WOOD PULP 67
with bleached sulphite or rag stock in present products being manufactured
at a paper or board mill. Bleached kraft by itself is ideally suited to extra-
strong tough papers which hitherto it has not been possible to make from
wood pulps. Progress will undoubtedly also lead to the development of
specialities which have not been economic with rag or hemp.
Bleached 'L. & S.' kraft is a name chosen for the slightly softer grade.
At yellowish-white colour the strength should approach that of bleached
kraft. The usual aim is best possible whiteness, with appreciable decrease in
burst and fold. The fibre is somewhat finer in texture, softer and more opaque
than that of bleached kraft, making it competitive with strong bleached sulphite
for various papers and boards.
Soft bleached sulphate is the grade made from easy bleaching sulphate.
Single-stage bleaching does not produce high white colour, but the brightness
can be raised close to that of bleached sulphite by multi-stage treatment. Opacity
is very high. Softness and absorbency are also high. In addition to paper and
board competition with soft bleached sulphite, this grade may be expected
to take a share of the chemical uses after suitable purification.
Bleached aspen sulphate is the corresponding grade of short-fibred pulp.
Other hardwoods will also find a pkce in this grade.
Bleached soda pulp is the well-established grade in America, based on
caustic soda cooking and bleaching of hardwoods for mixing with soft bleached
sulphite in papers of the book and magazine type. The brightness is com-
paratively low. The fibre is soft and bulky, and the pulp "resists hydration to
maintain freeness, very good opacity, and low shrinkage.
Bleached soda pulp is perhaps the nearest wood pulp approach to esparto,
although the latter has superior burst, tear, opacity and fineness of texture.
A variation of the soda process is sometimes practised by mixing only a
small proportion of sodium sulphide. In this way there is an appreciable gain
in strength over the regular soda process, and the bulking properties are better
than in the case of sulphate pulp. Softwoods are sometimes cooked in this
way for bleaching purposes.
PULP QUALITIES
Uniformity is the first on the list of quality requirements for all pulps,
regardless of process or grade. This refers to evenness of fibre qualities de-
manded by the particular use for which the pulp is needed. It applies not
only to the regularity of quality from one shipment to another, but also
throughout a bale or batch. For instance, in a self-contained newsprint mill
one of the main technical problems is continuous control at.the grinders, of