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O MECHANICAL DENTISTRY AND METALLURGY.
METHODS OF TAKING IMPRESSIONS OF
THE MOUTH.
Impressions in Plaster.—Before mixing the plaster a
siiitaljle impression tray or cup should be selected. In de-
termining this the cup should be tried in the mouth, and if
exactly adapted to the case proceed as follows :
For full upper impressions place a piece of softened bees-
wax across the rear of the palatine portion of the cup
(Fig. 3 shows the form of cup to be used with a layer of
wax in position) just sufficient to support the plaster at that
point, making it more certain to secure a correct impression
of the palate ; especially is this necessary where the arch is
high. It also aids in keeping the plaster from being forced
over the rear of the cup into the fauces. If the tuberosities
are deep a little wax should be placed around the posterior
corners of the cup, and when the alveolar ridge is unusually
deep, and it is desirable to obtain a high impression over the
region of the cuspids, place a layer of wax over the edge of
the cup at these points also, before mixing the plaster.
Method of Mixing Plaster.—The best method is that
which most perfectly excludes the air, prevents expansion,
and gives the sharpest and smoothest impression.
I. A medium-sized bowl (rubber is the most convenient)
should be partially filled w^ith water (^\•arm water is pleas-
anter for the patient and hastens the setting of the plaster)
;
to this should be added about ten grains of common salt,
to hasten the setting and to make the plaster more brittle.
It is better to add the salt at this time, that is, before adding
the plaster, as it gives it a better opportunity to become
uniformly diffused. Other agents, such as chlorate of pot-
ash, potassium sulphate, and alum, have been and are used
to hasten the setting of plaster, but salt is the least ob-
jectionable and answers every purpose.
O MECHANICAL DENTISTRY AND METALLURGY.
METHODS OF TAKING IMPRESSIONS OF
THE MOUTH.
Impressions in Plaster.—Before mixing the plaster a
siiitaljle impression tray or cup should be selected. In de-
termining this the cup should be tried in the mouth, and if
exactly adapted to the case proceed as follows :
For full upper impressions place a piece of softened bees-
wax across the rear of the palatine portion of the cup
(Fig. 3 shows the form of cup to be used with a layer of
wax in position) just sufficient to support the plaster at that
point, making it more certain to secure a correct impression
of the palate ; especially is this necessary where the arch is
high. It also aids in keeping the plaster from being forced
over the rear of the cup into the fauces. If the tuberosities
are deep a little wax should be placed around the posterior
corners of the cup, and when the alveolar ridge is unusually
deep, and it is desirable to obtain a high impression over the
region of the cuspids, place a layer of wax over the edge of
the cup at these points also, before mixing the plaster.
Method of Mixing Plaster.—The best method is that
which most perfectly excludes the air, prevents expansion,
and gives the sharpest and smoothest impression.
I. A medium-sized bowl (rubber is the most convenient)
should be partially filled w^ith water (^\•arm water is pleas-
anter for the patient and hastens the setting of the plaster)
;
to this should be added about ten grains of common salt,
to hasten the setting and to make the plaster more brittle.
It is better to add the salt at this time, that is, before adding
the plaster, as it gives it a better opportunity to become
uniformly diffused. Other agents, such as chlorate of pot-
ash, potassium sulphate, and alum, have been and are used
to hasten the setting of plaster, but salt is the least ob-
jectionable and answers every purpose.
