Page images
PDF
EPUB

chiefly to the faces and limbs of the Madonnas and saints, and many of these being produced in enamelled or flashed glass rather than pot-metal.

We may, perhaps, properly digress for a moment to say that the true cause of the superiority of these old cathedral windows, speaking paradoxically and yet truly, is found in the inferiority of the glass, its richness in the poverty of its constituents, its very perfection in its uneven thickness and the imperfections of its surface and its body, all covered as they are by the accumulating dust of ages and honeycombed by the corroding tooth of time. Like the facets of a diamond or ruby, each little wave and thread and blister becomes, by interference, refraction and reflection of the light which plays upon it, a new source of the gemlike brilliance, harmony, and beauty which distinguish the painted glass of former centuries.

The glassmakers of Berkshire and England aim to reproduce in some measure the perfection of this old glass, by reproducing its imperfections in the antique and rolled cathedral glass which are so extensively used by the artists and architects of to-day.

And now, what the wonderful alchemy of the sunlight is, and what the methods of its operations, are questions on which various opinions have been given by glassmakers and scientific men, but which only a careful consideration and comparison of the observations and theories of many practical and scientific observers can accurately decide. Some have attributed one or another of these colorations to the presence of oxide of iron, some to arsenic, and some to carbon or sulphates in the constituent materials of the glass.

The greater number think oxide of manganese, singular as it may seem, used as a decolorizer, to be the great colorist in all of these changes. In many colored and colorless glasses, and espe cially in those which assume after exposure any tint of purple or flesh color, it undoubtedly plays a very important part. But in some experiments with glass said to contain no manganese, decided changes of color from greenish to yellow have been produced. Perhaps the question cannot be accurately solved, until some glass manufacturer or lover of science and scientific truth shall make, with great care and for this special purpose, a series of colored and colorless glasses, which shall be exposed for months and years to the influence of sunlight. Knowing the exact con

stituents of each specimen, a good foundation would be laid for a thorough scientific investigation of the subject. This has never yet been done, and in the absence of such knowledge, we can only theorize upon the results which we witness.

While I may not theorize, I may help others to do so by stating the interesting part which some metallic oxides play in coloring, and the oxide of manganese in decolorizing glass. In almost all kinds of window glass and glass-ware, materials are necessarily used which are not perfectly and chemically pure.

The sand, the carbonate or sulphate of soda, the lime and other constituents, one or all, contain slight impurities, and almost always oxide of iron.

The protoxide of this metal gives glass a bluish or bluish-green tint, the peroxide a yellowish, and a mixture of the two, in which condition it is generally found in glass, produces the almost universal greenish tints which we witness in the glass of commerce. To correct in some measure the coloration by iron, a small proportion of oxide of manganese, called "glassmakers' soap" is put into the "batch," or glass mixture. The natural tint produced by manganese in a high state of oxidation is purple or violet, and we cannot produce these colors without its use. In proportion as it is deprived of oxygen, it loses its coloring power, and when it reaches the state of protoxide, it becomes nearly colorless.

Now in the reaction which takes place in the melting-pot, the manganese gives up a portion of its oxygen and its coloring power to the iron, which is converted into peroxide, which gives a yellowish color to the glass.

This yellow color is complementary to whatever of purple coloring power is left in the manganese, and is therefore neutralized, and the glass comes out of a light color. When the sunlight strikes this glass, the nice balance between the oxygen of the iron and the manganese is disturbed, and there ensues, even in so solid a body as glass, an interesting contest for the possession of the coveted oxygen, and the conquering element is known and announces its victory by a display of its special colors. Every change of color or tint involves the wonderful condition, invisible to human eyes, of a molecular or inter-molecular movement of the atoms of this solid body, and it is probably attended by two circumstances: an interchange or redistribution of the oxygen among the constituents of the glass, and the development, in consequence of this

redistribution, of the special coloring power of the metallic oxide which has the greatest affinity for oxygen, or stands in the greatest proportion to the other constituents.

And so in some specimens the yellow of the iron predominates, in some the purple of the manganese, and in others, the yellow at first appears, and afterwards a yellowish purple and purple fully developed.

While these suggestions may help to explain the sunlight coloration of the so-called colorless glasses, they may apply also to colored glass, inasmuch as all of the sensitive specimens contain a certain proportion of iron, or manganese, or both.

If this coloring power of the sunbeams has not been generally known, it is because it cannot readily be seen by ordinary observers, except in such as the purple-tinted windows of Beacon street, and even then it may sometimes require the background of a light colored curtain.

The discovery of the defect is an annoyance to both consumers and glassmakers. When our Beacon street friends could no longer obtain similar glass to replace broken panes, the insertion of colorless ones gave their windows the interesting checker-board appearance which we witness to-day. When the glassmakers found the defect a matter of scientific observation and experiment, and their results published at home and abroad, they remedied the difficulty in a measure by using purer materials, or reducing the proportion of manganese in their "batch," or giving up its use entirely, preferring to have the glass assume its natural and more permanent color, even if it be a little greenish or bluish, rather than by "doctoring" the mixture,-to adopt the glass-makers' term for the use of manganese, to have it light colored to-day and easily affected by the sunlight of to-morrow.

This improvement is of especial importance to photographers, who, in all operations requiring short exposures and all the light possible to obtain, would avoid the use of any glass in their skylights which, after a few months or years of exposure, will be robbed of a great proportion of its power to transmit the chemical influence of sunlight by a change to a yellow or purple tint, which in time might cut off almost as much actinic effect as if it had been ground or enamelled on one of its surfaces.

I have made some photographic experiments to show this deteriorating effect, by exposing sensitive paper under glasses of the

original colors, and those of the same kind, changed by sunlight exposure, and witnessing the perceptibly different shades of darkening produced. This action of sunlight must not be confounded with that called "rust," or "stain," which is occasioned in some glasses having an excess of alkali in their composition, by exposure to the atmosphere, and manifests itself in two ways; first, by a disintegration and roughening of the surface, sometimes producing all the effects of ground glass; and secondly, by an efflorescence and apparent formation of an infinitesimal coating of oxide upon the surface, on which the play of the sun's rays produces all the colors of the rainbow, as with the action of light on the infinitesimal grooves of mother-of-pearl. This is simply surface action, whereas the action of sunlight permeates the whole body of the glass wherever the rays directly strike it.

I might refer to many other points in connection with my subject, but my time will not permit and I hasten to exhibit the results of my experiments, asking my hearers, who are so much better versed in scientific studies than myself, to give me their light in ascertaining the causes and exact operations of this interesting power of the sun's rays to paint the products of art, as they do so beautifully and wonderfully the works of nature on the mountain, in the forest and field.

The large number of specimens of colored and colorless glasses here exhibited show the coloring effect of exposure to sunlight. A tangible illustration of this subject is found in the purpletinted windows of some of the Beacon-street houses, of which the specimens exhibited show the original color and the changes produced by sunlight exposures varying from one day to fifty years. These examples also show how this action can be turned to interesting account in impressing upon colored and colorless glasses the forms of leaves and ferns, and in printing inscriptions and mottoes. It is a species of photographic work with sensitive glass, instead of sensitive paper, the sun showing itself a most excellent printer and developer, and, indeed, the only true photographer in colors.

THE DISCOVERY OF OXIDE OF ANTIMONY, IN EXTENSIVE LODES, IN SONORA, MEXICO. By E. T. Cox, of Oakland, Cal.

Up to the present time, the antimony of commerce has been mostly obtained by the reduction of the sulphuret; and though this ore is widely distributed over the globe, it is, as a rule, associated with a variety of mineral substances that obstruct reduction and add to the cost of purifying the metal. These sulphurets are also found in such sparse quantities, that the metal usually commands from three to four times the price of lead and fully as much as that of tin or copper. At present, the principal supply of sulphuret of antimony, for the English smelters, is obtained from Algeria, Spain and Ceylon. Small quantities of oxidized antimony ores have been found in portions of Europe, and in Ceylon, but at no time in such quantities as to elicit special attention from the smelters. When, therefore, about a year ago, I called the attention of English metallurgists and smelters to the occurrence of vast lodes of almost pure oxide of antimony, in the district of Alta Sonora, Mexico, thirty miles from the Gulf of California, it seemed too marvellous for their belief. A company of gentlemen of Boston, Mass., now have control of these antimony mines, and the ore will soon be in the hands of the smelters.

The geological features of the country, where this ore abounds, is similar to that of southern Arizona. The mountains are in short, narrow ranges, having, for the most part, a northerly and southerly trend. Their crests are either rugged or well-rounded cones, according to the nature of the rocks forming their mass. Between these ranges, we have what is called mesa or table-land; the latter is formed of the débris of the mountains. This material is of so loose and porous a nature, that the small amount of rain which falls sinks through it and leaves the land dry and arid. As far as I have been able to make out the order of the rocks forming these mountain chains, we have first granite, and this is flanked by sub-carboniferous limestone, in most places so crystalline as to obliterate all traces of fossils. Protruding through these and forming the mountain peaks, we have porphyry, quartzites, basalts, diorites and trachites.

The country in the immediate vicinity of the antimony mines is quartzite and limestone. The ore lodes are from four to twenty feet wide, and exploration work, carried to a depth of thirty feet,

« PreviousContinue »