Barbara
At an early stage in my career I used chrome alum as a hardener. But very
very little;probably a tenth of a gramme in a litre. Then I found that if
you use a hard gelatine, available from B & S, you do not need the hardener
at all. As to the flaking this can arise from there being too much pigment
in the gum mix. But it is obviated if you use Gloy which, living in the
States, you cannot get !
Terry