> > Flash cooling isn't critical at all. Normally this is done when a
> >piece is taken to "visual maturity" and one wishes to halt the process
> >at a particular point.
> > Annealling *is* critical on a piece that size, but it really depends
> >on the insulating characteristics of, and thermal mass inside the kiln
> >as to whether or not a simple powerdown will allow slow enough cooling.
> There is a theory too, that flash cooling or the "kiln crash" prevents
> crystals from forming (devitrification) on the surface of the piece. Maybe
> this is more relevant at casting temperatures.
Yes, and sometimes even at fusing temps, depending on the glass.
In my experience, devitrification isn't generally a problem at
> The thin-ness of your piece will help it cool evenly, but small differences
> will probably build up stresses fast in a 12 x 12 sheet. I recommend using
> a pyrometer to ensure the ramp through the annealing range. Just turning my
> kiln off results in a cooling rate that is much too fast. I'm sort of
> working on a controller for my home-made kiln, but in the mean-time I can
> get a reasonable profile with manual control. It is very tedious, but it
> -Rod Christel
If your kiln won't give you a slow enough cooling rate with a simple
power-down, often a ceramic fiber blanket thrown over the top of the
kiln will slow cooling enough to give you a usable annealing rate for
a piece this size.
Metamorphosis Glassworks Page http://people.delphi.com/stackman
GLASS HOST & Assistant Manager of The Delphi Arts & Crafts Forum