Sizing is added to paper pulp or fully formed sheets of paper to prevent it absorbing colour like a sponge. Size allows paint or ink to partially sit on the surface
of the paper, allowing marks to appear crisp and vibrant.
Paper sizing explained
If you were to look at paper under a microscope, you’d see a mass of fibres interweaved every which way, chemically bonded with one another. In its natural, unsized state, this mass of fibres is as absorbent as the cotton, wood or linen it is made of. You can see this in action if you apply ink or paint to blotting paper – an example of an unsized paper, which will soak up any liquid colour almost instantaneously. It also impacts upon dry media such as graphite too, which can be more difficult to deposit onto unsized paper, because it is softer and the friction between drawing media and surface is less. Permeating paper fibres with sizing reduces their absorbency and allows for more of the colour to sit on the paper’s surface. Papers are usually sized internally with alkyl ketene dimer, a purpose made synthetic wax. Some papers are also externally sized which adds strength to the paper. While some are surface sized with a plant-based ingredient, gelatine (a by-product of the food industry, in most cases bovine derived) is usually used. This process is sometimes referred to as tub sizing, because the process involves immersing the formed sheets in a tub of size.
The difference between waterleaf, weak sized and hard sized paper
There are three degrees of sizing in paper. Unsized paper is sometimes known as waterleaf paper, and includes examples such as blotting or filter paper. Weak sized paper is also known as soft sized or slack sized and includes newsprint and non ‘waterleaf’ printmaking papers which have a comparatively high degree of absorbency. Hard sized papers include the majority of coated fine art papers.
How to test the absorbency of your paper
Some papers are deliberately over-sized to compensate for the loss of sizing that can happen if the paper is soaked for stretching purposes, a preparation that keeps paper flat even if it is saturated with large amounts of liquid. You can test the absorbency of a paper by putting a drop of water on it; if it sits high without sinking into the paper whatsoever then it is hard sized. Over-sized paper may cause the paint to gather in pools on its surface and give irregular results. You can stop this happening by lightly wiping the surface with a damp clean sponge.
It’s worth being aware that soap can dissolve paper size, so if you are stretching watercolour paper and intend to soak it, be sure that the container in which you immerse your paper is free from any soap residue, and always use a completely clean sponge to smooth your paper to the board you are stretching
the paper to.