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NON-PLASTIC FILLING MATERIALS 1 63

four walls, the missing wall is supplied by the use of a thin
metal band of steel, copper or German silver in the form of a
matrix. For technic purposes in the laboratory, a thin band of
German silver or copper, % or % the circumference of the

tooth, may be tied in position by means of floss silk, or bind-
ing wire, encircling the tooth several times. If the exercises
in filling are performed on natural teeth, with extensive cav-
ities, a measurement of the neck of the tooth may be taken,
and a soldered band made to completely encircle the tooth.
Technical Exercises.—Fill the cavities in the posterior
teeth prepared during the studies in cavity preparation.

NON-PLASTIC FILLING MATERIALS

Tin—Gold

TIN

Characteristics.—Tin, when pure and freshly cut, is co-

hesive (the separate particles clinging to each other), like gold.
It rapidly loses this property, though, on exposure to the
atmosphere, and gradually becomes non-cohesive. Until
recently it was worked almost exclusively after the non-
cohesive method. There have been introduced several

preparations of cohesive tin, one of which was known as
Duplex, a cohesive tin covered with cohesive gold. Another,
which received the trade name of Crystallia, was prepared
in coarse silver-colored shreds.
Tin is very little used as a filling material today. It may

be utilized in the technic laboratory for experimental purposes
as a substitute for gold. For working on the cohesive princi-
ple, it may be prepared for use after the --following method.
Take a corundum wheel, i^i to 2}^ inches in diameter,

and make a mold in sand or marble dust. After removing