Flint glass

“ It is probable that flint-glass was not invented, but gradually evolved, that potash-lead glasses were in use during the latter part of the 17th century, but that the mixture was not perfected until the middle of the following century.
The 18th century saw a great development in all branches of glass-making. Collectors of glass are chiefly concerned with the drinking-glasses which were produced in great profusion and adapted for every description of beverage. ”

“ Flint glass contains lead; the lead makes the glass dense, and gives it great refractive power, enabling it to bend and separate light in all directions. Cut glass and toilet articles are made of flint glass because of the brilliant effects caused by its great refractive power, and imitation gems are commonly nothing more than polished flint glass. ”

“ The tubes are so much the more easy of use, as the glass of which they are made is the more homogeneous. Under this point of view, the white glass, known in commerce by the name of crystal or flint-glass, is preferable to common glass: it is more fusible, less fragile, and less liable to break under the alternations of heat and cold; but it is dearer and heavier, and has the serious disadvantage of becoming permanently black when exposed to a certain part of the flame. ”

“ When flint glass is ready for working, the time required to work off a pot of it is much longer than that which is required for a pot of crown or sheet; and it is a matter of considerable importance, that the furnace-man should so manage his fires as to keep the glass in a proper working condition, that is, he should not let it get too cold (therefore too solid) nor too fluid. Flint glass is worked off by the blower into wine-glasses, tumblers, decanters, and other suitable vessels. Let us take a wine-glass as an illustration of the method of working. ”

“ Perhaps the principal charm of this native Indian glass arises from the violent contrast that it affords to the impeccable cristallo and to the flint-glass that have tyrannised over us so long in Europe. It is beginning at length to dawn upon us that there are other qualities than absolute transparency and absence of colour to be looked for in our material, and it is the attempt to bring these qualities into prominence that has led to the 347 development in France within the last few years of quite a new treatment of glass. ”

“ The applications of glass are so exceedingly extensive, that it is inconvenient, if not impossible, to manufacture every species of it at one glass-house or at one establishment. Some, therefore, confine their attention to the production of window glass, and such articles of hollow ware as may be made, with profit, from the same kind of paste. Others make [181] vials and other species of ware, employed by the druggist, apothecary, and chemist. And again, the efforts, at some factories, are confined entirely to the manufacture of flint glass, or to that of plate glass for mirrors. ”

“ Since considerable difficulty occurs in preserving unannealed drops of flint glass, the author has paid attention to the circumstances most favourable to their preservation, and recommends removing them from the water in which they have been plunged, as soon as the redness in the centre of the drop ceases to be visible. ”

“ 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. ”

“ Such striæ, due to want of homogeneity in the glass, are much more common in flint glass than in the soda-lime glasses used for sheet manufacture, but are not unknown in the latter. On the other hand, even if the glass be as homogeneous as possible, the gatherer can produce these striæ if he takes up his glass from a place close to the side of the fire-clay ring that floats in the furnace in front of his working opening. ”

“ Table-ware and Vases.—The varieties of glass used for the manufacture of table-ware and vases are the potash-lead glass, the soda-lime glass and the potash-lime glass. These glasses may be colourless or coloured. Venetian glass is a soda-lime glass; Bohemian glass is a potash-lime glass. The potash-lead glass, which was first used on a commercial scale in England for the manufacture of table-ware, and which is known as “flint” glass or “crystal,” is also largely used in France, Germany and the United States. ”

“ We here see that lead-glass or flint-glass has little greater refractive power on light than rock crystal or the ordinary plate and crown glass of commerce which belongs to the same family as the cristallo of the Venetians. In dispersive power, on the other hand, it stands apart from both these substances and rivals the diamond in scattering the component rays of white light. ”

“ Pressed glass is also essentially English, no other kind, save flint glass, being suitable for treatment in this way. It is, in the first place, essential to obtain a metal which has a low melting point, and one which does not shrink in solidifying, as that would draw it away from the sides of the mould, and so effectively spoil the design. The low melting point of the metal enables the product to be “fire polished.” In this process it is reheated to a point sufficient to melt a thin surface layer, and so remove any roughness due to the process of moulding, and leave a smooth bright surface. ”

“ Looking through the flat surface of window-glass, whether plate or blown, it appears colorless; but, if the sight is directed through the edge, it will disclose a sea-green tinge. Flint-glass proper is not so. It is absolutely colorless, except when cut into faces or prisms, when it reveals the colors of the spectrum. ”

“ With soda glass a very small irregularity will cause the joint to break when cold, but flint glass is much more long-suffering. It is easy to perform the above operation on small tubes. For large ones it will be found best to employ flint glass and use the clip stands as in the case of direct welds, treated above, but, of course, with suitable modifications. Never let the main tube cool after the hole is made until the work is done. ”

“ If you operate without taking this precaution, you run 28 the risk of decomposing the glass. Thus, for example, in the case of flint-glass, you may reduce the oxide of lead, which is one of its constituents, to the state of metallic lead. The consequence of such a reduction is the production of a black and opaque stain upon the work, which can only be removed by exposing the glass, during a very long time, to the extremity of the jet.
You must invariably take the greatest care to keep the flame from passing into the interior of the tube ”

“ Common paste, [6] which is met with in drapery goods and cheap ornaments in general—hat-pins, buckles, and so forth—is composed of ordinary crown-glass or flint-glass, the refractive indices being about 1·53 and 1·63 respectively. The finest quality, which is used for imitations of brilliants, is called ‘strass.’ It is a dense lead flint-glass of high refraction and strong colour-dispersion, consisting of 38·2 per cent. of silica, 53·3 red lead (oxide of lead) , and 7·8 potassium carbonate, with small quantities of soda, alumina, and other substances. ”

“ Artificial Gems.—The fact that pieces of suitably-coloured glass can be made to show a superficial, but sometimes more or less deceptive, resemblance to precious stones, has led to the manufacture of imitation jewels of all descriptions. The glass used for this purpose is usually a very dense flint-glass whose high refractive index facilitates the imitation which is aimed at. ”

“ Magnificent flint glass is made both in England and France. The English experimenter will probably prefer to use English glass, and, if he is wise, will buy a good deal at a time, since it does not appear to devitrify with age, and uniformity is thereby more likely to be secured. ”

“ The determination of the optical constants by means of a refractometer would at once reveal the true character of the imitation, but an even readier test is that of hardness. The dense flint-glass is naturally soft, and is readily scratched by most of the harder minerals, while the precious stones, more particularly garnets, rubies and diamonds, are very hard. ”

“ Chromatic aberration is easily cured by using an achromatic or compound lens made by cementing together two lenses of crown and flint glass respectively, as in Fig. 40.
It will be seen that the flint glass component by itself is a concave lens and therefore neutralises in part, or in whole, the convex crown lens. Flint glass has both greater dispersive power and also greater refractive power than crown glass, but fortunately not to the same degree ”

“ In what is now known as the older crown and flint glass the dispersion and refraction increased with the density, that is to say, the heavier the glass the more it scatters and bends light rays passing through it. With the new methods, glass is made which scatters the light rays very little, though bending them considerably and vice versa. The ordinary crown glass is composed of silicates of calcium and sodium or of calcium and potassium or a mixture of both and it is possible to make it colourless and free from defects, but its optical properties are never so valuable as those of Jena glass. ”

“ Where, however, large quantities of flint-glass are made every day, it is possible and proper to make more perfect arrangements for the mechanical handling and mixing of the lead with the other ingredients by the provision of suitable mixing and transporting machinery, so arranged as to be dust-tight. It is only fair to state, however, that partly under their own initiative, partly under pressure from the authorities, glass makers in this country are complying with these requirements in an adequate manner. ”

“ 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. ”

“ It is very unusual for the investigator to have to make complicated apparatus from this glass. Fused joints may be made between hard glass and flint glass without using enamel, and though they often break in the course of time, still there is no reason against their employment, provided the work be done properly, and they are not required to last too long. ”

“ This having been made entirely by hand, with no implement but a common cut nail, the process is of course too slow to be valuable; but the result attained may very probably afford useful hints and suggestions to inventors of weaving machinery.—I think the display of Flint Glass by the Brooklyn Company is equal in purity and fineness to any other plain Glass in the Exhibition, and only regret that the quantity sent had not been larger. ”

“ Those flakes of flint, my child, were split off by men; even your young eyes ought to be able to see that. And here are other pieces of flint—pear-shaped, but flattened, sharp at one end and left rounded at the other, which look like spear-heads, or arrow-heads, or pointed axes, or pointed hatchets—even your young eyes can see that these must have been made by man. And they are, I may tell you, just like the tools of flint, or of obsidian, which is volcanic glass, and which savages use still where they have not iron. ”

“ Flints form excellent building materials; because they give a firm hold to the mortar by their irregularly rough surfaces, and resist, by their nature, every vicissitude of weather. The counties of Kent, Essex, Suffolk, and Norfolk contain many substantial specimens of flint-masonry.
FLOSS, of the puddling furnace, is the fluid glass floating upon the iron produced by the vitrification of the oxides and earths which are present. ”

“ These may be taken to be glasses made cheaply for tavern and kitchen use; though not so attractive as the better qualities, they should not be neglected by the collector.
10. YARD OF ALE GLASSES
Evelyn tells in his diary that in 1683 the health of James II was drunk at Bromley “in a flint glass of a yard long.” Imitations of these are made, but the real old ones are excessively rare. In shape they rather resemble a coaching-horn, the mouthpiece being the foot, or the [65] mouthpiece being replaced by a bulb. ”

“ Frequently flints are found in all stages of formation: siliceous chalk with the corroded remains of sponge spicules may be found in the interior, black flint blotched with grey forming the mass of the nodule, while the exterior is completed by a thin layer of white porous flint. This layer must not be confused with the white layer which is frequently met with on the surface of weathered flints, which is due to a subsequent solution of some of the silica, so that by a process of unbuilding, the flint is brought back to the incompleted flint in its second stage. ”

“ Surely so much information has rarely been compressed into one print as we find in the main illustration to this part of Agricola’s text (Plate XXXVIII.) . Here at one working-hole (fenestrella) we see a workman gathering the glass at the end of his fistula or blowing-iron, another is shaping the gathering upon the marver at his foot, a third is vigorously blowing the paraison to the required size, and a fourth is swinging another round his head. On the ground lie scattered moulds of various forms, and here, too, we may discover the forceps (pucella) used in shaping the glass. ”