Page 119 - My FlipBook

placed on one of the supports, and by raising or lowering the arm, the
temperature is changed. The lower the arm is placed the more heat is
required to fuse the plug. While this furnace is accurate for low-fusing
porcelain it seems to be less reliable for high-fusing porcelain, unless
the plugs are made of a higher-fusing metal. The base of the furnace
becomes hot when used for high-fusing porcelain ; it retains so much
heat that the lead plug melts before sufficient heat is obtained to fuse
some high-fusing porcelains.
The muffle should first be heated to at least
technique 2,000° F. in order that the fireclay casing
of may become hot. The lever should be brought back
Tusing Porcelains. to the first button of the rheostat. When the work-
is ready to be placed into the muffle the current
should be turned off and the muffle permitted to cool until the redness
has disappeared. The porcelain should then be placed into the muffle and
the current turned on. The heat should be increased gradually. If an
electric pyrometer is used the needle should move slowly, and when all
the heat is obtained from a given resistance the lever should be moved
to the next button. When the needle of the milli-ammeter registers at the
proper point for the fusing of a given porcelain the current is shut ofif.
If the Garhart automatic pyrometer is used the secondary hand is set
at the point at which the porcelain is to fuse. The rheostat lever is
moved gradually until it has reached the button required to fuse the por-
celain, after which no further attention is required, for the current is
automatically cut out when the pyrometer hand and the secondary hand
come in contact. The author has used this automatic furnace more than
three thousand times and it has never failed to cut out the current at
the proper time.
It must be borne in mind that pyrometer furnaces are not accurate
unless they are manipulated under uniform conditions. The following
points should be observed in fusing porcelain. Always heat the muflle
previous to firing the porcelain. Never place the porcelain in an in-
tensely hot furnace. Heat the porcelain slowly. Run the pyrometer
under similar conditions for each firing. Do not try to fuse porcelain
and do other things at the same time unless an automatic pyrometer fur-
nace is being used.
A complex inlay should be tried into the cavity before the matrix
is removed ; for in many instances it is necessary to grind the inlay to
proper occlusion. This can be accomplished best by testing with car-
bon paper and grinding at such points as are indicated by spots on the
porcelain. The incisal edge should be ground to proper length and
contour. After this necessary grinding the inlay should be thoroughly
cleansed and then fired to a high biscuit.

115