ANNEALING, by the workmen called nealing, is
particularly used in making glass: it consists in placing
the bottles, &c. whilst hot, in a kind of oven or fur-
nace, where they are suffered to cool gradually: they
would otherwise be too brittle for use.—Metals are ren-
dered hard and brittle by hammering: they are there-
fore made red hot, in order to recover their malleabil-
ity; and this is called nealing.
The difference between unannealed and annealed
glass, with respect to brittleness, is very remarkable.
When an unannealed glass vessel is broken, it often
flies into a small powder, with a violence seemingly very
unproportioned to the stroke it has received. In gen-
eral, it is in greater danger of breaking from a very
slight stroke than from one of some considerable force.
One of those vessels will often resist the effects of a
pistol bullet dropped into it from the height of two or
three feet; yet a grain of sand falling into it will make
it burst into small fragments. This takes place some-
times immediately on dropping the sand into it: but
often the vessel will stand for several minutes after,
seemingly secure; and then, without any new injury,
it will fly to pieces. If the vessel be very thin, it does
not break in this manner, but seems to possess all the
properties of annealed glass.
The same phenomena are still more strikingly seen in
glass drops or tears. They are globular at one end,
and taper to a small tail at the other. They are the
drops which fall from the melted mass of glass on the
rods on which the bottles are made. They drop into
the tubs of water which are used in the work; the
greater part of them burst immediately in the water.
When those that remain entire are examined, they dis-
cover all the properties of unannealed glass in the high-
est degree. They will bear a smart stroke on the
thick end without breaking; but if the small tail be
broken, they burst into small powder with a loud ex-
plosion. They appear to burst with more violence,
and the powder is smaller, in an exhausted receiver than
in the open air. When they are annealed, they lose
these properties.
Glass is one of those bodies which increase in bulk
when passing from a fluid to a solid state. When it is
allowed to crystallize regularly, the particles are so ar-
ranged, that it has a fibrous texture: it is elastic, and
susceptible of long-continued vibrations; but when a
mass of melted glass is suddenly exposed to the cold,
the surface crystallizes, and forms a solid shell round
the interior fluid parts: this prevents them from ex-
panding when they become solid. They, therefore,
have not the opportunity of a regular crystallization;
but are compressed together with little mutual cohe-
sion: On the contrary, they press outward to occupy
more space, but are prevented by the external crust.
In consequence of the effort of expansion in the inter-
nal parts, the greater number of glass drops burst in
cooling; and those which remain entire are not regu-
larly crystallized. A smart stroke upon them commu-
nicates a vibration to the whole mass, which is nearly
synchronous in every part: and therefore the effort of
expansion has little more effect than if the body were
at rest; but the small tail and the surface only are reg-
ularly crystallized. If the tail be broken, this com-
municates a vibration along the crystallized surface,
without reaching the internal parts. By this they are
allowed some expansion; and overcoming the cohesion
of the thin outer shell, they burst it, and are dispersed
in powder.
In an unannealed glass vessel, the same thing takes
place. Sometimes the vibration may continue for a
considerable time before the internal parts overcome
the resistance. If the vessel be very thin, the regular
crystallization extends through the whole thickness;
or at least the quantity of compressed matter in the
middle is so inconsiderable as to be incapable of burst-
ing the external plate.
By the process of annealing, the glass is kept for
some time in a state approaching to fluidity; the heat
increases the bulk of the crystallized part, and renders
it so soft, that the internal parts have the opportunity
of expanding and forming a regular crystallization.
A similar process is now used for rendering kettles
and other vessels of cast iron less brittle: of it the same
explanation may be given. The greater number of me-
tals diminish in bulk when they pass from a fluid to a
solid state; iron, on the contrary, expands.
When cast iron is broken, it has the appearance of
being composed of grains: forged or bar iron appears
to consist of plates. Forged iron has long been pro-
cured, by placing a mass of cast iron under large ham-
mers, and making it undergo violent and repeated com-
pression. A process is now used for converting cast
iron into forged, by heat alone. The cast iron is placed
in an air furnace, and kept for several hours in a
degree of heat, by which it is brought near to a fluid
state. It is then allowed to cool gradually, and is
found to be converted into forged iron. This process
is
Anneley is constituted under a patent; although, if Reaumur's experiments upon cast iron be consulted, it will appear not to be a new discovery.
By these experiments it is ascertained, that if cast iron be exposed for any length of time to a heat considerably below its melting point, the texture and properties are not changed: but if it be kept in a heat near the melting point, the surface soon becomes lamellated like forged iron; and the lamellated structure extends farther into the mass in proportion to the length of time in which it is exposed to that degree of heat. When it is continued for a sufficient time, and then allowed to cool gradually, it is found to possess the lamellated structure throughout.
Cast iron, then, is brittle, because it has not had the opportunity of crystallizing regularly. When it is exposed to cold while fluid, the surface becoming solid, prevents the inner parts from expanding and arranging themselves into regular crystals. When cast iron is brought near to the melting point, and continued for a sufficient length of time in that degree of heat, the particles have the opportunity of arranging themselves into that form of crystals by which forged iron is distinguished, and by which it possesses cohesion and all its properties.
There appears, therefore, to be no other essential difference between forged and cast iron, except what arises from the crystallization. Cast iron is indeed often not sufficiently purified from other substances which are mixed with the calx. It appears also to contain a considerable quantity of calx unreduced; for during the process for converting it into forged iron, by heat alone, a pale flame arises from the metal till near the end of the process. This is owing to fixed air which the heat forces off from the calx. The expulsion of this air reduces the calx, and thereby frees the metal from that injurious mixture.
That this explanation of the annealing of iron is probable, appears also from the well-known fact of forged iron being incomparably more difficult of fusion than cast iron. A piece of forged iron requires a very violent heat to melt it; but when it is reduced to a small powder, it melts in a much lower degree of heat. Iron diminishes in bulk when it passes into a fluid state, while most other metals increase in volume. The expansion which heat occasions in bringing them to their melting point, will be favourable to their fluidity by gradually bringing the particles to the same state of separation in which they are when the mass is fluid; but the expansion of iron by heat removes it farther from that state, and keeps it in the state which is favourable to the continuance of it in a crystallized form. It will not melt till the heat expand it so much that the cohesion of crystallization be overcome. When it is reduced to a minute powder before it be exposed to the heat, it melts sooner. The crystals having been destroyed, that cohesion has no effect in preventing it from passing into a state of fluidity.
Upon the same principles may be explained the almost peculiar property of welding possessed by iron, and the conversion of forged iron into steel.
But perhaps they may also be applied to platinum, a metal which has lately gained much attention. It possesses some of the properties of iron. It is still more difficult of fusion than that metal. It is susceptible of
being welded. The natural grains of it can scarcely be melted in the focus of the most powerful burning glass; but when it is dissolved in aqua-regia, and precipitated by potash, it has been melted in small globules by the blowpipe. When precipitated by muriate of ammonia, it has been melted in a considerable mass in the heat of a furnace; but it is said to be hard and brittle.
Many attempts have been made to procure a mass of it in a malleable state, but without success. It is said that the process is now discovered by a chemist in Spain. The treatment of the metal is probably very simple. Perhaps it only consists in precipitating it in a minute powder from aqua-regia, exposing it to a strong heat which melts it, and keeping it for some time in a state nearly fluid, that it may, like iron, crystallize regularly; by this it will possess all its metallic properties.