What is it?
Ivo de Croock wrote to us from Antwerp, Belgium, and asked what we meant by the term “fire-polishing”.
In the first place, we don’t etch with hydrofluoric acid at our studio. (When we must use acid, a colleague lets us use their studio and their equipment.)
But we sometimes etch by sand-blasting.
Now the sand abrades the surface of the glass and leaves it rough where it has blasted through the layer of flashed colour i.e. where the glass is not protected by your stencil.
It’s a good idea to use as fine a grain of sand as possible. This is especially worth remembering if you’re using another person’s sandblaster; the sand they have might be so coarse it would blast your glass to bits, which you do not want.
It’s now that we “fire-polish” the sand-blasted glass.
- We put it in the kiln and fire the glass to about 710 Celsius / 1310 Fahrenheit.
- We soak it there for about five minutes before descending and annealing.
This has two effects.
It smooths the sand-blasted glass.
It also softens the line where the sand-blasted glass meets the glass which has not been sand-blasted.
All in all, the glass seems to us to appear more like a liquid than it did before.
After the fire-polishing, we paint and stain it as we normally would. Consider this small panel here: we’ve sand-blasted flashed pink-on-white, fire-polished it, then traced and fired, and finally stained and fired:
You will need to test things for yourself
When you use sand-blasting and fire-polishing instead of hydrofluoric acid, you will need to run some tests:
- How thick must your stencil be?
- How fine / coarse must your sand be?
- What is the top temperature of your own particular firing schedule?
- How long do you need to soak there (if at all) to achieve the effect you want?
I can’t give you answers to these questions: the answers depend on your sand, the strength of your sand-blaster, the thickness of your flash, and the hardness/softness of your glass, plus of course its size, and the particular kiln you’re using.