scale is left on, and this is reduced to powder or shaken loose in the
course of riveting and left between the plates, thus offering a
tempting opening for the steam to work through, and is really cause of
the heavy calking that puts so unnecessary a pressure on both plate
and rivet. A clean metallic joint can be secured by passing over the
two surfaces a sponge wet with a weak solution of sal-ammoniac and hot
water, an operation certainly cheap enough both as to materials and
labor required.

[Illustration: FIG. 19]

The above cut, Fig. 19, gives an illustration of calking done by
sharp-nosed and round nosed tools, respectively. It will be seen by
Fig. 20 that the effect of a round-nosed tool is to divide the plate
calked, and as the part divided is well driven toward the rivets, a
bearing is formed at _a_, from one-half to three-fourths of an inch,
which increases the strength of joint, and will in no way cut or
injure the surface of the under plate. A perfect joint is thus
secured.

[Illustration: Fig. 20.]

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