownish-black crucibles have been found there is (grey or black) as metals have to be blobs up to a few mm across which some suggestion that individual melted under reducing conditions to are quite soft and sometimes have types were preferred for particular stop them being oxidised and lost pale purplish powdery corrosion metals. However, there is also into the crucible slag. Because of deposits around them. Gold is evidence that whatever was the high temperatures at which they unaltered, though it is usually only available was used. In general, are used, crucibles become vitrified, present as tiny droplets, well under precious metals, particularly gold, like hearth lining. This vitrification 1mm in diameter. Crucibles used to were melted in small quantities :rod can be just a thin surface gloss or melt silver and gold often have so are usually found in smaller glaze but in more extreme cases the copper-red areas on them as the crucibles. whole of the thickness of the fabric precious metals usually contained

The Historical Metallurgy Society: Archaeology Datasheet No. 1 Downloaded from hist-met.org Moulds normally used at later periods only the metal droplets are usually firmly Molten metal was cast, either direct for larger, complex castings. The held by the slag layers. into objects, or into small ingots. moulds had to be broken to remove These could be hammered to the casting so fragments are usually However, if large quantities of metal produce rods, wire or sheet which small and have broken edges all are trapped in them it is best to store were in turn made into objects. Ingot round. The only original inner them as though they were metal moulds are usually made of stone surface is that of the matrix. objects. though some are brick or tile with shapes cut into them. The Unlike investment moulds, the Moulds are generally not well-fired commonest shape in ingot moulds is valves of piece moulds could be and so are fragile and friable. A a bar about one cm across, though taken apart, but this was only so the vigorous wash will certainly remove discs and other shapes are also pattern from which they were made almost all traces of metals -- which found. Surface finish was not could be recovered; they are not may not have been visible to the eye important as the metal ingot was to likely to have been used more than -- and can even destroy the moulds. be worked. The mould surface in once. Stone moulds were far more and around the cut-out shapes is durable and would have been used Further reading often blackened and traces of metal many times over. Clay piece moulds The following publications all may survive. can be identified by their original contain illustrations of a range of edges, the mating surfaces around crucibles and moulds of different Moulds for small objects were the matrix (often with locating dates, as do many excavation usually made of either fired clay or marks) and traces of luting clay on reports. (less-commonly) fine-grained stone. the outside, all of which are absent For casting lead or its alloys, wood from investment moulds. Bayley, J (1988) Non-ferrous or antler moulds could also be used. metalworking: continuity and Clay moulds are not very common Large objects such as cauldrons and change. In E A Slater and J O finds, partly because they are friable bells were also cast in moulds, Tate (eds) Science and and so do not survive well unless sometimes with a tallow model Archaeology, Glasgow, 1987, they were buried soon after being which had to be melted out, and 193-208. BAR Brit Ser 196. discarded. sometimes without. The process of Bayley, J (1990) Evidence for making these moulds is well known metalworking. Datasheet 12. Diagnostic features of moulds are from medieval documents such as Finds Research Group 700- the funnel-shaped in-gates or sprue Theophilus' De diversis artibus. 1700 AD. cups, runners (channels the molten They are normally found in small Bayley, J (1992) Metalworking metal ran through) and the matrix fragments as the mould was broken ceramics. Medieval Ceramics for the object being cast. They are to remove the casting and the 16, 3-10. usually reduced fired (grey or black) organic tempered fabric that was Tylecote, R F (1986) The in and near the runners and matrix usually used tends to be soft. prehistory of metallurgy in the but the rest of the mould is often Typical fragments of the cope (outer British Isles. oxidised fired (red or brown). part of the mould) are about 2-3 cm Youngs, S (ed) (1989) 'The workof thick while the core (inner part) can angels' Masterpieces of Celtic Two main types of clay moulds, be thicker. A zone up to about one metalwork, 6th-9th centuries investment moulds and piece cm from the modelled face is AD. moulds, are found. usually black or grey while the rest of the mould is oxidised fired. Investment moulds were made by Sometimes the inner surface is Justine Bayley modelling the object to be cast in coated with a fine white slip. Ancient wax and coating it thickly in clay. Monuments Laboratory The mould was then fired, the wax Handling and storage English Heritage melted or burnt out, and the molten Most crucibles are fairly robust and metal poured in. This type of lost do not need any special storage March 1995 wax (cire perdue) mould was in conditions. Even vigorous washing general use in the Iron Age but was is unlikely to do much damage as

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