Manufacturer and Builder 9, 1870
In our first volume we gave, on pages 147 and 302, a description of the manufacture of colored glass, to which we will now contribute a new chapter, on the manufacture of colored window-glass for ornamental purposes.
It has been said, on page 302, that the oxides of the heavy metals, with the exception of lead, have the property of making colored glass, and that the oxides of the light metals all partake of the quality of lead, and make with silica a colorless glass. The light metals used are potassium, sodium (Not "iodine," as was mentiopned by a typographical error, page 309,) cadmium, magnesium, etc.
There are two kinds of ornamental window-glass - white glass ground and cut in different figures, and glass colored with the metallic oxides. The first is chiefly used when the purpose is simply to admit light and to prevent persons from looking through the panes; it dispences with curtains, and besides, may be made highly ornamental and expensive. There are two ways of procuding it - the first by actual grinding, the second by the action of hydrofluoric acid. A pane of common or plate glass may be expeed to the cutting-edge of a revolving disk moistened with water and emery of fine sand; and in the way figures may be ground in which will be translucent, like ground glass on a perfectly transparent ground. More commonly, however, the glass is first ground all over, on one side, so as to change its perfect transparency into translucency, and then ground surface is cut or ground and the cut parts polished; by this operation we have transparent figures on a translucent ground. Such window-glass is extensively used and quite ornamental.
In order to understand how the same effect is produced by means of hydro-fluoric acid, it must only be known that the vaports of this acid attack glass in such a way that it leaves its polished surface so that it very much looks like ground glass, while the content with the fluid acid dissolces the surface, and gives its polished, perfectly transparent look. The most common way of applying this process is to cover the glass with a thin coat of wax or some suitable varnish, and design on this coat the figures intended to be etched in the glass, scraping it off so as to expose those parts of the glass on which the acid is to act. If we take ordinary glass with polished surface, and expose a plate so prepared to the fumes of hydro-fluoric acid, we shall obtain ground figures on a polished surface; while if we take a plate of ground glass, prepare it in the manner described, and expose it to liquid hydro-fluoric acid, (in the same way as metallic plates are etched with nitric acid for printing,) we shall obtain transparent figures on a ground surface.
In order to multiply regular figures on a large scale in a manufacturing way, stencil plates are used, by which the varnish is applied on those parts of the glass to be protected against the action of the acid, and in this manner the large ornamental glass plates often used in offices, doors, etc., are produced at a moderate price, which would be impossible if the figures were all drawn in detail by hand labor.
The colored ornamental window-glass is also of two kinds, painted and manufactured. In painted glass designs, such as church windows, etc., each specimen may be regarded as a picture, usually executed but once, and depending on the skill of the workman or artist for its beauty and effect. Such designs are not capable of reproduction unless by the same tedious operation that perfected the original; whereas, in the class of the ornamental specimens to which we wish to call the reader's attention, each design is capable of easy as well as cheap and rapid reproduction. Compared with a "painted window," it is as printing compared with writing, or as an engraving compared with a painting; and though the results thus far obtained have been somewhat imperfect, yet they possess so much decorative beauty as to promise large and full rewards for hte application of enterprise to the discoveries of art.
We have before, in the article on colored glass, (page 302, Vol. I.,) explained the difference between so-called pot-glass and flashed-glass; they can not be distinguished except when seen on edge, when the pot-glass will show the color all through, while the flashed-glass will show a white body with a thin layer of colored glass on one surface. In some of these the layer is very thin, as is always the case in ruby glass, and such glasses are very well adapted for producing colorless figures on them, by grinding the colored surface away in some places, or doing the same by means of hydro-fluoric acid, as above explained for white glass. This process has been brought to such perfection in Germany and England that it is impossible to give the remotest idea of its beauty, even by means of the most finished print.
The best staining for ready-made glass objects thus far discovered is made with chloride of silver, which produces yellow tints, from a faint lemon, through full yellow and orange, up to a brownish red, in proportion to the quantity of the silver and duration of the heat to which the objects were exposed. This stain is so easily applied that glass painters consider it very desirable that stains of other colors may be discovered of as easy application as this. It is, indeed, of so easy an application that it enables the window-painter to introduce this series of colors (yellow, orange, and brown) into designs executed upon the most common window-glass, and by using flashed-glass and partially removing the layer of color by etching with fluoric acid, he can "stain" the white glass laid thus bare, and so make a step nearer the solution of the great difficulty - the production of various clear and transparent colors in the same piece of glass.
The other colored glass paints do not possess the clearness and richness of the above. The blue and the green, particularly when applied with the brush, and burned in, as is done in flashing, require special precautions to obtain the clearness found in the same materials when made as pot-glas; and, as before mentioned, it requires artists who devote their lives to this art to produce effects as perfectly satisfactory as we find in some glass-windows in the old churches and castles of Europe.