Glass and Glass Ware, Paris 1878 - 07




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Classification and properties of glass.

Glass is an amorphous substance, hard and liable to break at ordinary temperatures, liquid or plastic at a high temperature, transparent or translucent, white or colored, having a peculiar brilliant and smooth fracture, called “vitreous.” It is composed of silica with some of the following bases: potash, soda, lime, magnesia, lead, iron, and alumina.

Several kinds of glass are known, such as window and plate glass, flint, white, and bottle glass, made up in different proportions of sand, soda, potash, lime, red lead, etc.

Bohemian glass, used in the making of ordinary and fine hollow ware, is a silicate, with potash and lime base. It contains, like all other kinds of glass, a small quantity of alumina from the pots and oxide of iron from the impurities contained in the materials used. Potash is often replaced by soda, owing to the lower cost of the latter.

Bottle glass contains — besides silica — soda or potash, lime, magnesia, alumina, and oxide of iron.

Flint glass, or crystal, is known as a glass with a base of lead and potash. This denomination, however, is not accepted by all nations, as, in Bohemia, lime-glass used for fine table ware is known as crystal. Glass used for optical purposes, with a great density, owing to the lead it contains, is called flint. Strass is another variety of lead glass, used for making imitations of diamonds and precious stones. Enamels contain, besides lead, oxide of tin or arsenious acid.

Colored glasses are produced by using various metallic oxides, charcoal, or sulphur. Oxide of manganese is introduced to correct the green coloration of glass by giving it a purple tint. In larger proportions it produces various colored glasses.

Glass at a white heat becomes almost as liquid as water, but when cold is quite rigid; however, at a cherry-red heat it is plastic and malleable. This property of glass enables the blower to work it with facility. At the cherry-red heat it is plastic enough to be blown by means of a pipe and shaped with tools. When it becomes rigid by cooling it may be reheated and worked until the proper shape is obtained. Glass rolled on a metallic table is made into plates; by blowing it into a mold all kinds of bottles are made. By pressing the plastic mass by means of a press, plunger and metallic mold, glass can be shaped into all kinds of wares, such as tumblers, goblets, vases, etc. By means of the glass-blower’s lamp this material can be drawn into very fine

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threads and reeled up like ordinary thread. Glass can also be reduced to almost impalpable threads, as fine as filaments of cotton, by means of a steam or air blast acting upon a very fine stream of molten glass.

Glass is a bad conductor of heat, and when heated and suddenly cooled flies to pieces. While being worked it cools very rapidly by the action of the ambient air; it becomes necessary to correct this defect by annealing. This operation consists in carrying the glass objects when still hot to a special furnace, where they are reheated to a low cherry-red, and gradually and slowly cooled. This operation, when improperly carried out, exposes the glass to break suddenly. An unpleasant experience of this nature often takes place in households when a lamp chimney, without any apparent cause, suddenly flies to pieces. Under the head of toughened glass we shall again refer to this unfortunate habit. Glass is also a bad conductor of electricity.


Potash. — Bohemian glass is made with carbonate of potash, as pure and as rich as possible. Hydrate of potash of 54 to 56 degrees is the best for use. Potash used in glass making is extracted from the residuum of beet-root sugar making. In Bohemia, a potash extracted from wood ashes, coming from Hungary. America also furnishes a good article. All potash used in glass making should especially be free from soda, as it tends to give glass a green color.

Soda. — This alkali, to a great extent, has taken the place of potash. It is used as a carbonate or sulphate. Carbonate is yet used in the manufacture of table ware, but in the making of plate and window glass, bottles, etc., the sulphate is almost entirely used, owing to its cheapness. By adding a small quantity of charcoal to sulphate of soda it is decomposed much more easily.

Lime. — In the manufacture of plate, window, and white glass, lime is used as a carbonate or slacked.

Oxide of lead. — In the manufacture of flint glass lead is used in the red oxide form, or sometimes as litharge, but the red oxide gives the best results. Red lead furnishes oxygen, which escapes in melting, and serves to refine the glass and burn the organic matters which may be contained in the mixture of materials. Red lead is prepared from the purest of leads coming from England and Spain, which are comparatively free from the oxides of iron, copper, etc. Lead is but little used in this country for glass making,

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with the exception of a few manufacturers who make a superior quality of glass.

Silica. — It is important that sands should be selected with care, as impure silica has the most detrimental effect upon the color of glass. This is especially important in the manufacture of fine table ware, plate, and window glass. All sands should be free from iron, as this is the fruitful source of the green coloration of all glasses. In Bohemia quartz is used instead of sand. It is first heated, then thrown in water; this breaks it into many pieces, which are subsequently reduced to fine powder by mechanical action in wooden mortars with quartz pestles. By this means the introduction of iron is avoided.

France and Belgium, for the manufacture of fine wares, use the Fontainebleau sand, not far from Paris; also the sands of Nemours, Chantilly, and from the province of Champagne. These sands are used for making flint, plate, and first quality window glass.

English sands contain a considerable proportion of iron. Silex is sometimes used instead of sand for fine wares. For plate-glass making sand from the Isle of Wight is used, but for the best qualities of glass the English manufacturer has to import sand from France and this country. Bottle manufacturers, on the contrary, seek sands containing iron and clay, on account of the elements they contain acting as a flux.

Silica is found in the shape of quartz, rock-crystal, sandstone, and quartz sand in the crystalline form; in silex or flint stone in the amorphous state. Silica is one of the most abundant natural substances; it is insoluble in water; and resists the action of most chemicals. Among the acids only one, hydrofluoric acid, is capable of decomposing it; this acid decomposes and dissolves glass entirely. Silica, although infusible at the highest temperature of furnaces, has nevertheless been fused by the use of the oxyhydrogen blow-pipe. Silica combines with all bases, alkalies, such as potash, soda, and with metals, lead and bismuth. These give it the property to form vitreous compounds. Lime, magnesia, alumina, form with it infusible compounds; the latter, however, mixed with silicates of potash, soda, or lead, furnishes compounds which are suitable for the work of the glass-blower. It is glass proper. If two infusible silicates are mixed together they nevertheless produce fusible glass. Fine plate-glass has been made with a compound of

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sand, slacked lime, and carbonate of baryta. This glass, as fine as any ever made, contains, on being analyzed —

Silica 46.5
Baryta 39.2
Lime 14.3
100. 00

To produce a glass requiring as little fuel as possible, the glass maker should introduce as many bases as possible in his mixture, such as potash, soda, lime, magnesia, alumina, oxide of iron; these, however, more particularly in bottle glass, where color is not so much an object as cheapness. Per contra, in making pots the clay should be as free as possible from a mixture of bases; simple silicates, such as silica and alumina, are preferable.

Nature has provided this country with an abundance of sand suitable for glass making. The Pittsburgh hollow-ware manufacturers use a very good quality of sand, procured in the Allegheny Mountains, in Mifflin County, Pennsylvania; it is worth about $5 per ton. The West Virginia glass makers use the same, worth about $6.50 per ton at Wheeling. New Jersey glass-houses, and several in Philadelphia, use a sand procured from the State of New Jersey, of very good quality, costing from $2 to $3 per ton. Pittsburgh window-glass works use a cheaper kind of sand, procured from the neighborhood, worth $2.25 to $2.50 per ton. Ohio works use a sand worth from $2.50 to $3.50 per ton. Manufacturers of hollow ware in the Eastern States, and those about New York City and Brooklyn, use a sand procured from Berkshire, Mass., worth about $8 a ton; this sand is the purest to be found in the world, and is nearly pure silica; it is found in fine grains and is quite fusible. Western manufacturers, in Illinois and Missouri, use sand procured in their respective States, worth from 60 cents to $2 per ton. At Grand Tower, Mo., there is a mine of very pure quality of sand. I know from personal experience that these sands also make a very good quality of glass. Those who are in the proximity of Minnesota use a sand worth $2 per ton, mined in that State from the Saint Croix sandstone, about 50 feet thick, and also at Red Wing, on the bluffs of the Mississippi River. These bluffs are traced all along the river to the southern line of the State. At Minneapolis and Saint Paul a sand rock is also found, furnishing a good quality of sand; this rock in some parts is 175 feet thick.

In Indiana sand suitable for glass making is found in abundance near New Providence, where a vein of 16 feet

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thickness exists. This sand is used in the glass works of New Albany, Louisville, Jeffersonville, and Cincinnati; it costs about $3.25 per ton at New Albany, exclusive of royalty.

In Maryland, in the neighborhood of Cumberland, an excellent quality of sand is found, which, as per analysis of Professor Chandler, of New York, is said to contain 98.25 per cent, of silica, the remainder being sesquioxide of iron, equivalent to 0.29 per cent, metallic iron. This sand is certainly very pure. In the glass works I had in operation in Washington, D. C., a few years ago I made glass repeatedly with this sand, and a better and more brilliant glass could not be made. I found it easy to melt, and containing but very little vegetable impurities. This sand can be had for $2 to $3 per ton. Cumberland has all the materials in abundance in its vicinity for a successful glass manufactory — coal, sand, and clay, all of first quality, besides timber for boxing and packing. The railroad facilities for shipping to different points are good.

Virginia has several sand mines of very good quality. In Rockbridge County, on the North River Canal, 25 miles from Lynchburg, is found a very good quality of sand; it has been analyzed by Messrs. Manard and Tieman, of New York, and Prof. John H. Appleton, of Brown University. The assay shows:

Silica       97.50
Alumina 1.02
Sesquioxid of iron 1.48

It is found in abundance, and is easy of access. This sand can be delivered at a very reasonable price by the James River and Kanawha Railroad, when it is finished. I have tried a sand sent from Richmond, said to have been obtained in the neighborhood of the city, but found it unsuitable for glass making, owing to the large proportion of mica it contained.

In West Virginia, near Romney, at a place called Hanging Rock, a very good quality of sand has also been found.

In Tennessee, on the line of Tennessee Coal and Railroad Company’s road, a short line running to the Suwanee mines, sand of a good quality has been found at several of the cuts on the road. This sand is the result of the disintegration of the sandstone of that locality. This region seems also to be favored for glass making, as coal, sand, and fire clay are found in abundance.

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Crystallization — devitrification.

When submitted for a long while to a high temperature, glass loses its transparence and becomes opaque, though it may preserve its shape; this is known as devitrification. Reaumur, in 1727, succeeded, by maintaining glass at a very high heat during twelve hours, in producing a hard opaque glass, resembling white porcelain. The attempt has been made repeatedly, but without success, to manufacture this glass as an imitation of porcelain; but the expense of fuel and the difficulty of preserving the shape of articles while under the influence of the long-continued fire has made it difficult ana unprofitable. Devitrified glass is harder than ordinary glass and is capable of scratching it readily; it does not break so easily, and cannot be cut any longer by the diamond. It has a noticeable fibrous texture and is a better conductor of electricity. Glass with a potassic base does not devitrify as well as with a sodic base. Bottle and window glass devitrifies easily, and unless the contents of the pots are rapidly worked off the glass is likely to get knotty and lose its transparency; in this state it becomes impossible to work it any longer.

Action of air and oxygen. — Dry air or oxygen has no effect on glass. Damp air, however, has a marked effect upon glass, owing to the action of the water contained in the air; the fact cannot be attributed to air, since it is only the medium through which water acts.

Action of light. — In connection with this subject it is my pleasant duty to call attention to the numerous and repeated experiments made by Mr. Thomas Gaffield, of Boston, who, as an ex-manufacturer, now an amateur, and a true lover of the art of glass making, has contributed many articles to the literature of glass making. Among others, I may cite a contribution to the Philadelphia Photographer of 1869, entitled “Photographic leaf prints,” a discovery made while pursuing his experiments on the coloration of glass by sunlight. In the same periodical Mr. Gaffield, in 1876, published his “Glass for the studio and dark room,” pointing out how to select glass properly for the photographic room. In the Boston Transcript of 1876 is a fine review of the absurdity of the “Blue glass mania,” and here, also, the unpardonable blunders contained in Wendell Phillips’s lecture on the “Lost arts,” are handled in a masterly manner. In the same journal, in 1875, Mr. Gaffield wrote a review of the “De la Bastie toughened or tempered glass,” about which so much nonsense has been written in our daily journals.

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The writings of Mr. Gaffield bear the imprint of an observing mind, and a fearless denunciation of absurdities where-ever found. It is with pleasure that I call attention to the researches of Mr. Gaffield, because of the fact that so far we have had comparatively nothing published in this country in regard to the scientific investigation of glass.

Mr. Peligot, in his late work, “Le Verre,” mentions favorably the researches of Mr. Gaffield. After a series of experiments upon the different kinds and qualities of glass from France, England, Germany, Belgium, and America, Mr. Gaffield has established the following results by exposing to sunlight pieces of glass during different periods of; time: The changes in colorless glass after exposure are from white to yellow, from green to yellow-green, from brown-yellow to purple, from green-white to blue-white, and from blue-white to darker blue, according to the length of time of exposure to light. Although colorless glass can be exposed to a furnace heat without changing color, when the same glass has been exposed to sunlight and has acquired the peculiar orange and purple lines, if reheated the original color will reappear. This change, therefore, cannot be attributed to heat. In pot metals of intermediate colors, such as brownish, yellowish, and rose or purple, the change was found to be quite rapid, a few days in some instances being sufficient to show a perceptible change. Mr. Gaffield draws the conclusions from his experiments that the particular shades of colors found in some old cathedral glass are not owing to peculiar mixtures which some writers on glass claim are a “lost art,” but must be the result of long exposure to the rays of the sun. This change of color has been attributed to a great extent to the use of manganese, singularly known as a decolorizer among glassmen. Many of the European manufacturers are believers in this theory, and have discontinued its use on that account, or have decreased the proportion used. M. Bontemps mentions that glass containing manganese has been found unfit for light-house lenses. Glass containing lead, however, even in a proportion as small as five per cent., he asserts remains perfectly colorless while exposed to the sun. Peligot believes that to the use of manganese these changes of color may, to a great extent, be attributed; the green tint of ordinary glass being attenuated by the action of sunlight, the peculiar purple color due to manganese becomes predominant.

Action of water. — Water at ordinary temperature and under ordinary contact with glass has but a slight or no perceptible effect.

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An increase of temperature and of surface of contact, however, tends to augment the dissolving action of water. The composition of glass has a manifest influence upon its solubility. Where glass contains an excess of alkali it is more apt to be altered by the action of water, while glass containing a predominant earthy silicate is freer from attack. A peculiar purple coloration is often noticed in panes of glass in places exposed to dampness. This is explained by the action of water being in contact with the surface of glass for more or less time, producing a solvent action upon the alkali contained in the glass. If the surface of such a glass is rubbed, small thin pellicles will be detached; they are composed of earthy silicates, the alkaline silicate having disappeared. When the action is continued for along period the peculiar iridescent coloration increases. According to Newton, this coloration is the result of the reflection of light upon the thin pellicles or pieces which become somewhat separated from the main body of the glass.

The following analysis, made by Mr. Haussmann, shows plainly the action of water upon glass:

Part having
Silica 59.2 48.8
Alumina 5.6 3.4
Lime 7.0 11.3
Magnesia 1.0 6.8
Oxide of iron 2.5 11.3
Soda 21.7 0.0
Potash 3.0 0.0
Water 0.0 19.3
  100.0 100.9

All glass when reduced to powder is subject to the influence of water, and gradually absorbs carbonic acid in such a quantity as to show quite an effervescence in contact with acids. Powdered glass boiled in water in contact with carbonic acid absorbs this gas in a few minutes and produces an instantaneous effervescence in acids. Powdered glass kept in boiling water for several hours with sulphate of lime produces a notable quantity of sulphate of soda. All glasses reduced to powder will bring back the blue color of test papers colored red; it is owing to their alteration by the absorption of water. Glass made with soda is subjected to a different alteration from that made with potash. Soda glass continues to become iridescent with time, sometimes to such an extent as to appear to be colored glass, and small pellicles gradually become detached. The same peculiarity

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has been noticed in ancient glass dug from the earth, and the iridescent coloration is attributed to decomposition by water. Potash glass is affected by water in producing small crystals upon its surface. This deposit of crystals depolishes glass, renders it rough, and seems to have covered the surface with a multitude of small cracks. These cracks appear to be the result of the small crystals acting upon the surface of the glass in a manner similar to that of cutting with a diamond. Flint and crown glass, for the particular purposes they are intended, are manufactured with a large proportion of alkalies. This excess has the tendency to make them damp on the surface, to make them lose their transparency, and, with time, to alter their shape. Crown-glass disks, piled one upon another, have been known to become cemented quite firmly together; this is caused by the silicate of potash they contain in excessive quantity attracting the dampness of the atmosphere. M. Daubrée, to illustrate the action of water upon glass under pressure and temperature, took some glass tubes and subjected them to a temperature of 572 degrees in contact with water. The result transformed the glass into a fibrous matter resembling Wallastonite (silicate of lime). Thus, it will be seen that manufacturers who may be tempted to produce glass with an excess of alkali, in order to save fuel in melting, are exposed to produce an inferior quality, which, after a comparatively short time, will show the peculiar objectionable iridescent coloration. This coloration, however much prized in fancy articles, is very obnoxious in window and plate glass.

Action of acids and alkalies.

Pulverized glass in contact with hydrochloric acid diluted with hot water, or even at ordinary temperature, is easily attacked. The same effect takes place with lead glass, the dissolution being in contact with hydrosulphuric acid. Bottle glass being made with a large proportion of bases, in order to produce a cheap glass, is very easily attacked by acids. If a bottle is filled with strong sulphuric acid, after a certain time small concretions of sulphate of lime will appear, while alumina and the alkali will be dissolved in the acid. Silica will fall to the bottom in the form of a jelly. Many bottles are attacked by the concentrated mineral acids, but resist the action of these acids diluted. Some bottles are even attacked by the bitartrate contained in wine, and decompose it and impart to it the taste of ink, also destroying its color. It has been ascertained that few bottles,

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if even made of a superior quality of glass, resist the action of wine in course of time. The discoloration of wines is attributed to the formation of a lake made up of gelatinous silica and the coloring matter of wine. Certain white wines will sometimes turn black when exposed to the air even for a few moments. These wines contain tannin, which, under the influence of a small quantity of iron extracted from the glass when exposed to the air, form a trace of tannate of peroxide of iron, the coloring matter of ink.

Certain bottles are rapidly attacked by acid liquors. Champagne bottles of apparently good manufacture have been known to alter the color of wine in a few days. Acidulated water, containing only 4 per cent, of sulphuric acid, has also been known to produce even in one day a thick crust of sulphate of lime and a dissolution of sulphate of iron and potash.

Peligot, in making experiments of this nature, found the glass to contain —

Silica 54.56
Lime 18.20
Alumina 10.43
Protoxide of iron 1.86
Magnesia 0.51
Potash 1.37
Soda 13.07

The number of bases contained in this glass explain the rapid effect that even the weakest acids produce upon wine.

Flint or lead glass resist much better the action of water and acids. Strong alkaline solutions preserved in lead-glass bottles extract oxide of lead. These alkaline sulphates in course of time produce a black deposit. It has been repeatedly noticed how the ground stoppers of bottles containing solutions of caustic soda or potash will with time become thoroughly cemented. This is owing to the formation of a soluble alkaline silicate, which has very strong adhesive qualities. Bottles intended to contain reagents should be made of a hard glass, free from lead. A study of the matters derived from glass by the effect of the solutions used should also be carefully made to avoid erroneous results in making analysis.

Hydrofluoric acid having a very strong dissolving effect upon glass, this quality is availed of for engraving glass and for making easy and reliable analyses of all kinds of glass. Hydrofluoric acid is made by introducing into a leaden still pulverized fluoride of calcium and concentrated sulphuric

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acid. The mixture is heated and the distillate is received in a leaden receptacle containing water. To manufacture this acid in large quantities a cast-iron still is substituted for lead. The acid is kept in gutta-percha or leaden bottles. It should be handled with great care, for if any of it should penetrate through the skin by an abrasion or a cut it produces painful sores which are difficult to cure. Rubber gloves should be used when it is handled.

Analysis of glass.

As it is frequently important for a manufacturer to ascertain the ingredients making up certain kinds of glass which he may want to reproduce, the following is given by Bontemps as one of the most reliable methods he has found during his long experience as a glass maker:

The glass must first be broken to pieces, then reduced to powder in a steel mortar. Sieve and take away all metallic particles by means of a magnet; further pulverization may be continued in an agate mortar.

Take two grams of the glass powder, put it into a platinum capsule. Take a leaden vase of about 25 centimeters diameter and 6 deep, having a cover of same metal: spread upon the bottom of this vase some pulverized fluoride of calcium; add a sufficient quantity of concentrated sulphuric acid to form a thick batter, which must not rise above two centimeters above the bottom. Place upon this bottom a leaden ring 5 centimeters wide by 0.025 high. Upon this ring place the capsule containing the glass, and add a little water to it; then cover the vase. Warm slightly over a sand bath; the vapor arising will condense into the water contained in the capsule, will attack the vitreous matter, and produce fluoride of silicium, which will volatilize. Stir the matter now and then with a spatula. At the expiration of twelve hours, when the matter has been completely reduced, pour sulphuric acid into the capsule to transform the fluorides of the bases into sulphates. Evaporate to dryness; expel the excess of sulphuric acid by means of heat; then treat the dry mass with water, ammonia, and carbonate of ammonia; boil, filter, and wash in warm water. The residuum may contain sulphate of baryta, carbonates of lead, bismuth, lime, some alumina, and oxide of iron. The filtered liquor contains the alkalies in the state of sulphates, traces of magnesia, and an excess of carbonate of ammonia, mixed with sulphate.

To separate the substances contained in the residuum, treat with dilute hydrochloric acid, which will dissolve all

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with the exception of the sulphate of baryta, which may be gathered upon a filter; wash it and burn the filter in a platinum crucible. 116.6 of sulphate corresponds to 76.6 of baryta. The weight of baryta is to be deducted from that of the sulphate of baryta, according to this proportion.

Pass sulphureted hydrogen through the acid filtered liquor; this will precipitate the sulphurets of lead and bismuth; then filter to separate them from the liquor. Burn the filter in a porcelain crucible, sprinkle the matter with nitric acid, containing a little sulphuric acid, to transform the lend into sulphate of lead, which is insoluble. The greater part of the free sulphuric acid is now driven off. The residuum is treated with water and filtered. Sulphate of bismuth is thus separated from the sulphate of lead. According to the weight of the sulphate of lead the quantity of oxide is calculated, 151.5 of sulphate of lead being equal to 111.5 of oxide of lead. Treat the acid sulphate of bismuth with carbonate of ammonia; the carbonate of bismuth thus obtained is calcined, and thereby produces oxide of bismuth, the weight of which is to be determined. The liquor, containing an excess of sulphureted hydrogen, is boiled in order to expel it; add a few drops of nitric acid to oxidize the iron if there should be any; then add caustic ammonia slightly in excess; this will produce alumina and protoxide of iron together. Ascertain the weight of this precipitate after having calcined it; then dissolve it in concentrated hydrochloric acid and add an excess of caustic potash. The iron is precipitated while the alumina is redissolved. Filter, wash with boiling water, and ascertain the weight of the oxide of iron; in subtracting it from the weight of the alumina and oxide of iron hereabove, the difference is the weight of the alumina. The liquor now contains only lime, with traces of magnesia, which may be neglected. Add to it oxalate of ammonia, which will produce insoluble oxalate of lime; filter, wash, and calcine at a high heat capable of transforming the oxalate into a carbonate, then into caustic lime, the weight of which is ascertained.

If zinc is contained in the glass, it will be found in the liquor containing the alkalies, and before dosing these alkalies the zinc should be precipitated by hydrosulphate of ammonia. The sulphuret of zinc is received on a filter, washed with water containing a few drops of hydrosulphate of ammonia; dry and burn the filter in a platinum capsule. Sulphate of zinc is then obtained by a careful roasting. Weigh the zinc.

To dose the alkalies, take the filtered liquor which contains them; evaporate to dryness; heat the residuum, in

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order to get rid of the ammonia salts, which are volatilized. Treat with water and hydrate of baryta, which is added in sufficient quantity to precipitate all the sulphuric acid of the sulphates, and a little magnesia, if there should be any. Filter, to separate the caustic potash and soda which are in solution with the excess of baryta; then add carbonate of ammonia. Carbonate of potash and soda and carbonate of baryta are produced; filter, to separate the latter. Evaporate to dryness, and calcine to expel the excess of carbonate of ammonia. Finally, saturate the residuum with a few drops of hydrochloric acid. Evaporate to dryness, calcine, and weigh the chloride of potassium and of sodium. The chloride of potassium must now be separated from the chloride of sodium.

Dissolve the two chlorides in a small quantity of water, and pour a concentrated dissolution of perchloride of platinum until the liquor turns a deep yellow. Evaporate the liquor to dryness, and operating with alcohol the double chloride of platinum and sodium is dissolved. There remains the double chloride of potassium and platinum, which is gathered upon a filter. Wash it with alcohol and weigh it after desiccation. 244 of the double salt are equivalent to 74.5 of chloride of potassium. Knowing the weight of chloride of potassium and that of chloride of potassium and sodium united, the weight of the chloride of sodium is deducted. We now calculate the weight of the soda and potash contained in the glass, knowing that 74.5 of chloride of potassium is equivalent to 47 of potash, and that 58.5 of chloride of sodium is equivalent to 31 of soda.

All the elements of the glass but silica have now been ascertained. It may be obtained by difference, but it is always best to get it by a direct dosing. To obtain it, mix 2 grams of glass with about 6 grams of dry and pure carbonate of soda; melt in a platinum crucible in a muffle fire. Put the crucible containing the well-melted matter in a large porcelain capsule, with water and hydrochloric acid (use nitric acid when the glass contains lead); the matter is dissolved with effervescence. When the dissolution is complete, take away the platinum crucible, wash it several times, then evaporate all over a sand bath; heat pretty high towards the end. Pour over the evaporated matter hot water acidulated with one of the two acids; leave it to digest and dilute with water. All the metallic oxides are dissolved, and silica only remains as an insoluble residuum. Gather it upon a filter, calcine it after a good washing, and weigh.

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To find sulphate of soda, take the liquor from which silica has been removed, and which must contain all the sulphate of soda which is to be found in the glass; add an excess of chloride of barium; boil for a long while, and let it settle for about twelve hours. Receive upon a filter the sulphate of baryta produced, and from its weight deduct the quantity of sulphuric acid, and, consequently, the equivalent quantity of sulphate of soda.

To obtain boracic acid, heat the pulverized matter with carbonate of soda to redness; boil the reddened mass with water. Filter, and by means of carbonate of ammonia the small quantity of alumina and silicic acid which has been dissolved by the alkaline liquor is precipitated. Evaporate to dryness, treat with sulphuric acid, and digest the residuum with alcohol, which dissolves the boracic acid. Saturate the solution with ammonia, redden the residuum which contains the boracic acid, and then ascertain the weight.

To obtain magnesia, use carbonate of soda. Magnesia still remains in solution after having precipitated the lime by means of oxalate of ammonia, since it cannot be precipitated by any of the reagents used until now. Concentrate the filtered liquor by evaporation; add phosphate of soda to it. After twenty-four hours all the magnesia is precipitated in the form of ammonia-phosphate of magnesia, which is gathered upon a filter and washed with ammoniated water. 112 parts of this precipitate, calcined, correspond to 40 of magnesia.

The analysis of glass is a very delicate operation, and it is always well to ascertain the right dosing by repeating the operations several times.

Pot making — bricks.

In Europe nearly every manufactory of glass has a potmaker to make pots, bricks, and the different articles of fire clay used about a furnace. In this country, in manufacturing centers like Pittsburgh and Philadelphia, factories for the making of pots and crucibles have been established. As every manufacturer, however, is aware of the importance of having good pots, there are but few who are willing to depend upon outside makers to supply them. The breaking of pots always causes serious losses, and unfortunately it is but too often the case that the success of a factory depends upon the skill of the pot-maker. There is no reason for this state of things, and every manufacturer, with a little study and attention, will soon learn how to judge of his clays, and mix them in a proper manner. The building

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of the body of the pot is a work that, with ordinary care, can be taught to an ordinarily intelligent man in a few days. The writer of this, when carrying on glass works in Washington adopted a mode of making clays which proved quite successful.

The clay should be selected from the sievings of different numbers of sieves, commencing from the finest to a certain degree of coarseness. The same method is to be observed for the cement or broken baked clay. In mixing the different selections, in proportions which will readily be indicated by experience, attention should be paid to the fact that too large an admixture of broken cement has the tendency to decrease the strength of pots, and too fine a grained clay exposes the pots to a large shrinkage and a tendency to break. When the clay is thoroughly mixed dry in small quantities at a time, spread it in thin layers, and wet sufficiently. Repeat this operation of mixing in small quantities until you have enough mixed. Cut thin layers across the box, turn them over, and tread with the feet thoroughly. Continue to cut widths and work them in the same manner until you reach the end of the box or trough, when the operation is reversed. The more the clay is worked the better it becomes. This method of mixing clay, observing the proper proportions, has invariably given me satisfaction. The matter of making, drying, and baking pots subsequently, when good materials have been used, mixed, and worked, is comparatively an easy task. To get good pots it is essential to have, 1st, good refractory clay; 2d, to mix sieved clays and cements of different qualities and degrees of fineness; 3d, to properly mix dry, by small quantities, the green clay and cement; 4th, to spread thin layers of the mixture, and wet sufficiently to make the mass plastic, and dampen evenly every particle; 5th, to work the mixture over by small quantities at a time, always intervening the order of layers; 6th, keep the clay well covered to keep it moist, and avoid adding water when once thoroughly mixed.

Clay breaking with a clear, smooth, and bright fracture is generally good; the color varies, but the dark gray is generally of a superior quality. When clay remains white after having been submitted to a violent heat, it is generally of a good quality, as the absence of coloration is an evidence of the absence of oxide of iron. Green clay should be unctuous to the touch and adhere to the tongue. A small piece chewed between the teeth will reveal the approximate quantity of sand contained, the silica being readily detected under the teeth.

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It is well known that the Chinese allow their clays to ferment or ripen for long periods before they use them, and the longer they are kept the better they claim them to be. Be that as it may, Bontemps, the celebrated glass manufacturer, holds, as the result of his experience, that clays which have been allowed to putrefy make a superior quality of pots.

Little or no attempts have been made to manufacture pots by pressure, although it must be self-evident that if we could increase the density of a pot it would add to its homogeneity and consequently to its life. The writer of this, however, some years since, succeeded in making small covered pots, all in one piece, molded and pressed by an improvised press and mold, which, when dry, showed a great hardness and sonority. Tapped all around, they gave the same sound, showing they were quite homogeneous.

Although, as I have said before, pot-making is not a difficult art to acquire, yet, from the number of factories that have had to succumb owing to badly made pots, it is to be hoped that the day is approaching when pots will be discarded entirely, and compartment or tank furnaces will be substituted in their place. The Siemens continuous melting furnace is a notable and worthy step in that direction.

Fire-bricks. — More care is devoted to the manufacture of fire-bricks for furnace use in Europe than in this country. The quartz is ground with more care and is freed from iron by sulphuric acid. Sand is sometimes used as a mixture with clay, but with this material it is difficult to make a cohesive brick, while quartz, when properly ground, not in too large grains, presents angular points which facilitate the cementing of the particles. I have often seen in this country bricks made with large — too large — and small pieces in the same brick; this variety of sizes cannot of course make a homogeneous brick, and manufacturers would do well to pay a little more attention to the regularity of the sizes of the ground quartz they use.

In the Welsh district of Great Britain, a very good quality of bricks is made with a mixture of ground quartz or silex, one per cent, of lime, and a sufficient quantity of water. The mixture is introduced into iron molds and is agglomerated by pressure. The lime is used as a flux and cements the quartz. These are excellent fire-bricks and stand a very high degree of heat. An increase of temperature expands instead of contracting them, as ordinary bricks.

Clay is to be found in many places in this country.

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It  has been discovered of good quality in several places in Virginia. At Washington, D. C., the writer has tried several qualities of clay found in the neighborhood; his trials prove that there is no necessity for manufacturers to continue to purchase clay from England and Germany, as the results are fully satisfactory. The discovery of clay near Washington has been the cause of developing the clay industry in the neighborhood, and a large drain-pipe factory is now in operation at Potomac City, on the Point of Rocks Railroad. Clay of a very good quality has also been found in Tennessee, on the Tennessee Coal and Railroad Company’s road, running from the Nashville and Chattanooga Railroad to the Suwanee coal mines.

Western glass manufacturers have long supplied themselves with clay obtained from Missouri, near De Soto, costing from $10 upwards per ton. Eastern manufacturers are using English and German clays, at prices varying from $23 to $29 per ton. Missouri clay, when first introduced, did not give much satisfaction until the owners of the mines understood its preparation; it is now equal to any imported clay and is extensively used. Several Western glass manufacturers are also using a clay procured from West Virginia and Ohio. In Clay and Warren Counties, in Indiana, clays, said to be of good quality, are found underlying the seams of coal in the mines. In Maryland, near Cumberland, in the coal fields, are found extensive deposits of fire-clay of very good quality. In this region the “Mount Savage” and the “Savage Mountain” fire-bricks are manufactured. There are also several other works making retorts and fire-bricks of well-known good quality. In the counties adjoining Cumberland, in Pennsylvania, are also found veins of very good-quality fire-clays, some of them measuring 53 feet in thickness. The surrounding country of Cumberland seems to be especially well supplied with glass-making materials.

Kentucky seems to be especially rich in fine qualities of fire-clays containing a large proportion of alumina, which has the effect of making articles manufactured with these clays very strong. In Carter County are found clays which on analysis are shown to contain —

Silica 45 to 64
Alumina 23 to 43
Oxide of iron A trace
Lime A trace
Magnesia A trace
Potash 0.212 to 2.093
Soda 0.283 to 0.728