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CHAP. XVI.

ON THE DEVITRIFICATION OF GLASS.

FIRST OBSERVED BY NEUMANN. EXPERIMENTS OF REAUMUR.

SUBSTANCE KNOWN AS REAUMUR'S PORCELAIN. - INAPPROPRIATENESS OF THIS NAME. -USES TO WHICH THE SUBSTANCE MAY BE APPLIED. —COMMON BOTTLE GLASS MOST PROPER FOR THIS CONVERSION. — METHOD OF EFFECTING THE CHANGE. PRODUCED SOLELY BY HEAT. - EXPERIMENTS OF DR. LEWIS. REVITRIFCATION. – EXPERIMENTS OF SIR JAMES HALL. PROPOSAL SUGGESTED THEREBY, OBSERVATIONS OF GUYTONMORVEAU. — ARTIFICIAL INTAGLIOS. MOCK ONYXES. — POWER OF DEVITRIFIED GLASS TO BEAR SUDDEN CHANGES OF TEMPERATURE. EXPERIMENTS WITH COLOURED GLASS. GLASS DEVITRIFIED BY BURNING LAVA. - THE PROCESS PROMOTED BY MULTIPLYING THE INGREDIENTS OF GLASS, -DEVITRIFIED GLASS CONDUCTS HEAT MORE PERFECTLY THAN WHEN VITREOUS. BECOMES A CONDUCTOR OF ELECTRICITY. RETAINS THIS PROPERTY WHEN REVITRIFIED.

It was observed very long since by Neumann, that some kinds of glass, if exposed during any considerable time to a high degree of heat, but below their point of fusion, are so far changed in their properties and texture as to become opaque, fibrous, and tough ; and so hard as to give abundant sparks if struck with steel, to cut any common glass readily, and to be scarcely susceptible of abrasion by filing. It has also been found, that in taking this form glass so far alters its nature as regards its qualities of expansion and conducting of heat, that it will bear a sudden transference from freezing to boiling water.

That indefatigable naturalist, M. Reaumur, made various experiments and observations on this phenomenon; and, in the year 1739, communicated the result of these to the Royal Academy of Sciences in Paris. The subject becoming by this means more generally known, glass, when thus converted, obtained, and has since kept the name, of Reaumur's porcelain ; a designation which it owes to its appearance rather than to its real propera

ties, which do not at all entitle it to be classed with porcelain. It is probably owing to the inappropriate name which the substance thus acquired, that so little has been done towards a true developement of the facts and circumstances attendant upon the devitrification of glass. Even the greater number of such scientific men as for a time entered upon the investigation, limited their labours to experiments with various cementing substances, that they might arrive at the discovery of that one which would ensure the concurrence of the greatest number of good qualities that should be found in porcelain. The futility of these experiments has since been made evident; and it must be regretted that the same amount of research as was thus unprofitably bestowed, has not been given to elucidate the actual properties of devitrified glass, and to render it practically serviceable to society.

Reaumur was of opinion that its quality of resisting alternations of temperature, its toughness, as well as the power it possesses of withstanding the action of acid liquids, render this porcelainous glass well qualified for the formation of chemical vessels. The same opinion has been equally held and declared by other philosophers who have brought their minds to the investigation of the subject; and it appears singular that their suggestion should not, consequently, have been very generally reduced to practice. This circumstance must further excite surprise, when it is considered in how many important operations of the laboratory such a substitute for metallic vessels would be advantageous. In operating upon any practical scale, the chemist is driven, for want of such a substitute, either to the employment of metals which are liable to be injuriously acted upon by the matters under process, or is compelled to adopt vessels of platinum, the expensiveness of which places them beyond the prudent reach of most persons. In one instance, a manufacturer of pharmaceutical preparations, who is exceedingly particular as to the absolute purity of his productions, has recently incurred the expense of constructing a pan of únalloyed silver, forty inches in diameter, wherein to

evaporate vegetable extracts, many of which, in some degree or other, act upon and are impregnated by copper.

Could vessels formed of this fibrous glass be adopted with safety, there would be nothing in their cost, especially when of moderate size, to prevent their general adoption. The only circumstance hitherto assigned against this adoption is, that although the inner texture of the glass is fine and white, the surface is coarse and of a dirty appearance; but this must be thought a very insufficient cause for foregoing such decided advantages as are apparently offered through its employment.

All kinds of glass are not equally qualified to undergo this conversion ; with some descriptions, indeed, it will not ensue. Not any vitreous compound seems altogether proper for it, with the exception of common green bottleglass, and perhaps also the ordinary kinds of window glass.

The method commonly employed for effecting the change is as follows: — The glass vessel is placed within a larger earthen vessel, in the same manner as is pursued for baking porcelain. The entire space unoccupied by the glass is next filled by pouring into the vessel fine white sand, or powdered gypsum, so that the glass shall not be allowed at any point to come into contact with the earthen case. The containing vessel is then covered down, securely luted, and the whole is placed within the furnace.

It was for some time generally imagined, that in this process, which is very similar to that which is known to chemists under the name of cementation, the glass owes the change which it undergoes to some chemical action of the gypsum upon its substance; but this has been proved erroneous. It is shown by Dr. Lewis, in the detail of his various experiments, that not only may the nature of the powder be almost infinitely varied, without in the least affecting the operation, as far as regards the altered texture of the glass, but that the change equally and identically goes forward in the absence of all cementing substance ; a fact which is conclusive upon the subject, and which proves that whatever may be the particular substance employed, whether sand, bone-ash, chalk, or gypsum, it acts merely by affording mechanical aid, sustaining the glass in its proper form during the period when it is softened by heat, and when, if deprived of such support, it would be liable to irreparable injury in falling together by means of its own gravity.

In the course of experiments, which are detailed by him at some length *, Dr. Lewis placed several pieces of common wine bottles into crucibles, pouring over them the requisite quantities of white sand, and placing them in a proper furnace, wherein they were heated during many hours. In order to ascertain the progress of the change, pieces were withdrawn from time to time for examination. Those pieces which were first taken out, after having been during several hours in the furnace, but without being heated to redness, exhibited no sort of change whatever. In a low red heat the change went forward very slowly, but still was quite perceptible; while in a strong red heat approaching to whiteness, and which only just avoided that degree of intenseness which would have melted the glass, the change went on rapidly, beginning at each surface, and spreading towards the middle; so that, in two hours, the substance had assumed throughout the appearance of porcelain.

The glass became first of a bluish colour on the sur. face, and exhibited a very sensible diminution of its transparency. After this, it gradually became white and more opaque; the texture no longer continued vitreous, but became fibrous, as already described; and these fibres were disposed nearly parallel to each other, and transverse to the thickness of the piece. The fibres from both surfaces meeting in the middle, formed there a kind of partition, in which cavities were occasionally perceptible. By degrees this opacity and fibrous change were completed, the blue colour disappeared, and was succeeded by a dull white or dun colour.

* Commercium Philosophico-technicum, p. 250.

When exposure to the same high degree of heat was continued after the production of this effect, the glass was seen to undergo a still further change of texture : the fibres appeared to be divided or cut into grains; beginning, as before, at the outer ends, and proceeding onwards towards the middle, until the entire substance assumed a granular form, similar to ordinary porcelain. A still further continuance of heat caused the grains, which at first were fine and glossy, to become enlarged and dull, and to change from a compact to a porous, and at length to a friable substance, resembling a slightly cohering mass of white sand, not easily distinguishable from that wherein it was embedded.

If glass, which has been withdrawn from the furnace at the time it has assumed the fibrous state, be afterwards subjected to a very strong heat, it will melt into a semi-transparent mass, and may be drawn out in strings, which on cooling are found to have lost their fibrous quality, and to have resumed their former vitreous state, being no harder than before the original cementation. This fusion of porcelainous glass cannot, however, be effected, without the application of a degree of heat considerably more intense than is required for melting glass in its more usual form ; and it is also found that the farther the process of cementation has been carried, the higher must the temperature be raised for its fusion; so that specimens which have been rendered granular are much more refractory than such as are simply fibrous.

Although, throughout the experiments of Dr. Lewis, no difference in the actual properties of Reaumur's porcelain followed the employment of different substances for embedding the glass, its internal colour, hardness, texture, and the regular succession of its changes being the same in all cases ; —yet considerable difference was occasioned in its outward colour. If charcoal or soot had been used, these produced a deep black colour, which was not affected by long exposure to heat in an open furnace. Clay or sand which was coloured, communicated

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