supply of air must not be so excessive as to reduce
the temperature of the flame sufficiently to prevent the thorough
softening of the glass, which will occur if the bellows is worked with
too much zeal.

In working upon lead glass with the highly oxidising brush flame, it is
a good plan to heat it in the reducing part of the flame _A_ for
thoroughly softening the glass, and to remove it to the oxidising flame
_B_ to burn away the reduced metal. In prolonged operations, in order
that reduction may never go too far, hold the glass alternately in the
hot reducing flame and in the oxidising flame. The inferiority of the
outer oxidising flame to those portions nearer the inner blue zone for
softening the glass, may perhaps be accounted for by the presence of a
larger proportion of unconsumed air in the former, which being heated at
the expense of the hot gases produced by combustion, thereby lowers the
temperature of the flame. At or near _A_ (Fig. 4) where the combustion
is nearly complete, but no excess of air exists, the temperature will
naturally be highest.

If a very large tube be rotated in the oxidising flame at _B_ (Fig. 4)
it may happen that the flame is not large enough to surround the tube,
and that as it is rotated those parts of it which are most remote from
the flame will cool down too considerably to allow all parts of the tube
to be simultaneously brought into the desired condition. This difficulty
may be overcome by placing two blow-pipes exactly opposite to each
other, at such a distance that there is an interval of about an inch
between the extremities of their flames, and rotating the tube between
the two flames. It may be necessary to provide two blowers for the
blow-pipes if they are large.

Again, if a very narrow zone of a tube of moderate size is to be heated,
two pointed flames may be similarly arranged with advantage.
Occasionally more than two flames are made to converge upon one tube in
this manner.

Another method of preventing one side of a tube from cooling down whilst
the other is presented to the flame, is to place a brick at a short
distance from the extremity of the flame. The brick checks the loss of
heat considerably. A block of beech wood may be used for the same
purpose, the wood ignites and thereby itself becomes a source of heat,
and is even more effective than a brick.

Fuller details of the management of lead glass under various
circumstances will be found in the subsequen