I48 OF GUN-METAL. .

flame that it is in small quantity, and a green flame that itis almost dissipated. Before the furnace is opened the com-position should be stirred ; and if there be the least reasonto be dubious of its quality, a litde should be taken out withan iron ladle, and when cold be carefully proved. If therecast metal contained any brass, the zinc will be totally dis-sipated (23); therefore when brass should form any part ofthe new composition, a proportion must be added before thefurnace is opened, not exceeding (28) half the quantityof tin.

no. The furnace is heated before the copper is put in(108) to give it the necessary degree of heat which it wouldnot otherwise attain; for if it were put in before, the partof the hearth covered by the copper and the copper itselfwould never be properly hot; therefore not to lessen thedegree of heat in the furnace or in the metal already fused,small quantities are put in at a time, that the liquid matter mayspread equally over the whole surface. The brass and tin areput in when the copper is thoroughly fusSd, and just beforethe furnace is opened, to avoid as much as possible the sub-limation of the zinc, and the calcination of the tin (22, 23,24) ; they are then well stirred, since the motion producedby the fire is not sufficient to mix them thoroughly.

When old ordnance containing very little tin is to berecast, the founders generally put two or three large gunsinto the furnace, in such a position that they may cover aslittle as possible of the base, and a moderate fire is kept upfor five hours that the furnace may be well heated before themetal begins to fuse. If on the contrary, the metal to berecast contain much tin, as bell-metal ; the founders know-ing that the tin accelerates the fusion (24) put a great quan-tity into the furnace before the fire is lighted, taking care soto arrange the pieces that the flame may penetrate betweenthem before they melt; and the heat is kept moderate tillthe fusion begins ; this in the language of the founders iscalled putting them into a bath.

in. The calcination of copper and tin may be partlyrepaired by the introduction of phlogistic substances', but it isnot judged expedient to have recourse to this method inlarge furnaces: the founders contenting themselves with drop-ping, while the melted matter is running into the moulds,some unctuous substances, as fat, suet, &e. to prevent thecalcination.

112. The