Wednesday, 13 December 2017

Annealing Large Pieces

A question was asked about how long to anneal a large piece in relation to smaller pieces.

“Large” is in relation to the size of your kiln.  A large piece for a 300mm square kiln would be something 250mm square.  For a kiln of 600mm square, 250mm would be a small piece.  It would contain a large piece of 500mm square as a large piece. 

Large also relates to the distance from the edge of the kiln.  Although some kilns have much more even heat than others, all have areas that are relatively cooler than others.  It is important to know where those are, so that you can avoid those cool areas, by placing pieces to avoid those spots or by altering the rate of cooling.  Bullseye has a tip on determining the relatively hot and cool temperatures are in your kiln

In a rectangular kiln, there are usually cool spots in the corners.  Front opening kilns often have cooler areas at the front of the kiln.  Knowing where these are will give you the information to know the area of the kiln that has even heat.   This area tells you what the size of a large piece for your kiln is.

You can alleviate many of the differences in temperature in your kiln by remembering that annealing is not simply a given temperature.  It is a range. 

The popular perception is that the soak at the annealing temperature is all that needs to be done to anneal.  The soak at the annealing point equalises the temperature throughout the glass. But it does not complete the annealing. That continues through the gradual cooling of the glass down the next 110°C.

Simply soaking longer at the annealing point, in the circumstances where the temperature in not equal all over the glass, “locks” the stresses of uneven temperatures into the glass.  Instead, a gradual, slower than usual annealing cool is required.

Of course, the rate of cooling is relative to the thickness of the piece and the degree of temperature variation in your kiln.  If you must utilise the area of the kiln with slightly cooler temperatures, the minimum requirement would be to use a cooling rate for a piece at least two times thicker than the thickness of the one you are annealing at present.

But, to answer the original question - how long to anneal a large piece in relation to a small one of the same thickness?   Given the precautions above, the size of the piece is not the major determining factor.  The thickness of the piece is the important dimension when considering annealing.