Manufacturer and Builder 7, 1875
Glass is colored with metallic oxids by two different methods; in one instance the sheet is colored throughout and blown as usual from what is technically termed "pot metal." In the other instance the workman dips his blowpipe, first into colored and afterwards into white glass; when the compound mass is blown out both glasses expand together and a sheet is formed of two layers, one (generally very thin) colored and the other white. On looking through this socalled flashed glass, there is no perceptible difference between it and the pot metals, but the distinction is easily seen by looking at the edge.
All the colors are occasionally made in this manner, but copper ruby glass is always flashed, because it is naturally much too deep in color to allow of its being blown of the ordinary thickness; the layer of ruby glass, or even the darkest sheets are seldom thicker than common letterpaper, the substance of the pane being white glass. These thin colored strata of the flashed glasses are easily removed from the surface by wellknown methods of glass-cutting and engraving, or by etching with the aid of fluoric acid. White patterns on colored grounds, or the reverse, are thus readily produced, many of them, especially in the finely cut and engraved ones, being of exceeding beauty. Unfortunately it is impossible, by any illustration, to give the remotest idea of the brilliancy and life of these designs.
The colors just referred to are produced while the glass is in the furnace, and of course before it is blown into sheets; but by the process of staining all the various tints of yellow, from a faint tint of lemon through full yellow and orange, up to a somewhat brownish red, can be imparted to the glass after it is blown. This process depends upon a peculiar property of silver, which is generally employed in the form of the chlorid, (horn silver,) and is mixed and ground with some inert substance, as oxid of iron or pipe-clay. The mixture or stain is floated over the article to be colored by time aid of seater or spirits of turpentine, and when dry the coating is about the thickness of cardboard. The glass is then brought gradually to a full red heat, and afterwards annealed; during the operation the silver penetrates .d actually dyes or stains time glass, the oxid of iron or clay remaining loose upon the surface. The color thus obtained is perfectly clear and brilliant, and the surface of the glass appears to have undergone no change, so that finished cutglass goblets and vases can be colored entirely or (by very simple and apparent modifications of the process) in any device that may be desired. The intensity of the stain is in proportion to the quantity of silver in the mixture and the degree and duration of the heat; but the darkest and richest tints can only be produced on glass made for the purpose.
This property of silver is invaluable to the ornamental window painter; it enables him to produce colors, although his choice is limited, into designs executed upon common windowglass, and by using flashed glass and partially removing the layer of color by etching with hydrofluoric acid, lie can stain the white glass thus laid bare, and is a step nearer the solution of his grand difficulty—the production of various clear and transparent colors in the same piece of glass.
The colored glass paints never possess the clearness and richness of the colors already described. These glass paints are made of very fusible glasses ground and mixed with oil of turpentine; they are laid on with a brush in the usual manner, and when sufficiently heated, melt and fasten to the glass, but never become quite clear and transparent. The difficulty. of insuring even moderate success in glasspainting is mainly owing to the great and often apparently capricious alterations which the colors under-go in baking; time shade or opaque colors always vary in tint as they are seen by reflected or transmitted light, and when to these obstacles is added the wellknown brittleness of the material, it is scarcely surprising that time longest experience and most practiced care can not always avoid disappointment.
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