Cracks in Lead Crystal with Copper Diffusion

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Thank you, Paul.  I know how difficult it is to diagnose a problem without being able to see it and with only limited information.  I had the occassion to discuss the problem and show the glass to John Williams, owner of Pacific Art Glass, this weekend.  His conclusion was “Compatibility!”

He thought my firing schedule was OK.  What he meant by “compatibility” was that the glass was not compatible with the kiln furniture dams and kiln shelf (all mullite).  I had done the right thing in lining the dams with fiber blanket, in that the fiber would provide the needed cushion for expansion and contraction against the fixed position of the mullite pieces which expand and contract at a much lower rate and amount.  But my dams had leaked in several places and everything was “glued” together by the solidifying glass.  As the crystal blank contracted during cooling, it was being pulled apart by these “anchors”.  Something had to give, and it was my crystal blank. 

As I thought about this it started to make sense.  I knew exactly where the dams had leaked, since I had to clean up the mess.  Every fracture and crack in the glass corresponded precisely where those leaks had occurred.  At this point, I think John Williams has nailed it!

BTW, all of the breaks were sharp and straight.  This is what had me thinking they were due to thermal shock.  I see now that the fractures are a symptom, not THE problem.  The problem I have is containing the glass at the high temperatures I am working at.   And the kiln was at room temperature after the first successful firing to make the blank, before it was opened and the copper powder applied.  That fact does seem to support John Williams assessment.  The glass was contained by the dams while full fusing at 1500 F to create the blank, and I had no fractures.  The leakage occurred during the second firing to 2100 F.  The fact that the breaks were sharp, indicate that the fractures occurred during cooling, after annealing, when everything was “glued” together on the kiln shelf. 

First I have to repair my kiln.  I procured some cement for that today.  Then I have to prepare a mold that will contain the glass.  I am going to try some “700” Vermiculite board, 2″ thick.  With a plunge router I will mill out a well 1″ deep, then cut 5 to 6 degree draft angles on the edges with a hand chisel.  I have some high temperature kiln wash used in glazing ceramics that is good to beyond cone 10 that I will use to coat the mold.  That should solve the problem, I hope.  Whether or not the vermiculite board will withstand more than one firing up to 2100 F is just something I will have to try and to discover. 

It will probably be a week or two until I have anything further to report, but I will follow up and let you know how it goes.  I will be taking photos of successful glass and I will share those with you when I have them. 

Thank you for all your help, comments, and suggestions.  They are greatly appreciated.

–Joseph 2bears, Lomita, CA

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