Sizing is an integral part of the paper-making process. Sizing is a liquid that coats the fibres of the paper.
Sizing paper helps to improve water-resistance whilst also allowing inks and pigments to sit on the surface of the fibres instead of being absorbed. This helps to create a more consistent and economical surface to print, paint and draw on.
Sizing also affects abrasiveness, creasibility, finish, printability, smoothness and surface bond strength.
Surface sizes typically consist of vegetable starch (potato, rice or wheat) or gelatin (from bones or hides). Gelatin size can improve the strength and flexibility of paper. Here is a guide to making your own gelatin sizing.
Popular alternatives to Gelatin for use in hand paper making are: Methylcellulose and Carboxymethylcellulose (CMC).
Here is a more detailed page on sizing paper which examines the two methods for sizing: internal and surface.