Size is a sort of waterproofing for paper. Unsized papers, like blotting paper, will just drink up ink and smudge it, so in order to get ink to sit on the sheet, various additives have been developed. These go under the name of size. The word apparently derives from the Latin “assidere” (to set next to, besiege) having come to us courtesy of early Italian papermakers who referred to it as “assisa”, slurred down to “sisa”.

The gifs below are from Paperslurry and show the effect of sizing. The paper towel on the right hand side of both pictures is designed to be absorbent, while the writing paper on the left has been sized to allow it to keep the ink on the surface.

 

 

 

 

 

Size coats the pulp fibers and makes them water resistant to a greater or lesser extent. It can be applied in the beater (internal or engine sizing) mixed up with the pulp along with dies, pigments and other chemicals, or it can be applied later, after the sheet has been formed, as a coating (surface or tub sizing). Size, essentially abietic acid, was made from rosin emulsified in water together with soda ash. Gum rosin is the natural pitch of the pine tree, while wood rosin is a by-product of turpentine production. For special purposes other sizes are used, animal glue, starch, casein, synthetic resins, polyvinyl alcohol, or wax emulsions.

There are four categories of sizing for papers: waterleaf — unsized, like blotting paper; slack sized — paper towels get a small amount of size to prevent them defibering while remaining water absorbent. Newsprint falls into this category too; medium sized — most printing papers to varying degrees; and hard sized — used for things like paper cups and butcher’s paper which need to be water resistant.

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