ults.

In the glass-works, where large quantities of ornamental and other glass
goods are made of lead or flint glass, the pots in which the glass is
melted are so constructed that the gases of the furnace do not come into
contact with the glass;[4] and as the intensely-heated sides of the
melting-pot maintain a very high temperature within it by radiation, the
workman has a very convenient source of heat to his hand,--he has, in
fact, only to introduce the object, or that part of it which is to be
softened, into the mouth of the melting-pot, and it is quickly heated
sufficiently for his purpose, not only without contact of reducing
gases, but in air. He can therefore easily work upon very large masses
of glass. In a special case, such a source of heat might be devised by
the amateur. Usually, however, the difficulty may be overcome without
special apparatus. It is, in fact, only necessary to carry out the
instructions given below to obtain a considerable brush flame rich in
air, in which the lead glass can be worked, not only without
discoloration, but with the greatest facility.

[4] See _Principles of Glass-making_, p. 31.

_To Produce an Oxidising Brush Flame._--The blower used must be
powerful, the air-tube of the blow-pipe must be about half as great in
diameter as the outer tube which supplies the gas. The operator must
work his bellows so as to supply a strong and _steady_ blast of air, and
the supply of gas must be regulated so that the brush flame produced is
free from every sign of incomplete combustion,[5] which may be known by
its outer zone being only faintly visible in daylight, and quite free
from luminous streaks (see Fig. 4, p. 9). When a suitable flame has been
produced, try it by rotating a piece of lead glass at or near the end of
the inner blue part of the flame (_A_ Fig. 4); the appearance of the
glass will quickly indicate reduction. When this occurs move the glass
forward to the end of the outer zone _B_, but keep it sufficiently
within the flame to maintain it at a high temperature. If all is right
the metallic reduction will quickly disappear, the glass will become
perfectly transparent once more, and will present the appearance
previously observed in the experiments with the pointed flame, or, if
very hot, assume a brownish-red appearance. If this does not occur, the
supply of air must be increased or the supply of gas diminished until
the proper effects are secured.

[5] Nevertheless the