328

MODERN PAPER-MAKING

tests van- widely on the same sample if tested under different conditions of
relative humidity and temperature. An observation of folding endurance
carried out at 70 per cent relative humidity may be double the value obtained
on the same sample at 60 per cent relative humidity; it has become common
practice, therefore, to 'condition paper specimens before tests are made. The
usual recognized standard atmosphere for the conditioning and testing of paper
is at 65" per cent relative humidity at 65° to 70° F. It is advantageous that the
paper-testing laboratory is a constant humidity" and constant temperature
room, having double doors with an air lock between and preferably double
windows. The walls and ceiling, besides being reasonably thick and

heat insulating, are better glazed or
gloss painted rather than plastered, so
as to reduce the water-absorbing surface
to a minimum. The disposal of the
paper-testing apparatus in the room
must be given careful consideration, so
as to avoid the room being unduly
large and giving rise to difficulties with
air circulation. Provision should be
made to supply conditioned air into
the room at the rate of about 30 cubic
feet per minute per man working in
the room. Temperature control of
the incoming air, through the medium
of a thermostat, does not present
any serious problems, electrical heat-
ing of the air in winter being
possibly the most convenient, and water cooling in summer where necessary,
although it may be found that this latter recourse is unnecessary by careful
choice of room site. Humidity control is usually brought about through the
length changes of a humidity sensitive hair or a sensitized parchment strip.
Constant humidity cabinets are also available in which paper specimens may He
for some time before testing in order that their moisture content may attain
equilibrium with the controlled atmosphere in the cabinet.

Strength.—There are several recognized appliances for testing the strength
of paper. The most popular, reliable and easiest to manipulate is the Mullen
paper tester (Fig, 140).

In this machine die paper is clamped over a circular diaphragm of thin
rubber, I square inch in area. By turning a wheel, a piston forces glycerine
against the under side of the diaphragm. The latter presses against the paper

FIG. 140.—THE MULLEN PAPER TESTER FOR TESTING
THE BURSTING STRENGTH OF PAPER