The grinding of both sides being completed, the glass is polished by bedding it with plaster upon a flat table, and rubbing on the surface with a polisher, which is a block of wood, covered on the lower side with woollen cloth. The workman keeps it supplied with fine po lishing powders, as tripoli and putty ; at first using the coarsest, and, towards the end of the operation, the finest. The block of the polisher for large plates is about ten inches square, and has two handles projecting from it. But to_•egulate the pressure, a springing pole is put upon the back of the block. and being bent to a curve, is sup ported in the ceiling of the workshop. When both surfa ces are polished, the glass is laid upon a table covered with a cloth, and any deficiences ate removed, by a small polisher applied by the pressure of the hand without a spring.
In the plate glass manufactory at Ravenhead in Lanca shire, which we have had occasion to examine, the opera tion of grinding and polishing is performed by appropriate machinery driven by two large steam engines. There is nothing very peculiar in the nature of this machinery:, ex cepting the ingenious contrivance for changing the path of the polisher in advancing and returning over the plate of glass. See ENGLAND.
In our article on GEMS, we have already had occasiort.to consider the subject of coloured pastes, or fictitious gems. We have described the method of Fontanieu of making a colourless base, and afterwards communicating to it any particular tint by metallic oxides ; but as we have followed that ingenious author only in so far as his experiments re late to the imitation of precious stones, such as the orien tal topaz, the amethyst, the hyacinth, and the beryl, we shaN here resume the subject, and give an account of his method of producing other colours.
1. From Gold.—To obtain the purple colour known by the name of precipitate of Cassius, Al. Fontanieu employed the following process.
Distil in a glass retort, placed in a bath) of ashes, some gold dissolved in aqua regia, made with three parts of ni trous, and one part muriatic acid ; when the acid is passed over, and the gold contained in the retort appears dry, leave the vessel to cool, then pour it into some new aqua regia, and proceed to distil as before. Replace the aqua regia twice upon the gold, and distil it. After these four operations, pour by little and little into the retort some nil of tartar per deliquium, which %vil I occasion a brisk effervescence ; when this ceases, distil the mixture till it becomes dry, and then put some warm water into the retort. Shake the whole, and put it into a cucurbit, when a precipitate is deposited, the colour of which is either brown or yellow. After hav ing washed this precipitate, dry it.
2. From Silver.—The oxide of silver being vitrified, pro duces a yellowish-grey colour. This oxide enters only into the composition of the yellow artificial diamond and opal. Mr Fontanien introduces it into the base in the form of horn silver (tuna cornea). In order to prepare it, he dis solves the silver in nitrous acid, and afterwards pours into it a solution of sea salt : a white precipitate is obtained, which, being washed and dried, melts easily in the fire, and is soon volatilized, if not mixed with vitrifiable matter.
3. From Copper.—The oxide of copper imparts to white glass the finest green colour ; but if this metal be not ex actly in a state of oxidation, it produces a brownish-red colour.
4. From Iron.—Though it is commonly believed that the oxides of iron communicate a very fine transparent red colour to white glass, M. Fontanicu could only obtain from it a pale red, a little opaque. The oxide of iron he em ployed, was in the proportion of the 20th part of the base. There are various ways of preparing the oxide of iron;call ed crocus martis. In general it is necessary that this me tal be so far oxidated that the magnet ceases to attract it. Thus one may use the scales of iron found upon the bars of furnaces, which serve to distil aquafortis. By digesting filings of steel with distilled vinegar, then evaporating and replacing the vinegar ten or twelve times upon these filings, and drying them alternately, an oxide of iron is obtained, which must be sifted through a silk sieve, and then cal cined. The oxide of iron, thus obtained by the vinegar, only introduced into the bases a green colour inclining to yellow. By the following process, one of the finest red co lours is obtained :—Let an ounce of iron-filings be dissolv ed in nitrous acid, in a glass retort, and distilled over a sand-bath to dryness. After having replaced the acid or the dry oxide, and rc-distilled it a second and a third time, it is then edulcorated with spirits of wine, and afterwards washed with distilled water.
5. From Cobalt.—The oxide of cobalt is made use of for introducing a blue colour into glass : but as this metal is rarely free from iron and bismuth, it is first necessary to separate them from it. This is clone by calcining the co balt ore in order to disengage the arsenic ; and next distill ing the oxide in a retort with sal ammoniac, when the iron and the bismuth are found sublimed with this salt. The distillation must be repeated with the sal ammoniac till this salt is no longer coloured yellow. The cobalt which re mains in the eornute is then calcined in a potsherd, and be comes a very pure oxide, which being introduced into the base, in the proportion of a 900th part, gives it a very fine blue colour.