A Small Gas Furnace
A small gas furnace for melting small quantities of gold, silver, or other metals, can be easily made as follows: Procure a cylinder of stovepipe iron of the desired size, say 4¹⁄₂ or 5 in. in diameter, and 6 in. long. Turn in the edge of one end so that it will hold loosely a disk of iron of the same thickness as the cylinder, to form the bottom. Punch ¹⁄₈-in. holes, about 1 in. apart, all over the cylinder and the bottom. This is to allow for the escape of moisture and air. Attach three legs to the sides with rivets, evenly spaced so as to raise the bottom 4 in. In the side of the cylinder cut a hole, 1¹⁄₄ in. in diameter, its center being 1¹⁄₂ in. from the bottom. Select a glass bottle whose diameter is 1 or 1¹⁄₂ in. less than that of the cylinder.
Make a putty of fire clay, and add about five per cent of plaster of Paris, and also add about a teaspoonful of vinegar, to prevent a too rapid setting. Place a mass of the putty in the bottom of the cylinder and press in the bottle after its surfaces have been well greased. The bottle should be well centered in the cylinder. Press the bottle well down, so that its bottom is on a level with the lower edge of the hole in the side. Fill in the putty to form the sides, tamping it well all around the bottle, and build it up a little above the metal of the sides, carefully smoothing off the edge. With the fingers, or a spatula of some kind, cut out the putty in the hole to form an opening, ³⁄₄ in. in diameter. Allow the putty to set for more than an hour, and then with a circular, or twisting, motion remove the bottle. To prevent a vacuum in the bottom that will prevent removing the bottle easily, puncture the putty that lies on the disk with a hatpin, to allow the air to pass.
A few places will be found where the putty has not filled in properly, and these should be patched up while the filling is yet soft. The clay will probably shrink after standing for a time, and should be pressed out against the sides while in a plastic state. When dry, the furnace is ready for use.
In furnaces of this kind the jet of a large gas blowtorch is to be used. If a standard is rigged up to hold the blowtorch, it will be much easier for the operator. The jet from the pipe is thrown in the hole at the side and a crucible is placed in the furnace. It is well to make a cover similar to the body of the furnace with a hole in the center a little larger than the hole in the side.
A more efficient furnace can be made with two holes in the side, opposite each other, for using a double jet from blowtorches.—Contributed by James H. Beebee, Rochester, N. Y.