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 it is 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 extra 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.