Wednesday, 15 April 2020

Texture Mould Firings

Texture moulds are essentially permanent kiln carving moulds.  These are moulds that use different levels within the mould to develop the imagery by giving different thicknesses to the glass. Temporary or single use moulds can be made from fibre paper, although not with the same subtlety as the ceramic ones.

Single layers
Many people wish to use a single layer in these texture moulds. For a single layer, a tack fuse is a high as you can take the temperature. This will not give you the definition that you could get with higher temperatures unless you use very long soak times. 

Good definition
To use higher temperatures, you need at least two layers (6mm) to avoid distortion, dog boning, possible bubbles and needling.  With two layers you can go to full fuse temperatures to get the best conformation to the mould.

Low temperature firings
You can get better definition at lower temperatures by going slowly to your target temperature. This slow rise in temperature – ca. 100C - or less - per hour – all the way to the lower end of the tack fuse range – ca. 730C to 750C -  needs to be combined with a long soak, possibly two or more hours.  This long soak allows the glass to sink into the depressions of the mould without shrinking, dog boning or needling.  This shows that the speed you use has a major effect on the target temperature.

Another element of difficulty in the use of these moulds is the separator used. Kiln wash is adequate, especially if you are using the lower temperatures.  Boron nitride is a popular choice for those going to higher temperatures.  Using iridescent glass with the coated side down to the mould provides an additional separator, allowing higher temperatures to be used.  It can enhance the appearance of the piece too. 

The way you schedule for texture moulds is an interaction between the thickness of the glass, the rate of advance, the target temperature and the kind of separators used. With these four factors interacting, the choices are not simple.

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