Lead glass

“ As it is frequently necessary to make additions, alterations, or repairs to purchased apparatus, it is best to provide supplies both of soft soda glass and lead glass, for though purchased glass apparatus is frequently made of lead glass, yet sometimes it is formed from the soda glass, and as it is a matter of some difficulty to effect a permanent union between soda glass and lead glass, it is desirable to be provided with tubes of both kinds. ”

“ Lead is one of the most widely-used ingredients of glass; the glasses containing this substance in notable quantity are all characterised to a greater or less degree by similar properties, such as considerable density and high refractive power, and are classed together under the name “flint glasses.” Lead is now almost universally introduced into glass mixtures in the form of red lead, although the other oxides of lead might be employed almost equally well. ”

“ On the other hand, glasses very rich in lead are always decidedly yellow in colour, and it has been shown that this coloration is due to the natural colour of lead silicates and not to the presence of impurities. What has just been said of lead applies, with only very slight modification, also to the rare metal thallium and its compounds, which have been introduced into glass for special purposes. Leaving these two exceptional bodies on one side, we now pass to a consideration of the elements in the order of their chemical grouping. ”

“ In the seventeenth century the whole process was revolutionised by the introduction of a large proportion of oxide of lead, making what is technically known as “flint” glass—a glass much more brilliant than any other, a quality due partly to its transparency and partly to its increased refractive power, which renders it specially fitted for “cutting”—a process which enhances its beauty by increasing the number of ways in which the light rays falling on the glass are dispersed. ”

“ Simple bulbs and joints do not even need this smoking; but thick articles, and especially those that are to be subjected to the stress of grinding, need more prolonged annealing in a special oven.
Use of Lead-Glass.—When lead-glass is to be used, the blowpipe flame should be in good adjustment and the glass should not be allowed to approach so near to the blue cone as to be blackened. Slight blackening may often be removed by heating the glass in the extreme end of the flame.
Lead-glass articles tend to be rather more stable than similar articles of soda-glass. ”

“ He can therefore easily work upon very large masses of glass. In a special case, such a source of heat might be devised by the amateur. Usually, however, the difficulty may be overcome without special apparatus. It is, in fact, only necessary to carry out the instructions given below to obtain a considerable brush flame rich in air, in which the lead glass can be worked, not only without discoloration, but with the greatest facility. ”

“ Again, if the silicate of lead be present in too large proportions, and if great care be not taken in the manufacture of lead glass, the silicate of lead, from its greater density, will sink lower among the molten silicates, and will therefore cause a larger proportion of lead to be in the glass at the bottom of the pot than there is at the top. We often notice in tumblers and decanters of the cheaper kind, that there are very distinct striæ running through the whole substance in some particular portion of the glass. ”

“ It is sometimes necessary to join soda glass to lead glass. In this case the edge of the lead glass tube may be bordered with white enamel before making the joint. Enough enamel must be used to prevent the lead and soda glasses from mingling at any point. The enamel is easily reduced, and must be heated in the oxidising flame. ”

“ It is quite possible to have the glass too heavy for beauty, since the dense lead compounds have a tendency to separate from the lighter silicates, and, consequently, if present in too large amounts, they make the glass streaky and mottled. In general, lead glass for domestic uses has a specific gravity of from three to four—that is, it is from three to four times as heavy as an equal bulk of water. The brilliancy given to the glass by its increased density has attached the name crystal to this particular product. ”

“ I should have told you, however, that in our finest and most brilliant glass, a quantity of the oxide of lead is put, which vitrifies with the other ingredients, and gives the glass more firmness and density. The blue, yellow, and red glasses are coloured with the oxides of other metals. As to the common green glass, it is made with an alkali that has a good deal of calcareous earth remaining with the ashes of the plant. But to understand all the different circumstances of glassmaking, one must have a thorough knowledge of chymistry. ”

“ This property does not belong to the glass containing lime, to anything like the same extent. Lead glass, too, is much heavier than lime glass, and is therefore unsuited to many of the purposes for which the latter is generally used, the principal of which is for the glazing of windows. ”

“ The product is obtainable in considerable purity, very small proportions of silica, derived from the furnace bed, and of iron derived from the tools with which the lead is handled, being the principal foreign substances found in good red lead. Silver would be an objectionable impurity, but owing to the modern perfect methods of de-silvering lead, that element is rarely found in lead products. Analytical control of red lead as used in the glass mixtures, and consequent adjustments of the mixture, are, however, necessary where exact constancy in the glass produced is desired. ”

“ When therefore the objects are light on dark, the lead-work will sink into the background, and although it may leave small space for the glass, it is sometimes surprising how efficiently that little lights up and expresses the colour. If the charges are inconveniently small for the leading, resort is had to what is called flash glass, which consists of two layers, of which one only is coloured, and is made by dipping a piece of molten white into a coloured glass, when the mass is about to be blown into the bulb which, shaped and expanded, ultimately becomes a sheet of glass. ”

“ The glasses which contain lead, suffer another kind of change in the air, if sulphuretted hydrogen be present; the oxide of lead is converted into a sulphuret, with the effect of rendering the surface of the glass opaque and iridescent. The more lead is in the glass, the quicker does this iridescence supervene. By boiling concentrated sulphuric acid in a glass vessel, or upon glass, we can ascertain its power of resisting ordinary menstrua. Good glass will remain smooth and transparent; bad glass will become rough and dim. ”

“ There remains, in the form either of iridescent scales or of an opaque pearly crust, a layer consisting not perhaps of pure silica, but of an acid silicate of lime, alumina, or lead as the case may be.
Now a piece of clear glass may appear to the eye to be devoid of internal structure. But the ‘metal’ has, we know, in every case been subjected during the manufacture to a complicated series of involutions and doublings, to say nothing of the subsequent inflation if the glass has been subjected to a blowing process. ”

“ In general terms it may be said that the lighter elements do not as a rule tend to the production of coloured glasses, while the heavier elements, so far as they can be retained in the glass in either solution or suspension, tend to produce an intense colouring effect. The element lead appears to form a striking exception to this rule, but this is due to the fact that while the silicates of most of the other heavy elements are more or less unstable, the silicate of lead is very stable, and can only be decomposed by the action of reducing agents. ”

“ The softness of lead which makes it the only practicable metal for joining pieces of glass of complicated shapes, has the disadvantage that a stained-glass window when leaded up has a considerable degree of flexibility, and, if held by the edges alone, would be quite unable to resist the pressure of the wind, which on a big window is enormous,—think of the power even of a fresh breeze on a boat's sail. ”

“ The uses of lead are numerous: it is employed in making the fine kinds of glass, enabling them to bear the sudden changes of heat and cold better; also to give glass a proper degree of weight, and render it more easy to be cut without breaking. Lead gives to glass a greater power of refracting the rays of light, and confers upon it a higher polish. ”

“ There is a third method of avoiding lead glazing. If little jewels of coloured glass be cut out of various sheets and placed upon white glass they become fused at a sufficient heat in the kiln, and adhere more or less firmly to the glass on which they are laid; but this process of “annealing” is not very safe. Still less to be depended upon is the fourth process of “enamelling.” In that case the coloured glass is applied in the form of a paint upon a sheet of white. Fusing at a comparatively low temperature, it rarely gets quite firmly fixed. Nor has it the depth of pot-metal colour. ”

“ The proportion of gold should be very small, and the heat very great, to produce a good effect. The glass must contain either the oxide of lead, bismuth, zinc, or antimony; for crown glass will take no colour from gold. Glass combined with this metal, when removed from the crucible is generally of a pale rose-colour; nay, sometimes is as colourless as water, and does not assume its ruby colour till it has been exposed to a low red heat, either under a muffle or at the lamp. ”

“ The difference of its colour depends on the degree of fire it hath undergone, and on the metalline substances vitrified with it.
Litharge alone is very fusible, and being exposed to the fire is easily converted into glass: but this Glass of Lead, made without additament, is so active, so penetrating, and so apt to swell, that it can scarcely be made use of when pure. ”

“ As regards quality, the chief kinds are crown glass, flint glass, plate glass, bottle glass, and crystal glass, and the differences in composition may be conveniently expressed in the form of a table:—
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Cheapest of all glass is bottle glass, where the base is mainly lime. The metal used for medicine bottles contains more potash and is purer and clearer. The use of potash and soda makes the glass more easily fusible; alumina has the opposite effect; lime makes a harder glass; lead gives lustre, increases fusibility, and heightens the refractive power. ”

“ Long experience has fixed the mixtures, so far as ordinary furnace temperatures are concerned, which produce the varieties of glass in common use. The essential materials of which these mixtures are made are, for English flint glass, sand, carbonate of potash and red lead; for plate and sheet glass, sand, carbonate or sulphate of soda and carbonate of lime; and for Bohemian glass, sand, carbonate of potash and carbonate of lime. ”

“ Glass, Merret tells us, is ‘a concrete of salt and sand or stones.’ This, in modern scientific language, we should express by saying that it is a combination of silica with an alkali. But these substances alone are not enough. You cannot make a glass fit for practical use from a pure quartz sand with the addition of nothing else than a salt of potash or soda. Such a glass—a simple alkaline silicate—would indeed be transparent, but it would be difficult to work and very fragile. In all cases there is need of a second base, and this, to speak generally, should be either lime or oxide of lead. ”

“ These, with few exceptions, are made of very pure, fusible, highly transparent, and dense glass, usually termed ‘PASTE’ or ‘STRASS,’ which is generally formed of oxide of lead, potassa, and silica, with small quantities of other ingredients to increase the brilliancy and clearness. The characteristic tints are imparted by the addition of metallic oxides. The beauty of artificial stones and gems depends, chiefly, upon the tint of the real stones being exactly imitated, and upon proper care and skill being exercised in the cutting, polishing, and mounting them. ”

“ It is possible, however, that some kinds of Bohemian glass may equal the latter mineral in hardness; it is indeed a common statement that certain Bohemian or German ‘combustion-tubes’ will strike fire with steel. On the other hand, the presence of lead tends to make a soft glass; our cut flint is perceptibly softer than common window-glass, and perhaps the most important defect of the paste used to imitate precious stones—such paste may contain as much as 50 per cent. of lead oxide—is to be found in its comparative softness. ”

“ In the glass-works, where large quantities of ornamental and other glass goods are made of lead or flint glass, the pots in which the glass is melted are so constructed that the gases of the furnace do not come into contact with the glass ”

“ Fig. 34 is an illustration of various kinds and sizes of lead; showing some with the glass inserted in its place. [Pg 90] By all means make your leads yourself, for many of those ready made are not lead at all, or not pure lead. Get the parings of sheet lead from a source you can trust, and cast them roughly in moulds as at fig. 35. Fig. 36 is the shears by which the strips may be cut; fig. 37 is the lead-mill or "vice" by which they are milled and run into their final shape ”

“ The electrical properties of glass are of much greater practical importance, glass being frequently used in electrical appliances as an insulating medium. The insulating properties of glass, as well as the property known as the specific inductive capacity, vary greatly according to the chemical composition of the material. Generally speaking, the harder glasses, i.e., those richest in silica and lime, are the best insulators, while soft glasses,30 rich in lead or alkali, are much poorer in this respect. ”