. Dental metallurgy, for the use of dental students and practitioners. Fig. 28 Muffle Furnace.—Browns assay furnace (Fig. 28) illus-trates this form of furnace; in it coke is used as a older furnaces used in continuous gum work were ofthis type. The muffle used is of a refractory material andduring the heating process the metals do not come in contactwith the products of combustion. If an alloy of gold andlead is placed in the muffle in a small dish composed of bone HEATING DEVICES 87 ash, the lead will be oxidized, leaving the gold as a process is called cupellation; the bo
. Dental metallurgy, for the use of dental students and practitioners. Fig. 28 Muffle Furnace.—Browns assay furnace (Fig. 28) illus-trates this form of furnace; in it coke is used as a older furnaces used in continuous gum work were ofthis type. The muffle used is of a refractory material andduring the heating process the metals do not come in contactwith the products of combustion. If an alloy of gold andlead is placed in the muffle in a small dish composed of bone HEATING DEVICES 87 ash, the lead will be oxidized, leaving the gold as a process is called cupellation; the bone ash dish is thecupel. The electric muffle furnace consists of a muffle wrappedwith platinum wire of about 28-gauge, and then covered witha mixture of refractory materials, such as fire-clay. Themuffle is then put into position in the outside casing, whichdiffers in the various makes of furnaces. The platinum-wire. Fig. 29 wrapping leads to binding posts. Then from the bindingposts it passes to some form of a resistance coil which regu-lates the amount of current passing through the resistance coil or rheostat gives us control of the tem-perature. Hoskins furnace is a good illustration of a furnacein which electricity is made use of for heating 29 illustrates this type of furnace. Chromel, a nickel-chromium alloy, is used instead of electric furnaces used in porcelain work, of which 88 METALLURGICAL APPARATUS there are many types in use, as a rule contain platinum wireand serve to illustrate an electric muffle furnace. Refractory Materials.—Refractory agents are those thatcan withstand the agency of heat without destruction. Theideal refractory material should not decompose or crackupon sudden changes of temperature, nor be attacked by themetal or flux used during the heating process. They areused: (a) In the construction of furnaces. Example: fire-clay.(
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