fritas

3. These two boron frits (Ferro 3124 left, 3134 right) have almost
the same chemistry. But there is one difference: The one on
the right has no Al2O3, the one on the left has 10%. Alumina
plays an important role (as an oxide that builds the glass) in
stiffening the melt, giving it body and lowering its thermal
expansion, you can see that in the way these flow when
melting at 1800F. The frit on the right is invaluable where the
glaze needs clay to suspend it (because the clay can supply the
Al2O3). The frit on the left is better when the glaze already has
plenty of clay, so it supplies the Al2O3. Of course, you need to
be able to do the chemistry to figure out how to substitute
these for each other because it involves changing the silica and
kaolin amounts in the recipe also.