THE YOUNG SCIENTIST. 
85 
of sulphuric acid, pure lime, and water, in 
the proportions of 46 of the first, 32 of tho 
second, and 22 of the third. 
That quality which renders plaster so 
valuable in the arts, is its power of setting 
into a solid body after being simply mixed 
with water. This, whilst the plaster is 
fresh from the kiln, it does in a few min- 
utes ; but with time and exposure to the 
air, it gradually loses this power. It sets 
less and less quickly, and when quite 
stale refuses to set altogether. Thus, bad 
plaster may involve total failure and dis- 
appointment, to guard against which it is 
well to buy of a dealer on whose word 
you can rely. 
If you feel any doubt with regard to 
your plaster, try a little before you begin 
your work, and prove whether it sets 
properly or not. If you find it necessary 
to keep plaster for any length of time, 
put it in a dry place, and exclude the air 
from it as much as possible. I have kept 
superfine perfectly good for three or four 
years by shutting it in a closely-fitting 
tin, and then well wrapping it in paper. 
Fresh plaster is always to be desired ; yet 
if very fresh, one difficulty attends it : it 
may set so quickly as scarcely to give you 
the necessary time to use it after you have 
mixed it. If so, you must bestir yourself 
accordingly. 
There are various methods of making- 
moulds for plaster casting. AVhen a clay 
model has to be dealt with, the process 
called waste moulding is emjDloyed, be- 
cause the mold is destroyed in the pro- 
duction of a single cast. This method I 
shall first describe. 
Waste Moulding in Plaster .—het us sup- 
pose that the amateur has made a model, 
such as a panel charged with rosettes or 
foliage. This will have been modelled on a 
board somewhat longer and wider than 
itself, tl]e model being, say, a foot square 
and periiaps from two to three inches 
thick. Plaster of a medium quality will 
do for this work ; and be sure and have 
enough of it. If the supplv fails whilst 
the work is on hand, the moulder will be 
in an awkward position. 
And here let me premise, that though 
plaster is among the most cleanly of sub- 
stances, odorless in itself, and a neutral- 
izer of all things that have bad smells, it 
has a tendency to get splashed about and 
trampled upon floors, in sucli a way as to 
make a mess, and to rouse the temper of 
good housewives; for which reason it is 
well to practice casting in some workshop 
or back-kitchen, rather than in an ordi- 
nary living room. 
Having laid the model flat on a table, 
its surface must be damped. Sculptors 
use a syringe, pierced with minute holes, 
for throwing water in the form of fine 
spray over their models. Such an instru- 
ment the amateur will not possess; but 
he will have what, for small models, does 
still better— his mouth. Fill your mouth 
with water, and after two or three trials,, 
you will find that you will be able to blow 
it out in a cloud of fine spray. Blow a 
mouthful of water in this way over your 
model. Nothing else will so well prepare 
it for the reception of the plaster mould. 
In no other way can you damp the^ 
whole surface so regularly, without over- 
damping it, and causing water to run and 
stand in deep cuttings and hollows, where^ 
mixing with the i:)laster which is to form 
the mould, it would tend to soften and in- 
jure it. 
For reasons which will appear by and 
by, it is desirable that the first or inner 
mould should be tinged with color. Vari- 
ous coloring matters are used, according 
to the whim or practice of the moulder. 
Many use yellow ochre ; but this and 
similar i)igments hnve a tendency to 
soften the plaster, which is objection- 
able. Nothing is really better than com- 
mon ink, the tendency of which is rather 
to harden the mould than otherwise, and 
of this less will suffice than of anything 
lighter in hue. 
Before you begin to mix plaster, it is ad- 
visable to tinge as much water as you will 
want for your inner mould—say a quart 
in the present instance. Eemember that 
it is merely to be tinged — that it is to have 
so much color only as will enable you to 
distinguish readily the plaster mixed 
with it from that mixed with plain water. 
If you put more color than this, you will 
run the risk of staining and disfiguring 
i the cast itself. 
[ We are now ready for mixing the plas- 
ter. Two-thirds fill a basin with the col- 
' ored water, and then begin to sprinkle in 
the plaster. Do this with the hand, that 
you may detect any lumps that it may 
contain ; and should you meet with any 
lumps that do not crumble readily, throw^ 
them aside, k.^ you sprinkle the*^ i)laster 
over the surface of the water, it will grad- 
ually become saturated and sink. This- 
metiiod of mixing is superior to any othei', 
because the air contained in the plaster is 
thus driven out, and not imprisoned, and 
so does not form bubbles, which would in- 
jure your work. Go on sprinkling till 
you see that the plaster no longer sinks^ 
i3ut stands ui> in hills above the water ; 
this is a proof that enough has been ])ut 
in. The water takes the proper quantity ^ 
and no more. 
The mixture must now be well beaten 
up with a spoon. For mixing superfine, 
and even fine, if the work is particular, 
use a silver spoon, as baser metal, partic- 
ularly iron, may discolor the plaster. 
Beat quickly but carefully. Beware lest 
in doing this you beat air into the plaster, 
and so cause bubbles. There is a way of 
beating by which the spoon is kept at thf^ 
