My GCSC Intro to Torched Fired Enameling class began yesterday.
As we got down to basics, some of the students ran into an issue of cracked enamel on their counterenamel pieces. Normally when I teach we might have one every once in a while, but this was about 7 to 8 pieces that cracked
On the Thompson Enamel site here is their explanation:
“When enamel cracks it is because there is too much stress in the glass and the glass has no choice but to relieve that stress by cracking.
This can happen with any enamel under certain conditions. Some of these conditions could also lead to enamel releasing from the metal or from itself, but the ‘chipping’ defect may or may not involve conditions that always apply to cracking. Under the conditions of heat the metal expands. The enamel flows and is carried with the metal. As the piece cools, the enamel becomes rigid between 1020 degrees F. and 1050 degrees F. The metal continues to contract. This puts a strain on the enamel and given certain conditions, the enamel will crack.
There are many reasons or combination of reasons why the glass/metal relationship fails. The reasons can include – wrong alloy or base metal; no counter enamel; the wrong expansions relationship; uneven or too thick enamel application; uneven enamel thicknesses back to front; non uniform base metal thickness; solder joints; weighting the enamel after firing; metal inclusions in the enamel; impact; the list can go on and on. There is not a general answer that will cover all situations. To understand what is causing the cracking involves looking at each instance of cracking individually, and looking at all the conditions that may be causing the cracking or contributing to it. The greater the thickness of the enamel in relationship to lesser thickness of metal is worth considering. Slow cooling is best as well as lower temperature when possible which would obviously decrease the amount of expansion which decreases the amount of contraction.”
We discussed possible causes. We explored the idea that it was a bad batch of black opaque(1995) and maybe the metal was contaminated. But everyone was using the same enamel and the metal and the cracking didn’t happen to everyone; so we eliminated these possibilities. We narrowed it down to these:
- Too heavy of an application of the enamel
- Cooled too quickly as our room was colder than usual
- The pickle was too strong
I had made the pickle in one of the later Encore sessions that ended on February 22. It was a little strong and then it sat in my studio till yesterday and probably evaporated. Some of the pieces that cracked looked like some of my pieces that were forgotten in my pickle for a week or more. I just not sure if that reaction could happen in 5 minutes or so, as it did in class. But to be safe I will make a new batch for the next class.
The best source I have found for information on pickle is on Nancy L.T. Hamilton’s site.