be removed. On
occasion, therefore, apparatus which has become stained with lead during
its production, may be rendered presentable by suitable treatment in the
oxidising flame. The process of re-oxidising a considerable surface in
this way after it has cooled down is apt to be very tedious, however,
and, especially in the case of thin tubes or bulbs, often is not
practicable. In working with lead glass, therefore, any reduction that
occurs should be removed by transferring the glass to the oxidising
flame at once.

Small tubes, and small areas on larger tubes of English glass, may be
softened without reduction by means of the pointed oxidising flame; but
it is not easy to heat any considerable area of glass sufficiently with
a pointed flame. And though it is possible, with care, to employ the hot
space immediately in front of the visible end of an ordinary brush
flame, which is rich in air, yet, in practice, it will not be found
convenient to heat large masses of lead glass nor tubes of large size,
to a sufficiently high temperature to get the glass into good condition
for blowing, by presenting them to the common brush flame.

It may seem that as glass which has become stained with reduced lead can
be subsequently re-oxidised by heating it with the tip of the pointed
flame, the difficulty might be overcome by heating it for working in the
brush flame, and subsequently oxidising the reduced lead. It is,
however, difficult, as previously stated, to re-oxidise a large surface
of glass which has been seriously reduced by the action of the reducing
gases of the flame, after it has cooled. Moreover, there is this very
serious objection, that if, as may be necessary, the action of the
reducing flame be prolonged, the extensive reduction that takes place
diminishes the tendency of the glass to acquire the proper degree of
viscosity for working it, the glass becomes difficult to expand by
blowing, seriously roughened on its surface, and often assumes a very
brittle or rotten condition.

When it is only required to bend or draw out tubes of lead glass, they
may be softened sufficiently by a smoky flame, which, probably owing to
its having a comparatively low temperature, does not so readily reduce
the lead as flames of higher temperature. But for making joints,
collecting masses of glass for making bulbs, and in all cases where it
is required that the glass shall be thoroughly softened, the smoky flame
does not give good res