Alum

alum, in powder ( left) and lump ( right) form

Alum 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.

The size - carrageen

The size in the body of fluid that the marbling colours are floated on . It is of a consistency thicker than water, but not too thick. A consistency like that of cooking oil, or olive oil is ideal.  Several sizes were formerly used. However, only one size is used today, by almost all professionals, carrageen moss. 

Carrageen, or Irish moss, is a type of red alga, which releases a sort of jelly-like thing called mulicage when boiled in water.  Now, it used to be prepared by taking the dried moss, and boiling it for up to half an hour with borax and hot water.The  modern marbler need not suffer this, as a extract of carrageen moss is readily avalible. 

To make the size, take ......

1 ounce of the extract, 

1 quart of hot water. 

There are 2 ways of proceeding from hence. 

The more common method involves whirring the ingredinents iin a blender, and the resulting foamy mess is then left to settle overnight .

The second method involves taking the ingredients, and whisking them in a bowl till all the extract is dissolved. In either case, the sollution is left to cool and diluted, till the desired concictency, as it is often found to be too thick . 

The amount of size that you make must be enough to fill your through to a deph of at least three fingers. 
 

Other sizes were ( and sometimes are) also used, and they will be covered in a subsequent post.