Colored Glass.

Manufacturer and builder 10, 1869

With few exceptions, the oxides of the heavy metals possess the property of producing with silica colored compounds, which may be combined with ordinary glass, tlee latter being, when pure, a colorless compound of silica with oxides of tlee light metals. The light metals are, potassium, iodine, calcium, magnesium, aluminium, etc. The heavy metals used to form colored compounds are, iron, copper, cobalt, antimony, gold, uranium, manganese, chromium, etc. Lead is an exception, as its oxide forms no colored compound with silica, but a perfectly transparent and colorless one; in fact, it transforms common glass into flint-glass, as was explained on page 98 of this journal. There are two methods of coloring glass; one is to mix the metallic oxide intimately with the material of common glass, or of the flint-glass, and put both together into the pot; this kind of glass is therefore called pot-glass, and is only used for the colors pro-duced by the cheaper metallic oxides. Tlee second method produces the so-called flashed glass, and consists in covering only tlee surface of colorless glass with a very thin layer of the colored glass. This may be accomplished in two ways. By leaving two pots, one with colorless and one with colored glass, and dipping a globe of hot colorless glass into the pot with colored glass, a layer of tlee latter will adhere, and by the dexterity of the workman may be extended over the whole surface of the object lee is making, be it a goblet or a window-pane. The other way is by means of a brush to cover the glass object after it is made with a cream-like mixture containing the coloring metallic oxide. After it is dry, it is placed in a suitable furnace and heated as highly as the glass can stand without melting. It is then slowly cooled, and the operation repeated if tlee layer applied has not been fused sufficiently to combine with the surface. It is evident that this coloring layer must be slightly more fusible than the original glass object, and this is a very delicate point. If the colored mixture be too fusible, it will melt and run down; if not fusible enough, tlee original glass itself may become soft before this combination has taken place.

The art of painting on glass consists chiefly in the preparation of tlee diverse metallic oxides, which, by previous tests, are known to produce certain colors. The operation is rendered peculiarly difficult from tlee fact that, at the time the colors are used by the artist, they all look nearly alike, being a dirty brown. The desired colors appear only after tlee pane of glass on which tlee painting leas been made is exposed in a furnace to such a heat as to melt the compound and muse it to combine, to a greater or lesser depth, with tlee surface of tlee colorless glass beneath. In olden times this art was highly esteemed, as is evidenced by the painted windows in many churches on tlee European continent, some of which are justly celebrated as containing master-pieces of tlee highest artistic merit. Among them stand foremost those in the Protestant cathedral in the city of Gouda, Holland, a Christian Mecca for lover's of peculiar art productions. Among the common people of Europe an idea prevails that some secret in regard to this art has been lost; this, however, is by no means the case. The manner and means of their production have always been perfectly known; but we no longer have the artists who devoted their lives to the practice of this very difficult and hazardous department of art.

In this kind of work, as in the preparation of colored glasses in general, the effects are calculated for transmitted light, the colors being transparent. On the other hand, enameled and opaline glasses are intended for reflected light, and in such cases, opaque or semi-translucent glass and colors are used.

In general, it has been found that it is easier to color glass when it contains lead, that is to say, flint-glass; in fact, all the imitations of precious stones, gems, etc., are made from a very soft lead-glass, its fusibility and aptitude to take the color being greater, and its brilliancy being more marked. Soda and lead oxides make glass more brilliant and fusible, but at the same time very soft, whence the name of paste, which is applied to this compound, such imitation stones being in reality as soft as a paste when compared with the genuine gems,whose hardness is so extreme that they never lose their polish and original lustre, as is the case with imitations. For this reason, the so-called doublets have been introduced, in which a thin genuine gem is pasted on the exterior or exposed surface of an imitation of the same color made of soft glass. This is extensively practiced in the East-Indies, and such stones will of course retain their polish, but can never be fully as brilliant as the genuine article.

The coloring materials for glass are the same as for tlee imitation gems, only in glass any variety of color may be used, while in the imitation of gems we can adopt only such peculiar colors as resemble special gems.

Yellow glass.
- This is produced as follows: 1st. A dirty yellow by charcoal, passing into a dark brown if the coloring agent be used in excess. 2d. A beauti-ful bright yellow by antimony, in the state of the so-called glass of antimony, or antimonite of potash. 3d. Silver in combination with alumina, in the state of chloride of silver and clay. 4th. Uranium, in the state of oxide, produces a beautiful but expensive canary yellow; this glass is very interesting to tlee scientist, as, by being exposed to electricity in the dark, it be-comes illuminated by a peculiar greenish fluorescence.

Red glass.
- 1st. Iron, used in the state of bloodstone ochre as derived from the nitrate, gives a cheap brownish-red color, whose quality depends on the purity of the sesquioxide of iron used; the protoxide gives another color, to which we shall refer hereafter. 2d. Copper, in the state of suboxide, gives a very brilliant red, which leas long been known. A peculiarity is that this glass looks nearly colorless, with a slight tinge of green, when leaving the furnace, and only becomes red when, after cooling, it is heated a second time. As this red is so intense as to make the glass opaque if not used in very small quantity, it is always flashed. 3d. Gold, in the form of purple of Cassius, gives a scarlet, carmine, rose, or ruby tint; as it is very expensive and intense, it is always flashed.

Orange glass is made in Bohemia, from a glass of antimony, red lead, and a little oxide of iron.

Violet glass.
- Manganese, in the state of peroxide; care is to be taken that no coal or soot shall come in contact with it during the melting, as the carbon would reduce the peroxide to a protoxide, which gives no color at all.

Blue glass.
- Oxide of cobalt, in its different forms as smalt, taffre, etc., is the only true blue color produced in glass; the shade and tone is modified by different quantities and admixtures.

Green glass.
- 1st. Protoxide of iron, in small quantity; the resulting glass has little brilliancy. 2d. Peroxide of copper gives a beautiful emerald green; if the glass contains lead, it is more brilliant still; if the glass is not transparent, but dull or only translucent, it becomes deep blue. 3d. Chromium, in the state of the sesquioxide, or genuine pure chrome green, gives a brilliant grass-green color. It bears a high price. 4th A mixture of the oxides of nickel and uranium; this is used in Bohemia, where the color produced is called modern emerald-green, to distinguis it from the peroxide of copper green, which they call ancient emerald-green.

Black glass.
- A mixture of forge-scales, (protoxide of iron,) bone ashes, (phosphate of lime,) and charcoal, (carbon,) n excess, added to the ordinary materials, makes a black glass, which in Bohemia is called jasper; it is perfectly opaque, very hard, and possesses a remarkable lustre. Its properties are such that it may be used for boiling liquids without risk of breakage. It is reported that in Bohemia basalt or lava is used, with or without the forge-scales.

Bronze colored glass.
- If, in the last recipe, lead slags are substituted for forge-scales, and opaque yellowish bronze-colored jasper is produced. Bottles of the opaque blackish kinds of glass are now extensively used by chemists and photograpjers, to protect many chemicals that are sensitive to light against its decomposing influence. These bottles are mostly imported from Bohemia.

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