64 MODEKN PAPER-MAKING
than can be reached by soft cooking, this being accomplished by starting with
bleadhable pulp.
Strong bleached sulphites include several varieties. Tough papers and
boards may call for fibre of yellowish-white colour which will stand up to
heavy beating. Bond pulps are adjusted to fairly quick beating and high
burst. Multi-stage bleaching has extended the range to the bleaching of com-
paratively hard sulphites, and in general has given better strength and purity at
a given whiteness, as well as yielding whiter pulps of excellent stability compared
with earlier days.
Soft bleached sulphites are ordinarily used for book, magazine, and other
printing papers. High-white colour, high cleanliness, good bulk, and opacity
become more important than strength. This is the type of pulp which the
pulp mills with older equipment can continue to make. Speciality qualities
are made for genuine vegetable parchment, photographic papers, absorbent
tissues, and other uses with control of highest whiteness and cleanliness, low
pitch, low content of iron and copper, low ash, etc.
Bleached hardwood sulphites have been coming into more prominence.
In Europe, bleached aspen sulphite of excellent whiteness, cleanliness, and
fineness of fibre has been supplemented by bleached beech. In America bleached
hardwood sulphites made from birch, beech, and maple are on the market
to give much better strength and quicker beating than with bleached soda
pulp.
Silk pulp for viscose rayon staple fibre and viscose films has reached such
a large tonnage that it has become almost an industry in itself. The brands
of the older type are not very different from soft bleached sulphites made for
paper-making, except that greater care is taken to control viscosity and good
swelling characteristics in caustic solution, and to reduce resin, ash, and other
impurities. The use of alkali and other special treatments has resulted in a
higher grade-of silk pulp with over 90 per cent alpha-cellulose content, less
degradation of the cellulose, better whiteness in alkali, and extremely low
impurities for conversion to stronger, brighter thread in the modern rayon
mills. This grade also involves special control of sheet size, moisture content,
basis weight, thickness, and absorbency. Although this grade has been largely
made from spruce and western hemlock sulphite pulps, there is a distinct
widening of die scope to include sulphite and sulphate pulps from pine and
hardwoods.
'Alpha* pulps of alpha-cellulose content well above 90 per cent are
specialised in their characteristics and limited in their sources of production.
Sulphite and sulphate pulps from, softwood and hardwoods are given special
treatments in addition to bleaching. In die range up to about 95 per cent