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.
CHEMICAL FORMULAE AND MOLECULAR WEIGHTS.
Materials. | Formulae. | Molecular Weight. |
|---|---|---|
| Alumina | Al2O3 | 102 |
| Antimony Oxide | Sb2O3 | 287 |
| Arsenic | As2O3 | 197 |
| Bismuth Oxide | Bi2O3 | 468 |
| Boric Acid | H3BO3 | 62 |
| Borax | Na2B4O710H2O | 382 |
| Calcined Borax | Na2B4O7 | 202 |
| Calcined Potash | K2CO3 | 138 |
| Carbon | C | 12 |
| Carbonate of Barium | BaCO3 | 197 |
| Carbonate of Magnesia | MgCO3 | 84 |
| China Clay | 2SiO2Al2O32H2O | 258 |
| Chrome Oxide | Cr2O3 | 153 |
| Cobalt Oxide | Co2O3 | 105 |
| Copper Oxide (Red) | Cu2O | 143 |
| Copper Oxide (Black) | CuO | 79 |
| Cryolite | 6NaFAl2F6 | 210 |
| Dolomite | CaOMgO2CO2 | 184 |
| Fluorspar | CaF2 | 78 |
| Gold Chloride | AuCl32H2O | 339 |
| Iron Oxide | Fe2O3 | 160 |
| Lime | CaO | 56 |
| Lime Spar | CaCO3 | 100 |
| Manganese Oxide | MnO2 | 87 |
| Nickel Oxide | NiO2 | 75 |
| Nitrate of Soda | NaNO3 | 85 |
| Phosphate of Lime | Ca3(PO4)2 | 310 |
| Potash Carbonate | K2CO3(2H2O) | 174 |
| Potash Felspar | 6SiO2Al2O3K2O | 556 |
| Red Lead | Pb3O4 | 683 |
| Saltpetre | KNO3 | 101 |
| Sand | SiO2 | 60 |
| Soda Carbonate | Na2CO3 | 106 |
| Sodium Fluoride | NaF3 | 61 |
| Sulphate of Soda | Na2SO4 | 142 |
| Tin Oxide | SnO2 | 150 |
| Uranium Oxide | UO2 | 272 |
| Zinc Oxide | ZnO | 81 |
CHAPTER III
THE CHEMICAL AND PHYSICAL PROPERTIES OF GLASS
The main essential and peculiar property of glass is its transparency. When subjected to a gradually increasing temperature, glass becomes softened, and whilst hot it is plastic, ductile, and malleable, in which state it can be cut, welded, drawn, or pressed. A thread of glass can be drawn so thin and fine that it can be twisted and bent to a remarkable extent, showing that glass is flexible.
The above properties shown by glass while softened under heat permit it to be shaped and formed by a variety of methods, so that in the manufacture of the different kinds of glass we find goods pressed, blown, drawn, moulded, rolled and cast from the hot metal. Upon cooling, the form given to them is retained permanently.