144 
THE YOUNG SCIENTIST. 
used in facing your cast, and stir in oclire. 
A slight tinge will suffice— much does not 
look well, and moreover softens the i^las- 
ter, and then, as there will be a sediment, 
pour off your colored water for use into 
another vessel. It is important to mix 
enough, as it would be difficult to make a 
second quantity of exactly the same 
shade. 
The directions given above, so far as 
they treat generally of the mixing and 
management of plaster, will equally 
apply to casting models of other shapes. 
I have taken a "relief" as an example, 
because it is the most simple form, and 
can be cast in a single piece. Such addi- 
tional instructions as may be required to 
enable the amateur to waste-mould a 
model in the "round," can be quickly 
given. 
Instead of a panel, let us suppose that a 
vase has been modelled. This may serve 
as an illustration of a simple ol)ject in the 
round." If we try to moukl this at a 
single operation, we should find it impos- 
sible either to extract the clay or to clean 
the mould. The mould must therefore be 
made in two pieces. 
To prepare this model for moulding, as 
shown in Fig. 5, we must make a strip of 
clay, the sixth of an inch thick, and about 
two inches wide, and stick it edgewise 
round the vase, so as to divide the latter 
into two equal portions. This collar can 
be supported by wire pins, as shown at a, 
A, A. One side of the vase can now be 
moulded, for the collar of clay will pre- 
vent the plaster from reaching the other 
side. When the first half of the mould is 
set, the collar must be removed, and 
conical holes made at intervals, with the 
end of the scraper in the edge of the 
mould thus left bare. These are " key " 
holes, and as when the other half of the 
mould is cast, corresponding " keys " 
will be formed to fit them, they will 
enable the two halves to be put together 
with the nicest precision. Some clay- 
water must be brushed over the edges, to 
prevent the two halves adhering too 
closely, and the remaining part of the 
mould can then be cast. When a model 
has to be cast in an upright position, as 
is the case with a bust, or with this vase. 
plaster cannot be poured upon it ; it must 
be thrown on with the spoon. 
The mould being finished, its two 
halves will easily be forced apart by the 
mallet and chisel . The clay can then be 
removed, the mould cleaned, and fitted 
together again. The keys will ensure the 
pieces fitting accurately. They then have 
to be tied together as tightly as is pos- 
sible. Perfect tightness is to be secured 
by tying a piece of string or cord, accord- 
ing to the size of the work, loosely round 
the mould, passing a piece of stick 
through it, and twisting the stick round 
and round. When you can screw it no 
farther, secure the stick from untwisting 
by tying it fast. You can then pour in 
the plaster gradually, and well shaking 
and turning the mould about whilst you 
do so. When the mould has been chipped 
off, a little seam will appear on the cast 
where the two pieces of the mould met, 
which will have to be cleaned off'. 
A bust might be moulded in the above 
manner ; but if so, the seam would neces- 
sarily pass down the ears and along the 
neck and shoulders, where it would be 
highly unsightly. The method shown in 
Fig. 4 is therefore generally preferred in- 
stead. By thus making a " pot-lid," the 
seam is formed only on the hair, in apart 
where it will be little noticed, and the 
opening thus made will allow of every 
purpose of emptying and cleaning the 
mould of the head. 
Sometimes a portion of the mould may 
project from the general mass of the work 
so far, that it would be a difficult matter 
to mould it in situ. If so, it may be cut 
off with a fine piece of wire, and moulded 
separately, and afterwards refixed in its 
place. 
As a cast becomes dry, a disagreeable 
yellow tinge, the result of some impurity 
in the plaster, will sometimes come to the 
surface. To obviate this, the cast should 
be placed with its back to a fire, and so 
dried. As the water evaporates, it will 
bring out the discoloration on the side by 
which it escapes, and the front of the 
cast will thus be uninjured. 
Wax Moulding. — The above is the ordi- 
nary and most useful method of waste- 
moulding ; but there is another, which is 
