LEHRS AND ANNEALING
Owing to the peculiar structure of glass, and its liability to fly or
collapse when exposed to sudden changes of temperature, a process of
annealing becomes necessary in order to produce a more equal
distribution of the tensions throughout the structure of the glass;
otherwise, glassware of any thickness would be in such a state of
tension as to be extremely liable to fracture when passing through any
sudden change in the atmospheric temperature, especially in frosty
weather. In this state it is useless or dangerous for general purposes.
On this account most glasswares undergo a form of annealing at some time
during the process of their manufacture. And in the case of certain
goods, such as table glass, lamp glasses, optical glass, etc., special
care and time are devoted to this process of annealing. Often in the
case of improperly annealed glass, instances are known where its
unhomogeneous structure has suddenly given way as the result of
derangements set up by internal tension. Friction, or rough handling
whilst cleaning, at the ordinary temperature of the atmosphere, is
sufficient to cause a rupture. Therefore annealing cannot be too
carefully attended to.
For annealing the glass manufacturer uses a lehr, which is an arched
tunnel with a fully exposed opening at the exit end and partially closed
at the entrance end, where the goods are introduced. The lehr is heated
at the entrance end to a temperature of about 350° Cent., which
temperature is gradually diminished towards the exit end, which is quite
cool. The hot end, or entrance, should be constantly at a temperature
just short of the actual deformation or softening point of the glass
introduced; usually the entrance is in a position near, or convenient
to, the glass furnace around which the glass blowers make the goods.
In old-fashioned works coal-fired lehrs are used, but they are very
unsatisfactory and difficult to regulate. The heat of the lehrs in
modern works is maintained and regulated by a series of gas burners
situated under the floor of the tunnel or lehr. Along this floor are
placed iron trays linked up with each other to form a continually
travelling track, which gradually moves towards the cold end of the
lehr; these trays are operated by a mechanical jack and gears. As each
tray of goods comes out of the cooler end of the lehr, they are taken
off and conveyed to the warehouses for cleaning and packing, and the
empty tray is sent back to the entrance end to be linked up and refilled
again with fresh goods.
These tunnels, or lehrs, are about 40 ft. long, and as the glasswares
travel through on the trays they are subjected to the gradually
diminishing heat, until they are ultimately removed at the cooler end in
an annealed condition, in which state they are less liable to fracture
in use. The time occupied in travelling through the lehr is usually
about three days. But this period varies according to the nature of the
ware being manufactured. In special glasses, and in the annealing of
optical glass, the glass may undergo a process of annealing that takes
as long as ten days, and in other cases, where the glassware is made
very thin, no annealing at all is necessary. Usually the thicker and
heavier articles require the longest time in annealing. Table glass
which is made thick and heavy for cutting or decoration requires a
little more care and time in the lehr than ordinary plain glassware, as
the abrasive action of cutting quickly develops any latent strains and
causes fracture.
In some works, especially on the Continent, several small
externally-heated kilns are used for annealing, in which the hot
glassware, as it is made, is packed in tiers; when full, these kilns are
closed up and then allowed to cool of their own accord; after which they
are opened and the goods taken out to the warehouse. This is an
intermittent process of annealing, and is quite satisfactory for certain
classes of goods, such as lamp shades, which are usually of equal
thickness throughout their form.
The travelling or continuous form of lehr admits goods of more unequal
thickness in form and variety. Thus, wine-glasses, jugs, and bowls may
be annealed together with less risk of malformation in their shape than
would be present if they were annealed together in kilns. The
manufacturer can, by suitably arranging the temperature of the gas
burners, give more heat to one side of the lehr than to the other. He
then places the heavier goods on the hotter side and reserves the other
for lighter goods, such as wines, etc. They then travel down together
side by side under the most suitable conditions for the annealing of
each class.
Many physical changes take place in the glass passing through the lehr.
One remarkable effect is the slight change in colour which occurs in
glass decolorized with manganese. It is noticed that the glass becomes a
greener tint in passing through the lehr when the decolorization is just
on the margin of efficiency.
The state in which the structure of glass exists when quickly cooled and
the action of annealing might be explained. When glass is quickly
cooled, being a bad conductor of heat, insufficient time is allowed for
the middle or interior portions of the glasswares to settle down and
assume their normal state of solidification. The outer portion, or
crust, will first cool and contract with an enormous strain upon the hot
interior. This difference in the state of tension between the outer and
interior portions gives a want of uniformity, and stresses of tension
and thrust are developed, which cause the whole to collapse with the
slightest external scratch or heat change. In annealing, this strained
or forced condition in the structure of the glass is relieved by
subjecting the glass to a pre-heating, and gradually diminishing the
temperature, allowing a sufficient time for the different layers
mutually to adjust themselves to their comparative normal positions, and
thus relieve the strains within the mass. Much depends upon the
pre-heating temperature and the rate at which the diminution of the
temperature takes place. If this is properly provided for, the best
results are obtained in the stability of the resulting glass. The
presence of any stress can be determined by using a polariscope.
The average British glass manufacturer has little knowledge of the value
of a polariscope, or stress viewer, in ascertaining the physical state
of his glasswares, and until he adopts its use there is little prospect
of an improvement in his annealing methods. Much faulty annealed glass
is being turned out which the glass manufacturer is not aware of, and
which could be avoided by the intelligent use of such a simple
instrument, which detects badly annealed glass at once by the aid of
crossed nicols and a selenite plate.
Owing to the unequal densities of the various silicates present in the
heavy lead and barium glasses, they are more subject to striation and
require more careful annealing than the soda-lime glasses, in which the
silicates present are of more equal density. However, much depends upon
the proper “founding” and melting of such glasses. The use of a larger
proportion of cullet assists in breaking up striation. The presence of
striae or cords in glass disqualifies it for most purposes, as it is
usually found that, apart from their defective appearance, they tend to
produce stresses within the glass.
Transparency, brilliancy, stability, and homogeneity are important
factors in producing perfect glassware, and the proper development of
these distinguishing properties requires considerable skill on the part
of the glass manufacturer, alike from a technical, physical, and
practical standpoint.
------------------------------------------------------------------------