GLASS-Porcelain, the name given by many to a modern invention of imitating the china-ware with glass.
The method given by Mr Reaumur, who was the first that carried the attempt to any degree of perfection, is shortly this: The glass-vessels to be converted into porcelain are to be put into a large earthen vessel, such as the common fine earthen dishes are baked in, or into sufficiently large crucibles; the vessels are to be filled with a mixture of fine white sand, and of fine gypsum or plaster-stone burnt into what is called plaster of Paris, and all the interstices are to be filled up with the same powder, so that the glass vessels may no where touch either one another, or the sides of the vessel they are baked in. The vessel is to be then covered down and luted, and the fire does the rest of the work; for this is only to be put into a common potter's furnace, and when it has stood there the usual time of the baking the other vessels, it is to be taken out, and the whole contents will be found no longer glass, but converted into a white opaque substance, which is a very elegant porcelain, and has almost the properties of that of China.
The powder which has served once will do again as well as fresh, and that for a great many times: nay, it seems ever so often. The cause of this transformation, says Macquer, is probably that the vitriolic acid of the gypsum quits its basis of calcareous earth, and unites with the alkaline salt and saline earth of the glass, with which it forms a kind of salt or selenites, different from the calcareous selenites, by the interposition of which matter the glass acquires the qualities of porcelain. See further on this subject the article CHEMISTRY, n° 591—594.
Glass-Pots, the vessels in the glass trade used for melting the glass. Those for the white glass works are made of a tobacco-pipe clay, brought from the Isle of Wight, which is first well washed, then calcined, and afterwards ground to a fine powder in a mill; which being mixt with water, is then trod with the bare feet till it is of a proper consistence to mould with the hands into the proper shape of the vessels. When these are thus made, they are afterwards annealed over the furnace. Those for the green glass work are made of the nonfuch, and another sort of clay from Staffordshire; they make these so large as to hold three or four hundred weight of metal. And besides these, they have a small sort called piling-pots, which they set upon the larger, and which contain a finer and more nice metal fit for the nicest works.
The clay that is used for this purpose should be of the purest and most refractory kind, and well cleansed from all sandy, ferruginous, and pyritous matters; and to this it will be proper to add ground crucibles, white sand, calcined flints duly levigated, or 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 exposed to a great heat, is not absolutely determined, but depends on the quality of the crude clay, which is more or less fat. M. D'Antic, in a memoir on this subject, proposes the following method of ascertaining it: the burnt and crude clay, being mixed in different proportions, should be formed into cakes, one inch thick, and four inches long and wide. Let these cakes be slowly dried, and exposed to a violent heat, till they become as hard and as much contracted as possible, and in this state be examined;
and the cake, he says, which has suffered a diminution of its bulk equal only to an eighteenth part, is made of the best proportions. He observes, in general, that most clays require that the proportion of the burnt should be to the fresh as four to five.
Tin Glass, the same with Bismuth. See the article BISMUTH; also CHEMISTRY, N° 1250.
Vessels of Glass used in Chemical Experiments. See CHEMISTRY, n° 556.
GLASSES are distinguished, with regard to their form, use, &c. into various kinds, as, drinking glasses, optical-glasses, looking-glasses, burning-glasses, &c.
Drinking Glasses, are simple vessels of common glass or crystal, usually made in form of an inverted cone.
Each glass consists of three parts, viz. the calyx or bowl, the bottom, and the foot; which are all wrought or blown separately.
Nothing can be more dexterous and expeditious than the manner wherein these parts are all blown; two of them opened, and all three joined together. An idea is only to be had thereof, by seeing them actually at work.
The glasses chiefly used in England are made of the ashes of fern; crystal glasses being less frequent in use. The exceeding brittleness of this commodity, notwithstanding the easy rate of each glass, renders the consumption thereof very considerable. For the method of gilding the edges of drinking-glasses, see Gilding on Enamel and Glass.
Optical-Glasses. See OPTICS.
The improvements hitherto made in telescopes by means of combining lenses made of different kinds of glass, though very great, are yet by no means adequate to the expectations that might reasonably be formed if opticians could fall on any method of obtaining pieces of glass sufficiently large for pursuing the advantages of Mr Dollond's discovery. Unfortunately, however, though the board of longitude have offered a considerable reward for bringing this art to the requisite perfection, no attempt of any consequence has hitherto been made. Mr Keir is of opinion, that the accomplishment of this is by no means an easy task; as it requires not only a competent knowledge of the properties of glass fitted for the purpose (the faults not being evident to common inspection), but a considerable degree of chemical knowledge is also necessary in order to invent a composition by which these faults may be avoided; and lastly, a kind of dexterity in the execution of the work, which can only be acquired by practice. Our author, however, thinks, that if the subject were more generally understood, and the difficulties more fully pointed out, for which purpose he makes the following remarks, the end might be more easily accomplished.
1. The rays of light passing through a glass lens or prism, or through any other medium of unequal thickness, are refracted; but not in an equal manner, the blue, violet, &c. being more refracted than the red.
2. Hence it happens, that the rays of light, when refracted by a common lens, do not all unite in one focus, but in reality form as many different foci as there are colours; and hence arise the prismatic colours, or irises, which appear towards the borders of the image formed by the common convex lenses, and which render the vision extremely indistinct.
3. The indistinctness of vision produced by this cause, which
which is sensible in telescopes of a small aperture, increases in so great a proportion, viz. as the cubes of the diameters, that it seemed impossible to increase the power of dioptric telescopes greatly, without extending them to a very inconvenient length, unless this confusion of colours could be corrected.
4. It was known that different transparent bodies possessed different degrees of refractive power; and, until Mr Dollond discovered the contrary, it was supposed, that the refractions of the coloured rays were always in a determinate ratio to one another. On this supposition it seemed impossible to correct the faults of refracting telescopes: for it was supposed, that if the dispersion of light produced by a convex lens were counteracted by another lens or medium of a concave form, the refraction would be totally destroyed; and this indeed would be the case, if the two mediums were made of the same matter; and from some experiments made by Sir Isaac Newton, this was supposed to be actually the case in all substances whatever.
5. From considering that the eyes of animals are formed of mediums of different colours, it occurred first to Mr David Gregory, the celebrated professor of astronomy at Oxford, and then to Mr Euler, that, by a combination of mediums which had different refractive powers, it might be possible to remedy the imperfections of dioptric telescopes. It does not, however, appear, that either of these gentlemen understood the true principle on which these phenomena depend. Mr Euler executed his idea by forming a compound object lens from two glass lenses with water interposed; but his attempt was not attended with success. Mr Dollond, however, was led by some arguments adduced by Mr Klingefern, to repeat one of Sir Isaac Newton's experiments, and which had induced even that great philosopher himself to suppose that the improvement afterwards executed by Mr Dollond was impossible. This experiment was made by Sir Isaac Newton, by placing a glass prism within a prismatic vessel filled with water, in such a manner that the rays of light which were refracted by the glass prism should pass through and be refracted in a contrary direction by the water prism. In this manner the refraction of the light was entirely destroyed. But when Mr Dollond repeated the experiment, he found, that, contrary to his own expectations, when the angles of the two prisms were so proportioned that they counteracted each other's mean refraction, then colours appeared; and on the other hand, when they were so proportioned that the dispersion of the coloured rays was counteracted, the mean refraction still subsisted; which evidently proved, that the mean refractive and dispersive powers of glass and water were not proportional to one another.
6. To apply this to the proposed improvement, Mr Dollond examined several kinds of glass. Crown-glass was found to possess the smallest dispersive power in proportion to its refraction; while flint-glass possessed the greatest dispersive power in proportion to its refraction, which was also very great. On comparing these two exactly together, he found, that a wedge of white flint glass whose angle was about 25 degrees, and another of crown-glass whose angle was 29 degrees, refracted very nearly alike. He found also, that, when the wedges were ground to such angles, the refraction produced by the flint-glass was to that produced
by the crown-glass nearly as two to three, the refracted light was then free from colour. On measuring the general refracting powers of these two glasses, he found, that in flint-glass, the sine of incidence of the rays was to the sine of mean refraction as 1 to 1.583; and that, in crown-glass, the sine of incidence was to the sine of mean refraction as 1 to 1.53.
The methods of determining the different refractive powers of glass are given under the article Optics. Here we shall only observe, that two kinds of glass are necessary for the construction of achromatic telescopes; one of which shall possess as small, and the other as great, dispersive powers, relative to their mean refracting powers, as can be procured. The difference of glasses in this respect depends on the quality of the ingredients employed in their composition. Crown-glass, which is composed of sand melted by means of the ashes of sea-weeds, barilla, or kelp, both which fluxes are known to consist of vegetable earth, alkali, and neutral salt, is found to give the smallest dispersive power. Plate glass, which consists of sand melted by means of fixed vegetable alkali, with little or no vegetable earth, gives a greater dispersive power; but both these give much less than flint glass, which consists of sand melted by means of minium and fixed alkali. It appears, therefore, that the dispersion of the rays is greatest when minium, or probably other metallic calces, are made use of; and that alkalies give a greater power of dispersion than vegetable or other earths. Mr Zieher of Peterburgh, however, informs us that he has made a kind of glass, much superior in this respect to flint-glass; but it does not as yet appear whether it be more fit for optical purposes than that commonly made use of. There seems no difficulty in augmenting the dispersive power, as that is found to depend on the quantity of minium or other flux; but thus we unfortunately increase also the capital fault to which flint glass and all compositions of that kind are subject; namely, the being subject to veins or small threads running through it. By these, even when so small as to be imperceptible to the naked eye, the rays which fall on them are diverted from their proper direction, and thereby render the images confused. This is owing to the greater density of the veins, as appears by their image being received on white paper, when the glass is held between the paper and the sun or a candle, at a proper distance. The rays of light being then made to converge by the superior density of the veins, their images will appear as bright lines bordered with obscure edges on the paper. Flint-glass is so much subject to this kind of imperfection, that it is with difficulty the opticians can pick out pieces of the size commonly used from a large quantity of the glass. It is farther to be regretted, that the minium which produces the greatest dispersive power, is likewise the very substance which renders flint-glass much more subject to these imperfections than any other. The reason is, that the sand and earthy matters mix uniformly in fusion; and, having not only a considerable degree of affinity towards each other, but also being not much different from each other, they are not apt to separate. On the other hand, when such an heavy substance as minium is added to these earthy substances, though it has a pretty strong tendency to unite with the earthy substances, it has none with the fixed alkali, which
Glas. is another ingredient in this glass. Hence some parts of the glass will contain more metallic matter than the rest; particularly that near the bottom of the pot, which is so full of large veins as to be applied only to the making of wares of little value. The veins in this case are formed by the descent of the minium to the bottom, which in its passage forms threads or veins by dragging other parts of the glass along with them.
The correction of this fault appears therefore to be very difficult. M. Macquer informs us, that he had in vain tried to remove it by very long fusion and a fierce fire; which indeed others have found by experience not to correct, but to augment the evil. Mr Keir is of opinion that some new composition must be discovered, which, along with a sufficient refractive power, should possess a greater uniformity of texture; but he is likewise of opinion, that scarce any alteration in this respect could be made without injuring the colour of the glass. For optical purposes, however, our author does not think that an alteration in the colour of the ingredients would be very detrimental. "I am convinced (says he), that glasses sensibly tinged with colour, might transmit as much or more light than the best flint-glass. For the colourless appearance of flint-glass is an optical deception. The minium gives it a considerable tinge of yellow, and the alkali inclines it to a bluish cast, besides the colour arising from a greater or less impurity of the materials; so that the glass would actually be very sensibly coloured, unless by the addition of manganese, which is known to give a purplish red. Thus the other tinges are counteracted, but not effaced or destroyed as has been frequently imagined. By the mixture of the three principal colours, red, yellow, and blue, more or less exactly counterpoised, a certain dark shade is introduced, in which, as not any one of the colours predominates, no coloured tinge appears, but the effect is merely a diminution of the transparency of the glass, which, however, is too small for ordinary observation." Mr Keir is even of opinion, that a certain tinge of yellow would in many cases be of service, because it would exclude some of the blue rays, which being most refrangible are most injurious to the distinctness of vision.
Very considerable difficulties, however, must arise in attempting improvements of this kind; as the experiments must all be tried on a very large scale. This is not only attended with a very heavy expense in itself on account of the quantity of materials employed, but from the heavy duty of excise which is rigorously exacted whether the glass be manufactured into saleable articles or not. It is observed in the manufacture of every kind of glass, that the glass in the middle of the area or transverse section of a pot is much purer and freer from veins and other imperfections than the part which is near the sides, and that the glass at the bottom is the worst of all. Consequently it is chiefly in large pots, such as are used in manufactures, that there is a probability of success. Very fine and beautiful glasses, called paste and artificial gems, may be made in smaller pots or crucibles; but this glass is suffered to cool and subside in the vessel, by which means the contiguous parts are more uniform in their texture than can be expected in a piece of glass taken out of the pot while hot in the common way, by making it adhere and twist round an iron rod or pipe. But although
the method of allowing the glass to cool in the pots is very advantageous for the purposes of the jeweller, it is by no means applicable to those of the optician. Glass cooled in that gradual manner, suffers some degree of crystallization or peculiar arrangement of its parts; the consequence of which is, that the rays of light undergo certain refractions independent on the form of the glass, which greatly affect the distinctness of vision in telescopes.
Musical GLASSES. See HARMONICA.
Looking GLASS. See LOOKING GLASS, MIRROR, and FOLIATING.
Burning GLASS. See BURNING GLASS.
Weather GLASS. See BAROMETER.
Cutting GLASS. See SURGERY.
Hour GLASS. See Hour GLASS.
Watch GLASS. See WATCH.