The above may be varied by gently concentrating the liquid filtered from the precipitate of alumina and oxide of iron (see d), and precipitating it with dilute sulphuric acid; the mixed precipitate is exhausted by digestion in water holding chloride of ammonium in solution; the undissolved residuum (sulphate of barytum,) is washed, dried, and otherwise treated as before; whilst the solution with the washings is treated with a solution of carbonate of ammonium; the precipitate is carbonate of calcium, which is to be washed, &c., as directed under f. The liquor, &c., filtered from the lime, is lastly tested for magnesia. (See g.)
i. A second 100 gr. of the powdered glass (see a) is mixed with 200 gr. of fluor spar, also in powder; the compound is placed in a platinum or leaden capsule, 500 gr. of strong sulphuric acid are added, and the whole cautiously stirred together with a silver stirrer or spoon, care being taken to avoid inhaling the fumes; the heat of a spirit lamp is next applied, and at first is kept at about 212° Fahr., but towards the conclusion of the process is raised to 300° Fahr., or even higher, and is continued for at least 2 hours, or until fumes entirely cease to be evolved; 5 or 6 fl. oz. of distilled water are next poured on the residuary mass, and, after thorough agitation, the whole is thrown on a filter, more water being at last poured on to wash out any remains of soluble matter; to the filtrate, carbonate of ammonium is added in excess, and after a time the earthy salts are removed by filtration; the filtered liquor is now evaporated to dryness, and ignited to dull redness for 2 or 3 minutes; the residuum (sulphate of potassium or sodium, or of both), after being weighed (the weight being carefully noted down), is redissolved in distilled water; a solution of chloride of barium is then added as long as it disturbs the liquor, and after a time the whole is again filtered; the filtrate is concentrated by evaporation, and solution of bichloride of platinum added in excess; the whole is now gently evaporated to dryness, mixed with alcohol, collected on a filter, carefully washed with weak alcohol, dried at a temperature under 212° Fahr., and weighed. The weight, in grains, multiplied by ·1940, gives the per-centage of potassa sought.
k. The weight of sulphate of potassium in the ignited residuum in i is calculated from that of the potassium last found (47 parts of the one being equal to 87 parts of the other), and this weight is deducted from the gross weight of the ignited sulphates; the remainder represents the quantity of sulphate of sodium present. The weight of the latter, in grains, multiplied by ·4367, gives the per-centage of pure soda required.
Concluding Remarks. One of the chief points to which the skilful glass manufacturer directs his attention, is the quality of the materials. Great care is exercised in the selection of the sand for all the finer varieties of glass. The usual practice is to test it before using it, by exposing it to a very high temperature. The purest sand is that which is the whitest and freest from iron, and which, consequently, suffers the least alteration by this treatment. The alkalies (potash, soda) employed are purified by solution and crystallisation. The red lead and litharge must be pure and absolutely free from oxide of copper (a common contamination), which gives a green tint to the glass. The former, which is the most costly, is preferable to the finest crystal. Care must also be taken that the lime, clay, &c., are respectively of proper purity; and that the ‘cullet,’ or broken glass, which is almost always remelted with the other materials, is of proper quality, and of the same kind as that to which it is added. Potassa produces a better glass than soda, although the latter is now very generally employed, from its lower price. It is, however, quite inadmissible as an ingredient in the manufacture of the better class of crystal and plate glass, as, however pure it may be, it imparts to the product a slight greenish tinge more or less destructive of its beauty. When sulphate of soda (Glauber salt) is used as a source of soda, it is gently calcined to dissipate its water of crystallisation, and requires the addition of about 8% of charcoal to effect its reduction in the melting-pot. Common salt is also employed as a source of soda in the same manner. Sometimes native sulphide of lead (galena) is used to decompose the sulphate of soda, and in lieu of part of the
oxide of lead; in which case about 5 parts of the sulphuret are taken for every 9 parts of the calcined sulphate.
To anticipate the results of his processes, and to carry out with certainty his various intentions, the glass manufacturer, perhaps more than any other person, requires the aid of science and experience. All his most essential operations depend on chemical principles. The products of his furnaces are not formed by the mere mechanical admixture of their several ingredients whilst in the state of fusion, but result from the play of delicate affinities which only act under certain conditions, and when the materials are presented to each other in uniform and definite proportions. Chemically speaking, the glasses are mixed super-silicates of the respective bases which enter into their composition (potassium, calcium, lead, &c.), and, like all other compounds which are formed by elective attraction, obey the common laws of combination, as developed by Dalton, and now so successfully applied in almost every department of industrial art. It has been shown by the most careful analysis, that in all the more valuable and beautiful commercial glasses the relative proportions of the materials are conformable to these laws, and that several of them are true atomic compounds, as perfect in this respect as the crystalline bodies commonly denominated salts. In some of the harder glasses of Bohemia the number of atoms or equivalents of silica are to each of the bases with which it is united, nearly as 5 to 1; whilst in a softer glass of German manufacture the proportions of the two are found to be as 4 to 1. The celebrated plate glass of St. Gobain is an atomic compound formed of 1 equivalent of trisilicate of soda united to 1 equivalent of trisilicate of lime, with a small per-centage of alumina in combination with silicic acid, also in atomic proportion. Glasses in which the ingredients bear no atomic ratio to each other are never homogeneous, but always more or less striated and of unequal colour and refractive power. The absence of atomic proportion between the substances entering into its composition appears to be the only reason why the best English plate and mirror glass is so greatly inferior to that of France and Germany, that comparison of the two becomes absurd. The only variety of glass in the production of which the English manufacturer excels is flint glass or crystal, and here he certainly surpasses all his numerous competitors. The subject is doubtless involved in difficulty, owing to the precise temperature necessary to effect the perfect combination of the bases with the silicic acid, varying with the character of the compound, and not being satisfactorily settled by observation or experience. The modifying influence of temperature is shown by the fact that the lower the heat employed in the process, the smaller the quantity of silica which enters into the composition of the resulting glass; whilst at higher temperatures a part of the base is dissipated in fumes, until such proportions of base and acid result as are required to produce a permanent atomic compound corresponding to the temperature employed. If the heat is excessive or improperly continued, the loss of base produces an opposite effect, and an opaque, semi-vitrified mass is formed, resembling ‘Reaumur’s porcelain.’ The quality of the resulting glass depends on this change being more or less complete. If the furnace yields the right temperature, and the duration of the exposure to its action is neither too short nor too prolonged, nature makes up for the unskilful conduct of the operative, and removes the stumbling blocks which his ignorance had placed in the way of his own attempts at excellence. The proceedings and their results are accidental; but being once obtained, the first are repeated without further trouble or inquiry. This accounts for the same mixture of materials yielding products of different qualities at different times, and in different works, which the operative contents himself with referring to the ‘going of the furnace.’ The common plan in this country is to regulate the proportions and firing by experience only, rather than by theory and practice combined. Now, although the chemist has much yet to learn on the precise constitution of the glasses, and although theory may not be able to ensure unvarying success, it is nevertheless certain that, in all cases, it can afford much valuable assistance in that direction. Indeed, it has been asserted by one of the leading Continental chemists, that ingredients that will yield the proper equivalent proportions in the melting pot cannot produce a bad glass, if exposed to such a temperature as to permit of perfect combination taking place.
It is found that those glasses which contain a predominance of alkali are acted on by water, and when this is in great excess they are perfectly soluble in that fluid. Ordinary flint glass is affected by long coction in water, whilst crown glass, which contains less alkali, is unaltered by that trial. Glass which contains any considerable quantity of lead is acted on by sulphuretted hydrogen. This is the cause of the surface of flint glass, under certain circumstances, becoming opaque and iridescent. Glasses made of silica and alkali alone are incapable of permanently resisting the action of water. The addition of lime or oxide of lead appears to be necessary to give them this quality. Glasses that have a slight greenish or bluish tint may be often whitened, or rendered colourless, by exposure to light and air. This arises from the peroxidation of the iron, to whose protoxide they owe their tint. Other glasses become purpled by exposure, owing to the peroxidation of the manganese.
Different colours are communicated to glass by the addition of metallic oxides. Thus, oxide of manganese gives an amethyst; oxide
of cobalt, a blue; oxide of iron, a brown; black oxide of copper, a green; oxide of gold, a purple; suboxide of copper, a ruby-red; oxide of tin, a white; oxide of silver, a yellow, &c. These substances are either added to the melted contents of the glass-pot, as in preparing artificial gems, &c., or they are applied in a thin layer to the surface of the object, which is then heated until fusion of the coloured compound occurs, as in enamelling and painting on glass.
Glass is FORMED or FASHIONED into articles by the processes of blowing, casting, drawing, rolling, or spreading. In the process of BLOWING GLASS the workman begins by collecting a proper quantity of glass in a soft, pasty state, at the end of his blow-pipe (an iron tube, five or six feet in length, terminated by a mouth-piece of wood), which he then commences blowing through, by which the lump is expanded into a kind of flask, susceptible of having its form modified by the position in which it is held, and the velocity of rotation continually given to the iron tube. If an open-mouthed vessel is to be made, an iron rod, called a ‘pontil’ or ‘puntil,’ is dipped into the glass-pot and applied to the bottom of the flask, to which it thus serves as a handle, the blow-pipe being removed by the application of a cold iron to the neck. The vessel is now re-heated, and the aperture enlarged, and the vessel otherwise altered in figure by the aid of a few simple tools until completed. It is then detached, and carried to the ‘annealing oven,’ where it undergoes slow and gradual cooling during many hours. In this way bottles, flasks, carboys, and an almost infinite variety of other articles, are formed. The large circular tables of CROWN-GLASS are made by a joint process of BLOWING and SPREADING. The globular flask at first produced, transferred from the blow-pipe to the ‘pontil,’ is suddenly made to assume the form of a flat disc by the centrifugal force of the rapid rotatory movement given to the rod. Spread or BROAD GLASS is formed into sheets in a nearly similar manner. Plate-glass is cast upon a flat metal table, and, after very careful annealing, is ground and polished by suitable machinery. Tubes are made by rapidly drawing out a hollow cylinder; and from these a great variety of useful small apparatus are constructed with the help of a lamp and blowpipe, or, still better, the bellows-table of the barometer-maker. Glass beads are made from small tubes chopped into pieces of suitable lengths, which are stirred first in a mixture of sand and wood-ashes, in the cold, and afterwards in an iron pan over the fire until they assume a rounded form. Small tubes are bent in the flame of a spirit lamp or gas-jet, and cut by a file, a scratch being made, and the two portions pulled or broken asunder in a way easily learned by a few trials. Large tubes require the heat of a powerful blowpipe and lamp, or that of a furnace.