Wednesday, 31 August 2011

Glass Stuck to Moulds

Glass that is stuck to moulds needs more care in removal than removal of glass from kiln shelves does.
The major element in removal is to get the mould to release the glass. This requires some diagnosis of why the glass is stuck.
  • Is the glass trapping the mould? This happens most often when the glass is draped, especially over ceramic moulds.
  • Has the glass been fired high enough to fuse to the mould? If you have fired the glass to tack fusing temperatures, you may find more occasions when the glass sticks slightly or firmly to the mould.
  • Is the mould trapping the glass? This can happen when slumping into a steep sided steel mould. Occasionally a steep sided ceramic mould will show the same effect.
  • Has the separator been too thin or failed? If none of the previous elements apply, it may be that the separator was too thin or has been fired to tack fusing temperatures in a previous firing.
You can use mechanical methods to free the glass from the mould by inserting a thin pallet knife between the glass and the mould. This works better for items where the glass is inside the mould. If the glass is outside the mould, the chances are that you will break the glass. Using mechanical methods for any glass that is more than lightly stuck will most often lead to breakage of the glass, and often both items.

Where the mould is trapping the glass, you can put the item back into the kiln, but upside down and supported a centimetre or so above the kiln shelf. Heat the glass gently toward slumping temperatures. The glass should fall from the mould at 300 or 400C, but you need to keep watch to make sure you do not over heat the glass.

Where the glass is trapping the mould and you are using mechanical methods, you normally need to decide which is more important – the mould or the glass. It is just possible to break a ceramic mould and leave the glass, if the glass is 6mm or more. If the mould is more indestructible, you will probably lose the glass.

With ceramic moulds you can try heating the two to 300C or 400C and reach in with appropriate protection to try to lift the mould out of the glass. If the mould is steel, it will expand faster than the glass and break it.

Once you have the glass off, you may need to repair the mould.