Steel Moulds

Credit: Slump and Fuse

Do steel moulds need to be kiln washed for slumping?

Some prominent people in the kilnforming milieu like to promote the extremes of the craft. The argument seems to be that the glass does not get hot enough to stick to the metal at slumping temperatures. It could be argued in the same way that it is possible to slump glass on bronze or unglazed ceramic moulds.

This proposal may come from glass blowing where glass is pressed into metal moulds. The difference is that the glass is in contact with the metal for a short time. And in industrial processes the steel is water cooled.

Also, the higher the slumping temperature, the “softer” and “stickier” the glass becomes. The metal is also heating up and expanding, unlike in glass blowing. If the glass adheres to the metal at all, the greater contraction of the metal during cooling will ensure the glass is stressed and likely break. Therefore, it is usable only in low temperature slumping – below about 630˚C/1167˚F – or for short times. The break patterns that occur when slumping on bare steel show glass most often sticks to the steel and becomes crazed or even shatters on cooling.

Bare steel as a slumping mould is always a risky practice. Just because it can be done - or is done - in glass blowing and other industry settings, does not mean it should or can be done in studio settings. The practice comes with high risks of failure.

To be safe, a separator needs to be used between any supporting structure and the glass. Why risk glass into which you have put significant amounts of work for a few pennies worth of kiln wash, fibre paper, or boron nitride?