The Penny Cyclopædia...: Glass.

The Penny Cyclopædia of The Society for the Diffusion of Useful Knowledge.
Volume XI.
Fuego, Tierra Del - Haddingtonshire.
London: Charles Knight and Co., 22, Ludgate Street.

GLASS, a transparent and impermeable substance, exceedingly brittle while cold, but which by the application of a high degree of heat is rendered so flexible and tenacious that it array with the utmost facility be moulded into any form. It is so ductile while heated, that it may be spun into filaments of the greatest conceivable fineness, and these when cold are pliant and elastic in a high degree. The time at which glass was invented is very uncertain. The popular opinion upon this subject refers the discovery to accident. It is said (Plin, Nat. Hist., lib. xxxvi., c. 26), that some mariners, who had a cargo of nitrum (salt, or, as some have supposed, soda) on board, having landed on the banks of the river Bolus, a small stream at the base of Mount Carmel in Palestine, and finding no stones to rest their pots on, placed under them some masses of nitrum, which, being fused by the heat with the sand of the river, produced a liquid and transparent stream : such was the origin of glass.' The antient Egyptians were certainly acquainted with the art of glass-making. This subject is very fully discussed in a memoir by M. Boudet, in the Description de l'Egypt,' vol. ix., Antiq. Memoires. The earthenware beads found in some mummies have an external coat of glass, coloured with a metallic oxide ; and among the ruins of Thebes pieces of blue glass have been discovered. Tne manufacture of glass was long carried on at Alexandria, from which city the Romans were supplied with that material ; but before the time of Pliny the manufacture had been introduced into Italy, Frame, and Spain (xxxvi., c. 26). Glass utensils have been found among the ruins of Herculaneum.

The application of glass to the glazing of windows is of comparatively modern introduction, at least in northern and western Europe. In 674 artists were brought to England from abroad to glaze the church windows at Weremouth in Durham ; and even in the year 1567 this mode of excluding cold from dwellings was confined to large establishments, and by no means universal even in them. An entry then made in the minutes of a survey of Ainwick Castle, the residence of the Duke of Northumberland, informs us that the glass casevents were taken down during the absence of the family, to preserve them from accident. A century after that time the use of window-glass was so email in Scotland that only the upper rooms in the royal palaces were furnished with it, the lower part having wooden shutters to admit or exclude the air.

The earliest manufacture of flint-glass in England was begun in 1557, and the progress made in perfecting it was so slow, that it was not until near the close of the seventeenth century that this country was independent of fo-reigners for the supply of the common article of drinking-glasses. In 1673 some plate-glass was made at Lambeth, in works supported by the Duke of Buckingham, but which were soon abandonod. It was exactly one century later that the first establishment of magnitude for the production of plate-glass was formed in this country, under the title of 'The Governor and Company of British Cast Plate-glass Manufacturers.' The members of this company subscribed an ample capital, and works upon a large scale were erected at Ravenhead, near Prescot in Lancashire, which have been in constant, and successful operation from that time to the present day.

At an early period of its history in this country the glass manufacture became an object of taxation, and duties were imposed by the 6 and 7 William and Mary, which acted so injuriously, that in the second year after the act was passed one half of the duties were taken off, and in the following year the whole was repealed. In 1746, when the manufacture had taken firmer root, an excise duty was again imposed, at the rate of one penny per pound on the materials used for making crown, plate, and flint-glass, and of one farthing per pound on those used for making bottles. In 1778 these rates were increased 60 per cent. upon crown and bottle-glass, and were doubled on flint and plate-glass. These rates were further advanced from time to time in common with the duties upon most other objects of taxation, and in 1806 stood as follows: - on plate and flint-glass, 49s. per cwt.; on crown and German sheet-glass, 36s. 9d. per cwt. ; on broad glass, 12s. 3d., and on common bottle-glass, 4s. 1d. per cwt. In 1813 those rates were doubled, and with the exception of a modification in 1819 in favour of plate-glass, then reduced to 31. per cwt., were continued at that high rate until 1825. In that year a change was made in the mode of taking the duty on flint-glass, by charging it on the weight of the fluxed materials instead of on the articles when made, a regulation which did not affect the rate of charge. In 1830 the rate on bottles was reduced  from 8s. 2d. to 7s. per cwt. The only further alteration hitherto made in these duties occurred, in 1833, when, in consequence of the recommendation contained in the thirteenth Report of the Commissioners of Excise Inquiry, the rate upon flint-glass was reduced two-thirds, leaving it at 2d. per pound, a measure which was rendered necessary by the encouragement given under the high duty to the illicit manufacture, which was carried on to such an extent as to oblige several regular manufacturers to relinquish the prosecution of their business. The number of establishments for the manufacture of glass in the United Kingdom, in 1833, was 126, of which 106 were in England, 10 in Scotland, and 10 in Ireland. The principal seat of the manufacture in England is at Newcastle-upon-Tyne and the neighbouring town of Shields ; next in importance stands Stourbridge ; then the works in and near Liverpool, including the Plate-glass Company's establishment at Raven-head ; next follow Bristol, Warrington, Birmingham, and Leeds ; in London there were only three glass-houses, yielding to the revenue about 2 per cent. of the whole amount of duty collected upon this material. In Scotland five out of the ten houses are in and near Glasgow, two are in Leith, the remaining three are at Cartadike,Portobello, and Alloa. In Ireland four manufactures are in Dublin, two each in Cork and Belfast, and one each in Waterford and Newry.

There are five distinct kinds of glass, which differ from each other in regard to some of the ingredients of which they are made, and in the processes of manufacture. These kinds are, glass, or crystal ; crown-glass, or German sheet-glass ; broad-glass, or common window-glass; bottle, or common green glass ; and plate-glass.

The principal ingredients used for the production of each of these kinds of glass are silex, or flint, and an alkali. The differences in the various kinds result from the description of alkali employed, and from the addition of certain accessary materials, usually metallic oxides. The form in which silex is now universally used in this country for glass-making is that of sea-sand, and care is required to select those kinds which are free from foreign matters and impurities. The port of Lynn in Norfolk, and Alum Bay in the Isle of Wight, have long furnished the greater part of the silex used in our glass-houses. Flint-glass derives its name from the practice in former times of using flints calcined and ground in the manner now employed for making porcelain, but this has long been discontinued. Of late there has been some apprehension of a scarcity of sand suitable to the manufacture, and a good idea may be formed as to the importance attached to the purity of this chief ingredient from the fact that sand has been imported for the purpose from New South Wales. The alkali employed fix making fine flint-glass is pearl-ash, purified by solution and subsidence, in which process impurities to the extent of one-third of the weight are removed. Barilla, kelp, and wood ashes, combined with many impurities, are used for making inferior kinds of glass : the impurities even assist towards fusing the silex. Coarse alkaline substances all contain iron in some degree, and it is to the presence of this metal that the green colour of common glass is owing.

Fliint Glass, known in other countries under the name of crystal, is the most generally useful, the most brilliant, and the heaviest description of glass. This last quality it owes to the large quantity of oxide of lead which it contains, and which is used sometimes in the form of minium, but more frequently in that of litharge. This metallic oxide acts as a flux, and promotes the fusion of the other materials at a comparatively low temperature. The greater density which it imparts to glass gives to it a grater power a. refracting the rays of light, and it is this quality which renders flint-glass of so much importance for optical purposes. Nitre in a small proportion is used for the destruction of any carbonaceous matter in the other ingredients The oxygen which it gives out in the furnace further serves to maintain at their highest degree of oxygenation the metallic oxides that are present. Black oxide of manganese in minute proportion is also used to remove any foul colour that might otherwise remain through the impurity of the alkali used its cleansing property occasioned this oxide to be known formerly under the name of glass-soap. Any undue proportion of manganese would impart a purple hue to the mass, and if any considerable quantity be used that colour will be deepened almost to black. When through inadvertence the glass has been made purple, the I colour will be almost instantly discharged by thrusting a  piece of wood into the melted mass. The cause of these changes is as follows the purple colour given by oxide of manganese arises from its being in a high state of oxygenation , the wood when thrust into the heated mass becomes speedily carbonized, and the carbon, combining with the superfluour oxygen, is driven off in the form of carbonic acid gas ; if by the addition of nitre the quantity of oxygen is again increased, it will combine with the manganese, and restore the purple colour. It will be seen from these circumstances how much skill and experience are necessary for the due mixture of ingredients so as to produce glass of the best quality. The manufacturers of flint-glass are generally unwilling to disclose the precise proportions in which they employ the requisite ingredients, and our knowledge on the subject must consequently be derived from scientific men who are not commercially engaged in the manufacture. Mr. Arthur Aikin, who has given much attention to the subject, recommends the following proportions :—
120 parts fine clean white sand,
40 " well-purified pearl ash,
35 " litharge, or minium,
13 " nitre ; and a small (undefined) quantity of the black oxide of manganese.

The French chemists recommend a much larger pro-portion of oxide of lead, but this is found to make the glass inconveniently soft. Where less metallic oxide is used, more nitre is required as a flux, and vice versa : the French chemists recommend only 2 to 3 parts of nitre, while Mr. Aikin recommends 13 parts.

The ingredients must all be intimately mixed together before they are put into the crucibles, or pots, which are previously placed in the furnace. As the bulk decreases by fusion, fresh portions of the ingredients are added until the pots are full of melted glass. A very strong and long continued heat is necessary, not only for the perfect fusion and amalgamation of the materials, but also for the discharge of the impurities which they contain. The chief of these, known under the name of sandivir, or glass-gall, consists of salts existing in the alkali which have but small affinity for silex, and from their specific levity rise in the form of a white porous scum to the top of the crucible, whence it must be removed before it is volatilized by the excessive heat of the furnace. This glass-gall is used as a powerful flux by refiners of metals. When the whole of the impurities have been thus thrown off by the action of heat and are removed, and the glass, or metal as it is called, appears colourless and translucent, the vitrification is known to be complete. The temperature of the furnace is then lowered by preventing the access of air until the glass loses apart of its fluidity, and assumes that pasty character which is the most convenient for the workmen, it being sufficiently consistent to be tenacious, but soft enough to yield to the slightest pressure without cracking or losing its tenuity. The material is usually brought to a perfect state of vitrification in about forty-eight hours from the first application of heat. There is perhaps no process of manufacture which excites so much the surprise and admiration of a stranger as that of fashioning flint-glass into all the various objects of convenience and ornament for which it is employed. To see a substance, proverbially brittle, blown with the human breath, pulled, twisted, cut, and then joined again with the greatest facility, never fails to strike with astonishment those who are unaccustomed to the sight. The tools with which all these operations are performed are of the most inartificial description, and do not appear to have received any improvement from the earliest records of the manufacture.

Glass of every kind would be oven much more brittle than it is, so brittle indeed as to crack and break at every comparatively small variation of temperature, if it were not subjected, immediately after it is fashioned, to the pro-cess of annealing. [ANNEALING.]

Crown Glass.

This is the best description of window-glass. It is made without any mixture of metallic oxide, and is both specifically lighter and much harder than flint-glass. Many receipts have been given for the production of this kind of glass. At the great works of St. Gobain, in France, the mixture of ingredients is said to be —
Fine white sand 100 parts,
Carbonate of lime " 12 "
Carbonate of soda, calcined " 48 "
 Clippings of crown-glass 100 "
with minute portions of manganese and cobalt to correct  impurities, and to remove the colour which those impurities would impart : they are not therefore at all times necessary. In England the ingredients are mostly sand, kelp, and slaked lime, in the ploportions of 200 pounds weight of the first, 330 pounds of the second, and 15 pounds weight of lime, to which is added about half the weight of the three materials in broken crown-glass, called by the makers cullet. The perfect fusion and refining of these materials are usually accomplished in about forty hours. Crown-glass of very superior quality is composed of
120 parts by weight of white sand,
60 " purified pearl-ash,
30 " saltpetre,
2 " borax,
1 " arsenic,
with the addition, if needed to correct the colour, of a mi nute quantity of manganese. Crown-glass is made by blowing, in the form of circular plates of 60 to 60 inches diameter. A quantity of glass in the pasty state is collected upon the end of a hollow iron tube, five feet long, similar to the tube used for blowing flint-glass. This lump of glass is then converted, by blowing through the tube, into a hollow globe of the requisite substance. This globe is flattened at the side opposite to the tube by pressing it upon a hard plane surface, and a solid rod of iron hating a small quantity of melted glass at the end is applied, and adheres to the centre of' the flattened side opposite to the tube, which is then removed by wetting the glass near to the point of union with the tube, leaving a small circular hole. To arrive at this stage the glass must have been several times re-heated, by placing it, when connected with the tube, within a small opening left for the purpose in the wall of the furnace. When transferred from the tube to the solid rod, called a punt, it must be again heated in the same manner, and is then twirled round by the workman somewhat in the manner that a mop is twirled to drive off the moisture ; with this twirling the softened material is continually driven off from the centre by the centrifugal force; the hole just mentioned expands, and at length forms an annulus of a few inches wide, when suddenly, and in a most unaccountable manner, it flies open, and the whole substance is converted into a flat disc of circular form, and, except at the centre, where it is attached to the rod, of a uniform thickness. These centre parts are used for the commonest purposes, such as glazing outhouses and the like.

Broad Glass is an inferior kind of window-glass, made with a cheaper kind of alkali. The usual materials are three measures of sand, the same quantity by measurement of kelp, and six measures of soap-boilers' waste. This mixture, when vitrified and brought to the proper consistency, is collected upon the hollow rod, or pontil, and blown to the requisite size, when it is cut open with a pair of shears, and spread into a flat plate.

Bottle Glass is still inferior in quality to broad-glass, the alkali employed being the cheapest that can be procured, with the addition of a portion of lime to assist fusion. Considerable manufactures of bottle-glass are carried on at Newcastle-upon-Tyne, encouraged by the low price of the title (small coal) which is used in the furnaces. The ingredients are usually nothing more than lime and sea-sand, the latter article having been frequently wetted with sea-water, and allowed to dry, in order that the salt may be allowed to deposit itself in the sand ; the soda contained in the salt is the only alkali, properly so called, that is used. Bottle-glass is fashioned by blowing, much in the same manner as flint-glass.

Plate Glass is both blown and cast. Plates which are blown are limited in dimensions, while those that are cast are made of very at size, the limit being caused by the expensiveness of the machinery required for the manage-ment of very large masses of the material. Plate-glass is necessarily costly, because of the numerous and laborious operations which it undergoes, and of the risks of fracture while subjected to them. The ingredients are chosen with the greatest care, and every possible amount of skill is brought to bear on the manufacture. The ingredients used are sand of the purest and whitest quality, and soda produced by the decomposition of common salt and lime : manganese and oxide of cobalt are added for the purpose of discharging colour. Soda is preferred to potash or pearl-ash because the glass that is made with it flows better when in fusion, a quality of much importance where large quantities employed for the production of the same piece. The lime acts as a flux, and is used in proportions varying from 1-24th to 1-16th of the whole materials employed. Besides these ingredients it is necessary to use a large proportion of broken plate-glass, or cullet. The following proportions are given by Parkes: -
Lynn sand, well washed and dried  720 parts
Alkaline salt, containing 40 per cent. of soda 450 "
Lime, slaked and sifted 80 "
Nitre 25 "
Broken plate-glass  425 "
1700 parts.

It requires 40 hours' exposure to the full heat of the furnace to reduce the materials to the proper state of fusion and vitrification. When this is accomplished, the glass is transferred from the melting-pot, by means of copper ladles, to a large vessel called a cuvette, previously heated to a very high degree ; when filled, it remains some hours in the furnace, to disperse the air that may have been introduced into the mass by the operation of ladling. When this effect has been produced, the cuvette is withdrawn from the furnace and taken to the casting-table, over the upper end of which it is raised and suspended by means of a crane. It is then thrown into an inclined position, and the contents are allowed to flow out upon the table, and are distributed by means of a roller over the whole surface of the table, bars of metal being placed at each side along its entire length, and across the bottom, in order to prevent the glass from running upon the floor. The casting of large plates of glass is one of the most beautiful processes in the arts : the large mass of melted glass, rendered in a high degree luminous by heat, which is poured forth, exhibiting changing colours in the sheet after the roller has been passed over it.

In the manufactory at Ravenhead, where the workmen are well trained and experienced, this operation is conducted with celerity and in silence, each of the twenty men engaged knowing well the part in the operation which he has to perform. Previous to the casting, the table is placed with one end against the mouth of an annealing oven, and as soon as the plate is set, it is carefully slipped from the surface of the table to the floor of the annealing oven, and when the oven has received as many plates as it will contain urn its floor, the door is closed and its crevices are stopped with mortar or clay, to insure the gradual cooling of the plates. They remain in the oven during a fortnight, after which the ovens are opened and their contents are with-drawn. The plates are then squared by means of a glazier's i diamond, then ground and polished, and when intended for mirrors they are silvered. In order to their being ground they are imbedded in plaster of Paris, and first powdered flint is rubbed steadily and evenly over the surface by machinery worked by steam power, both sides of the plate being grolind in succession. Emery powder is then substituted for ground flint, coarse at first, but finer afterwards as the rougher inequalities of the surfaces are removed : that part of the operation in which emery powder is used is called smoothing. The polishing is also performed by steam-machinery. The plates are firmly fixed upon large tables, and the polishing instruments, which are of wood covered with many folds of woollen cloth, having carded wool between each fold, are passed to and fro over the surface. The polishing substance used is colcothal, an oxide of iron which remains in the retorts after the distillation of acid from sulphate of iron : the two surfaces are polished in succession. or silvering glasses an amalgam of mercury and tin-foil is used, and this by means of considerable and long-continued pressure is made to adhere to one of the surfaces of the plate.

The processes here described are those used for the manufacture of cast plate-glass. Plates which are blown are made in the manner described for making broad-glass ; the after processes of squaring, grinding, smoothing, polishing, and silvering, are the same whether the plates are cast or blown.

Paste. - Artificial gems, familiarly known under the name of paste, are glass into the composition of which a large proportion of metallic oxide enters, such proportion being in almost all cases greater than that of the silex with which it is combined. The production of these mock jewels was formerly considered of much greater importance than at present, and a large part of every old treatise upon glass-making is made up of instructions for producing the best imitations of different precious stones. The processes recommended are in general tedious, and the directions given are very minute, several preliminary operations being de-scribed for purifying the ingredients used. The propriety of adopting different mixtures, independently of the colouring ingredients, which must of course be different for the imitation of different gems, is enforced by the fact that the different refractive and dispersive powers of those gems depend upon their specific gravity, and that in order to imitate each successfully the glass or paste employed should be of the same specific graft), as the stones to be imitated. The softness of all these compounds, when compared with that of the real gems, makes it impossible that any person resorting to such a test can be deceived with regard to their genuineness.

The foregoing description is confined to the explanation of those branches of the glass manufacture which, from their magnitude, are of the most importance. It would require a long treatise to explain minutely all the conditions necessary to be attended to in the processes, and to describe the variations which must be made in these conditions for producing the peculiar qualities of glass that are best adapted for other numerous purposes to which the material is applied.

The effect of high duties upon the consumption of articles of convenience is strikingly exemplified in the history of the duty upon glass in this country. In 1793, the year in which the war of the French revolution was begun, and when taxation was comparatively low, the quantity of all kinds of glass made and retained for use in the kingdom was 407,203 cwt., and the amount of revenue obtained from it 177,408l. The average rate of duty was therefore 8s.8½d. per cwt. upon the whole quantity. In 1834, the rate of duty was by progressive additions fourfold what it was in 1793, the average being 35s. 7½d. per cwt. upon the aggregate quan-tity used ; and although the population had in the meantime increased more than 60 per cent., the quantity of glass which was taken for use was only 374,351 cwt., or one-twelfth less than was so taken in 1793. If the quantity used in proportion to the population had continued the same, that quantity would in 1834 have amounted to 663,740 cwt., and a revenue equal to what was realized would have resulted from an average rate of 20s, instead of 35s. 7½d.

The precise rates of duty charged upon each kind of glass at the two periods were as follows :—
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 In 1835 the duty upon flint-glass was reduced from 6d. to 2d. per. lb., as already mentioned : the ultimate result to the revenue from this partial reduction cannot yet be fairly estimated ; but it may well be doubted whether it can ever be judicious to extract revenue from an article of domestic manufacture, the ingredients for which are so cheap and so abundant as those from which glass can be made, and where the processes of manufacture are so simple in themselves that any person of ordinary talents may produce it illicitly, as it is well known many do in this country, in an attic or cellar. The quantity of each description of glass brought to charge by the excise, in each of the three years from 1834 to 1836, was as follows:—
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The real value of glass-ware exported from the United Kingdom, in each year from 1827 to 1836, was:-
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The greater part of these exports waionade to India and America. In 1836 the value of the shipments to various quarters of the world was as follows: -
To the north of Europe £22,210
To the south of Europe 14,440
To Africa, including the Mauritius 18,412
To the East India Company's territories and Ceylon 129,756
To Arabia, China, and the Eastern Islands To Australian Settlements 7,239
To British N American Colonies 103,481
To British West Indies 69,550
To Foreign West Indies 10,833
To United States of America 98,045
To Mexico, Brazil, and other parts of S. America 34,325
To Guernsey, Jersey, Alderney, and Man 6,943

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