STAINED GLASS The process of coloring glass requires particular care. When it is to be of full color (as commonly for windows of rich decorative effect) it is called 'pot metal ;' hut, while colored glass of many hues and tints is used in which the whole mass of the glass is equally stained throughout, certain colors are prepared by covering one side of a piece of col orless glass with pot metal of the color required. Crimson and ruby glass for windows is made in this way, the process being called 'flashing.' Opaline glass, much used for windows in America since 1885, though known and used long before, when very much clouded or stained so as to be nearly opaque, for utilitarian purposes, has some of the opal's power of giving at once the general color and its complementary color; thus glass of smoky blue shows also a spark of orange or of pinkish light. It has been found practicable to give the opaline character to glass of the full color of pot metal; and upon this depends much of the splendor of recent American windows. Glass so colored is often not of the same degree of translucency throughout. Some pieces of opaline and other glass prepared for windows vary much in the same piece with regard to this matter of translucency. The designer, while di recting the choice of glass for a rich mosaic win dow, will sometimes be in doubt just where to cut off the sheet, and will experiment with rays of sunlight falling upon the glass to see where it becomes too nearly opaque for his use. Some of these sheets of glass prepared for rich win dows have much change of color in their mass, clomlings, and veinings of different hues, the contrasts being enhanced by the varying thick ness of the parts of the same sheet.. The com poser of a rich window knows how to utilize these contrasts and gradations.
Iridescence, as found on the surface of ancient glass vessels and fragments which have been long buried in the earth, is the result of a curious process of decay, in the course of which the sub stance of the glass becomes disintegrated, form ing thin lamina:, which are no longer brittle as a thin piece of undecayed glass would be, but have rather the crumbling texture of flakes of mica or the like; that is say, this part of the glass has lost its vitreous qualities. Modern attempts to produce iridescence are, of course, not in that direction ; the attempt has been to give the changing play of colored light to the surface of perfect glass. They are of different character, according to the greater or less degree of translucency desired. The ancient and partly
decayed glass is of course mainly non-transpar ent, but still allows some light to pass through it. Thus in a phial or plate of that pale green trans parent glass so common in the Greco-Roman pe riod, parts will he brilliantly iridescent when the piece is found, and still other parts will have a whitish degeneracy which is incipient change of mass. These parts will be comparatively opaque, not allowing a small object to be clearly seen through their substances, while other parts of the same vessel will be entirely transparent as soon as the earthy deposits have been removed. The most common iridescent glass of modern manufacture is, however, entirely clear, and an iridescent effect of the surface, like that of the surface of a soap-bubble, is given by some process which is often kept secret. During the last two decades of the nineteenth century a very brilliant glass was made in America by a single establish ment. In this glass, the uniting of successive layers of the material, one imposed upon another while hot, allows of giving to one or both sur faces a brilliant. iridescence which, when fully developed, makes the whole mass entirely non translucent. It is full of the play of brilliant color, nearly like a rich piece of mother-of-pearl. This process differs wholly from enameling (q.v.). in that the glass to be made iridescent is not allowed to lose its high degree of heat. It is continually returned to the furnace at very brief intervals, and the layers of glass become a solid mass, although close examination will often reveal their once separate existence and their number. The idea may havebeen taken from the Greco-Roman pieces, which are often amber or violet in the natural state of the glass. like mod ern pot metal, and rendered opaque and iridescent by the gradual decay of the exterior surface.
In the course of experiments upon iridescent and lustrous surfaces, much was done in the way of depositing iron, tin, and other metals in a nearly pure state. By way of producing tours de force for exhibition pieces, these metallic de posits were sometimes carried to surprising ex tremes; thus about 1890 a vase was exhibited which was of glass within, but had its outside almost wholly non-vitreous and showing only an opaque and rough surface of warm brown ; and this piece was described by the exhibitor as con taining as much iron as glass could possibly re ceive and retain.