Chap. II.

COMPOSITION AN1) USE OF MINERALS.

689

the statue, which was performed with great care, and constituted a perfect model. Thewhole statue was then covered with smaller pieces of wax, in the direction intended for thechannels of the metal, and of the necessary size ; some being, as shown, considerably largerthan the others; the whole was then coated with similar clay to that which formed thecore. The great channels met at the top of the model, and formed apertures where themetal was to be poured in, but there were others provided by which the air could escapewhen the metal entered; at the bottom, or at the feet of the statue, a hole or two was left,where the great channels and waxen statue join; the wax forming the covering and thechannels then ran out, and the mould was again submitted to a red heat: after this it wasplaced in a pit, and the same process adopted as for casting bells: to ascertain the quantityof metal, it is only necessary to weigh the wax, and compare it with the weight ofmetal to be used.

The statue of Lord Hopetoun, erected in 1834 at Edinburgh , was cast in a somewhatsimilar manner at the Royal Arsenal at Woolwich : a model made in plaster of Paris wascovered over with a thin shell, composed of a number of pieces fitted nicely together, andwhich could be taken asunder, and after the model was removed again built up ; the workwas commenced at the bottom by covering a portion of the shell with sand to about1 or 1;] inches in thickness, to which was added about 12 inches of plaster of Paris, whichuniting with the sand, formed one block ; the remaining part of the statue being com-pleted with similar blocks, so arranged that they could be removed readily without disturbingthe sand when the whole was finished : tubes were introduced with the plaster for the ad-mission of the metal and for the escape of the air, and iron rings let into the blocks for theconvenience of removing them. After the shell was complete, the whole was taken to piecesand removed to the casting pit, where it was carefully rebuilt, and the interior filled upwith the material to form the core ; it was then a second time taken to pieces, leaving thecore of the shape and dimensions of the original statue; from this sufficient was scraped offto allow for the intended thickness of the metal ; the shell was then put together as before,and a space left between it and the core for the operation of casting.

Green or dry Sand Castings .—In thisprocess after the mould is made, thetlask is placed in the stove, and therekept till the sand becomes perfectlydry : before it is used a fire of char-coal is made all round it to heatthe sand previous to the metal beingpoured into it, the casting of which isalways of a superior kind, in conse-quence of its not being cooled downtoo rapidly ; all castings intended tobe turned or filed should be made bythis process, moist sand rendering thesurface of the iron refractory and hard,as well as injuring its quality.

Ix)am work is employed where largecisterns or cylinders are required,and is effected without moulds orpatterns, by modelling in loamthe object required. Supposingthe work to be that of a steam-enginecylinder, upon a plate of metal is builta mould with very soft bricks laid inbeds of loam ; this being completed,it is plastered over with loam andhair, about an inch in thickness,

dressed perfectly even by a striking _

l>oard, which works on a pivot, and Fig. 591- loam work casting.traverses freely round the whole cir-cumference : when dry, this is dressed over with a coating of coal and charcoal powdermixed with water.

.The intended thickness of the metal is then set out, and another coat of loam withouthair is put on, and being worked true by the striking board, which is adjusted for thepurpose, represents the place of the metal ; any bands or ribs may be moulded upon it. bycutting out their profile in the striking board. When this coat is dry it receives its blackwash, which prevents one coat of loam from adhering to the other.

Iwo semicircular plates of iron, having projecting arms and their insides made to thecurvature, are now placed on the foundation plate; on these are built the external case ofthe mould, or the jacket, in two cylindrical halves, in the same manner as the core, but of

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