Page 701 - My FlipBook

THE CEMENTS. 359

robbing it of a part that is necessary for the most complete
hardening of the mass. If, however, the harder cements have
been used, tlie shrinkage will bo much greater if the filling be
kept dry. The adding of water to the surface soon after the
cement begins to harden limits the shrinkage very materially,
therefore moisture should ])e added soon after the cement begins
to stiffen enough for a good finish to be made. The rubber dam
should be in place and pure water added for wetting the filling.
The salts in the saliva seem to form some injurious combinations
when admitted to the filling before the setting is well advanced.
Prom what is now known it would seem that for setting inlays,
the hardest and most impenetrable cements should be used.
With the very thin film remaining between the inlay and the
walls of the cavity, the shrinkage can not be of much importance.
The more important point is that the adhesive property of the
cement, which is depended upon to retain the inlay, is lost if
the cement is penetrated by moisture. The cement which will
retain this property longest is the best for this purpose. The
stickiness of the cements has, however, been very disajapointing.
Every dental student soon learns that the cement that hardens
on his glass mixing slab is easily removed after soaking for a
short time in water. The same loosening from the walls of cav-
ities is the general rule. This loosening occurs with the most
impenetrable cements. Therefore, after all has been said, the
adaptation to the cavity walls, which is in a large degree per-
fected by the cement, is the chief dependence in holding the inlay.
These general principles should Ije observed in all uses of
cements. Softer masses must be used in setting crowns and
some other operations requiring a plasticity approaching a
fluid condition, but it should be remembered that this renders
the cement less strong and more soluble, and is to be avoided
whenever the conditions of the case in hand will allow.