Size

Size is essentially glue. The name grew out of the use of hide glues including rabbitskin glue used to prepare canvases for painting. The application of the glue causes the canvas to shrink considerably. Thus it was said to be ‘sizing’ as it pulled tighter and gradually the word was applied to the glue itself. As gelatin is closely allied to hide glue in its properties and preparation it was also called size. So the addition of gelatin to the pulp when making paper became to be called sizing the paper. All paper except blotting and tissue papers have small amounts of sizing to make them less absorbent (these days not always gelatin), but the thing that makes watercolor so suitable for watercolor washes is the larger quantity of gelatin size in the paper. The best papers are sized in the pulp (‘in the mould’) but some cheap water color papers have a (cheaper to produce) layer of size rolled on one side, and are called surface sized. They should be avoided.