There are several things that can be done to reduce the number and size of bubbles in casting.
- Fire higher - to 830ºC instead of 815ºC - and soak for at least four hours. This allows more bubbles to rise to the top and burst. If there are still more bubbles than wanted, increase the soak time.
- Stack the glass in the centre of the mould, allowing a few centimetres from the mould walls. This allows the glass to spread and flow from the bottom and up the sides, reducing the likelihood of trapping air. If you have more than one stack, keep the same space between the stacks as the mould walls.
- Make sure that the way you stack the billets or casting plates so there is a smaller space at the bottom of any cavity than at the top. The reverse allows the glass to soften and seal in the air in the space.
- You can construct a mould to make billets of the general shape of the final object. This of course, is much more work, needing two moulds.
- A major thing to avoid is the use of frit, especially at the bottom or deep in the mould as bubbles will collect around each piece and lead to a multiplicity of bubbles throughout the casting.