If you over heat the shellac while melting it, it will be brittle.
It's pretty easy to overheat it too. If it is possible, use the
flame of an alcohol lamp to melt it. I use a torch, but it's held
quite a distance from the shellac so that it only blows heat in the
direction of the shellac, it doesn't dirrectly contact it. I've not
tried a heat gun, but I suspect that might be the best method.
I've also found that the "setter's cement" that Stuller carries is a
lot more forgiving and tougher too. It's sold in a flat block, so
it's easier to manage than the shellac flakes. It's more opaque than
shellac, and is a lighter, more orange color. The only drawback is
that it solidifies quickly, which makes it difficult to mold it into
place inside a ring before it gets too stiff. It dissolves in
alcohol, actually a bit faster than shellac. So, when I need to mold
a material and pack it into an article, I use shellac, and for all
other cases I use the setter's cement. I seldom use thermoplastic
for setting work as it has too much bounce to it and obscures my
sensitivity to the movement and resistance of the metal over the
stone and deadens that higher tone one gets when the metal makes
contact (at which point you STOP hammering in that area).
David L. Huffman