2H2

FLUIDITY AND PLASTICITY

material entails IOHH physical and mental fatigue on the
part- of the* plasterer, and ho in thereby led unwittingly to produce
u better quality of work/1
Rmley pointB out that the method of Hlaking the lime has much
to do with the development of pla«t,ieity, but that quite ae impor-
tant IK the* Hotiree, and by informer, the chemical cornponition of
the lime. A lime high in mapicBimn oxide is capable of develop-
ing a high plantleity more readily than one which is low in the
dolomttie oxide. The growing practice of buying Ohio finishing
lime, already hydra ted, even when local lime may be purchased
for about one-half the price is a reflection of the above facts and
IB a demonstration of the indiiHtrial importance of plasticity.
In handling roud-building and roofing materials, a knowledge
of the principles of plastic flow might enable UB to avoid losses.
The first principle of road building is to secure proper drainage,
which is in accord with the theoretical requirement of keeping
the yield value* as high as practicable. The "metal" of the rail-
road is made up of coarse crushed stone of uniform size which
glve« excellent drainage and a very high yield fxrint. Where
liquid hydrocarbons are used as binder, a considerable amount of
fine! material must be UBcd in order to raise the yield point sxiffi-
cictntly to sustain the contemplated loads. In order to be able
to apply the material the mobility is greatly increased by raising
the temperature.
Paints and Pigments.—Paint must have a yield value high
enough HO that it will not run under the influence of gravity
hut the mobility must also bo high so that the painter may spread
it without undue fatigue. Other thingn being equal, these ends
are, both achieved by the u»e of finely-divided materials, and at
the time the covering power is augmented. Perrott (1919)
hm a study of the plasticity of "long" and "short" carbon
blacks.
tip to 1914, Austrian ozokerite was thought to be essential
in the wax uned in making electrotypes. Research has shown
that a good impreftBion can be obtained and held with waxes
which do not contain the Austrian material
TeictHes and Belting.—If a cotton window cord is ran over a
free pulley a certain number of times under a load which is small
in comparison with the tensile strength of the cord, it may fail