Holes on texture moulds

Holes are blown through single and even double layers on texture moulds when the temperature is taken too high.

Image credit: Debi Scoggins Johnson

This piece was taken to a full fuse at 804°C/1480°F. The bubbles were blown near the edge, causing the glass to fall over the edge of the mould, rather than the blank being too big to begin with. At full fuse temperature the glass viscosity is too low to resist the air pressure from underneath. The air could not escape from the edge as the glass had conformed tightly to the mould surface.

Image credit: Debi Scoggins Johnson

Although it may be possible that the glass draped over the edge of this texture mould during the same full fuse, it is possible the air gathered closer to the centre, again because the full fuse sealed the edges, restricting air flow out to the edges.

These two examples reinforce the requirement that all texture mould schedules should use slow ramp rates, long bubble squeezes, low temperatures and long hold/soaks at the top temperature. This keeps the viscosity of the glass high, helping to resist bubble formation. This post gives information about the rate, bubble squeeze and top temperature.

Be patient. Let the glass have a long time to conform to the mould.

Many bubbles are blown simply because the top temperature is too high and reached too fast.

Carbon for moulds

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.