There are three types of sizing.  Natural sizing, external sizing which involves applying a sizing agent to a dry sheet of paper andf internal sizing when the sizing agent is added during the formation of the sheet.

Natural Sizing occurs when the original fibres themselves are coated with the remnants of a waterproofing substance. A degree of natural sizing can be present paper is made from old rags where there is a certain amount of grease still present or where lanolin soap has been added to  make the sheet more supple. The sizing strength though is usually weak.

External sizing is the application of a sizing liquid to a dry sheet of paper. Starch, Gum Tragacanth, egg albumen, and Gum Arabic were used by Arabic & Islamic calligraphers as far back as the 12th Century. From the sixteenth century on, gelatin was frequently used in concert with additives or preservatives, especially alum (historically, potash alum or aluminum potassium sulphate.).  

Tub Sizing in a Paper Mill

Internal sizing eventually superceeded the expensive tub sizing process .When papermaking became mechanised in the 19th C.  the rosin alum sizing method became widely adopted. It was introduced in Germany (1807) and later adopted elsewhere in Europe .Internal sizes are usually added whilst the pulp is being beaten to ensure even mixing and are incorporated into the sheet structure during formation. The sizing effect is achieved by retarding the rate of penetration of a fluid, usually water  through capillaries formed both within and between the fibre. All internal sizing agents attempt to modify the fibre surface by introducing hydrophobic molecular groups onto the fibre surface: this  has to be done in a uniform way through the body of the sheet but in such a way as not to disturb inter-fibre bonding.   As the paper dries the size molecules migrate and undergo molecular reorientation. This reorientation continues to happen even after the paper has dried. This gradual development of the sizing effect is called curing.

Neutral sizing. This the internal sizing of paper in an alkaline based system. Some of the most successful types of neutral sizing agents are alkyl ketone dimmer emulsions (AKD).This is the type that we use at Griffen Mill. 

Three methods of telling whether a paper is sized:

 1. Lick it. Useful only if you do not have any water handy! If the paper changes colour you know there is a degree of liquid penetration. The effect is most noticeable if you like unused toilet paper or blotting paper.

2. Place a droplet of water carefully on the surface of the sheet and observe whether the droplet shape disappears rapidly or whether it remains as a rounded drop or some intermediate shape in between. When the water droplet disappears very quickly the paper is highly absorbent and is described as waterleaf: when the shape of the drop is "relaxed" and disappears  slowly the paper is called medium sized  and is called highly sized when the water droplet remains on the surface as a bead.

3. Perform a Cobb Test.This essentially measures the amount of water absorbed in a set number of seconds using a Cobb Tester.