Of Copying By Casting





105. The art of casting, by pouring substances in a fluid

state into a mould which retains them until they become solid, is

essentially an art of copying; the form of the thing produced

depending entirely upon that of the pattern from which it was

formed.



106. Of casting iron and other metals.--Patterns of wood or

metal made from drawings are the originals from which the moulds

for casting are made: so that, in fact, the casting itself is a

copy of the mould; and the mould is a copy of the pattern. In

castings of iron and metals for the coarser purposes, and, if

they are afterwards to be worked even for the finer machines,

the exact resemblance amongst the things produced, which takes

place in many of the arts to which we have alluded, is not

effected in the first instance, nor is this necessary. As the

metals shrink in cooling, the pattern is made larger than the

intended copy; and in extricating it from the sand in which it is

moulded, some little difference will occur in the size of the

cavity which it leaves. In smaller works where accuracy is more

requisite, and where few or no after operations are to be

performed, a mould of metal is employed which has been formed

with considerable care. Thus, in casting bullets, which ought to

be perfectly spherical and smooth, an iron instrument is used, in

which a cavity has been cut and carefully ground; and, in order

to obviate the contraction in cooling, a jet is left which may

supply the deficiency of metal arising from that cause, and which

is afterwards cut off. The leaden toys for children are cast in

brass moulds which open, and in which have been graved or

chiselled the figures intended to be produced.



107. A very beautiful mode of representing small branches of

the most delicate vegetable productions in bronze has been

employed by Mr Chantrey. A small strip of a fir-tree, a branch of

holly, a curled leaf of broccoli, or any other vegetable

production, is suspended by one end in a small cylinder of paper

which is placed for support within a similarly formed tin case.

The finest river silt, carefully separated from all the coarser

particles, and mixed with water, so as to have the consistency of

cream, is poured into the paper cylinder by small portions at a

time, carefully shaking the plant a little after each addition,

in order that its leaves may be covered, and that no bubbles of

air may be left. The plant and its mould are now allowed to dry,

and the yielding nature of the paper allows the loamy coating to

shrink from the outside. When this is dry it is surrounded by a

coarser substance; and, finally, we have the twig with all its

leaves embedded in a perfect mould. This mould is carefully

dried, and then gradually heated to a red heat. At the ends of

some of the leaves or shoots, wires have been left to afford

airholes by their removal, and in this state of strong ignition a

stream of air is directed into the hole formed by the end of the

branch. The consequence is, that the wood and leaves which had

been turned into charcoal by the fire, are now converted into

carbonic acid by the current of air; and, after some time, the

whole of the solid matter of which the plant consisted is

completely removed, leaving a hollow mould, bearing on its

interior all the minutest traces of its late vegetable occupant.

When this process is completed, the mould being still kept at

nearly a red heat, receives the fluid metal, which, by its

weight, either drives the very small quantity of air, which at

that high temperature remains behind, out very through the

airholes, or compresses it into the pores of very porous

substance of which the mould is formed.



108. When the form of the object intended to be cast is such

that the pattern cannot be extricated from its mould of sand or

plaster, it becomes necessary to make the pattern with wax, or

some other easily fusible substance. The sand or plaster is

moulded round this pattern, and, by the application of heat, the

wax is extricated through an opening left purposely for its

escape.



109. It is often desirable to ascertain the form of the

internal cavities, inhabited by molluscous animals, such as those

of spiral shells, and of the various corals. This may be

accomplished by filling them with fusible metal, and dissolving

the substance of the shell by muriatic acid; thus a metallic

solid will remain which exactly filled all the cavities. If such

forms are required in silver, or any other difficulty fusible

metal, the shells may be filled with wax or resin, then dissolved

away; and the remaining waxen form may serve as the pattern from

which a plaster mould may be made for casting the metal. Some

nicety will be required in these operations; and perhaps the

minuter cavities can only be filled under an exhausted receiver.



110. Casting in plaster. This is a mode of copying applied to

a variety of purposes: to produce accurate representations of the

human form--of statues--or of rare fossils--to which latter

purpose it has lately been applied with great advantage. In all

casting, the first process is to make the mould; and plaster is

the substance which is almost always employed for the purpose.

The property which it possesses of remaining for a short time in

a state of fluidity, renders it admirably adapted to this object,

and adhesion, even to an original of plaster, is effectually

prevented by oiling the surface on which it is poured. The mould

formed round the subject which is copied, removed in separate

pieces and then reunited, is that in which the copy is cast. This

process gives additional utility and value to the finest works of

art. The students of the Academy at Venice are thus enabled to

admire the sculptured figures of Egina, preserved in the gallery

at Munich; as well as the marbles of the Parthenon, the pride of

our own Museum. Casts in plaster of the Elgin marbles adorn many

of the academies of the Continent; and the liberal employment of

such presents affords us an inexpensive and permanent source of

popularity.



111. Casting in wax. This mode of copying, aided by proper

colouring, offers the most successful imitations of many objects

of natural history, and gives an air of reality to them which

might deceive even the most instructed. Numerous figures of

remarkable persons, having the face and hands formed in wax, have

been exhibited at various times; and the resemblances have, in

some instances been most striking. But whoever would see the art

of copying in wax carried to the highest perfection, should

examine the beautiful collection of fruit at the house of the

Horticultural Society; the model of the magnificent flower of the

new genus Rafflesia--the waxen models of the internal parts of

the human body which adorn the anatomical gallery of the Jardin

des Plantes at Paris, and the Museum at Florence--or the

collection of morbid anatomy at the University of Bologna. The

art of imitation by wax does not usually afford the multitude of

copies which flow from many similar operations. This number is

checked by the subsequent stages of the process, which, ceasing

to have the character of copying by a tool or pattern, become

consequently more expensive. In each individual production, form

alone is given by casting; the colouring must be the work of the

pencil, guided by the skill of the artist.





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