he reducing flame on lead is not altogether a
disadvantage, however; because a little care in adjusting the blowpipe
and a little care in holding the glass in the right position will enable
the student to work lead-glass without producing the faintest trace of
blackening. This, in addition to being a valuable exercise in
manipulation, will teach him to keep his blowpipe in good order, and
prove a useful aid in his early efforts to judge as to the condition of
the flame. It prevents discouragement if the student does his
preliminary work with the soda-glass, but he should certainly make
experiments with lead-glass as soon as he has acquired reasonable
dexterity with soda-glass.

_Annealing._--Annealing is a process by which any condition of strain
which has been set up in a glass article, either by rapid cooling of one
part while another part still remains hot, or by the application of
mechanical stress after cooling is relieved. Annealing is carried out by
subjecting the article to a temperature just below the softening point
of the glass, maintaining that temperature until the whole article has
become heated through the thicker part, and then reducing the
temperature very gradually; thus avoiding any marked cooling of the
thinner and outer parts first.

For thin glass apparatus of the lamp-blown or blowpipe-made variety in
which there are no marked difference of thickness, such as joins on
tubes, ordinary seals, bulbs, etc., there is little need for annealing;
and even those having rather marked changes of thickness, such as
filter pumps, can be annealed sufficiently by taking care that the last
step in making is heating to just below visible redness in the blowpipe
flame and then rotating in a sooty gas flame until covered with a
deposit of carbon. The article should then be allowed to cool in a place
free from draughts and where the hot glass will not come in contact with
anything.

A few of the blowpipe-made articles, such, for example, as glass
stopcocks, need more careful annealing, and for this purpose a small
sheet-iron oven which can be heated to dull redness over a collection of
gas burners will serve. Better still, a small clay muffle can be used.
In either case, the article to be annealed should be laid on a clean,
smooth, fireclay surface, the temperature should be maintained at a very
dull red for two or three hours and then reduced steadily until the oven
is cold. This cooling should take anything from