Prepare Materials: Mix 3 tablespoons alum per quart of hot tap water in a bin; let cool. Soak leather or prewashed fabric in mixture for 20 minutes, or brush onto paper. Let dry flat. (Reserve alum mixture in bin to use over several days.)
Prepare “size,” or base, for paint: In bucket, mix 4 tablespoons Methocel per gallon of warm distilled water. Add 1 tablespoon ammonia per gallon and mix vigorously with whisk.
Pour Methocel mixture into second bin. Let sit until all bubbles dissipate. (This should take several hours; mixture will have a sheer, viscous consistency, slightly thinner than egg whites.)
Squeeze paints into paper cups (mix some, if desired). Thin each with a drop of dispersant and slowly add water until paint has consistency of whole milk.
Marble: With a coffee stirrer, drop paint onto several spots on surface of size; let pigment spread out 2 to 3 inches.
Add another color, dropping paint into middle of previous color, if desired.
Continue in this fashion, adding color over color and letting paint spread out and form random shapes. Repeat sequence in several spots over surface.
Some paints will spread out more than others; the more concentrated the color, the more it will stay put.
Drag stirrer through paint to add swirls and create designs; work over whole bin, or focus on specific areas.
Gently lay medium flat onto size, and pat to remove air bubbles.
Let sit until fully saturated.
Pick medium up by two corners.
Gently and evenly pull up. Rinse thoroughly under running water until soapy film is gone. Hang to dry. Lay paper towels on size surface, then lift to remove paint. Reuse size as desired for several days (cover bin with lid or plastic wrap when not in use to protect from dust).