he fact
that ebonite sometimes springs a little out of shape when the skin is
removed.

Turned work in ebonite, if well done, requires no sand-papering, but
may be sufficiently polished by a handful of its own shavings and a
little vaseline. The advantage of using a polished ebonite surface is
that such a surface deteriorates more slowly under the influence of
light and air than a surface left rough from the tool. If very highly
polished surfaces are required, the ebonite after tooling is worked
with fine pumice dust and water, applied on felt, or where possible by
means of a felt buff on the lathe, and finally polished with rouge and
water, applied on felt or cloth.

Ebonite works particularly well under a spiral milling cutter, and
sufficiently well under an ordinary rounded planing tool, with cutting
angle the same as for brass, and hardened to the lightest straw
colour.

It is not possible, on the other hand, to use the carpenter's plane
with success, for the angle of the tool is too acute and causes the
ebonite to chip.

In boring ebonite the drill should be withdrawn from the hole pretty
often and well lubricated, for if the borings jam, as they are apt to
do, the heat developed is very great and the temper of the drill gets
spoiled. Ebonite will spoil a drill by heating as quickly as anything
known; on the other hand, it may be drilled very fast if proper
precaution is taken.

It is advisable to expose ebonite to the light as little as possible,
especially if the surface is unpolished, for under the combined action
of light and air the sulphur at the surface of the ebonite rapidly
oxidises, and the ebonite becomes covered with a thin but highly
conducting layer of sulphurous or sulphuric acid or their compounds.
When this happens the ebonite may be improved by scrubbing with hot
water, or washing freely with alcohol rubbed on with cotton waste in
the case of apparatus that cannot be dismounted.

A complete cure, however, can only be effected by scraping off the
outer layer of ebonite so as to expose a fresh surface. For this
purpose a bit of sheet glass broken so as to leave a curved edge is
very useful, and the ebonite is then scraped like a cricket bat. In
designing apparatus for laboratory use it is as well to bear in mind
that sooner or later the ebonite parts will require to be taken down
and scraped up. Rods or tubes are, of course, most quickly treated on
the lathe with rough glass cloth, a