GLASS

Syn. Vitrum , L. This well-known substance is e entially a mixture of silicates with an exce of silica or silicic acid. It generally contains the silicates of pota a, soda, lime, baryta, magnesia, alumina, and lead, coloured by small portions of iron, manganese, cobalt, uranium, copper, or gold. In its usual form it is brittle, transparent, noncrystalline, insoluble, and fusible; but it sometimes exhibits other properties. The manufacture of gla is one of the highest beauty, and, considering the comparative worthle ne of the materials of which it is made, and the various purposes of a useful, ornamental, and scientific nature which it subserves, it may be regarded as, perhaps, the most important in the history of inventions. The principle of its production is very simple, although great skill and experience are nece ary to ensure its excellence. Silica (commonly under the form of sand) is heated with carbonate of pota a or of soda, and slaked lime or oxide of lead, until the mixture fuses, and combination takes place. After a time the melted ma becomes perfectly limpid and free from air-bubbles, when it is allowed to cool until it a umes the peculiar tenacious condition proper for working. The operation of fusion is conducted in large crucibles of refractory fire-clay, which, in the case of ‘lead-gla ,’ are covered with a dome at the top, and have an opening at the side by which the materials are introduced, and the melted gla withdrawn. The manufacture of gla is only conducted on the large scale, and the precise character and proportions of the ingredients used by the gla -maker must nece arily greatly depend upon the nature of the raw materials furnished by his locality, or otherwise at his command. The attention of the manufacturer should be directed to the use of his materials in such proportions as will furnish, in the melting-pot, the proper quantities of the e ential ingredients, as determined from the known composition of the best commercial samples. The purity of the raw materials and the accuracy of his proportions and quantities are proved or disproved by the excellence of the product; and the cause of error (if any) may be at once determined by carefully ascertaining the quality of the ingredients employed, and the composition of the defective gla . A writer (in ‘Chem. Centr.,’ 1872, 528) points out that very generally the soda used in gla making, contains sulphate, and that when this is so a poor gla is produced. The addition of ·75-1 part of wood charcoal for every 100 parts of true soda—improves the quality of the gla . Prep. The following formulæ exhibit the composition of the leading commercial gla es, as shown by chemical analysis, together with the proportions of the raw materials used in their production. Bottle gla . Sp. gr. 2·700 to 2·735.— a. Composition by analysis:— 1. Silica, 53·55%; lime, 29·22%; mixed alkali, 5·48%; alumina, 6·01%; oxide of iron, 5·74%. Dark green. 2. Silica, 52%; baryta, 21·6%; soda, 26·1%; oxides of iron and manganese, ·3%. Pale green; very superior. b. Raw materials used:— 1. Yellow sand, 20%; kelp, 8%; lixiviated wood-ashes, 30%; fresh wood-ashes, 8%; pale clay, 16%; ‘cullet’ (broken gla ), 18%. This is the common mixture for coarse bottles, in Belgium, France, and Germany. 2. To the last add of black oxide of manganese, 2 1 ⁄ 2 to 3%. Has a rich yellowish colour; used for Rhenish-wine bottles. 3. Pale sand, 51%; lixiviated wood-ashes, 33%; pearl-ashes (dried), 8%; common salt, 7 1 ⁄ 2 %; white arsenic, 1 ⁄ 2 %; charcoal, q. s. Very pale green. 4. Siliceous sand (pale), 68 1 ⁄ 2 %; potash (or its equiv.), 4%; lime, 23 1 ⁄ 2 %; heavy spar, 2 1 ⁄ 2 %; peroxide of manganese, 1 1 ⁄ 2 %. This forms the celebrated ‘flask-gla ’ of St. Etienne. Gla , Broad , Spread window gla . Sp. gr. 2·642.— a. By analysis:— Silica, 69·70%; lime, 13·30%; soda, 15·25%; oxide of iron (and lo ), 1·75%. b. Materials used:— 1. White sand, 50%; dried sulphate of soda, 22%; charcoal (in powder), 9%; ‘cullet,’ 41%; peroxide of manganese, a little. Pale. 2. White sand, 60%; potashes (good), 24%; common salt, 10%; nitre, 5%; white arsenic, 1%; peroxide of manganese, a little ( 1 ⁄ 12 to 1 ⁄ 10 %); pale ‘cullet,’ at will (10 to 30%). Very pale. This is the ‘spread’ or ‘sheet window-gla ’ in common use. Gla , Chemical. Sp. gr. 2·390 to 2·396.— a. By analysis:— 1. Silica, 72·80%; pota a, 16·80%; lime (with a trace of alumina), 9·68%; magnesia, 40%; traces of oxide of manganese and iron (and lo ) ·32%. This is the difficultly fusible ‘Bohemian tube-gla ,’ so valuable in chemical manipulations. 2. Silica, 69·3%; pota a, 15·8%; soda, 3%; lime, 7·6%; alumina, 1·2%; magnesia, 2%; oxide of iron, ·5%; oxide of manganese (and lo ), ·6%. English chemical gla (without lead). More fusible than the last. b. Materials used:— 1. Quartz (hyalin, in powder), 60%; calcined purified pearlash, 30%; fresh-burnt lime (very pure), 9%; nitre (dried), 3 ⁄ 4 %; arsenious acid or peroxide of manganese, 1 ⁄ 4 %. Said to be the proportions used in the production of a , 1 ( above ). 2. (M. Peligot.) Quartz, 71 1 ⁄ 2 %; carbonate of pota a (or its equiv., dry), 20%; quicklime, 8 1 ⁄ 2 %; (manganese, a little). Said to be the formula for the hardest and least fusible ‘Bohemian tube-gla .’ It is very intractable and infusible, except at a very high temperature; but the addition of an exceedingly small quantity of boracic acid, borax, or arsenious acid, causes it to flow into a gla po e ing great brilliancy and hardne , and capable of being wrought at the highest heat of the ordinary furnace. Gla , Crown , White window-gla . Sp. gr. 2·486 to 2·488.— a. By analysis:— 1. Silica, 62·8%; pota a, 22·1%; lime, 15·5%; alumina (with traces of oxide of iron and manganese), 2·6%. Crown-gla of Bohemia, according to Dumas. Very beautiful. 2. Silica, 72·5%; soda, 17·75%; lime, 9·75%. English crown-gla ; excellent quality, but not so white as the last. b. Materials used:— 1. Finest white siliceous sand, 64%; purified potashes (dry), 23%; lime, 12%; white arsenic, 3 ⁄ 4 %; oxide of manganese, 1 ⁄ 4 %. Said to be used in Bohemia. 2. (Schweigger.) Pure sand, 57%; dry sulphate of soda, 28 1 ⁄ 2 %; quicklime, 11 1 ⁄ 2 %; powdered charcoal, 3 or 4%. Corresponds to a , 2, above (nearly). 3. Pure sand, 40%; soda ash, 24%; lime, 5%; white ‘cullet,’ 31%, Rather superior to the last. Crystal , Crystal gla . The ‘crystal gla ’ of England is flint gla ’ of superior quality; that of Bohemia is noticed under Table gla . Gla , Flint , Crystal . Sp. gr. 3·000 to 3·620.— a. By analysis:— 1. (Berthier.) Silica, 59·19%; oxide of lead, 28·68%; pota a, 12·13%; oxides of iron and manganese, traces. Finest colourle English crystal. 2. (Brande; Faraday.) Silica, 52%; oxide of lead, 34%; pota a, 34%. Crystal. 3. (Faraday.) Silica, 44·30%; oxide of lead, 43·05%; pota a, 11·75%; alumina, ·50%; oxides of iron and manganese, ·12%; (lo 28%). Heaviest of three samples of flint gla examined. b. Materials used:— 1. Finest Lynn-sand (calcined, sifted, and washed), 51%; litharge (purest), 28% (or red lead, 29%), refined pearlashes (calcined before being weighed), 16%; nitre (purified), 4 3 ⁄ 4 % arsenious acid and peroxide of manganese, of each, 1 ⁄ 8 %. Very fine crystal. 2. (M. Payen.) Fine sand, 46%; red lead, 31%; purified carbonate of potash, 23%. French crystal. 3. (Geddes.) White Lynn-sand, 51%; red lead or litharge, 33%; refined pearlashes, 13%; nitre, 3%; a very little arsenious acid and peroxide of manganese. Ordinary English flint-gla . Crystal ‘cullet’ may be added at will to the above. This gla was originally prepared from powdered flints, a fact to which it owes its common name. Gla , Optical. 1. (Crown gla .) Purest siliceous sand, 55%; carbonate of soda (dry), 12%; chalk (dry), 11%; carbonate of baryta, 22%. 2. (Flint gla .)— a. By analysis:— Silica, 44·30%; oxide of lead, 43·05%; pota a, 11·75%. This is Guinand’s ‘dense optical gla .’ b. Materials used:— 1. Purest quartz, 42%; red lead (finest), 42%; purified potash, 14 3 ⁄ 4 %; purified nitre, 1 1 ⁄ 4 %. These are the proportions used for the last. 2. (Korner.) Finest quartz (reduced to powder, treated with hydrochloric acid, washed, and dried), 47 1 ⁄ 2 %; red lead, 38 1 ⁄ 4 %; cream of tartar, 14 1 ⁄ 2 %. The above are used by opticians in the construction of achromatic object-gla es. Gla , Plate. Sp. gr. 2·488 to 2·600.— a. By analysis:— 1. (Dumas.) Silica, 75·9%; soda, 17·5%; lime, 3·8%; alumina, 2·8%. French mirror-gla . 2. (Mitscherlich.) Silica, 60%; pota a, 25%; lime, 12·5%; lo , 2·5%(?). Finest Bohemian plate. b. Materials used:— 1. Finest siliceous sand, 45%; dried carbonate of soda, 25%; lime, 5%; nitre (purified), 2%; plate-gla cullet, 23%; peroxide of manganese and cobalt azure, a very little. Ordinary English plate. 2. Whitish quartz sand, 60%; purified carbonate of soda (dried), 20%; lime (slaked by exposure to the air), 9%; plate-gla cullet, 11% (or more). Sometimes as much cullet as sand is used; but in all cases 1% to 1 1 ⁄ 2 % of its weight in carbonate of soda is added with it, besides that ordered in the formula, to compensate for lo of alkali by remelting. Used at the celebrated plate-gla works at Saint-Gobain, France. The product po e es an amount of excellence which British manufacturers have yet failed to equal. Gla , Table Bohemian crystal. Sp. gr. 2·6 to 2·8.— a. By analysis:— 1. (M. Berthier.) Silica, 71·7%; pota a, 12·7%; soda, 2·3%; lime, 10·3%; alumina, ·4%; oxides of iron and manganese (and lo ), 2·6%. Very white, hard, and beautiful table gla . 2. (Dumas.) Silica, 70%; pota a, 20%; lime, 4%; alumina, 5%; oxide of iron, ·6%; peroxide of manganese, ·4%. A beautiful white winegla . b. Materials used:— 1. Finest sand, 50%; purified potashes, 25%; chalk, 10%; nitre, 2%; crystal cullet, 27%; manganese, a little (say 1 ⁄ 16 %). Used in England recently for table gla . 2. Quartz (hyalin, in powder), 63%; purified potashes, 26%; slaked lime (carefully sifted), 11%; manganese, a little; crystal cullet, at will. Used in Bohemia. 3. (M. Perdonnet.) Powdered quartz, 44%; carbonate of pota a, 33%; quicklime (in fine powder), 22%; nitre, 1%; and a very small quantity of arsenious acid and peroxide of manganese. Said to be the formula used at Neuwelt for the gla a , 1 ( above ). Qual., . These are denoted by its hardne , transparency, homogeneity, strength, and power of resisting the action of water, air, light, and the stronger acids and alkalies. The power of gla to resist the action of menstrua is readily tried by exposing it to boiling oil of vitriol, and hot but dilute solution of caustic pota a. Neither of these tests should cause the gla to lose its transparency or to become dim. Swallowed gla . Gla and enamel, both in fragments and in powder, have occasionally been swallowed, with different results. These bodies are insoluble in the fluids of the body, and, consequently, any injurious action they may exert upon the system whilst they are retained in it must entirely depend upon mechanical attrition or irritation. As treatment, we must administer an emetic, and a ist its action by thick mucilaginous liquids, and afterwards have recourse to antiphlogistics, if nece ary. Anal. — a. A portion of the sample for examination is heated to dull redne , and then suddenly thrown, whilst still hot, into a ve el of cold water. It is next dried, and reduced to fine powder in an agate or hardened-steel mortar. b. 100 gr. of the prepared powder is thoroughly mixed with 200 gr. of pure pota ic hydrate, and the whole is exposed to heat in a silver or platinum crucible or capsule until perfect fusion takes place; when cold, the crucible and its contents are boiled in about half a pint of distilled water; nitric acid is added to the resulting solution, in exce , and the mixture, together with any sediment, is evaporated to dryne , after which the heat is gradually increased to 400° or 500° Fahr.; the dry residuum is next reduced to powder, and digested in water acidulated with nitric acid, until exhausted of soluble matter; the insoluble portion is then carefully dried, gently ignited and weighed. The weight in grains represents the per-centage of silica in the sample examined. c. The mixed liquid and washings of b is next acidulated with nitric acid, and treated to a stream of sulphuretted hydrogen, which, if it produces a precipitate, is continued for some time; the precipitate is collected on a very small filter, washed, and dried; the filter with the precipitate next placed in a beaker gla , and strong fuming nitric acid is cautiously added, drop by drop, until complete solution takes place; after boiling the solution for a few minutes, diluting with distilled water, and allowing it to cool, it is precipitated with sulphuric acid, in exce ; this precipitate (sulphate of lead) is washed, dried, slightly ignited in a porcelain crucible, and weighed. The weight in grains, multiplied by ·7369, gives the per-centage of oxide of lead or litharge. d. The filtered liquid from c is evaporated to dryne , and redi olved in water acidulated with hydrochloric acid, and treated with a solution of ammonium chloride, and afterwards with ammonia, in exce ; the precipitate (alumina and oxide of iron) is collected, washed, and boiled in a solution of pota ium hydrate; the undi olved portion is collected on a filter, washed with boiling water, ignited, and weighed. This gives the per-centage of peroxide of iron. e. The liquid filtered from the oxide of iron holds the alumina (if any) in solution; a solution of carbonate of ammonium is dropped in; the resulting precipitate is washed, dried, ignited, and weighed. This gives the per-centage of alumina. f. The filtrate from the alumina and oxide of iron (see d ), after being evaporated to dryne , is redi olved in a large quantity of distilled water, and is treated with a solution of oxalic acid (a solution of oxalate of ammonium is preferable when no baryta is present); the precipitate is washed, dried, gently ignited, and weighed. The weight of the resulting carbonate of calcium, in grains, multiplied by ·56292, gives the per-centage of lime required. g. The filtrate from f is now mixed with carbonate of pota ium, in considerable exce , and boiled for a long time; the resulting precipitate (if any) is then collected on a filter, slightly washed with hot water, dried, and exposed to a full red heat for some time (say 2 hours); the residuum of the calcination is then weighed. This furnishes the per-centage value of the sample in magnesia. h. The filtrate from f is treated with dilute sulphuric acid or the solution of a sulphate, as long as a precipitate falls; the precipitate (sulphate of barytum,) is washed, dried, gently ignited, and weighed. The weight, in grains, multiplied by ·6589, gives the per-centage of baryta in the sample. 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 undi olved 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, ., as directed under f . The liquor, ., filtered from the lime, is lastly tested for magnesia. (See g .) i. A second 100 gr. of the powdered gla (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 proce 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 ma , 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 exce , and after a time the earthy salts are removed by filtration; the filtered liquor is now evaporated to dryne , and ignited to dull redne for 2 or 3 minutes; the residuum (sulphate of pota ium or sodium, or of both), after being weighed (the weight being carefully noted down), is redi olved 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 exce ; the whole is now gently evaporated to dryne , 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 pota a sought. k. The weight of sulphate of pota ium in the ignited residuum in i is calculated from that of the pota ium last found (47 parts of the one being equal to 87 parts of the other), and this weight is deducted from the gro 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 gla 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 gla . 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 gla . The former, which is the most costly, is preferable to the finest crystal. Care must also be taken that the lime, clay, ., are respectively of proper purity; and that the ‘cullet,’ or broken gla , 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. Pota a produces a better gla than soda, although the latter is now very generally employed, from its lower price. It is, however, quite inadmi ible as an ingredient in the manufacture of the better cla of crystal and plate gla , as, however pure it may be, it imparts to the product a slight greenish tinge more or le destructive of its beauty. When sulphate of soda (Glauber salt) is used as a source of soda, it is gently calcined to di ipate 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 proce es, and to carry out with certainty his various intentions, the gla manufacturer, perhaps more than any other person, requires the aid of science and experience. All his most e ential 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 gla es are mixed super-silicates of the respective bases which enter into their composition (pota ium, calcium, lead, .), and, like all other compounds which are formed by elective attraction, obey the common laws of combination, as developed by Dalton, and now so succe fully 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 gla es 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 gla es 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 gla of German manufacture the proportions of the two are found to be as 4 to 1. The celebrated plate gla of St. Gobain is an atomic compound formed of 1 equivalent of tri silicate of soda united to 1 equivalent of tri silicate of lime, with a small per-centage of alumina in combination with silicic acid, also in atomic proportion. Gla es in which the ingredients bear no atomic ratio to each other are never homogeneous, but always more or le 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 gla is so greatly inferior to that of France and Germany, that comparison of the two becomes absurd. The only variety of gla in the production of which the English manufacturer excels is flint gla or crystal, and here he certainly surpa es all his numerous competitors. The subject is doubtle involved in difficulty, owing to the precise temperature nece ary 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 proce , the smaller the quantity of silica which enters into the composition of the resulting gla ; whilst at higher temperatures a part of the base is di ipated 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 exce ive or improperly continued, the lo of base produces an opposite effect, and an opaque, semi-vitrified ma is formed, resembling ‘Reaumur’s porcelain.’ The quality of the resulting gla depends on this change being more or le 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 gla es, and although theory may not be able to ensure unvarying succe , it is neverthele certain that, in all cases, it can afford much valuable a istance in that direction. Indeed, it has been a erted by one of the leading Continental chemists, that ingredients that will yield the proper equivalent proportions in the melting pot cannot produce a bad gla , if exposed to such a temperature as to permit of perfect combination taking place. It is found that those gla es which contain a predominance of alkali are acted on by water, and when this is in great exce they are perfectly soluble in that fluid. Ordinary flint gla is affected by long coction in water, whilst crown gla , which contains le alkali, is unaltered by that trial. Gla which contains any considerable quantity of lead is acted on by sulphuretted hydrogen. This is the cause of the surface of flint gla , under certain circumstances, becoming opaque and iridescent. Gla es 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 nece ary to give them this quality. Gla es that have a slight greenish or bluish tint may be often whitened, or rendered colourle , by exposure to light and air. This arises from the peroxidation of the iron, to whose protoxide they owe their tint. Other gla es become purpled by exposure, owing to the peroxidation of the manganese. Different colours are communicated to gla 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, . These substances are either added to the melted contents of the gla -pot, as in preparing artificial gems, ., 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 gla . Gla is FORMED or FASHIONED into articles by the proce es of blowing, casting, drawing, rolling, or spreading. In the proce of BLOWING GLASS the workman begins by collecting a proper quantity of gla 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 ve el is to be made, an iron rod, called a ‘pontil’ or ‘puntil,’ is dipped into the gla -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 ve el is now re-heated, and the aperture enlarged, and the ve el 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 proce of BLOWING and SPREADING . The globular flask at first produced, transferred from the blow-pipe to the ‘pontil,’ is suddenly made to a ume 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-gla 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. Gla 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 a ume 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. The following hints respecting the MANAGEMENT OF GLASS may prove useful to the inexperienced:— Annealing. The proce of annealing gla has been briefly referred to before. The extreme brittlene of imperfectly annealed wrought gla may generally be remedied on the small scale by immersing the articles in a bath of oil, or a concentrated solution of chloride of calcium, or common salt, and heating the whole gradually and cautiously to the boiling-point, and letting it again cool—the slower the better. By this treatment the gla will be enabled to bear any alterations of temperature between the two extremes to which it has been exposed. Blowing. By the ingenious art of GLASS-BLOWING and GLASS-DRAWING , as practised on the small scale, with a blowpipe lamp furnace, a variety of articles of ornament and utility may be made, their number being limited only by the ingenuity of the artist. The details of the various operations are, however, too lengthy to describe here. Cleaning. 1. Windows, looking-gla es, ., may be quickly cleaned as follows:—Dip a slightly moistened rag or flannel into whiting, fuller’s earth, wood-ashes, or rotten-stone, in impalpable powder, with which smear the gla , and wipe it off with a dry, soft cloth. This does well when the surface is very dirty. In other cases, a little thumb blue, whiting, or chalk, in fine powder, tied up in muslin, may be dusted on the gla , which should then be cleaned off with chamois leather. This gives a fine polish. 2. The ve el to be cleansed, is filled, or, if large, rinsed, with a moderately dilute solution of permanganate of potash, contact being prolonged till a film of hydrated manganic oxide has been deposited; the solution is then poured away, and the gla ve el rinsed with some strong hydrochloric acid. Cutting. Gla may be easily cut with a common well-hardened steel file, provided it be moistened with oil of turpentine, or plunged under water. It may be also perforated with a common steel brad-awl in the same way. Gla ve els , as bottles and tubes, may be readily cut or shortened by placing a heated iron ring over the spot, or a piece of loose string or cotton dipped in oil of turpentine and set on fire, and immediately on the withdrawal of either applying cold water to the part. Gla ve els or tubes thus treated will generally crack round, and may be readily divided into two parts. In this manner a common Florence oil-flask may be converted into an evaporating dish and a funnel. By a little practice a crack may be led in almost any direction, or a new one made, by the point of a red-hot poker or a spring coal (an ignited crayon of prepared charcoal). The parts may then be separated by a little force or a smart rap, and the divided edges smoothed by the flame of a blowpipe, or by grinding them with powdered emery and water on a flat stone. In this way many broken articles in gla may be converted into others scarcely le useful. Etching on gla has been already noticed under the head of Etching. Gilding of gla . Gold chloride is di olved in boiling water; the solution is filtered, and the filtrate so far diluted, that 200 cubic centimètres contain 0·0648 gram of the metal, and it is then made alkaline with soda. The reducing agent is alcohol saturated with marsh gas; this is diluted with its own volume of water. 25 cubic centimetres of this solution are mixed with the alkaline gold chloride solution, and this mixture is poured between the perfectly well-cleaned plate to be gilded, and another sheet of gla placed at a distance of 3 mm. under the first. After two to three hours’ rest the gilding is effected. The plate is removed and washed. (‘Dingler’s Journal.’) Grinding. This, on the large scale, like gla -cutting, forms a distinct occupation. On the small scale, gla may be roughed or ground by friction with powdered emery and water and a flat rubber of wood; care being taken that the article, if a plate, is laid on a perfectly flat surface, or, if hollow, is supported by a core of cement or plaster. The frosted appearance of ground gla is given to the panes of windows by gently dabbing the gla over with a piece of glazier’s putty, stuck on the ends of the fingers. When applied with a light and even touch, the resemblance is considerable. Another method is to dab the gla over with thin white paint, or flour paste, by means of a brush, but the effect is much inferior to the above. Gla , packing. This subject will be considered under the general head of Packing . Writing on gla may be performed by a piece of French chalk or crayons prepared for the purpose; or even with a common pen held nearly perpendicular. Indian ink, or, when the article will be exposed to damp, shell-lac ink or varnish, thickened with a little Vermillion, or lampblack, is best adapted to this purpose. Common ink is not sufficiently opaque. Gla , to prevent the cracking of, by boiling water. When new, all gla and earthenware should be placed in cold water in a saucepan, and after some hours the saucepan containing the ve el or ve els, should be placed over the fire, until the water reaches the boiling point.