June, 1921
T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
56 1
Electric Muffle Furnaces for Laboratory Use’ By H. C. Kremers CHEMICAL
LABORATORY, UNIVERSITY
Most research chemists have found the small electrically heated muffle furnace a very valuable asset t o their laboratory equipment. Rather high costs and limited varieties, however, have restricted their use. Many chemists have found frequent occasion to builti their own furnaces.2 Here again the limit has been set by the size and shape of muffle a t hand. Special sizes and shapes of muffles made t o order can usually be obtained only after considerable delay and expense. The author has had frequent occasion t o use electric muffle furnaces of special shapes and sizes, and by following the procedure outlined below these furnaces were constructed with minimum delay and expense. CONSTRUCTION
O F MUFFLE
An ordinary glass bottle of low melting glass is selected of a size t o correspond t o the inner dimensions of the muffle desired. This bottle is first covered with a single layer of asbestos paper (a, Fig. 1). By moistening the asbestos paper it will be found t o adhere very well. Next in order, a 0.25-in. layer of alundum cement, c, of the consistency of a stiff mortar is applied. The built-up muffle is then dried in a n air bath a t a temperature of approximately 150’ C. The heating element d is next wound on in the regular way. The author’s furnaces were all wound with a nickel-chromium alloy wire. The data for calculating the size and quanFIG 1 t i t y of wire required can usually be obtained from the manufacturers of these products. It has been found very desirable t o double the lead-in wires and also t o have part of the first coil doubled. A reference t o the drawing will make this point clear. It has been the general experience t h a t a furnace will frequently burn out first a t the point of the lead-in wire; thus, by daubling this wire, the life of a furnace is much prolonged. Considerable difficulty is frequently met with by the novice in holding the winding in place. It was found t h a t the first and last coils of the winding can be held well in place by tying them in position with asbestos cord. This asbestos cord can simply be cemented over, and left intact. A second coating of alundum cement, e, is then applied over the winding. Here a 0.125-in. layer is usually sufficient. The completed Received March 28, 1921. z A n arrangement essentially similar t o that herein described was used by F. P. Venable and J M Bell, J . A m . Chem. SOC.,39 (1917), 1602.
O F ILLINOIS, U R B A N A . ILLINOIS
muffle is again heated t o 150” C. in an air bath. It will usually be found t h a t numerous very small cracks will result, but a wash of thin alundum cement will remedy this. The completed with the glass still in place is mounted as shown in ~ i 2 . ~ ~h~ . upper edge of the is usually mounted flush with the upper edge of the container g. A 0 . 2 5 - i ~layer . of d u m cement,j , will efiectively seal the upper opening of the container. Calcined magnesia or sil-o-cel make an effective lator, j . The wires FIG. 2 can be very The latter Of attached to binding Posts’ course, be well insulated from the container g. The current can now be turned on and the furnace allowed t o come u p t o a red heat slowly. For the first heating this should take a t least 3 or 4 hrs. As the furnace comes up t o red heat, the glass core will gradually begin t o soften, and can in most cases be withdrawn. If this is not possible, the glass may very easily be shattered b y a small spray of cold water and then easily removed. Upon allowing the furnace t o cool the single layer of asbestos paper b can be peeled off, and t h e muffle given a wash of alundum cement. A furnace thus constructed will satisfactorily maintain a temperature of 950Oto 1000° C., and will have a life equal t o any furnace using a nickel-chromium heating element. I n place of the cement seal f a ring of asbestos millboard is found very satisfactory. A slight modification in construction will provide a very satisfactory covering for the muffle, as follows: The muffle c is set enough lower SO t h a t a porcelain water-bath ring may be cemented in position on the upper edge of the muffle, in such a manner t h a t the upper surface of the porcelain ring will be flush with the cement f. When in use, the opening of the furnace can be partly or entirely closed by nesting in one or more rings. COMBUSTION
B U R N ACES
Furnaces of the combustion type with both ends open may be constructed along the same lines. A furnace of this latter type was recently constructed, having a muffle with a n opening 2 in. in diameter and 18 in. long. An ordinary 2-in. soft glass tube was used as t h e core, and the same general plan of construction as outlined above was used. This furnace has been in use for several days, and has been found entirely satisfactory for temperatures up t o 950 C.
