WOOD PULP 69
for high white plastic products. In the field of ordinary paper and board the
colour after beating must also be considered.
Strength, breaking length, burst, tear and fold are very useful in controlling
the uniformity of cooking and bleaching. Breaking length and burst may be
said to depend mainly upon the fibre-bonding properties developed by the
process. Tear depends more on fibre length. The cooking process has an
important bearing on tear, kraft being inherendy much stronger than sulphite.
Fold, though hard to measure accurately, gives a further insight into the chemical
effects of cooking and bleaching. It is always well to remember that burst
and tear develop in opposite directions in both the cooking and beating pro-
cesses, which means that attention should be paid to the overall strength of
a pulp.
Beating does not concern mechanical pulps to any extent, but the grinding
process is really a form of beating to develop hydration and fibrillation of the
fibres. Unbleached sulphite pulps can be controlled by the cooking process
within Hmits to give quick or slow beating as required. Sulphate cooking is
flexible in this respect, and in general the pulp requires more beating time and
power. The bleaching operation can be considerably varied in relation to
beating properties, the use of caustic soda in the second stage being a modern
example of improving the resistance to hydration.
Fibre size, opacity, porosity, absorbency, oil penetration, blister, bulk and
other characteristics can be controlled within limits both in pulp mill and
paper mill.
Moisture content is a question whicl^ does not arise in self-contained mills
using slush pulp. In the case of pulps for shipment the problem is largely an
economic one. Mechanical pulp does not lend itself to drying, except die free
grades for board-making. Unbleached sulphite and kraft pulps shipped in
die moist condition break up more easily and beat more quickly to somewhat
higher burst, but suffer to some extent from danger of picking up dirt and of
developing infection. Dry pulp is an advantage for long storage, better
opacity, and more uniform control. Bleached pulps, except for greaseproof
and a few other uses, are shipped in the dry condition to ensure best cleanliness
and brightness.
Ash content tends to be high in sulphate pulps, but is not of particular
significance. The ash in sulphite is largely due to calcium compounds pre-
cipitated during cooking or bleaching, and whoa necessary can be reduced
by acid treatment, as is practised widi bleached grades for chemical uses. Pitch
content has been given thorough study in die pulp mills. Mechanical pdps
seldom give running trouble, because the normal character of die wood resin
has not been altered by the grinding process. Sulphate and soda pulp ate