PAPER CONDITIONING 325
accepted as being an atmosphere of 60° F. temperature and 60 per cent relative
humidity. In an atmosphere such as this there will be approximately 345
grains of water as vapour in a cubic foot of air, and this corresponds to a moisture
content in the paper of between 6 and 7 per cent. Paper which is composed
principally of cellulose is hygroscopic—/.*., it will take in or give up moisture
according to the conditions of the air in which it is placed—and it will alter
in moisture content comparatively quickly. This fact is responsible for wavi-
ness on the edges of the finished paper. In the majority of mill-finished
Fourdrinier papers the moisture content of the finished paper as it is reeled
up varies considerably, and can be as low as 1.5 per cent and as high as 4.5 per
cent according to the surface given—a very highly finished paper which has
been run over hot calender rolls may contain very little moisture; on the other
hand, an offset paper which has had less rolling may contain the higher per-
centage. It is impossible to give any definite figures on this because of the
factors which govern this content at the reel end. It will be apparent that the
paper is deficient in the moisture necessary to bring it up to the balance with
normal atmosphere mentioned earlier. Provided that a normal atmosphere
has free access to the paper over the whole of the sheet for about 15 minutes
the sheet will of its own accord absorb sufficient moisture for it to arrive at
its correct moisture content. Under general mill conditions, however, this
free access is not given to the paper; it is reeled, cut, sorted, and packed, and
during the whole of that time it is in bulk, and in consequence moisture can
be absorbed only by the outside edges of the paper. In the case of a ream it
will be obvious that the centre of practically every sheet of that ream will be
unable to get air. The outside edges, however, are exposed to the atmosphere
and absorb moisture. The absorption of moisture causes the paper to expand,
and because that expansion is held back by the unexpanded centre portion of
the sheet, the edges develop into a series of waves, and it is this waviness and
irregular tension in the sheet which causes the printer so much trouble. Paper-
conditioning machines are now available for dealing with this, and all work
to the same principle—viz., of passing the continuous web through a number
of passes in an atmosphere which will impart sufficient moisture to the paper
in its passage through the machine to bring it 'into balance' with the normal
atmosphere.