One
description of casein is: principal albuminous constituent of milk, in
which it is found as a calcium salt, obtainable by curdling. From this
we can safely infer that curdled milk contains similar substances to those
found in egg white. Its only use as far as we are concerned is as a substitute
for the gum or glue in making gum prints. Its use was patented in 1908
for photo-printmaking but appears to have been neglected ever since. Making
a casein print is exactly the same as making a gum print with one difference.
We curdle milk to make the gum, which we then mix as usual with dichromate
and pigment.