Ox Gall Liquid Clarified with Alum and Salt, Ure 1839

Ox gall makes watercolours spread evenly and fixes them, but raw it stains with its own green. Andrew Ure (1839): boil and skim the gall, dissolve alum in one half and salt in the other, let both settle three months, then mix the clear parts in equal measure: the colour falls out and leaves the gall clear. The picture: our diagram of the two settled halves mixed to a clear liquid.