34]
[Illustration: Fig. 35]
FANCY SCREWHEADS.
There are two easy methods of removing the blue from the flat part of
the screwhead at _a_. (1) Make a special holder for the screw in the end
of a cement brass, as shown at _E_, Fig. 36, and while it is slowly
revolving in the lathe touch the flat surface _a_ with a sharpened
pegwood wet with muriatic acid, which dissolves the blue coating of
oxide of iron. (2) The surface of the screwhead is coated with a very
thin coating of shellac dissolved in alcohol and thoroughly dried, or a
thin coating of collodion, which is also dried. The screw is placed in
the ordinary polishing triangle and the flat face at _a_ polished on a
tin lap with diamantine and oil. In polishing such surfaces the thinnest
possible coating of diamantine and oil is smeared on the lap--in fact,
only enough to dim the surface of the tin. It is, of course, understood
that it is necessary to move only next to nothing of the material to
restore the polish of the steel.
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