On the Economy of Machinery and Manufactures - Charles Babbage (interesting books to read in english .TXT) 📗

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.

 

Of copying by moulding

 

112. This method of producing multitudes of individuals

having an exact resemblance to each other in external shape, is

adopted very widely in the arts. The substances employed are,

either naturally or by artificial preparation, in a soft or

plastic state; they are then compressed by mechanical force,

sometimes assisted by heat, into a mould of the required form.

 

113. Of bricks and tiles. An oblong box of wood fitting upon

a bottom fixed to the brickmaker’s bench, is the mould from which

every brick is formed. A portion of the plastic mixture of which

the bricks consist is made ready by less skilful hands: the

workman first sprinkles a little sand into the mould, and then

throws the clay into it with some force; at the same time rapidly

working it with his fingers, so as to make it completely close up

to the corners. He next scrapes off, with a wetted stick, the

superfluous clay, and shakes the new-formed brick dexterously out

of its mould upon a piece of board, on which it is removed by

another workman to the place appointed for drying it. A very

skilful moulder has occasionally, in a long summer’s day,

delivered from ten to eleven thousand bricks; but a fair average

day’s work is from five to six thousand. Tiles of various kinds

and forms are made of finer materials, but by the same system of

moulding. Among the ruins of the city of Gour, the ancient

capital of Bengal, bricks are found having projecting ornaments

in high relief: these appear to have been formed in a mould, and

subsequently glazed with a coloured glaze. In Germany, also,

brickwork has been executed with various ornaments. The cornice

of the church of St Stephano, at Berlin, is made of large blocks

of brick moulded into the form required by the architect. At the

establishment of Messrs Cubitt, in Gray’s Inn Lane, vases,

cornices, and highly ornamented capitals of columns are thus

formed which rival stone itself in elasticity, hardness, and

durability.

 

114. Of embossed china. Many of the forms given to those

beautiful specimens of earthenware which constitute the equipage

of our breakfast and our dinner-tables, cannot be executed in the

lathe of the potter. The embossed ornaments on the edges of the

plates, their polygonal shape, the fluted surface of many of the

vases, would all be difficult and costly of execution by the

hand; but they become easy and comparatively cheap, when made by

pressing the soft material out of which they are formed into a

hard mould. The care and skill bestowed on the preparation of

that mould are repaid by the multitude it produces. In many of

the works of the china manufactory, one part only of the article

is moulded; the upper surface of the plate, for example, whilst

the under side is figured by the lathe. In some instances, the

handle, or only a few ornaments, are moulded, and the body of the

work is turned.

 

115. Glass seals. The process of engraving upon gems requires

considerable time and skill. The seals thus produced can

therefore never become common. Imitations, however, have been

made of various degrees of resemblance. The colour which is given

to glass is, perhaps, the most successful part of the imitation.

A small cylindrical rod of coloured glass is heated in the flame

of a blowpipe, until the extremity becomes soft. The operator

then pinches it between the ends of a pair of nippers, which are

formed of brass, and on one side of which the device intended for

the seal has been carved in relief. When the mould has been well

finished and care is taken in heating the glass properly, the

seals thus produced are not bad imitations; and by this system of

copying they are so multiplied, that the more ordinary kinds are

sold at Birmingham for three pence a dozen.

 

116. Square glass bottles. The round forms which are usually

given to vessels of glass are readily produced by the expansion

of the air with which they are blown. It is, however, necessary

in many cases to make bottles of a square form, and each capable

of holding exactly the same quantity of fluid. It is also

frequently desirable to have imprinted on them the name of the

maker of the medicine or other liquid they are destined to

contain. A mould of iron, or of copper, is provided of the

intended size, on the inside of which are engraved the names

required. This mould, which is used in a hot state, opens into

two parts, to allow the insertion of the round, unfinished

bottle, which is placed in it in a very soft state before it is

removed from the end of the iron tube with which it was blown.

The mould is now closed, and the glass is forced against its

sides, by blowing strongly into the bottle.

 

117. Wooden snuff boxes. Snuff boxes ornamented with devices,

in imitation of carved work or of rose engine turning, are sold

at a price which proves that they are only imitations. The wood,

or horn, out of which they are formed, is softened by long

boiling in water, and whilst in this state it is forced into

moulds of iron, or steel, on which are cut the requisite

patterns, where it remains exposed to great pressure until it is

dry.

 

118. Horn knife handles and umbrella handles. The property

which horn possesses of becoming soft by the action of water and

of heat, fits it for many useful purposes. It is pressed into

moulds, and becomes embossed with figures in relief, adapted to

the objects to which it is to be applied. If curved, it may be

straightened; or if straight, it may be bent into any forms which

ornament or utility may require; and by the use of the mould

these forms may be multiplied in endless variety. The commoner

sorts of knives, the crooked handles for umbrellas, and a

multitude of other articles to which horn is applied, attest the

cheapness which the art of copying gives to the things formed of

this material.

 

119. Moulding tortoise-shell. The same principle is applied

to things formed out of the shell of the turtle, or the land

tortoise. From the greatly superior price of the raw material,

this principle of copying is, however, more rarely employed upon

it; and the few carvings which are demanded, are usually

performed by hand.

 

120. Tobacco-pipe making. This simple art is almost entirely

one of copying. The moulds are formed of iron, in two parts, each

embracing one half of the stem; the line of junction of these

parts may generally be observed running lengthwise from one end

of the pipe to the other. The hole passing to the bowl is formed

by thrusting a long wire through the clay before it is enclosed

in the mould. Some of the moulds have figures, or names, sunk in

the inside, which give a corresponding figure in relief upon the

finished pipe.

 

121. Embossing upon calico. Calicoes of one colour, but

embossed all over with raised patterns, though not much worn in

this country, are in great demand in several foreign markets.

This appearance is produced by passing them between rollers, on

one of which is figured in intaglio the pattern to be