38 MODERN PAPER-MAKING

The rags rest on a perforated plate at the bottom, and steam is admitted into
the space between the plate and the bottom.

The steam blowing upwards agitates the rags and forces the liquor through
the mass. The agitation is, however, very imperfect and the rags frequently
pack into dense masses, which then get little contact with the boiling liquor.

This type of boiler is very economical for use with good rags which do
not require very severe boiling. There is no loss of fibre through friction,
and little expense for upkeep as compared with rotaries, which require power
to drive, oil, packing for glands, and repairs to running parts. The blow-off
steam may be used to heat water for rag washing, etc. It is safer to use, as
there is no mechanical strain apart from the boiling pressure. The rags may
be filled through an opening at the top, but they have to be dug out by hand.

There are still some mills, making hand-made papers, which boil their rags
in open tanks with wooden lids, but only rags requiring slight treatment can
be dealt with in this way.

For esparto, straw, etc., vomiting stationary boilers are u&d. These are
made much larger than rag boilers, a common size holding 5 or more tons of
esparto. When larger sizes are used, more elaborate and expensive devices
are necessary for washing. The stock rests on a perforated false bottom
through which the liquor drains and collects over the steam inlet pipe. The
latter is extended either outside or inside the boiler, so that the liquor and
steam are forced upwards to the top of the boiler and spread over the charge
by a spreader plate. A constant circulation of liquor is maintained by means
of this arrangement.

The boiler has the "usual pressure gauge and steam, fittings, and also a run-off
pipe to convey the spent liquor to the soda recovery plant.

It is usual nowadays to cut out esparto or straw from a Sinclair boiler by
means of a jet of water at high pressure.

BLEACHING
In preparing half stuff for white printings and high-grade writing papers,
.the colour obtained by boiling and washing the stock, whether it is wood,
esparto or rags, is dull or brown, and must be further whitened and made
pure. This is effected by the process known as 'bleaching5.
The laundry and washerwoman1 bleach collars and other white garments
after washing. Bleaching has been in vogue for many years and the earliest
method is that known as sun bleaching; this consists in damping the material
to be bleached and placing it out in tie sun, where 'oxidation', which is the