VITRIFICATION, the production of glass, one of the most curious and most important operations in chemistry. See GLASS, (Encycl.)
The hardness, beauty, and perfect transparency, of glass render it very extensively useful in ordinary life, in natural philosophy, in chemistry, and in many arts. Almost any form may be given to glass, and all kinds of vessels and utensils may be formed of it, which are capable of resisting the most powerful corrosives and solvents. Glass is capable of receiving and preserving the finest polish. Its properties of transmitting and refracting the rays of light, and of reflecting these rays when one of its surfaces is covered with a metallic coat, render it very useful for constructing mirrors, telescopes,
microscopes, and all kinds of optical glasses; for collecting, separating, and decomposing the rays of light; and for exhibiting many wonderful catoptrical and dioptrical appearances and experiments. If to these admirable properties we add that of imitating the many brilliant precious stones, opaque or transparent, we shall easily be convinced that perfect glass is one of the most beautiful and excellent productions of human industry.
Perfect glass, or the most beautiful facitious crystal, is perfectly similar in appearance to the whitest and most transparent natural vitrifiable stones. The only sensible difference between these natural and artificial productions is, that the latter are much less hard and more fusible than natural crystal or any other vitrifiable stone. In fact, pure glass or artificial crystal is essentially of the same nature as vitrifiable stones; or we may rather say, that it is vitrifiable earth itself rendered more fusible only by certain additions, as we shall soon show.
The most simple and elementary of all earths, called by chemists vitrifiable earth, forms the hardest and most transparent of all natural bodies when its integrant parts are united together in a due state of aggregation: but the parts of vitrifiable earth cannot form this aggregation, unless they have been previously so much separated from each other that they possess a mobility, which enables them to unite together according to their natural tendency: the integrant parts of elementary earth may acquire this mobility by two methods; namely, by the interposition of the parts of water or of those of fire.
Almost all the vitrifiable stones that we know, as diamonds, rock-crystal, and other hard transparent stones, appear to have been once dispersed in infinitely small molecules through much water, from which they have afterwards been deposited, and then have united together and formed the very hard transparent masses which we now see. Possibly some of these stones may have been formed by fire, and by a true fusion. Of these two ways, that of the division and elaboration by water cannot be practised by human art, on account of the many ages required for the operation.
By the second method, namely, fusion, the time required for which is better adapted to the shortness of our lives, we certainly might, with a sufficient degree of heat, melt vitrifiable earth, and all other kinds of earths; since these, when sufficiently purified, are reduced to the nature of vitrifiable earth; and we might convert these earths into masses of crystal, or of hard transparent stones, as perfect as the natural precious stones. In a word, nothing but a heat sufficiently intense is required to melt any earthy matter, and to convert it into a diamond as brilliant and as hard as the most perfect natural diamonds: but here a great, and hitherto unsurmountable, difficulty occurs, namely, to produce a heat sufficiently intense to melt the most pure and simple vitrifiable earth; for the most violent heat that art has ever produced is insufficient to melt even many of the common impure earths and stones; and is therefore much less capable of melting that earth, which, because it is the simplest and purest, is also the most unfusible of all natural bodies.
Hence we conclude, that although we know by what means the parts of the purest earthy principle may be so united as to form bodies similar to the natural precious stones, yet we cannot possibly put these
means in execution: but if we cannot artificially produce solid masses of pure earth exactly similar in all their properties to the natural precious stones, we can however very well imitate these beautiful natural productions, not in their hardness and unfusibility, but in their transparency and lustre; that is, in their most obvious and striking qualities, by means of compositions of glass or factitious crystal. The solution of this problem is founded on the following principles.
First, pure and elementary earth, which makes the principal basis of all vitrifications, and therefore called vitrifiable earth, is indeed unfusible with regard to us: but we know, that certain very fusible substances are capable of uniting intimately with this earth, and of communicating to it some of their fusibility; so that by their means we may reduce it to a perfect fusion.
Secondly, the substances capable of acting upon the earthy principle are the inflammable principle or phlogiston, and several saline matters: but we may remark, that as these substances do only act as fluxes by uniting intimately with the vitrifiable earth, and while they themselves make part of the glass; and also, as the integrant parts of these fluxes are not capable of forming either with each other, or with the parts of vitrifiable earth, so strong an adhesion as that which can be formed between the parts of the pure vitrifiable earth; it follows, that all factitious crystal must be much less hard than natural crystals or other hard stones, since these contain only pure vitrifiable earth.
Thirdly, in consequence of the principle, that all compound bodies partake of the properties of their component parts, factitious crystals ought to partake so much more of the properties of inflammable and saline matters, and ought to be removed so much more from those of pure vitrifiable stones, as they contain a larger quantity of those matters. Accordingly we find, that the smaller quantity of flux that is contained in glass, the greater hardness, lustre, and resemblance to natural stones it has, provided the fusion has been complete. Thus glass is so much more perfect, as it possesses more of the properties of vitrifiable earth.
These principles, which are incontestable, being once understood, are easily applicable to the art of making glass; since, if we could produce a fire sufficiently intense, and had furnaces and vessels capable of sustaining it, we should make glass equal to the precious stones: it is therefore evident, that the most important object in vitrification, is to have furnaces capable of producing the greatest heat, and pots or crucibles capable of resisting, during a long time, this heat, and also the action of the vitreous matter that they contain. These pots, as has been elsewhere observed, ought to be made entirely of the most refractory and purest clay, well washed and cleansed from all sandy, ferruginous, and pyritous matters, and mixed with a certain proportion of the same clay baked, and pounded not very finely. The quantity of baked clay that ought to be mixed with the crude clay, to prevent the pots from cracking, when dried or when exposed to violent heat, is not absolutely determined, but varies in different glass-houses, according as the crude clay employed be more or less fat, as it is called.
The ovens and furnaces are different, according to the quantity of materials to be vitrified. See the article GLASS, (Encycl.) In large works, wood or fossil coal are used; and the furnaces are so constructed that the flame of the fuel circulates and burns within very
intensely. As these furnaces vary much in different countries and glass-houses, and as a description would not be very intelligible without figures, we shall refer the reader to other works in which these several furnaces are described and represented; particularly to the French edition of Neri's Art of making Glass, with Notes by Merret and by Kunkel; and to an excellent Memoir by Dr D'Antic of Paris. We shall here say only in general, that these furnaces are placed under large and lofty buildings called halls; that they are covered with a vaulted roof; and that they have no chimney, but only lateral openings, through which the melted glass is drawn from the pots. Under these openings is built a kind of platform or banquette, on which the pots containing the glass stand, one of which is opposite to each opening. In the middle of the furnace, below the banquette, is a space, across which iron bars are laid. In this space, which communicates with a large cavity below called the ash-hole, the fuel is placed. In these furnaces, which have not any chimney, as we have said, the fire is nevertheless very intense. This intense heat is occasioned by the great capacity of the furnace, by means of which a strong draught of air passes continually from the ash-hole: besides, the flame being vividly agitated by air, and not being drawn out of the furnace by any chimney, circulates in every direction within the furnace, and produces a very violent heat. Most glass-house furnaces have also hollow towers that communicate with the cavity of the furnace, and which therefore increase the capacity of the whole. In these the heat is not nearly so intense as in the furnace. They are employed to contain the frit, or the pieces of manufactured glass that are to be annealed. Such is the general disposition of the furnaces of glass-houses.
Next to the good condition of the pots and furnaces, the most important and essential matter is the consideration of the kinds and proportion of the fluxes. Of these we may observe two kinds, namely, the inflammable and the saline. As we cannot obtain the pure inflammable principle, we must, when we intend that it should enter into the composition of glass, choose for this purpose some of the earthy substances with which it is found naturally combined; and these are particularly metallic earths, which are best adapted for conveying phlogiston into vitreous compositions: but all these earths are not equally fit for this purpose. All metals are composed of an earthy matter intimately combined with a sufficient quantity of the inflammable principle to be very fusible, and at the same time very opaque. Some of these metals, namely, those called perfect, are unalterable by fire, and cannot be entirely, or even partly, deprived of phlogiston: but as every metallic substance is incapable of uniting with earthy matters while it retains its metallic state, therefore no perfect metal can be vitrified. The case is different with those metallic substances, the phlogiston of which may be burnt, or otherwise destroyed. The earths or calces of most, perhaps of all combustible metals, when not perfectly deprived of all their phlogiston, are capable of being reduced to a vitreous state, by means of their remaining inflammable principle. As they are not then in a metallic state, they can facilitate the fusion of vitrifiable earths, may contract with these an intimate union, and reduce them to a state of perfect vitrification: but these phlogisticated calces of imperfect metals promote vitrification more or less, according
Vitrification cording to their particular natures. Some of these, as, for instance, tin, cannot be without great difficulty brought to that precise degree of calcination that is necessary to vitrification; because the heat required for this purpose deprives them of all their phlogiston, and renders them in a high degree refractory. Others, in whatever manner they be calcined, either retain too little phlogiston to be sufficiently fusible, although they still retain enough to give them colour; or if they be not calcined so much as to lose their fusibility, they cannot be melted but into opaque masses, so nearly in a metallic state that they cannot be united intimately with vitrifiable earths. Such, especially, are the earths of iron and of copper.
Of all metallic earths, that of lead is fittest for vitrification. This metal, which contains a large portion of phlogiston, is quickly deprived of so much of it, that it loses its metallic state, and is easily melted into a transparent vitreous mass; but it has, at the same time, this remarkable property, that when once it has lost as much phlogiston as is necessary to dispose it to vitrification, its calx strongly retains as much of it as is necessary to give to it its greatest degree of vitreoscibility; and that it may be sooner dissipated into vapours by the continued action of a very intense fire, than it can be changed into an earth totally dephlogisticated, and consequently refractory, like the calces of tin and of regulus of antimony. Besides, the fusible earth of lead is one of these that retain the least colour. All these qualities render it preferable to any other metallic earth for the purposes of vitrification. The earth of bismuth, which in the above-mentioned properties resembles the earth of lead, may probably be employed with equal success: but as the calces of lead are much more common than those of bismuth, the use of this semi-metal is not known to manufacturers of glass.
In whatever manner the calx of lead has been prepared, provided it be really in a calcined and not in a metallic state, it may be used in vitrification. Accordingly, the grey calx, or ashes of lead, massicot, red-lead, litharge, cerus, and all the precipitates of lead, separated from acids by unmetallic intermediate substances, being mixed with sand or any other vitrifiable stone, and exposed to a sufficient degree of fire, do always promote the fusion of these matters, and form with them glasses more or less hard and transparent, according to the strength of the fire and the proportion of the ingredients.
The earth of lead constantly retains, as we have said, enough of the inflammable principle to preserve its fusibility. When it is exposed singly to the fire, it vitrifies with a very moderate heat. It has more phlogiston than is requisite for its vitrification. The fusibility, fluidity, and activity of this glass of lead, when pure, are so great, that it cannot be contained in any crucible, all which it easily pervades; therefore pure glass of lead is never made. But as the calx of lead has more phlogiston than is required for its own vitrification, it may divide this excess of inflammable principle with any unmetallic earths with which it is mixed, and thus may produce their fusion and perfect vitrification. The glasses formed by a mixture of calx of lead with unmetallic earths, have more consistence, hardness, and less fusibility, than pure glass of lead. The proportions of calx of lead and of sand employed in these kinds of glass are from one part to two
of calx of lead, to one of sand or of ground flints.
We may observe upon the subject of glasses that contain no other flux than phlogiston or metallic earths, of lead, or of any other metal, that none of them are perfectly white, but are all more or less coloured; because phlogiston is, as chemists know, the principle of colours. Secondly, these glasses have a greater density or specific gravity than any natural crystalline stone, all metallic earths being heavier than any that are not metallic.
Thirdly, metallic glasses are generally somewhat less brittle, are less liable to be broken by the alterations of heat and cold, and have more of a certain smoothness, or, as it were, unctuousity, not easily to be described, than glasses made altogether of unmetallic earths. These properties can only be attributed to the inflammable principle, a pretty considerable quantity of which is united with them. As these latter qualities of glass are valuable, a certain quantity of calx of lead generally enters into the composition of most fine glasses, which are distinguished from ordinary glass by the name crystal-glass.
From what we have said concerning the properties of metallic earths in vitrification, we may perceive, that the more calx of lead, or other metallic earth, enters into the composition of any glass, the more fusible, soft, coloured, and dense this glass is; and reciprocally. The colours given to glass by calces of lead are shades of yellow.
Saline substances are the second kind of fluxes used in vitrification: but all these substances are not equally fit for this purpose; not that they are not all very fusible, but for several reasons hereafter to be mentioned.
First, neither the pure and disengaged acids, nor volatile alkalis, nor ammoniacal salts, can be employed as fluxes in vitrification; because none of these saline matters is sufficiently fixed. Their volatility is so great, that they may be totally dissipated by fire before they could act in any degree upon vitrifiable earth.
Secondly, none of the neutral salts with basis of fixed alkali, containing either vitriolic acid, or marine acid, can be employed as fluxes in vitrification. This proceeds, not from their want of fusibility, or of the necessary degree of fixity, but from the union of the acid and the alkali; which is so strong that they cannot act with sufficient force upon other substances, and particularly upon vitrifiable earth. The saline matters fit for vitrification are, fixed alkalis, vegetable and mineral; nitrates with basis of fixed alkali; sedative salt, and borax; fusible salt of urine, or rather phosphoric acid.
Of all saline matters, fixed alkalis, vegetable and mineral, are most frequently used in vitrification. These alkalis are fusible with a moderate heat; they are so fixed that they can resist during a sufficient time the heat of ordinary vitrifications; and they act powerfully upon flints, sands, and other vitrifiable stones. The proportion of alkali to sand, in order to make good glass, is, from one to two parts of the former ingredient, and two parts of the latter.
Nitre produces in vitrification nearly the same effects as fixed alkalis, although it be a neutral salt; the alkali and acid of which are united together nearly as they are in common salt, which however does not produce in vitrification similar effects. The remarkable difference in this respect betwixt these two salts, and betwixt
Vitrification betwixt the nitre and the vitriolic salts, must be attributed to the great affinity of nitrous acid to the inflammable principle; which affinity is so strong, that when nitre is exposed to fire in vessels not perfectly close, the acid quits its alkaline basis to unite with the phlogiston of combustible matters, even though these be not sensibly in contact with it. Hence nitre exposed during a certain time to the action of a strong fire, is gradually alkalisified, and then becomes capable of dissolving very effectually vitrifiable earth. This alkalisification is produced so much more easily in most vitrifications, that the ingredients employed generally contain some inflammable matter. Common salt cannot be alkalisified in this manner, because it cannot be decomposed by the contact of inflammable bodies, and therefore is not used in vitrifications: but as vitriolic acid has a strong affinity with phlogiston, we might be inclined to think, that vitriolic salts with basis of fixed alkali, which also are never used in vitrification, might perhaps be employed along with sand or other vitrifiable matters containing a larger proportion of phlogiston than the ingredients now commonly used do.
Fixed alkalis or nitre cannot be formed into transparent glass by being melted singly; because these salts contain too little of the earthy principle; for they form true glasses when they are mixed with a sufficient quantity of this principle, as with sands and other earthy matters: but borax, sedative salt, and the fusible salt of urine, may be melted without any earthy addition into transparent vitreous masses: and hence we may conclude, that these salts contain a larger portion of the earthy principle than nitre or fixed alkali. They nevertheless very powerfully promote the vitrification of other substances. These salts are not employed in the manufacture of large quantities of glass, because they are too dear. Borax is sometimes used for making small quantities of some particular kinds of fine glass.
Arsenic may be enumerated among vitrifying fluxes, as it is fusible and vitrescible singly, and is also capable of promoting the fusion of vitrifiable earths; for which purpose it is frequently employed as an ingredient in vitreous compositions. As arsenic partakes both of the metallic and saline properties, it probably acts in vitrification both as a salt and as a metallic earth. The quantity therefore of arsenic necessary to promote vitrification, is intermediate betwixt the quantities of calx of lead and of saline substances that are necessary for that purpose. But we must observe, that as arsenic is very volatile, a great part of it escapes in vapours upon the first application of heat, and that the quantity remaining is always uncertain. Arsenic cannot well be employed as the sole flux for vitrifiable earth. When we intend that a certain quantity of this matter should remain in the composition of a glass, one of the best methods that can be used for this purpose, is at the same time to add nitre to the ingredients of this glass; because the arsenic uniting with the alkaline basis of the nitre forms the neutral arsenical salt in which the arsenic is considerably fixed. But as this neutral arsenical salt is easily decomposed by contact of phlogiston, therefore no calx of lead or other substance containing the inflammable principle ought to be mixed with arsenic in the composition of glass. It has been found by experiment, that this neutral arsenical salt is very difficultly manageable in vitrifications, not only for the reason now given, but also from the property it has of powerfully corroding and pervading crucibles and pots.
In many mixtures, it quitted the vitrifiable earth with which it was mixed, and acted upon the crucible, which it penetrated and dissolved. These experiments seem to show, that arsenic has a greater affinity with clays than with vitrifiable earths.
From Mr Pott's experiments, chemists know, that vitrifiable, calcareous, and argillaceous earths, each of which are singly infusible, do reciprocally promote the fusion of each other, when mixed together nearly in equal proportions, and exposed to a very violent heat; and that from these mixtures matters perfectly vitrified are formed. But as we do not know the cause of this singular fusibility, we cannot determine whether it is produced by a phlogistic or by a saline substance, or perhaps by both.
Glasses that contain no other fluxing ingredient than phlogistic matters or metallic earths, partake of the properties of these metallic earths; and also glasses that contain only saline fluxes, partake of the properties of salts. The latter, or saline glasses, when pure and well proportioned, are less heavy, less dense, harder, whiter, more brilliant, more brittle, than the glasses containing calx of lead: and glasses containing both saline and metallic fluxes do also partake of the properties of both these substances. In general, glasses too saline are soft, and easily susceptible of alteration by the action of air and water; especially those in which alkalis prevail: which latter glasses are also liable to be attacked by acids, as we evidently see from the properties of the vitrified matter that is made with an excess of alkali, for the preparation of liquor of flints. Glasses containing too much borax and arsenic, although at first very beautiful, do quickly tarnish and become opaque when exposed to air.
From what we have said concerning the properties of fluxes, phlogistic or saline, we may know how to adjust the proportions of these to the sand or powdered flints for the various kinds of glass. Thus, if we require a glass that is dense, fusible, and not saline, one part and a half of red-lead or litharge may be mixed with one part of sand, and fused together. If equal parts of sand and of calx of lead be employed, a glass somewhat less dense and harder will be produced.
If a glass be required of very little density, only saline fluxes must be employed. A glass of this kind may be composed of six parts of salt of tartar, or of potash, or of purified soda, mixed with eight parts of sand or of flints; or of four parts of any of the above-mentioned alkalis, mixed with two parts of nitre or of borax, and eight parts of vitrifiable earth. These glasses must be left long in the fire, for the reasons hereafter to be mentioned.
When a crystal-glass is required which shall be of an intermediate quality betwixt the metallic and saline glasses, it may be made from a mixture of one part of the above-mentioned salts, one part of calx of lead, and two parts of sand or other vitrifiable earth. By varying the proportions of these ingredients, many different kinds of glasses may be produced, each of which may be good, if the quantity of each of the fluxes employed be proportionable to its vitrifying power. Several good receipts for glass and facitious crystal may be found in Neri's Art of Making Glass, with Notes by Merret and Kunckel; to which work we refer for many interesting particulars. See also the article Glass, (Encycl.) We shall however observe, that the proportions of the fluxes necessary to produce any required kind
Vitrification kind of glass cannot be precisely ascertained, for the following reasons.
First, the sands, flints, and other stones commonly employed for making of glass, are not all equally fusible. Thus the quartzose sand obtained by washing an earth found near Nevers, known to manufacturers of glass and of pottery by the name of sand of Nevers, may be almost entirely melted when exposed to a good vitrifying heat; and by a moderate heat its grains may be considerably rounded. There are some other hard stones, which in a violent fire are still more fusible, and convertible into an almost transparent glass without addition. The fusibility of these vitrescible stones is caused by some unknown heterogeneous matter united with them. These fusible sands and stones require a much less quantity of flux to promote their vitrification than other sands or vitrifiable stones which are much purer, and therefore more refractory.
Secondly, although the phlogistic and saline matters employed as fluxes in vitrification are sufficiently fixed to support the degree of fire necessary for the fusion of glass, they are nevertheless far from being so fixed as vitrifiable earth. The fire necessary for the perfect fusion of glass, is even sufficient to evaporate them entirely. Accordingly, in glass-houses where the pots are uncovered, a vapour or smoke continually rises from their surface, which is nothing else than the saline and phlogistic fluxes in a state of continual exhalation. Hence the longer glass remains melted in the fire, the harder and more difficultly fusible it becomes, and the more it partakes of the properties of pure vitrifiable earth. Accordingly, even when a very hard glass is required, such a quantity of flux ought to be added as shall at first promote a perfect fusion; and this fusion ought to be continued a long time till a considerable quantity of the flux is gradually dissipated, and till the glass has acquired the requisite degree of hardness, provided that the fire be sufficiently strong to maintain the fusion notwithstanding the loss of flux. From these observations it appears, that we cannot precisely ascertain the proportion of flux to vitrifiable earth, unless we knew the fusibility of the sand to be employed, and the degree of heat which can be excited in the furnace.
The saline fluxes, and especially the fixed alkalis employed in vitrification, are generally rendered impure by a mixture of several heterogeneous matters, and especially by neutral salts not vitrifiable, and by a certain quantity of inflammable principle. In manufactories of bottles and other common glass, the alkalis employed are not previously purified, but are even mixed with the earth of the ashes of the burnt plants, which earth is also much disposed to vitrification. Accordingly, to make glass of this kind, sand is mixed with common wood-ashes, sometimes even with those which have been lixiviated, together with some unpurified potash, soda, or kelp; and from this mixture a brown, dusky, not very transparent glass is produced, which is preferred in common sale for wine bottles to clearer and more transparent glass. But when a fine, white, and very transparent glass is required, the alkali must be perfectly purified from all heterogeneous matter, by lixiviation and calcination.
As a too large quantity of the inflammable principle is the chief cause of the colours and of the opacity of glass, when a perfectly colourless and transparent glass is required, not only the alkalis must be deprived of all their superabundant phlogiston, but also the sands
or flints employed must be purified from any of this principle which they may contain. The method used for this purpose is by mixing together the due proportions of sands and salts, by exposing this mixture during a considerable time to a red heat, not intense enough to melt it. By this calcination, the phlogiston of these matters is burnt and dissipated, all colour is destroyed, and the glass produced is also more clear and brilliant. This first mixture of materials of glass, when calcined, is called the fritt; and this fritt is used in large manufactories not only for the finer glass, but also for the common brown glass; not with an intention to render this latter kind of glass colourless, but because, during this calcination, the salts and earth begin to act upon each other, and to incorporate in a certain degree; by which a great part of the effervescence and swelling occasioned by the reaction of these matters, which happen when they are at once exposed to a melting heat, are avoided. Accordingly, when matters not previously fritted are employed in small experiments, the heat must be applied gradually; otherwise they so swell, that frequently the greatest part of the mixture runs over the crucible.
The due degree of heat is an essential point in making of glass; it ought not only to be very strong, but also maintained during a long time. In great manufactories, the glass is kept fused during ten or twelve hours before it is taken out of the pots. Accordingly their glass is always more perfect than that which is hastily made in small quantity in two or three hours. Good glass, although kept in fusion in a very great heat, is not perfectly liquid. It is always somewhat thick, and when taken from the crucible, it may be drawn out into fine wire or threads; which shows that it has a certain consistency and a very sensible tenacity when it is red-hot. It is not transparent while it remains red-hot, not even when it has become perfectly hard. Another remarkable circumstance is, that glass which is so brittle when it is cold and transparent, is very ductile when it is so heated as to be opaque. We might be induced to believe, that the disengaged fire with which the glass is filled when it is red-hot, produces upon it the same effect that phlogiston or combined fire produces upon metals. The ductility of red-hot glass is very useful; for by its means all imaginable shapes may be given to glass, and numberless vessels and utensils may be easily formed of it.
As soon as glass-vessels have received their intended form, they must be cooled very gradually, otherwise they would have no solidity, and would be of little use, as they would be liable to be broken by the smallest stroke, or by a slight change of heat and cold. This inconvenience is prevented in glass-houses by carrying the glass vessels as soon as they are formed, and while yet red-hot, into an oven too little heated to destroy their form, but in which they may be very gradually cooled. This is called annealing the glass.
Notwithstanding all the care taken in the manufactories of the finer kinds of glass, as crystal-glass and plate-glass, to make them perfectly good, they are nevertheless seldom found quite free from faults. The principal faults in glass are, colours, bubbles, and veins. The colours which generally hurt glass, especially that kind which contains saline fluxes, are shades of green, olive, and blue. These colours are destroyed by manganese; which being added in small quantities, clears the glass, and is therefore sometimes called by artists
Universe, the soap of glass. This effect of manganese cannot easily be explained, for it has the property of tinging glass with a purple colour. Mr Montamy, in his Traité des Couleurs pour la Peinture en Email, has a very fine and ingenious thought upon this subject; which is, that the manganese destroys the above-mentioned colours, by adding to these a purple tinge, and by the mixture producing a blackish brown colour; and that as blackness is caused merely by an absorption of the rays of light, therefore the blackish tinge given to the glass by the above mixture of colours, prevents the reflection of so many rays, and thus renders the glass less coloured than before. The causes of the bubbles and of the veins in all glass, even when most carefully prepared, and the methods of preventing these faults, are not sufficiently known.
We shall observe, in concluding this article, that several causes lessen or entirely prevent the transparency of glass, which is one of its most necessary qualities. As we cannot melt vitrifiable earth into transparent masses but by means of fluxes, and a sufficiently strong and long-continued heat, therefore when the vitreous mixture contains too little flux, or is exposed to too little heat, some parts of the vitrifiable earth cannot be entirely fused, and therefore injure more or less, according to their quantity, the transparency of the glass. The same fault may be perceived in glass, when it contains some earthy matter not susceptible of the action of fluxes, as vitrifiable earth is; such as, for instance, most metallic earths that are too much dephlogisticated, particularly the earth of tin. Accordingly, these earths are employed to make opaque or semi-transparent glasses, as enamels, artificial opals, and other such stones. Another remark may be made concerning the transparency of some kinds of glass, that it is destructible by a too long exposure to violent heat. As all fluxes, phlogistic or saline, are much less fixed than vitrifiable earth, and as some fluxes are less fixed than others, or less capable of becoming fixed by being mixed with vitrifiable earth; the cause therefore of the loss of transparency which some glasses suffer by a too violent fire is, that a part of their flux is dissipated, so that these glasses are decomposed, and that they contain so much earth that the flux is incapable of keeping them completely fused. It has been observed that glasses formed by a mixture of argillaceous and gypseous or calcareous earths, are more liable than any others to this loss of transparency.