Franks definition is right on. Unfortúnately, I do not agree with his assessment that many crack at least not
in the Swedish factories.
If they crack, the gaffer has not annealed them properly or discovered whether or not the colors are compatabile.
If not, they will crack because of the different coefficients
of the colors and the clear glass.
Paradise Paints in California developed a paint which can be used to paint a blank and then overlay it with clear without oxidizing the paint. David Hopper, glass artist, owns this company. In other words, the painted image remains. Ulrica Hydman Vallien has used this paint when designing glass.