Scientific Notices. On the coloring of glass by means of metallic oxides.

The London Journal of Arts, Sciences, and Manufactures, and Repertory of Patent Inventions.

Conducted by Mr. W. Newton, of the Office for Patents, Chancery Lane. (Assisted by several Scientific Gentlemen.)

VOL. XXXVI. (Conjoined Series.)

London: Published by W. Newton, at the office for patents, 66, Chancerylane, and Manchester; t. and W. Piper, Paternoster Row; Simpkin, Marshall, and Co., Stationers' Court; J. McCombe, Buchanan St., Glasgow; and Galinani's Library, Rue Vivienne,

Paris. 1850

By M. G. Bontemps.

[Translated for the London Journal of Arts and Sciences.]

In consequence of the demand which has arisen within the last fifteen years for stained glass windows and colored flint glass in Bohemia, Germany, France, and England, considerable attention has been directed to the coloring of glass by metallic oxides. For this purpose, there was most probably tried, in the first in stance, the processes indicated in the works of Neri, Merret, Kunckel, Ferrand, Haudiquer de Blancourt, and others, with frequent ill success, leading to the conclusion that the authors had not obtained the results they pretended to have produced;—the truth of the matter being, perhaps, that they did not operate under the same circumstances. At all events, these recipes only appear to have an empirical value, as chemistry had not yet become a science, but was merely an agglomeration of facts without a systematic arrangement, —the phenomena observed not being then explicable by any physical laws. At a later period, the science of chemistry admitted of the metallic oxides, as well as their various combinations with acids, being readily analyzed. Glass having been considered, by analogy, to be a salt, either with a simple or compound base, general axioms have been admitted in coloring that substance by means of metallic oxides. It has been said, for instance, that silicates of potash and soda are colorless; that the silicate of potash or soda and manganese is purple; that the silicate of potash or soda and cobalt is blue; that the silicate of potash and deutoxide of copper is blue; that the silicate of potash and protoxide of copper is red; that the silicate of potash and gold is purple, &c. Such axioms as these are sufficient for purposes where a superficial knowledge only is required; but, on entering upon a further investigation of the phenomena produced by the employment of metallic oxides in the manufacture of glass, it will be at once seen what a wide field is open for experiment, and how unsatisfactory is the information afforded.

The following statement of some of the phenomena produced by a few of the metals will, perhaps, prove interesting; as, although those mentioned are the metals generally employed for coloring glass, the phenomena they present do not appear to have been made the subject of close investigation.

I. Iron. —It is generally admitted that oxide of iron imparts a green color to glass; but the truth is, that this is only the case under peculiar circumstances. It is a fact well known to porcelain and earthenware manufacturers, that oxide of iron is the coloring matter of a fine purple red enamel, after firing in the muffle (and it may here be mentioned, that enamel has been proved to be merely glass). If the temperature were raised too high, this enamel would lose its purple tint, and incline to orange. The oxide of iron, therefore, produces three of the prismatic colors, even at what may be called low degrees of temperature, when compared with that of the glass furnaces presently to be mentioned.

If, in a pot containing white or flint-glass in a state of fusion, a small portion of iron be introduced whilst working, it will, by reason of its gravity, fall to the bottom; and, on taking the pot from the oven, a portion of glass, which has become of an orange or yellow color, will be found near the iron, which has become partially oxidized.

Another example of the yellow color produced by oxide of iron is found in the manufacture of artificial avanturine. It is known that this avanturine is produced by exposing soft glass, containing a large proportion of the oxides of iron and copper, to a temperature below its point of fusion; —the copper is converted into metallic crystals; and the glass, which is colored by the oxide of iron only, acquires a brownish-yellow color,—the yellow color increasing in proportion to the degree of reduction of the copper. To return to the ordinary circumstances of the coloration of glass by oxide of iron;—it will be seen that, at a moderate tem perature, such as that of covered pots for flint-glass, the oxide of iron gives a green color, approaching nearer to yellow than blue. It is generally by combining oxide of iron with oxide of copper (which gives the blue) that all shades of green are produced. The greenish color of bottle-glass must also be attributed to oxide of iron, combined with the carbonaceous matters contained in the mixture. But, on melting at a high temperature (in the manufacture of window-glass, for instance), it is remarked, that the addition of a small proportion of oxide of iron to the mixture produces a glass of a blueish color. Manufacturers of bottle-glass are also aware that, upon the glass cooling in the pot, it becomes blue and opaque before becoming devitrified.

From the foregoing observations it will be understood that glass receives from the oxide of iron all the prismatic colors; and that these colors are produced in their natural order, and in proportion to the elevation of the temperature to which the glass is submitted.

II. Manganese. — It is generally known that oxide of manganese imparts to glass a purple color, which property is not only applied for obtaining purple glass, but more especially for making what is called " glass-makers' soap," which is used for neutralizing the slight greenish color produced by small proportions of iron and carbonaceous matters existing in the materials which are employed in manufacturing white or flint-glass; but what is very remarkable is, that the slight purple color produced by oxide of manganese is very liable to become weaker; for, if the glass remains too long in the melting furnace and afterwards in the cupola, the purple first changes to a light brownish-red, then to yellow, and, afterwards, to green.

There is also a remarkable fact relative to the presence of manganese in the composition of glass. White glass, in which a small proportion of manganese has been employed, is liable to become yellow on being exposed to the light. On the glass being melted, for the purpose of making the celebrated Augustin Fresnel's polyzonal lenses, for which it was desirable to have the glass of the greatest possible purity, those prismatic pieces of glass became yellow in a short time, without losing their transparency or polish. This yellow color, it would seem, was due to the presence of manganese; as, on the use of that substance being discontinued, the yellow color no longer appeared. Moreover, in order to shew that the coloration was the effect of light, a prismatic ring, which had been recently manufactured from glass containing manganese, was broken into two pieces; one of which, on being exposed to the light during several weeks, became yellow; whilst the other, which was kept in the dark, un derwent no alteration, but retained its whiteness.

It is also well known that some squares of glass (particularly the Bohemian glass) acquire a slight purple tint, after having been exposed, for a length of time, to the action of light. The same effect is produced in window or flint-glass, containing a small proportion of manganese, when allowed to remain in the flatting or annealing oven a sufficient length of time, to allow devitrification to commence; —in this case, the interior of the glass becomes an opaque white, while the outside acquires a purple tint.

It is admitted that certain of the facts above mentioned, relating to coloration, might be explained by attributing them to various degrees of oxidation; and that the manganese, for instance, loses a portion of its oxygen when the glass passes from purple to yellow; but there appears to be a doubt whether this suffices to explain the phenomena (which may be termed photogenic) which take place when the glass is in a solid state.

III. Copper. — This metal, at its highest degree of oxidation, imparts to glass which is entirely free from iron, a sky-blue tint, inclining more to green than purple; and, at its lowest degree of oxidation, it gives a ruby color.

It has always been the custom to color window-glass red by means of protoxide of copper; but that color is not easily obtained, as it is fugitive, and must be watched for, and arrested the moment it appears;—the production of this color, therefore, gives rise to a number of interesting and curious observations. When the red glass is in a proper state for being blown, on being dropped into water, a yellowish-green globule will be produced; on this yellowish globule being heated to the point of fusion and cooled slowly, the red color will gradually appear as the glass cools, until it assumes the most beautiful ruby tint, inclining more to orange than purple. In Rome instances, this color is so delicate that the cooling process, which takes place in the ordinary mode of manufacture, is opposed to the production of the red tint; and it is necessary to expose the manufactured glass to the temperature of a reverberatory furnace;—in which case, the red color will be seen to increase gradually in intensity, until it attains the maximum. If the temperature be too high, or if the ruby glass already produced be placed in a muffle at too great a heat, the light orange-red color soon becomes changed,—first, to crimson, and, finally, to purple. If the heat be increased, it will first assume a blueish tint, and afterwards become colorless. It is therefore ascertained that ruby glass should be submitted to the lowest possible temperature, in order to obtain the brightest tints. From these observations it may be concluded that glass, which contains copper in the state of protoxide, by the addition of tin or carbonaceous matters, will assume successively all the colors of the spectrum, under circumstances which do not appear to be the effect of modification by means of oxygen.

IV. Silver.—Oxide of silver is not often added to the com pound which is melted in glass furnaces, but is generally employed for staining glass a transparent yellow color; for which purpose it is spread upon the surface and burnt in. This color is produced without the addition of any flux, by simply spreading upon the surface of the glass a small proportion of oxide, or any salt of silver in a very comminuted state, mixed with any neutral excipient, such as pulverized argil or red oxide of iron, and exposing the glass to the heat of a muffle. The excipient is then removed by scraping or brushing the surface of the glass, which will then be found to be colored yellow, varying between lemon color or greenish yellow and deep orange, according to the quantity of silver, and, especially, according to the quality of the glass. A red color may be produced by exposing the glass to the muffle twice.

M. Dumas found, by careful analysis, that glass which is susceptible of taking deep tints is composed of elements which approximate the most nearly to definite proportions; this agrees with the observation that glass must be entirely deprived of its excess of alkali by prolonged fusion at a high temperature, in order to enable it to take deep orange and red tints.

It is important not to raise the muffle to too great a heat, otherwise the surface of the glass would become opalescent; although, on looking through it, it still presents a yellow or orange color. On looking at it obliquely it reflects an opaque blue color; and at a still higher temperature it has a tendency towards a purple red when looked through, although the opacity of the surface is augmented, and the color changed to a brownish yellow. If, instead of tinting the glass in a muffle, the silver, added to a mixture of flint glass, be melted in covered pots as quickly as possible, the result is an agate-like semi-opaque substance, which, by the combined effects of refraction and reflection, presents all the colors of the spectrum. This is the more apparent if the surface of the glass, which is generally an opaque yellowish green, be cut at different depths. These effects result from irregularities in the cooling, as has been seen to be the case with manganese and copper.

V. Gold. — Oxide of gold imparts to glass a purple color, which may, by increasing the quantity, be converted into purple red. For this purpose, a small proportion of the purple precipitate of Cassius is added to the flint-glass composition; and on the first fusion the composition furnishes merely a colorless transparent glass; but, on being again heated, its purple color is brought out. Thus, if a small solid cylinder be formed of the glass first fused,it will, on cooling, be quite white; but, on being afterwards ex posed to the heat of the apertures of the furnace, it will gradually, as the heat penetrates, assume a red tint; and this color will become fixed by again gradually cooling in the annealing furnace. It has also been observed, that by varying the degrees of temperature to which a piece of glass of a certain length is heated, and cooling it several times, many tints are produced, varying from blue to purple, red, opaque yellow, and green. It does not, however, appear certain whether this effect may not be attributed to some fragment of silver mixed with the gold employed; the only point which seems positive being the existence of the purple color, which is developed by a second heating, in glass containing gold in its composition. To these results upon coloration by metallic oxides, may be added an effect produced in the coloring of glass by means of carbon, which is of the same nature as those mentioned with respect to the coloration of glass by means of copper and gold. An excess of carbon in the composition of a silico-alkaline glass furnishes a yellow color, which is not so brilliant as silver-yellow, but is nevertheless sufficiently fine to be employed for church windows. This yellow color may also, sometimes (according to the nature of the wood from which the charcoal is made, and the period at which it is cut), be converted into deep red, by a second exposure to the action of the fire.

It may be doubted whether all the facts above mentioned can be explained by the various degress of oxidation of metals.

The multiplicity of colors, which exceed in number those of the oxides attributed to each metal, should lead to an enquiry as to whether these phenomena may not be the results of physical laws. It seems to be a peculiar feature of the present time, and doubtless the result of the great progress made in chemistry and physics, so to unite the study of these two sciences as to render them inseparable from each other. The various facts observed in the coloration of glass, and which are specially produced by the influence of different temperatures, may probably be attributed to some modification in the arrangement of the molecules, giving rise to modifications in the reflection and refraction of the luminous rays; it may, moreover, be observed, that many of the results above mentioned are produced under certain circumstances "which appear to place the glass in a condition of crystallization.

During the last century, Edward Hussy Delaval, acting upon Newton's experiments upon the coloration of thin plates, undertook some researches as to the causes of the changes of color in bodies; but the science of chemistry was not at that period in a sufficiently advanced state to enable him to found his observations upon experiments which could be relied on. At the present time, we have only to collect a sufficient number of facts to enable us to deduce therefrom scientific explanations, leading most probably to further improvements in the manufacture.

With regard to glass, the observations to be made concerning the form and arrangement of its molecules, are of an extremely delicate nature. This is demonstrated by the difference in the action of the light upon it, according to the degree to which it has been annealed. It is known that even a very slight degree of pressure, acting upon a portion of its surface, will suffice to produce thereon the power of double refraction; which is also communicated to it by imperfect annealing. This effect takes place, not only with glass which has been suddenly cooled down from a red heat to the ordinary temperature, and is therefore liable to fly, but also with lumps of glass which had been considered to be well annealed. These specimens of glass will present the phenomena of polarization, which increases the difficulty (already sufficiently great) attending the manufacture of glass for optical purposes. This difficulty, which is considerable even for lenses of three or four inches diameter, is much more formidable for those of ten or twelve; it has, however, been overcome even with a diameter of twenty-two inches. It would, nevertheless, be well for opticians to throw some light upon the various stages of the processes they employ for the construction of achromatic telescopes, as there is no certainty that the glass, which is now considered free from blemish, might not, under very strong magnifying power, be found to have imperfections which had not hitherto been supposed to exist.

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