In all glasses a proportion of "cullet," or broken glass scrap, is used. This cullet is usually of the same composition as the glass mixture or " batch." The use of cullet facilitates the melting, and assists in giving homogeneity to the resultant glass by breaking up the cords and striae which tend to develop in most glasses.
In the commoner varieties of bottle glass Basalt and other igneous rocks are crushed and used. These are naturally occurring silicates containing lime alumina, alkalies, iron, and other elements in varying proportions. They are used more on account of their cheapness, and produce dark, dirty-coloured glasses, which in the case of common bottles are not objected to. In some instances iron manganese or carbon is added to produce black bottle glass.
Of the various silicates used in glassmaking, the silicate of alumina is the most refractory. The silicates of lime and barium are rather refractory, but under a strong heat and in the presence of other silicates they can be readily formed. The silicates of the alkalies, lead, and many of the other metals are formed at much lower temperatures. In the case of the silicate of iron, manganese, or copper, a strong affinity is shown between the metal and the silica, and a black or dark-coloured slag with a very low melting point is formed. Such slags are very active in corroding the fire-clay masonry and pots of the furnace.
No single silicate is entirely free from colour. Each gives a slight distinctive coloration, the lead silicate being yellowish and the soda silicate greenish, but by the judicious mixture of different silicates and the use of decolorizers, such as manganese, nickel, etc., compound silicates are obtained, giving less perceptible colours or crystal effects. In optical glassmaking the use of the ordinary decolorizers is not permissible, and the purity of the materials used becomes the most important factor.
The raw mixture of the various materials used in making glass is termed a " batch." The mixing is usually done by hand, but in many cases mechanical batch mixers are used. If the mixing is done by hand, the materials are first weighed out in their correct proportions by means of a platform weighing-machine. As they are weighed out, one by one, they are introduced into a rectangular wooden arbour or box, large enough to hold the whole unit weight of the batch and allow of its being mixed and turned from side to side. The batch is then sieved, and all the coarse materials reduced or crushed to a size not coarser than granulated sugar. By sieving and turning the batch several times a thorough mixture of the ingredients is obtained. A few ounces of manganese dioxide are then added, according to the unit weight of the batch weighed out, and the proportion of decolorizer necessary; which varies according to the heat of the furnace and the amount of the impurities present.
The whole batch is then put into barrels and conveyed to the glass house, where the furnace is situated. Here it is tipped into another arbour or box in a convenient position near to the melting pot, and, a proportional quantity of " cullet " being added, the mixture is then ready for filling into the pots. The stopper of the pot mouth is taken away and placed aside, and a man shovels the mixture or batch into the hot pot until it is full. He then replaces the stopper, and, after a few hours, when the first filling has melted and subsided, another filling of batch into the pot takes place until it becomes full of glass metal in its molten state. The batch melts with considerable ebullition, owing to the chemical reactions taking place under the heat of the furnace, giving off at the same time large quantities of gas. By the evolution of these gases the batch shrinks in volume so that it becomes necessary to fill a pot more than once with the batch before it becomes full of molten metal. The capacity of the pots varies between 250 and 1,200 kilogrammes, according to the type of glass and nature of the goods made.
Much care is required in mixing and sieving batches containing lead and other poisonous ingredients, to prevent the inhalation of the dust by the mixer. Therefore, where such materials are used, exhaust fans and ventilating ducts should be provided and fitted in the mixing rooms. A proper respirator should be worn by the mixer in charge to prevent any absorption into his system of the poisonous dust. Cases of poisoning are not unknown, but these are due to gross carelessness. A small regular weekly dose of Epsom salts should be taken by the mixers who have to prepare lead batches. This salt tends to remove any lead salts absorbed in the system by converting them into insoluble lead sulphate.
Carbonate of Barytes
Carbonate of Magnesia
Copper Oxide (Red)
Copper Oxide (Black)
Phosphate of Lime
Sulphate of Soda