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Thanks so much for your input. Will pass that along. Mostly my students are probably putting to much enamel on at once. It's impossible to be there watching each one as they sift.
You can enamel with one thin layer without counter enamel, but if you add more it will crack. Just counter-enamel it.
Sometimes things crack if you layer together enamels with different hardness (COE for the people that do glass fusing), usually if you are using different brands of enamel together. In that case, you must test the 'hardness' by firing a pinch of all the colors on one penny, and seeing if they melt at different temperatures. Obviously, the enamels that don't melt right away are 'harder', and should be used together, and the ones that melt easily go together. If you mix them up in layers, you are going to get cracking.
Most likely, counter enameling will solve your problem.
Lillian
One classroom I taught in had a chilly draft blowing right down in the only logical place to put the enamels fresh out of the kiln.
Not likely to be the culprit for only one student out of a class, but stranger things have happened...
, but stranger things have happened...
That's true!! One experience from one of my workshops.
I taught a group of absolute newbies. I told them: "It is very important to clean the metal surface very well". They showed me the cleaned surfaces and I said: "That is very well". They enameled the pieces and the enamel pops off. They cleaned again, enameled again but the enamels pop off again. I had at first no explanation! Then I watched the cleaning process. And because the newbies would be especially thorougly, they clean the surface at the end with a household-cleaner. Because I'm a chemist I know, that the most cleaner a detergent contain, that, after drying deposits onto the surface of the porcelain, to make the surface more glossy.
I say they should not clean the surface with household-cleaner. And after they let away the household-cleaner, the enamel don't spalling of again!
Edmund
In Linda Darty's class after using a cleaner (sometimes Comet) she would use baking soda as a final wash. I started wondering if the baking soda, not washed off well, would also be the culprit. Any thoughts on baking soda? It is so difficult to see the sheeting action of the water on a small piece.
Sondra, We use Penny Brite in my class for cleaning copper. It is true that the small pieces are hard to clean, but if they are first annealed to 'green', that should be enough to burn off most of the oily dirty surface of pennies. If you also pickle them you should neutralize with bakingsoda. Then a brushing with a little Penny Brite and a good rinse with water. Besides that I would still question the alloy of the pennies.I had people bring in copper to enamel that just was not pure, and caused more trouble than it was worth.
I need to get more information on why a student of mine is having so much trouble with her enamels cracking. I have talked to her about cleaning the metal, firing temps, and counter enamel effects. She is using pennies (before 1980 so they are copper) and not using counter enamel. Would it be needed when there is such a small layer of enamel on the top? Her other work is experiencing the same issues. She uses a bee hive kiln that of coarse doesn't have a way to know what temps she is firing at.
Cracks can have some causes .
1) The enamel is too hard;
2) The layers are too thick;
3) lack of counter-enamel;
4) the fired enamelled pennies are cooled down to quick;
5) the pennies are not from pure copper
(what I guess is the main cause, because the coins are world wide very seldom from pure metal. Pure metals whether it is copper, silver or gold are too soft and would wear too quickly in use.
Your student can make a simple test. They should apply the same enamel in the same manner applying on a penny-like of electrolyte copper and fire and cool down it as they allways do. She should fire the samples, until the enamel surface is glossy. Than the firing is o. k. If they get cracks, are one of the first 4 points the cause, if not, the pennies are not from pure copper.