66 MODERN PAPER-MAKING
Easy-bleaching aspen sulphate is an example of short-fibred hardwood pulp
for sale to paper mills having their own bleaching plants.
Semi-bleached grades may become more important as time goes on.
Present methods make it much easier to reach yellow colour with sulphate
than to attain the high whiteness of bleached sulphite, and very little strength
is lost.
Semi-bleached kraft represents the initial stages of chlorination and hypo-
chlorite treatment applied to the best brands of strong kraft. This grade is
intended for the toughest papers of manilla colour as a substitute for rope and
hemp. The fibre has maximum strength, is less harsh than kraft or bleached
kraft, and lends itself to a wide range of beating.
Semi-bleached CL. & S/ kraft is a softer grade. This is the kind of pulp
which may be economically made from southern pine to compete with
unbleached sulphite.
Semi-bleached soft sulphate may be a suitable name for easy-bleaching
sulphate which has been given preliminary single-stage bleaching. In this
case the pulp can be readily bleached at the paper mills to give better bulk,
opacity, and tear than is possible with easy-bleaching sulphite.
The most important advance has been made by the application of elaborate
multi-stage bleaching to sulphate pulps, with the result that good white colour
is reached without serious loss of strength properties.
Bleached kraft is the greatest achievement, and a number of well-standardised
brands are now available, mainly from the northern districts of Europe and
America. Colour equal to that of strong bleached sulphite is available if
maximum strength is not required, or a slightly yellowish colour can be ob-
tained with strength characteristics approaching those of kraft Opacity of
bleached kraft appears to be much the same as with strong bleached
sulphite at equal freeness of fibre, but higher at equal strength of sheet. For-
tunately, in beating time and power consumption, bleached kraft is inter-
mediate between unbleached kraft and strong bleached sulphite. Although
the fibre can be successfully mixed with sulphite in the beater, separate beating
naturally gives better results whenever convenient. The fibre tends to give fuzz
and inferior sheet appearance with ordinary light beating, but sharp tackle and
heavy roll improve the texture and opacity without seriously affecting the burst
and tear of the sheet. Longer beating increases transparency and toughness,
and the treatment can be continued to a very high end-point of bursting strength
without losing too much tear or freeness. Filler fibres such as bleached hard-
wood sulphite can be mixed with bleached kraft to improve formation,
surface appearance and opacity without undue loss of strength. Experience
with bleached kraft can conveniently be gained by using various proportions