Page 396 - My FlipBook

394 RESTORATION OF TEETH BY CEMENTED INLAYS.

reservoir or labor with the foot-bellows. The electric furnaces arc based
on the principle that platinum wire submerged in fire-clay will become
red-hot when a current is passed through it. The fire-clay stores up
this heat indefinitely, so that any degree of temperature below the fusing-
point of platinum may be obtained. This fusing-point is said to be
about 4500° Fahrenheit. The best of these furnaces for the high-fusing
bodies that require a temperature of from 2500° to 3000° F. are the
Timme (Fig. 379), the Custer crown (Fig. 378), and the McBriar crown.
Fk;. .-^77.

Downie gas crown furnace.
Any one of these will give satisfactory results, but the Timme furnace
has the important advantage of being made in sections which can be
readily taken apart, thus rendering a burn-out easy to locate and mend.
With the other furnaces it is difficult at times to find and twist together
the fused or broken wires ; and when the break is located, it has to
be reached by cutting down through fused fire-clay that is sometimes
so dense as to require the use of a hammer. Should electric furnaces
be used, it is well to have two, so that if a burn-out occur the opera-
tion need not be delayed while the defect is being located and repaired.