>> How do you feel about a
>> How do you feel about a short hold at 400F on the way
>> up to stabilize temperatures as one method of avoiding
>> thermal shock on the way up?
With few exceptions (like casting where there are considerations for the mold), I think that holds/soaks below 900F are one of the first warning signs that the schedule might not be a good one.
The goal (during intial heating) is to heat the glass evenly. If you have done that, there is no need for a hold. If you haven’t done that, you should have.
>> I have what I think are thermal shock cracks in 1/2" thick leaded crystal
>> blanks that I am making for a beveled panel. At least they fit the description
>> and pictures in Boyce Lundstom’s books for thermal shock. I controlled the
>> ramp down from 800F to 300F at 100 DPH. From Bullseye Tech Notes
>> and System 96 Firing schedules, 100 DPH over this range should have been
>> a safe approach. If I have deciphered the Labinski schedules in Shop
>> Notes correctly, 75 DPH would be the recommended rate. Can you make
>> any sense from all this conflicting information? I am thinking of trying 50 DPH
>> from 800F down to 100F on my next attempt. Comments?
If you are working with lead crystal I would trust Libensky.
Bullseye and Spectrum schedules are based on the Glass Engineer Handbook by Shand which assumes that you are firing from top and bottom. I don’t know about Spectrum, but Bullseye specifies this in the footnote in the technote on annealing thick glass. If the shelf was a perfect insulator (which it obviously isn’t) you would have to anneal for twice the thickness of the glass. In reality it is somewhere between the published schedule and twice the thickness. Not much help eh? As a rule I always anneal for at least "one thickness" more on Bullseye’s chart than the actual glass. That’s a lot of extra time for thick glass — but I haven’t lost one yet.
>> I should add that I am first doing a full fuse with lead crystal coarse frit
>> to make a clear 1/2" blank…[snip]…lightly (sparsely) sprinkle the surface with
>> elemental pure copper powder. …[snip]…When the kiln reaches room temperature
>> I open it and find broken glass.
Are you not letting the glass cool to room temp before applying the powder? Could the glass simply be breaking from not letting it cool sufficiently during the first firing? Is it possible it isn’t annealed properly from the first firing (it would be more likely to thermal shock on the 2nd firing. Is it breaking on the way up or down (are the edges sharp or fire polished)? Pure metal can create a heat sink which might thermal shock the glass. Also, at 2100F I have no idea what the copper might be doing to the compatibility of the glass.
I’d love to see pictures of the glass.
Helios Kiln Glass Studio