STRAW 71

cut out by a high-pressure water jet and washed, blown or pumped to the
washers or concentrator.

Washing and bleaching follow the same lines as for rag, but the potchers
have no plates, and the blades are blunt and waved, so as to prevent any cutting
or drastic beating taking place. Even then a very great loss of fibre takes
place in washing and bleaching through the washing drums; about 7 per cent
bleach is used and heat is permissible up to 90° F. Very dull-coloured
stuflf may with care be heated up to 100° F., but the risk of destroying
the fibre is very great, and only highly experienced men may be allowed to
take it.

Spanish esparto requires less bleach to bring up the colour. From the
potchers, after the bleach has been washed out and the last traces have been
killed with anti-chlor, the pulp is emptied into the presse-pdte chests. The
presse-pdte is the wet end of a Fourdrinier machine. There are long and deep
sand traps through and along which the grass flows with the water, loosening
sand, metal or any other heavy substance that may sink and be caught by the
felts on the bottom. Sufficient strainers to take the pulp through without
forcing are provided. These keep back all knots and thick untreated fibres
and lumps. The wire, of rather coarse mesh, then receives the pulp, and the
water drains out, assisted by the suction boxes* After passing the couch rolls,
the water is still further extracted by a felt and press rolls, and the pulp falls
into boxes ready for the beaters, or may be conveyed to the beater chest. The
yield of fibre varies according to the country of origin of the grass and other
factors, but it may be taken to be from 38 per cent for Oran grass to 45 per
cent for best Spanish. The amount of bleach required is usually between
7 and 10 per cent calculated on the weight of the raw grass.

STRAW
Up to the present time, in Great Britain, at least, it has been the custom
to treat straw in much the same way as esparto—namely, to boil it in stationary
boilers with caustic soda, then wash in tanks of potchers, bleach and run the
fibres over a presse-pdte.
Boiling is carried out at from 20 to 80 Ib. pressure per square inch, with
from 10 to 20 per cent caustic soda, according to whether yield or colour is
more important. When the boiling process is drastic, the yield is small and
the pulp is capable of being bleached very white, but its strength is greatly
reduced, and it produces a *wet* condition in the furnish owing to the formation
of oxycellulose.