Sunday, 21 August 2011

Aperture Drops Annealing

The soak at annealing temperature will need to long to accommodate the temperature variations within the thick and thin parts. The thin parts will be able to cool much faster than the thicker parts.

The objective in annealing is to keep all parts of the glass cooling within a 5C range, so the soak will need to accommodate those differences. I suggest a minimum soak time is 90 minutes for a 9mm thick blank, 2 hours for 12mm and 3 hours for a 15mm blank to be certain all the glass reaches the same temperature.

Annealing the drop has two main considerations – the variation in temperature over the length of the piece and the variation of thickness of the glass. These two in combination make it difficult to find a rapid annealing and cooling schedule. So having spent quite a bit of time so far on the piece, choosing a conservative schedule is sensible.

The variation in temperature between the top and bottom of the kiln can vary quite a bit, maybe as much as 20C for some kilns. So you can see immediately that the annealing will need to be slow if you are going to keep the thick and thin glass within 5C of each other. It would be possible to use schedules for annealing thick pieces just as they are published for the thickness of your blank, but it is more conservative to use a cooling schedule for the next size up to ensure a good anneal.

Thus, for a 9mm piece I would anneal at 55C/hr for the first 55C below annealing, then 99C/hr for then next 55C. After that you can go much faster. For a 12mm piece I would go at 25C/hr for the first 55C, 45C/hr for the next 55C and 150C/hr to room temperature. For a 15mm piece I would go at 15C/hr for the first 55C, 27C/hr for then next 55C, and 90/hr to room temperature.
It may be possible to go faster than this in annealing, but this is cautious to make sure the variations in both thickness and temperature are considered.