Page 125 - My FlipBook

MAKING AND SETTING OF A GOLD INLAY 121
When Using- Pin or Pure Gold Matrix. When casting an inlay
to a mold Avhich contains a pin or a pure gold matrix, the tem-
perature of the mold should be considerably higher. Particularly
is this true Avhen the pin is large or the amount of gold to cover
the matrix is thin as it may be close to the margins.
Quantity of Gold Used in the Cast. When using the suction or
pressure machines it is quite necessary to have a large sprue left,
as when the amount of gold is near the size of the inlay, failure is
liable to result owing to the philosophy of the force used in cast-
ing. However, with the centrifugal machine, it is not absolutely
essential that there be any considerable sprue left. Yet if Ave try
to guess too closely, many failures will result from having too lit-
tle material. Owing to the law of the shrinkage of the metal to-
wards the larger body, the sprue Avhich is left should never Aveigh
{IS much as the inlay cast. A large sprue left is of advantage, as
there is a tendency to hold the Avhole body of gold at a tempera-
ture sufficient to give it time to thread its Avay through the sprue
hole into the mold. It is also of advantage Avhere there is a large
pin or matrix present, as the high temperature is maintained longer.
The large sprue is particularly at a disadvantage Avhen casting
the base to pin croAvns. The Ioav fusing pin is liable to be melted.
There is also more danger of checking the porcelain.
Size of the Opening. The size of the hole leading to the mold is
of importance for a number of reasons. As a general rule the
larger the inlay, and the loAver temperature of both the hole and
the material at AV'hich you cast, the larger should be the hole; it
necessarily folloAvs that the hole should be smaller Avith the reverse
conditions.
A small hole lengthens the time required for the stream of molten
gold to pass to position. Hence, if the mold is cold and the mate-
rial is not extra Avarm in casting a large body, the material is lia-
ble to become chilled and the mold not entirely filled. HoAvever,
if Ave are casting a small inlay, in a rather Avarm mold Avith the
gold extra hot, the small hole is preferable as there is less liability
of a backAvard shrinkage of the gold to the sprue, Avhen cooling.
Better results are obtained Avhen the Avax pattern is immediately
invested, burned out and casting completed Avithout alloAving the
mold, either Avith the pattern in position or burned out, to lay over
night. If it must lay over night, it is best to burn out the Avax and
thoroughly heat the mold, as less change takes place thereafter in
the investment. In this connection your attention is called to the