Luted Crucible

Casting metal requires a crucible: a moveable and shock-resistant vessel with a higher melting point than the metal being melted that will safely contain it whilst you heat it to temperatures that are often over 1200°C. Because of the requirement to withstand these very high temperatures, as well as sudden and extreme changes in temperature (thermal shock), hot gases and often abrasive particles and chemicals, crucibles are normally made from refractory ceramics. These ‘refractories’ are more resistant to heat and chemical attack than ordinary ceramics. 

Crucibles have been used for thousands of years, all over the world, wherever high temperature processes were carried out. The first metalsmiths would have used ordinary, locally available silica-rich clay with added organic tempers, but from the late Iron Age onwards these ordinary clays were superceded by special refractory clays that were selected to be alumina-, silica- or carbon-rich and iron oxide-poor, giving them superior strength and thermal performance. In more recent history materials such as graphite and aluminium oxide (corundum) have been used by goldsmiths, chemists and industrialists to produce crucibles that melt metals faster and last longer than their predecessors.

The refractory layer of this crucible is made from a mixture of white clay (high fire stoneware) and rice husks. The addition of the vegetal matter opens up the clay and creates silica-rich voids when it burns out, making the crucible more porous and allowing gases to escape from the casting vessel whilst adding to its strength at high temperatures. 

This particular peanut-shaped crucible is a luted crucible, made as part of a lost-wax bronze casting masterclass run by artist Piers Watson at the Institute of Making in 2015. The object  to be cast is first modelled in beeswax, then encased in a sand and clay mould (investment). This mould is then joined or ‘luted’ to  the crucible, which is filled with a mix of metals (in this case 10% tin and 90% copper to  achieve  bronze). The whole mould and crucible ‘peanut’ is then covered with the heat and impact-resistant refractory layer. Once the crucible  has been heated to a high enough temperature to melt the metal inside, it is turned upside down so that the molten metal runs into the cavity left by the wax. This technique therefore avoids the need for the uncontained pouring of molten metal.

Once the metal object has been cast you can use a hammer to break open the luted crucible, knock away the refractory and investment layers, and reveal the metal object inside. A UCL medical student who took part in our luted crucible bronze casting masterclass commented on her experience of this process: “The moment I cracked open the investment was like Christmas morning: you go through all these steps of making the wax model, the investment, and the refractory layer and drying them out but you have no idea what it’s going to be like. The element of surprise is so great!”