3. Small Muffle Furnace, with three feet chimney. This requires about eighteen inches longer chimney than the small crucible furnace to obtain the same temperature in the same time, owing to a slight loss of heat by radiation from the stoppers.
4. a. Large Muffle Furnace. This is identical in design and construction with the smaller one. The clear working space inside the muzzle is 37⁄8 by 5 inches, by about 3 inches deep. This is recommended as a useful furnace for watch dial enamellers, assayers, photo-enamel burning, and for all purposes where exact temperatures are required not exceeding the fusing point of cast iron.
The burner of this furnace is twice the size of the small laboratory furnace, and requires a gas supply from a pipe and tap of half-an-inch bore. The burner is the same shape as the muffle, and is unfit for crucible work.
b. Extra Large Muffle Furnace 41⁄2 by 33⁄4 by 7 inches clear inside working space. This will take a No. 3 plumbago pot, and with half an inch gas pipe, giving a supply of about 35 feet per hour, will, it is affirmed, melt 3 or 4 lbs. of brass in about 25 minutes, and the same quantity of cast iron in 60 or 70 minutes
from the time the gas is first lighted, without the slightest trouble or attention.
5. Ladle Furnace. This takes ladles up to 61⁄2 inches diameter, and will melt 6 or 8 lbs. of zinc in about 15 minutes, or the same quantity of lead, tin, &c., in about half the time. It is said to be a convenient and powerful arrangement for dentists, heating soldering-irons, making granulated zinc, sand baths, &c.
6. Small Laboratory Furnace, complete for crucibles, muffles, ladles, and sand baths.
7. Fletcher’s Injector Gas Furnace (with Blast). This furnace is intended for general purposes, and for the treatment of refractory substances at high temperatures. The patentee states “that it will burn perfectly in the same space any available gas supply from 10 to 50 feet per hour, or more, if required, giving temperatures in exact proportion; and any operation may be repeated at any time by taking a note of the position of the air slide which governs the combustion of the gas.”
Mr Fletcher gives the power of the small furnace as follows:—With an 1⁄2 inch gas supply-pipe, day pressure, starting with the furnace cold, it will melt silver in 3 minutes, cast iron in 8 minutes, cast steel in 25 minutes.
With a supply of 50 feet per hour, the same results are stated to be obtained in a little over half the time, and so on in proportion with a greater or less gas supply. It is also said to work satisfactorily for gold, &c., melting it with a supply of gas too small for any other furnace, and the maximum temperatures obtained are limited, only by the available gas supply and the fusibility of the casing. The highest temperature as obtained by measuring by Wedgwoods’ Pyrometer, is said to be 9000 Fahrenheit. This furnace is stated to be particularly suited for gold and silver melting, and refining, iron assays, and general crucible work, and safe in the hands of the most careless workman. It is adapted for crucibles not exceeding 4 inches by 21⁄4, 5 inches by 31⁄2, 71⁄2 inches by 5. For further information respecting furnaces intended for use in the laboratory and assay office, the reader is referred to ‘Watt’s Dictionary of Chemistry,’ also to ‘Ure’s Dictionary of Arts, Manufactures, and Mines,’ for description of the furnaces employed in the different metallurgical operations; and to the ‘Chemical News’ (June 30th, 1876, and February 2nd, 1877), for a description of a new decomposing furnace. See Assaying, Chimneys, Copper, Crucible, Fuel, &c.