68 MODERN PAPER-MAKING
freeness, strength, and running properties of the mechanical pulp, because
uniformity means so much on the paper machine for high speed, fewer breaks
and steady quality of newsprint.
In a mill using purchased pulp smooth operation is greatly aided by being
able to rely on the running properties of a brand. The difficulty of controlling
uniformity is a story in itself. Remembering the variations in wood, seasonal
conditions, and the many other factors in a pulp mill, the problem of ending
up with uniform pulp can be readily appreciated. Modern pulp mills have
elaborate organisation of staff and instruments towards this end, and it is a
tribute to their efficiency that pulp for sale is nowadays a remarkably uniform
product.
Cleanliness is also an item of vital importance in relation to most grades
of pulp. The very fact that paper and board meet the eye naturally brings
cleanliness to the front. The main problem of dirt is in unbleached sulphite
pulp, due to black knots softening in the acid process and spreading through
the pulp. Great precautions are taken to clean the wood as carefully as possible,
to blow the digester gently, and to remove the larger dirt particles by settling
and screening. Circulation in the digesters has helped to remove shives and
dirt. The mills are not resting content with the present methods, and there
is some hope that a centrifugal separation of dirt will prove economic. In
the case of groundwood, ordinary dirt is seldom criticised because the mechani-
cal process has the advantage of finely grinding the hard knots and other dis-
coloured parts of the wood, but the shives and small fibre bundles are naturally
prominent because of the very nature of the process (grinding process).
Sulphate pulps gain by the cooking liquor digesting the knots and bark specks,
leaving shives as the main item. Grit or metallic particles may find their way
into mechanical pulp from the grinders, into sulphite pulp from the digester
linings, and into pulp in general from wood, piping, and other sources. The
fact that pulp bales pick up extraneous dirt during transport and storage adds
to the demand for extra-clean pulp sold in the open market, especially as the
paper machine screens are mainly for protection of the Fourdrinier wire.
" «The general demand for whiteness and brightness has led to much attention
along the lines of colour control. The brightness of unbleached sulphite and
kraft has been improved by studying the cooking conditions. Multi-stage
bleaching of chemical pulps has produced whiter products of wider range.
Furthermore, the chemical purity and stability of the bleached pulps are
higher, giving better resistance to light and heat. The colour after waxing
depends on tie physical and chemical nature of the fibre as well as on the
apparent colour of the pulp. Pulp for rayon must retain good white colour
in caustic soda. An extreme case is the need for highly purified chemical pulp