alum, in powder ( left) and lump ( right) formAlum is the third most important substance in marbling, after the gall and the size. It makes the colours adhere to the paper.
Marbling on tragacanth does not requore alum but the paper must be dampened slighly to accept the colour. The reasons behind this I have not been able to find out,( but it might, historicaly have something to do with the fact that the paper used being more absorbent). However, marbling on Carrageen requires the substance to be dissolved in water, and sponged on the paper. The process imparts some degree of acidity to the paper . As a result, some binders tend to avoid them. However, for odinary uses, the acidity is negligable.
There are two kinds of alum , potash alum (potassium aluminium sulfate), or just plain alum ( aluminium sulafte). Both work well.
Take a heaped tablespoon of powdered alum , and dissolve it in a cup of hot water. Stir to dissolve. ( this has a tendency to heat up the water, so be careful) .
Now, take the sheets of paper you intend to marble, and mark the side you do not want to marble with a pencil. This helps you distinguish which side to lay on the size.
Soak a sponge in alum, and apply it to the side that is not marked. Lay it flat to dry.
The paper can be marbled when it is dry, or slightly moist.