62 MODERN PAPER-MAKING
the process involves the extra step of softening in addition to grinding. The
steaming results in a dark-brown colour which limits the^ use of the pulp to
products of the leather board class. The drainage rate is in the range of very
high freeness. The fibres are comparatively long and. flexible, with much
better tear than with ordinary groundwoods, and about the same bulk.
Various methods of heating pulpwood blocks under pressure with weak
reducing liquors of the sulphite class have been tried to produce extra-strong
groundwood of bright colour. The fibre strength approaches that of unbeaten
sulphite, but the drainage properties tend to be slow. Cleanliness is excellent
because the laundering of the blocks removes adhering dirt and brightens the
exposed layers. Yields can be 80 to 90 per cent by weight of the dry wood,
as the screenings are easily refined. This general grade gives promise of finding
some economic use for low and medium grade white papers, and even suggests
the possibility of a straight line process for newsprint mills by treating all the
pulpwood without pressure.
In the intermediate range the most common example is short, brown,
bulky fibre produced by mild alkali cooking of extracted chestnut chips followed
by refining to brush out the fibre bundles for the manufacture of corrugating
board.
Close to the border-line of chemical pulps is the quick cooking of southern
pine at high yields for the manufacture of bulky kraft liner board. Only part
of the inherent fibre strength of the kraft need be developed by the refining
treatment, and the product does not require thorough brushing out of fibre
bundles.
High-yield unbleached sulphite has recently been developed for use in the
self-contained type of newsprint mill. The digester is blown before the fibres
are thoroughly separated, and the washed pulp is brushed out in a modern
type of refiner before going to the screens. Uniformity and cleanliness are
not the highest, but unbeaten strength and bulk are satisfactory, and an all-over
yield of 55 to 60 per cent means cheaper furnish.
Sulphite pulps are classified as unbleached, bleached, and semi-bleached.
The general characteristics are excellent whiteness, quick hydration, medium
strength, reasonable cleanliness, and low opacity of the fine flexible fibres.
Extra-strong sulphites represent minimum cooking to reach fibre separation
and best possible strength. Careful screening has to be practised to reduce
the shives and dirt. Depending partly on the wood, cooking can be adjusted
to give either quick beating, maximum burst, and maximum tear. Ordinary
.uses are for M.G. papers and other extra-strong hard sulphite sheets.
Medium strong grades include many well-known brands of sulphite, and
the uses cover a wide range. Brightness, uniformity, and cleanliness have