Mortenson used a variety of techniques to modify prints. Abrasion of regions of the print, the use of pencils to modify the image. etc. The mixture you specify sounds like it was used to provide "tooth" to a print to allow it to take pigment or pencil easily. He often made extensive changes to a print so that the final result was more illustration than photograph. Google on "abrasion tone" a technique he developed. AFAIK he did not make his own emulsions. I have never seen any reference to doing so in his books.