Clay & Ceramics in “On Divers Arts” – Medieval Crucibles Part 1

by Jamie Hall on March 12, 2016

Ceramics are important for the metalworker, and for other high-temperature crafts. This was no less true in the middle ages than it is today; clay was an essential natural product that was found in most localities, and whether used to make containers, as a refractory, or in other forms, it cannot be ignored. Modern ceramics are mass produced, ubiquitous, and may be invisible to the casual observer, hidden away as components inside equipment. Based on the descriptions by Theophilus, ceramics were a visible and ever-present necessity in the medieval workshop.

My current project is setting up an Anglo-Saxon workshop, and ceramic crucibles are an unavoidable requirement, in which metal is melted for casting, or chemical reagents are processed. The next blog post will look at the archaeological evidence for these crucibles, but first, I want to look at written evidence. The only appropriate text is Theophilus, and even this is tenuous – a bit too late in history, a bit too far away, and a bit too organised. But it’s the only option – the other extant text for this period is the Mappae Clavicula family of manuscripts, but that is a collection of recipes, and not a workshop handbook. With Theophilus, we have a description of intent, even if the details are not always precise.

Book I (on painting) has scant reference to clay, using it only for earthy pigments. Book II (on glass) has more detail, as there are several glassworking furnaces described. In Book III, it begins to feel as if the workshop would be severely limited by the absense of clay. Numerous uses are described, and more than one type of clay is specified.

The most obvious purpose is refractory – clay mixed with dung, or with stones, is used for various furnaces, some of which are temporary, and built in situ (eg. the furnace for heating the bell mould in III-85). Their size and lifespan suggests that they are not fully fired ceramic vessels, but they do need to stand up to the heat, and their own weight. The type of clay is never specified by colour or other properties, and the temper (materials mixed into the clay) seem more important. An Anglo-Saxon metalworker might have used specialised hearths, but the small volumes of metal required for most tasks would have been achievable in a simple hearth, and the more elaborate furnace designs can be ignored, for my purposes.

Medieval crucibles from Wales, via crucibles from Wales, via

The second key type of refractory is the crucible, and this is harder to avoid. True, there are other ways to melt metals, such as on a block of charcoal, but these are troublesome and unreliable compared to the shape and strength of a crucible. The descriptions of crucibles are very different to the furnaces; “white clay” is mentioned in III-22, and III-65 says to “take clay from which earthenware pots are made – there are two types of this, one white, the other gray; the white is good for colouring gold, the other for making these [brassworking] crucibles”. In both cases, these are tempered with old crucibles. Elsewhere, “fire-tested” dishes are mentioned, (eg. III-33, and III-37), suggesting either that they had been previously used, or perhaps had been fired to a high temperature, to ensure that they could stand up to the heat. Crucible strength is particularly important, as they hold molten metal – or other hot chemicals – which would harm the practicioner, or result in loss of metal (which is the real terror of the goldsmith, believe me). It is unsurprising that they are held to a high technical standard than mere furnace linings, and their working temperatures could be in excess of 1000ºC. For larger volumes of metal, such as the refining of copper in III-67, an iron pan is covered inside and out with clay. There are some other crucible-like objects, such as the “burnt brick” used as a scorifier in III-50, and some are not ceramics at all – the cupellation process uses bone ash cupels, which trade strength and longevity for absorbtion; they must be porous, to soak up metal oxides when refining gold and silver. With enough temper to make them porous, clay cupels may be possible – it is unclear when bone ash was first used for the process, and may have been antedated by ceramics or other materials.

The third type of refractory ceramic is the mould, which is something of a hybrid between a furnace and crucible; it must contain the heat of molten metal, and also it’s own weight, when empty, and when filled. In an Anglo-Saxon context, the moulds are small, but Theophilus describes a range of sizes, from small components up to large bells, which involved some impressive engineering. When there are are cores within the moulds (eg. Inside a bell), they are usually made either from clay, or clay mixed with dung; clay alone is used for the outer shells of the mould, where it is thinner, and probably at more risk of cracking. A mould must be dry, it must not decompose into gas when exposed to high temperatures (which causes porosity), it must not crack, and the ceramic material has to regulate the temperature of the metal as it hardens – too fast or too slow, and problems can occur. In the worst cases, the entire casting could be rejected, and the process started from scratch. More often, a partially failed casting might be repaired by soldering, or by the “casting on” process for copper-alloy bells (using clay), as described in III-85.

Another use for clay is as a barrier. For example, III-19 describes the case hardening process for steel, which is covered in organic material followed by clay, to create a carbon-rich reducing atmosphere. In III-92, “a little clay” is placed around iron that is being brazed with copper, to prevent oxidation of both metals. Another method sees a thin layer of clay used to prevent the flow of metal. III-90 makes use of this when casting a spout in tin, with a removable iron core. III-84 uses it in a very ingenious way, so that a organ wind chest can be made airtight using molten lead, while the movable parts are in situ. III-74 uses a mixture of beer dregs, salt and clay as a gilding resist.

The last category is broken or powdered ceramics. In this category, the clay is always fired. Various mentions are made of “powdered” tile, used to temper a pitch mixture in III-58, and more typically, it is used as an abrasive, eg. III-95, for polishing rock crystal.

Often, it seems that an element of recycling is involved. Several of the crucibles specify that they should be tempered with pieces of old crucibles, and broken tiles can be made into abrasives. Even when damaged, ceramics remain useful to the workshop.

Theophilus provides wide-ranging uses for clay, and clearly has a knowledge of the different types, and what they are good for. A monastic environment probably supplied fresh clay, and plenty of old or broken ceramics for the workshop. It seems almost surprising that there is no mention of practical pottery throughout the books – the craft must have been considered essential to the work of the goldsmith, but perhaps not important enough to be described. In the same way, Theophilus does not discuss morphology – the shapes of the crucibles are rarely mentioned, and must be inferred based on their function. The archaeological record will be more rewarding for this side, and I’ll be looking at early medieval and Anglo-Saxon crucibles in the next blog article.

Descriptions in On Divers Arts: III-3, “freshly dug clay” and clay “that has been kneaded and mixed with horse-dung” for making a hearth; III-19, “kneaded clay” for case hardening; III-22, “white clay,” and “old pots in which gold or silver has previously been melted,” or “pieces of white earthenware pot” which can be heated without cracking, used as crucibles for precious metals; III-27, kneaded clay used to seal an iron ingot mould; III-28, casting crucibles used for the manufacture of niello; III-30, as moulds for lost-wax casting; III-33, “fire-tested earthenware dishes” are used as crucibles, while “a tile or piece of burnt and reddened furnace-clay” is one of the ingredients for the cementation method of refining gold, and a special furnace of “stones and clay” is also needed; III-34 is the same again, except on a smaller scale, with a “new pot” and a “low clay tripod” used instead of the furnace; III-35, one of the crucibles for gold and silver “but which for this work ought to be thicker than those” is used for grinding gold.mercury amalgams; III-37, “a large fire-tested earthenware dish” is used in a hot process for making gold/mercury amalgams; III-38, “a large earthenware dish” is used during surface preperation of objects to be amalgam gilded; III-40, green vitriol is cooked in a “clean fire-tested earthenware dish; III-50, a burnt brick is used as a crude scorifier; III-55, polishing an enamel on a “potsherd of the kind that are found broken from ancient pots”; III-58, chaser’s pitch, made from “pitch, wax and [powdered] tile”; III-59, a detailed description for making the chaser’s pitch, including “Grinding a piece of brick or tile very small”; III-61, “Take some clay, mixed with dung and well kneaded, and let it dry in the sun. When it is dry, break it up small and carefully sift it” for use as a core for lost wax casting, which is covered with wax; “carefully smear with thin clay” for the outer shell. A crucible is then used to melt the brass. A passage describes smearing the hot moulds with wet clay before casting takes place (?). Clay is also used for ‘casting on’ a repair to a failed area of a casting; III-64, “well-kneaded clay mixed with horse-dung” is used to build a brass cementation furnace; III-65, instructions for making crucibles from “fragments of old crucibles that have previously been used for melting copper or brass and crush them into tiny pieces on a stone. Then take clay from which earthenware pots are made – there are two types of this, one white, the other gray; the white is good for colouring gold, the other for making these crucibles”; III-66, instructions for using the brass-making crucibles; III-67, an iron pan is “plastered” inside and out with “well-kneaded and mixed clay”; III-74, recipe for a gilding resist, “two parts of finely ground common clay and a third part of salt, and mix them in a pot with fairly thick dregs of beer”; III-84, “kneaded clay” is used for the cores and moulds for pipe organ components, and a “thin coat of clay” is used as a sort of resist when molten lead is poured into the wind chest, to make it air-tight; III-85, “clay”, “stones and clay” “well-kneaded clay”, and “sifted and carefully mixed clay” are used in a detailed chapter on bell casting; III-88, clay is used as moulds for casting tin cruets; III-90, casting a tin spout, an iron core is “thinly coated with clay”; III-92, “a little clay” is put around an iron joint being brazed with copper; III-95, powdered tile is used as an abrasive for polishing rock crystal.

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