Recently there has been a flurry of interest in “carbon” moulds. These are actually graphite moulds, a particular form of crystalline carbon. Whether these are as good materials for kilnforming as they are for glass blowing can be understood by considering some of the characteristics of graphite.
Graphite is crystaline carbon and it has a planar structure that allows slipping between planes just as kaolin and boron nitride do. Graphite and boron nitride share similar physical structures and continue to be slippery at high temperatures.
Graphite's high thermal stability and electrical and thermal conductivity facilitate its widespread use as electrodes and refractories in high temperature material processing applications. However, in oxygen-containing atmospheres graphite readily oxidises to form carbon dioxide at temperatures of 700 °C and above. https://en.wikipedia.org/wiki/Graphite
The consequence of this instability in the presence of heat an air is the creation of large bubbles from the decomposition of graphite into carbon dioxide when used above that decomposition temperature of 700°C/1292°F. Therefore, big bubbles are created in graphite texture moulds. They may not occur on the first firing because the exposure time was not long enough. By the second use, the graphite has already begun the decompose, and suffers large enough decomposition to produce the carbon dioxide.
Graphite might be useful as low temperature slumping moulds. The surface is easy to carve and shape to make smooth mould surfaces that do not require a separator. The disadvantages as a low temperature mould are the surface will erode quickly and mark easily, so detail will deteriorate and become damaged. Care is required to preserve it from marks and sharp materials. In addition, the slow, unnoticed decomposition will produce a rough surface over a number of uses.
The rapid deterioration of graphite moulds at low temperatures, make them unsuitable even for low temperature texture mould firings.
The expense of a thick slab of carbon thick enough to carve a decent depth and diameter of bowl needs to considered against a short life span. Small lamp working graphite moulds work well because the short times they are exposed to the heat do not give time for the decomposition to begin. And in any case, most often the moulds are open face, allowing any gases to escape without any distortion to the glass.
