Understand and Avoid Thermal Shock

If you've ever had a piece of work break in the kiln then you might have been the victim of thermal shock.

In this tutorial we explain what thermal shock is, what causes it, and – most importantly – how to avoid it.

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Thermal Shock

Administrator's Note:  I have copied Joseph2Bear's comment to the forum and am responding there.


Great, thorough, down-to-earth explanation, Paul.  Thank you. 

May I suggest the addition of a page describing the Thermal Shock temperature range and tips on traversing this range on the way up, and the way down.  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?  I've seen schedules with fast ramp rates (turn the kiln off) below 800F for small pieces, controlled ramps down to 300F (for thicknesses less than 1/4", and controlled ramps down to 100F, or even 75F (for thicknesses of 1/2" or more).  Can you explain some of this that would aid my understanding? 

Paul, 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?

I should add that I am first doing a full fuse with lead crystal coarse frit to make a clear 1/2" blank.  I have used the 800F to 300F at 100DPH schedule for this first firing without any problems.  I do have "seeds" in the blank since I am not fining it for any length of time at fusing temperature.  I am only holding it long enough to get a smooth even surface.  Once the blank has reached room temperature, I leave it in the fiber blanket dams backed by kiln furniture, and lightly (sparsely) sprinkle the surface with elemental pure copper powder.  I then fire it to 2100F for an hour.  (100 DPH to 400F, hold 30 minutes, 100DPH to 1000F, hold for 1 hr, AFAP to 2100F, hold for 1 hr, AFAP to 1100F, hold for 1 hr,  300DPH to 1000F hold for 2 hrs, 100 DPH to 900F, hold for 3 hrs, 100 DPH to 800F hold for 2 hrs, 100DPH to 300F, kiln off.)  When the kiln reaches room temperature I open it and find broken glass. 

I am getting the copper diffusion I want: the most intense, striking, magnificient blue-green wisps and streaks throughout clear crystal that you have ever seen!!!  (And no seeds in the final result!)  This glass is absolutely STUNNING!!!  At least the half dozen pieces of it are stunning.  Now if I can only get it to come out in one piece.  Any suggestions? 

I should also say that at 2100F the viscosity of the glass is very low, like hot honey is my guess.  (I have to get beyond copper melting temp so the atoms of copper will diffuse into the glass to create the color effects.  This is not a copper inclusion, but a partial diffusion.  Not a full diffusion, like in a crucible melt to make a teal colored glass, but a partial diffusion to create wisps and streaks of intense color dispersed within a clear crystal base.)  I have sprung leaks under my dams, ruined a kiln shelf and have a few golf ball sized divots in the kiln floor that I have to repair.  I have to go to a closed sided mold to contain the glass.  Simple dams are not sufficient for containment.

Paul, I realize that few have tried what I am attempting to do.  There just is not much information to guide me, and it has been a long trail of guesswork, trial and error to reach this point.  I have finally achieved the color effects I envisioned.  The right mold material will solve the containment and kiln damage problem, and I think I have a handle on that for the next experiment.  But the broken glass is disheartening.  Any insights you may have would be appreciated and most welcome.  Thank you.

--Joseph 2bears, Lomita, CA

 

Thermal Shock

Paul,  

You've cleared up so many vague ideas I've had about thermal shock.

An outstanding explanation. Thank you.  And for you great pot melt calculator.

Another  generous gift to us fusers.

Bea

 

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