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.