Edward Weston used an india ink and gum arabic mixuture. The formula is reproduced in the original edition of Adams "The Print":

"use equal parts (by weight) of Chinese (stick) ink and gum arabic; dissolve in enough water to cover these ingredients, and mix. Let dry out and mold to suit. Moisten a brush in water (to which a wetting agent may be added), wipe on a piece of paper until the proper gray shows, then apply to the print. A dry brush works much better than a "wet" brush. The amount of gum arabic may be increased 2 or 3 time to increae the sheet of the spotted area of the print."

For what it's worth, a commercial alternative is a set of spotting colors from Marshalls (the folks who make the oil coloring materials).

In my case, I bought a set of SpotTone colors about 30 years ago. I figure that I'm likely to run out before they will.