HOW TO USE A CEMENT CHUCK.
The shellac is melted in an iron vessel, and the ultramarine added and
stirred to incorporate the parts. Care should be observed not to burn
the shellac. While warm, the melted mass is poured on to a cold slab of
iron or stone, and while plastic made into sticks about 1/2" in
diameter.
[Illustration: Fig. 178]
[Illustration: Fig. 179]
We show at Fig. 178 a side view of the outer end of a cement chuck with
a cylinder in position. We commence to turn the lower pivot of a
cylinder, allowing the pivot _z_ to rest at the apex of the hollow cone
_a_, as shown. There is something of a trick in turning such a hollow
cone and leaving no "tit" or protuberance in the center, but it is
important it should be done. A little practice will soon enable one to
master the job. A graver for this purpose should be cut to rather an
oblique point, as shown at _L_, Fig. 179. The slope of the sides to the
recess _a_, Fig. 178, should be to about forty-five degrees, making the
angle at _a_ about ninety degrees.
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