Elevation of Moulds in the Kiln

The placing of the mould may have a significant effect on the outcome of a slump. The ideal placing is in the centre of the kiln to ensure it has the most even temperature. This avoids any uneven temperature that may exist within the kiln.

Hot and Cool Spots

Sometimes this is not a practical use of kiln time or space, but if the heat distribution in the kiln is uneven, the placing may be critical. If the cool areas are known, avoid them in the placing of the larger moulds. Simpler moulds, or those which do not require as much heat can go in the cooler areas of the kiln. A good and simple method to test for the heat distribution within your kiln is given in Bullseye’s Tech Note no.1.

Effect of Elevation of Mould

Elevation of the mould by a centimetre or two is often recommended to help evenly distribute the heat under the bottom of the mould as well as the top. This is viewed as a way of avoiding breakage or uneven slumps. There are differences between moulds on the shelf and those elevated. Recordings show differences up to 49°C/88°F. The differences on the cool down ramps are minimal and do not interfere with annealing. These differences appear to have no effect on breakages in the mould.

ΔT Shelf vs. Elevated Moulds (Celsius)

	Max. ΔT	Average ΔT
Rate / hour	on Rise	Start of slump	End of 30 min slump	On cool
150	49	41	30	8
120	39	31	24	5

 

These differences should be put in context. The air temperature is approximately three times any difference between the two arrangements of moulds. Much more important in breakage is the ramp rate, as it creates significant differences in expansion between the top and the bottom of the suspended glass. This much larger difference has the potential for greater effects than whether the moderately sized mould is elevated or not. This table demonstrates the air and mould temperature differences.

ΔT Difference Between Air and Elevated Mould (Celsius)

Ramp Rate	Air Minus Mould Temperature (ave)
240	138
150	112
120	97

 

Large, Heavy, Wet Moulds

The elevation of large, heavy, or wet moulds is very important. It is needed to protect the supporting shelf from breaking. The amount of shading of heat from the shelf that these kinds of moulds can do is large. Wet moulds, especially, can cause large temperature differences in the shelf. Always elevate moulds that are large relative to the kiln, contain thick glass, are heavy, or are damp to avoid difficulties with the shelf.

 

More information is available in the ebook: Further information is available in the ebook: Low Temperature Kiln Forming.