Conservation Science

36 Chapter 3

absorbent for inks and has little wet strength. Generally, paper is sized to make
it more hydrophobic (water repellent). Sizing also changes the surface texture
and both the wet- and dry-strengths of the paper. Traditionally, a hot solution
of gelatine was brushed onto the paper in its unsized state (water leaf) called
tub sizing. Papermaker’s alum (aluminium sulfate) was sometimes added to
the gelatine. Chemists will realise that this is a misnomer as this chemical is
not a true alum, however, papermakers consider its properties similar to alu-
minium potassium sulfate, which is an alum and also used by papermakers.
Papermakers say that alum is added to “harden the size” but it is possibly of
much value as a biocide to stop the stored gelatine solution from developing
fungal colonies while it is being stored cold. A popular sizing agent from the
mid-19th to the mid-20th century was alum/rosin size. Rosin is a natural prod-
uct derived from coniferous trees and is chemically a mixture of rosin acids.
It can be used in conjunction with alum to produce a size that produces opti-
mum results on paper. In water, the trivalent, positively-charged aluminium
ion is attracted to the negatively-charged fibre surface and there forms a link
to attach the negatively-charged carboxylic acid group on the rosin acid.
There are many ways in which rosin can be used as a paper size and several
theories on how they work.
Modern papers are often sized with an alkyl ketene dimer that reacts with
hydroxyl groups on the cellulose, and the large alkyl group imparts water-
repellency (see Figure 3). The product is a white solid that is emulsified with
water for application onto the paper.

2 The Directional Properties of Paper

In most processes for the manufacture of paper, the fibres may align them-
selves in a preferred direction. In machine-made paper, the orientation is par-
ticularly marked, as the fibres are oriented in the direction in which the web
is moving, the machine direction (MD). The MD of a piece of paper is of par-
ticular importance in determining the properties of a piece of paper. The
direction across the paper at right angles to the MD is called the cross direction
(CD). When paper is made from pulp, the smallest particles will penetrate
furthest into the paper as the water drains through it. So for this and several

R

C

H

C

O

C

C

O

HR

RCH 2 CCCOcellulose

OH

R

HO cellulose O

Figure 3The reaction of alkyl ketene dimer with cellulose