Читать книгу Glass and Glass Manufacture - Percival Marson - Страница 5

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The term “glass,” in a general sense, is applied to the hard, brittle, non-crystalline, transparent, opaque or translucent vitreous substance which results from fusing silica with active mineral solvents or fluxes, such as the alkalies, earthy bases, or metallic oxides. Silica exists in great abundance, in a free natural state, in the form of flints, quartz, and sand; and in the latter form it is now most generally used for glassmaking. When sand alkali and lead oxide are heated together to a high temperature, the sand is dissolved by the solvent action of the fused alkali and lead oxide until the whole becomes a molten mass of glass. The solvent action of the alkalies, soda potash or lead oxide, is very energetic whilst being heated, and the mass boils with evolution of gases until, at last, the solution, becoming complete, settles down to a clear quiescent molten liquid metal, which is quite soft and malleable, after the nature of treacle. In this condition it is ready for working. The time and temperature necessary for melting such mixtures vary according to the proportions and composition of the ingredients.

Silica, combined with alumina and other oxides, is freely distributed in nature in the form of clays, granites, and feldspars, which are also available for use in glassmaking. Originally glass was made by using crushed and ground flint stones as the source for the silica: hence is derived the old name of “flint” glass; but now the large extensive deposits of white sand present a much more convenient and less expensive source, and sand has become universally used. Fine white sand is obtained from Fontainebleau, near Paris; other sources are Lippe, Lynn, Aylesbury, Isle of Wight, Holland, and Belgium.^[2] These are the sources preferred by crystal glass manufacturers and makers of fine quality glass, such as chemical ware pressed glass, tube, cane, and medical bottles, on account of their greater purity. The commoner varieties of sand from Reigate and Bagshot and even red sand are being used in the manufacture of the lower grades of glass such as beer bottles and jam jars, where a greater latitude in the chemical impurities present is permissible. Only the best and purest silica sands are used for making cut crystal and optical glasses. In these trades the sand is always cleaned by washing it in water to clear it from any salt, chalk, or other impurities which may possibly be present. The sand, after washing, is heated to redness, or “burnt,” in order to burn off any organic or vegetable matter, and when cold it is sifted through a fine screen to take out any coarse grains or lumps. In this prepared state, the sand is ready for weighing out into the proportions desired for mixing with the other materials, and is stored for use in covered wooden compartments situated in or near the mixing rooms, along with the other materials which may be used in the glass mixtures.

2.See “British Glass Sands” (Boswell), “British Glassmaking Sands” (Peddle); papers read at the third meeting, Society of Glass Technology, Sheffield, for further information.

The alkalies, potash or soda, or a mixture of both, are commonly used in making glass in the form either of carbonates, sulphates, or nitrates. The soda and potash silicates form very fusible glasses, but they are not permanent, being soluble in water; therefore they cannot be used alone. In making glassware for domestic use, other bases, such as lead oxide, barium, or lime, have to be added to form more insoluble combinations with the silica or sand.

Carbonate of Potash or Pearlash, which before the war was imported into this country by glass makers from Stassfurt, is much prized by crystal glass makers on account of the colourless silicate it forms when fused with the best white sand. It is now very expensive and difficult to get, and is less used on this account. Potash carbonate is very hygroscopic and absorbs much moisture from the air; therefore it is necessary to keep it within sealed chests while in store.

Potash and soda each have an influence upon the colour of the resulting glasses in which they are respectively used. The potash silicate gives better and clearer glasses than the soda silicate.

Carbonate of Soda, or Soda Ash, is now more generally used. Being a less expensive form of alkali, it constitutes a base in most of the commoner varieties of glassware. Carbonate of soda is manufactured in England from common salt, of which there are large deposits in the Midlands. This common salt, or chloride of sodium, is treated chemically and converted into the carbonate, in which form it is supplied to the glass manufacturers as soda ash.

Sulphate of Soda (Salt Cake) is the form of alkali used in window and bottle glassmaking. In mixtures containing sulphate of soda it is necessary to use a small proportion of carbon in some form, such as charcoal or coal, in order to assist the decomposition of the salt and the formation of the sodium silicate. Sulphate of soda is used in this class of glassware on account of its cheapness. Glasses made from sulphate of soda mixtures are not so clear and colourless as those in which the source of alkali is potash or soda carbonate. On this account, the best crystal glasses cannot be made from sulphate of soda.

Potash Nitrate (Saltpetre) is used in glass mixtures to oxidise the molten metal and improve the colour of the glass. In fusing it disengages oxygen gas, which purifies the glass while melting, and assists the decolorizers in their action by keeping up an oxidising condition within the molten mass.

Sodium Nitrate, or Chili Nitre, is the corresponding soda salt to potash nitre. It is much cheaper, but less pure; it has a similar but not nearly so powerful an oxidising action in the glass as potash nitre. It is exported from Chili, where it exists naturally in a crude state as “Caliche,” from which the nitrate is refined by recrystallisation.

Boric Acid acts as an acid in glass, as does silicic acid. It renders glass more fusible and brilliant; it has a searching action upon the colourising properties of certain metallic oxides when they are dissolved in the glass. It is an expensive ingredient, but is considerably used in optical and special chemical glassware in replacing a portion of the silicates ordinarily used and forming borates. It cannot be used in large amounts, as an excess produces glass of a less stable nature.

Borax, or Borate of Soda, consists of boric acid combined with soda. It is a very useful glassmaking material and is an active fluxing agent. If used in excess in glass mixtures it causes considerable ebullition, or boiling of the metal. In moderate proportions it is used in the manufacture of enamels for glass, as it helps to dissolve the colorific oxides and diffuse the colouring throughout the enamel mass.

Tincal, and Borate of Lime, are other forms in which borates may be introduced into glass.

Carbonate of Lime, Limespar, Limestone, Paris White, or Whitening are all forms of Calcium Carbonate. It is an earthy base and is added to the simple alkaline silicates and borates to form insoluble combinations or double silicates of soda and lime. By the use of lime, glasses are rendered more permanent and unchangeable when in use. Lime forms a very powerful flux at high temperatures. The quantity used must be carefully regulated according to the proportion of other bases present; otherwise an inferior or less stable glass may be produced. In excess it causes glass to assume a devitrified state.

Dolomite is a Magnesium Limestone, and is a natural stone which is available for use in making glass in tank furnaces.

Fluorspar, or Fluoride of Lime, is used in giving opacity and translucency to glass. It can only be used in small amounts, as the presence of any large proportion attacks the clay of the pots, causing serious damage by the sharp cutting chemical action due to the evolution of fluorine gas.

Phosphate of Lime is another material which produces opacity and translucency, but does not seriously attack the pots. Bone ash is a form of phosphate of lime, and is procured by calcining bones until all organic matter is consumed.

Carbonate of Barium, or Witherite, is a very heavy, white powder, and is a form of earthy base available for use in glassmaking. It can be used to replace lime, with similar results. By replacing other elements in the glass which are of lower density, barium can be used to increase the density of glass. Like lime it is a very powerful flux in glass at high temperatures. It gives increased brilliancy and little coloration. For this reason it is very useful in the manufacture of pressed glassware, giving a glass which leaves the moulds with better gloss than is found to be the case with lime glasses.

Magnesia and Strontia are other bases which are less used in glassmaking.

Zinc Oxide is a base used in the manufacture of many optical glasses. With boric acid it gives silicates of a low coefficient of expansion and special optical values. Used with cryolite, it forms a very dense opal suitable for pressed ware. It is rather more expensive than the other bases used.

Cryolite is a natural opacifying ingredient used in making opal glasses. It consists of a combination of the fluorides of aluminium and sodium, and is one of the most active fluxes known to glass and enamel makers. Its cutting chemical attack on the pots is very intensive. It is imported from Greenland. An artificially manufactured form of cryolite is known, which is a little cheaper than the natural variety and gives similar results in opacifying glass.

Alumina. This is sometimes present to a small extent in glass makers’ sands. As such it is not a dangerous impurity. It exists in combination with silica and potash to a large extent in feldspars, china clays, and granites. Alumina, when used, has a decided influence upon the viscosity and permanency of glass. In large proportions it noticeably diminishes the fusibility of glass, and makes it more or less translucent. Owing to the refractory nature of alumina it is with difficulty that it can be diffused in alkaline silicates, borates, or lead silicates; consequently any considerable proportion present in glass may cause cords or striae, which are objectionable defects in the glass.

Oxide of Lead. Red Lead, or Minium, is much used in the manufacture of enamels, table glassware, and heavy optical glass. It gives great brilliancy and density to all glasses in which it is used, but if used in excess the glass is attacked readily by mineral acids and becomes unstable. Red lead is a powerful flux, even at low temperatures, and forms the chief base in making best crystal ware and enamels. The red oxide of lead used by glass manufacturers is a mixture of the monoxide and peroxide. Glass manufacturers, in buying red lead, should realise that it is the peroxide present which is the active oxidising agent, and that at least 27 per cent. should be present. A dull, dark red oxide shows a low percentage of peroxide; a bright orange red a high percentage. Impure red oxides of lead may be adulterated with barytes, finely divided metallic lead, or added water. Such impure varieties should be avoided. The red oxide of lead is preferred to the other oxides and forms of lead for glassmaking, on account of its greater oxidising action, which is desirable in producing crystal glassware.

Tin Oxide and Antimony Oxide are used as opacifiers. When used they generally remain suspended in a finely divided form in the glass. Used in small quantities they have a favourable influence in the development of ruby-coloured glasses.

Manganese, Arsenic, and Nickel Oxides are used in glassmaking as “decolorizers,” which will be treated in a later chapter.

Cullet. In all glasses a proportion of “cullet,” or broken glass scrap, is used. This cullet is usually of the same composition as the glass mixture or “batch.” The use of cullet facilitates the melting, and assists in giving homogeneity to the resultant glass by breaking up the cords and striae which tend to develop in most glasses.

In the commoner varieties of bottle glass Basalt and other igneous rocks are crushed and used. These are naturally occurring silicates containing lime, alumina, alkalies, iron, and other elements in varying proportions. They are used more on account of their cheapness, and produce dark, dirty-coloured glasses, which in the case of common bottles are not objected to. In some instances iron, manganese or carbon is added to produce black bottle glass.

Of the various silicates used in glassmaking, the silicate of alumina is the most refractory. The silicates of lime and barium are rather refractory, but under a strong heat and in the presence of other silicates they can be readily formed. The silicates of the alkalies, lead, and many of the other metals are formed at much lower temperatures. In the case of the silicate of iron, manganese, or copper, a strong affinity is shown between the metal and the silica, and a black or dark-coloured slag with a very low melting point is formed. Such slags are very active in corroding the masonry and pots of the furnace.

No single silicate is entirely free from colour. Each gives a slight distinctive coloration, the lead silicate being yellowish and the soda silicate greenish, but by the judicious mixture of different silicates and the use of decolorizers, such as manganese, nickel, etc., compound silicates are obtained, giving less perceptible colours or crystal effects. In optical glassmaking the use of the ordinary decolorizers is not permissible, and the purity of the materials used becomes the most important factor.

The raw mixture of the various materials used in making glass is termed a “batch.” The mixing is usually done by hand, but in many cases mechanical batch mixers are used. If the mixing is done by hand, the materials are first weighed out in their correct proportions by means of a platform weighing-machine. As they are weighed out, one by one, they are introduced into a rectangular wooden arbour or box, large enough to hold the whole unit weight of the batch and allow of its being mixed and turned from side to side. The batch is then sieved, and all the coarse materials reduced or crushed to a size not coarser than granulated sugar. By sieving and turning the batch several times a thorough mixture of the ingredients is obtained. A few ounces of manganese dioxide are then added, according to the unit weight of the batch weighed out, and the proportion of decolorizer necessary; which varies according to the heat of the furnace and the amount of the impurities present.

The whole batch is then put into barrels and conveyed to the glass house, where the furnace is situated. Here it is tipped into another arbour or box in a convenient position near to the melting pot, and, a proportional quantity of “cullet” being added, the mixture is then ready for filling into the pots. The stopper of the pot mouth is taken away and placed aside, and a man shovels the mixture or batch into the hot pot until it is full. He then replaces the stopper, and, after a few hours, when the first filling has melted and subsided, another filling of batch into the pot takes place until it becomes full of glass metal in its molten state. The batch melts with considerable ebullition, owing to the chemical reactions taking place under the heat of the furnace, giving off at the same time large quantities of gas. By the evolution of these gases the batch shrinks in volume so that it becomes necessary to fill a pot more than once with the batch before it becomes full of molten metal. The capacity of the pots varies between 250 and 1,200 kilogrammes, according to the type of glass and nature of the goods made.

Much care is required in mixing and sieving batches containing lead and other poisonous ingredients, to prevent the inhalation of the dust by the mixer. Therefore, where such materials are used, exhaust fans and ventilating ducts should be provided and fitted in the mixing rooms. A proper respirator should be worn by the mixer in charge to prevent any absorption into his system of the poisonous dust. Cases of poisoning are not unknown, but these are due to gross carelessness. A small regular weekly dose of Epsom salts should be taken by the mixers who have to prepare lead batches. This salt tends to remove any lead salts absorbed in the system by converting them into insoluble lead sulphate.

CHEMICAL FORMULAE AND MOLECULAR WEIGHTS.

Materials.	Formulae.	Molecular Weight.
Alumina	Al2O3	102
Antimony Oxide	Sb2O3	287
Arsenic	As2O3	197
Bismuth Oxide	Bi2O3	468
Boric Acid	H3BO3	62
Borax	Na2B4O710H2O	382
Calcined Borax	Na2B4O7	202
Calcined Potash	K2CO3	138
Carbon	C	12
Carbonate of Barium	BaCO3	197
Carbonate of Magnesia	MgCO3	84
China Clay	2SiO2Al2O32H2O	258
Chrome Oxide	Cr2O3	153
Cobalt Oxide	Co2O3	105
Copper Oxide (Red)	Cu2O	143
Copper Oxide (Black)	CuO	79
Cryolite	6NaFAl2F6	210
Dolomite	CaOMgO2CO2	184
Fluorspar	CaF2	78
Gold Chloride	AuCl32H2O	339
Iron Oxide	Fe2O3	160
Lime	CaO	56
Lime Spar	CaCO3	100
Manganese Oxide	MnO2	87
Nickel Oxide	NiO2	75
Nitrate of Soda	NaNO3	85
Phosphate of Lime	Ca3(PO4)2	310
Potash Carbonate	K2CO3(2H2O)	174
Potash Felspar	6SiO2Al2O3K2O	556
Red Lead	Pb3O4	683
Saltpetre	KNO3	101
Sand	SiO2	60
Soda Carbonate	Na2CO3	106
Sodium Fluoride	NaF3	61
Sulphate of Soda	Na2SO4	142
Tin Oxide	SnO2	150
Uranium Oxide	UO2	272
Zinc Oxide	ZnO	81