Page 26 - My FlipBook

14 MECHANICAL DENTISTRY AND METALLURGY.
such a purpose. Many partial impressions can be readily
taken by simply preparing the cups as directed for full im-
pressions. The illustration also shows the position and
amount of wax to be placed across the palatal edge. The
simplest method, however, is to first take an impression in
wax and then remove a layer from its surface, that is from
the surface representing that portion of the mouth or ridge
where the plate and teeth are to rest ; place sufficient plaster
at these points to replace this layer of wax, and carefully
return it to the mouth.
In taking partial impressions it is important that the
surface of the w^ax should be roughened by cutting grooves,
so that the plaster will be thoroughly anchored to the cup.
It is also important, when a layer of w^ax is removed,
especially in the interdental spaces wdiere the teeth are to
go, that it be of considerable thickness, so that the plaster,
should it break, will have sufficient strength to allow of
accurate replacement. Again, it is important that the
plaster should not set as hard as in full cases, so that the
impression will not adhere to the mouth and have to be
broken away in pieces; this can usually be avoided by mov-
ing the tray up and down, just enough to crack the plaster
when it is beginning to ,set. After fracturing it, the im-
pression should be pressed firmly into place again and held
there for a short time — possibly a half a minute—when
it may be removed.
Manner of Obtaining Impression in Wax or Compound.
—The first step, of course, is to select a properly shaped
cup for the case in hand; then look to the secretions of the
mouth. If the secretions are abundant, thick or viscid,
they should be removed by rinsing the mouth with salt and
w^ater. The impression material, whether w-ax or com-
pound, may then be softened by gently heating over a
lamp or Bunsen burner, or, as some prefer, by immersing