Glass is a funny thing. One piece can look just like another - flat, clear, same thickness - and they may still be as different as a cat and a cow. It's all in the "Coefficient Of Expansion" or COE. The majority of glass artists who are firing (fusing) their glass in a kiln are the happy recipients of years of trial and error by artists and glass companies who painstakingly developed ranges of glass with the same COE.
I personally use a line of glass made by Spectrum Glass Co. and another by Uroboros Glass Co. that all have the COE number of 96. Now, having the same COE means that this glass will expand and contract in the heating and cooling processes of the kiln at the same rate (or close enough to not cause stress on the glass - assuming the artist takes the proper precautions with their time and temperature rates).
The line of glass paints that I use has a very wide range of compatibility due to the fact that it is so finely ground that it can move more freely as the glass expands and contracts. However - I have found it's breaking point. Literally.
I am always coming up with new ideas to make miniature glass paintings to be used in jewelry pieces. The challenge is that I would like to make the glass round or oval, but cutting those pieces is more difficult and making them the exact shape and size to fit into a pre-made metal bezel is super difficult. That is because when the glass heats and cools it generally gets smaller and I have yet to perfect the knowledge of what that size difference will be.
I found a company that sells small domed glass pieces that are meant to be used as covers over miniature paper pictures for jewelry. I thought "Aha". I could paint on the flat sides of those and then mount them in the bezel settings. However, these domed glass pieces are not made of "tested compatible" glass and are actually very hard glass compared to that which is usually used in fusing.
I chose 4 pieces and successfully painted and fired the lining and shading stages of my black work. I was thrilled. Then, I added the paint colors. Huge failure. The COE is too far off to do my usual method of painting. The paint was too thick.
4 Failures of Compatibility:
The sparkling effect you see is the crazing or cracking where the paint and glass meet. Almost looks pretty, but it's definitely not right.
In this last image, you can actually see where a piece of the painted glass chipped off of the glass piece.
So, back to the drawing board - I knew that the first two paint steps worked just fine, but they use thinner layers of paint. I tried another piece and fired the lining and shading again with no issues, then mixed up some paint with my clove oil (I usually use water for this step) and then painted the colors on in thin layers. I crossed my fingers and into the kiln it went again.
This, apparently, is the way to go. The colors are brilliant and there is no crazing between the paint and glass. Yay!
"The Power of Three"
This one is mounted in an antiqued silver bezel with a 24" chain. The antiqueing process brought out some copper color in the metal which is just the right touch for this painted scene. It's available in my Etsy store now and there will be more to come.