Glass is a combination of sand, flint, spar, or some other silicious substances, with one or other of the fixed alkalies, and in some cases with a metallic oxide. Of the alkalies, soda is commonly preferred; and of the silicious substances, white sand is most in repute at present, as it requires no preparation for coarse goods, while mere washing in water is sufficient for those of a finer quality. The metallic oxide usually employed, is litharge, or some other preparation of lead. Iron is used in bottle-glass.
The silicious matter should be fused in contact with something called a flux. The substances proper for this purpose are lead, borax, arsenic, nitre, or any alkaline matter. The lead is used in the state of red-lead; and the alkalies are soda, pearlash, sea-salt, and wood-ashes. When red-lead is used alone, it gives the glass a yellow cast and requires the addition of nitre to correct it. Arsenic, in the same manner, if used in excess, is apt to render the glass milky. For a perfectly transparent glass, the pearlash is found much superior to lead; perhaps better than any other flux, except it be borax, which is too expensive to be used, except for experiments, or for the best looking-glasses.
The materials for making glass must first be reduced to powder, which is done in mortars or by horse mills. After sifting out the coarse parts, the proper proportions of silex and flux are mixed together, and put into the calcining furnace, where they are kept in a moderate heat for 5 or 6 hours, being frequently stirred about during the process. When taken out the matter is called frit. Frit is easily converted into glass by only pounding it, and vitrifying it in the melting pots of the glass furnace; but in making fine glass, it will sometimes require a small addition of flux to the frit to correct any fault. For, as the flux is the most expensive article, the manufacturer will rather put too little at first than otherwise, as he can remedy this defect in the melting pot. The heat in the furnace must be kept up until the glass is brought to a state of perfect fusion; and during this process any scum which arises must be removed by ladles. When the glass is perfectly melted, the glass-blowers commence their operations.
For the best flint-glass, 120 lbs. of white sand, 50 lbs. of red-lead, 40 lbs. of the best pearlash, 20 lbs. of nitre, and 5 oz. of manganese; if a pound or two of arsenic be added, the composition will fuse much quicker, and with a lower temperature.
For a cheaper flint-glass, take 120 lbs. of white sand, 35 lbs. of pearlash, 40 lbs. of red-lead, 13 lbs. of nitre, 6 lbs. of arsenic, and 4 oz. of magnesia.
This requires a long heating to make clear glass, and the heat should be brought on gradually, or the arsenic is in danger of subliming before the fusion commences. A still cheaper composition is made by omitting the arsenic in the foregoing, and substituting common sea-salt.
For the best German crystal-glass, take 120 lbs. of calcined flints or white sand, the best pearlash, 70 lbs, saltpetre, 10 lbs.; arsenic, 1/2 lb., and 5 oz. of manganese. Or, a cheaper composition for the same purpose is 120 lbs. of sand or flints, 46 lbs. of pearlash, 7 lbs of nitre, 6 lbs. of arsenic, and 5 oz. of manganese. This will require a long continuance in the furnace; as do all others where much of the arsenic is employed.
For looking-glass plates washed white sand, 60 lbs.; purified pearlash, 25 lbs.; nitre, 15 lbs.; and 7 lbs. of borax. If properly managed, this glass will be colorless. But if it should be tinged by accident, a trifling quantity of arsenic, and an equal quantity of manganese, will correct it; an ounce of each may be tried first, and the quantity increased if necessary.
The ingredients for the best crown-glass must be prepared in the same manner as for looking-glasses, and mixed in the following proportions: 60 lbs. of white sand, 30 lbs. of pearlash, and 15 lbs. of nitre, 1 lb. of borax, and 1/2 lb. of arsenic.
The composition for common green window-glass is, 120 lbs. of white sand, 30 lbs. of unpurified pearlash; woodashes, well burnt and sifted, 60 lbs.; common salt, 20 lbs.; and 5 lbs. of arsenic.
Common green bottle-glass is made from 200 lbs. of wood-ashes and 100 lbs. of sand, or 170 lbs. of ashes, 100 lbs. of sand, and 50 lbs. of the slag of an iron furnace; these materials must be well mixed.
The materials employed in the manufacture of glass, are by chemists reduced to three classes, namely, alkalies, earths, and metallic oxides.
The fixed alkalies may be employed indifferently; but soda is preferred in this country. The soda of commerce is usually mixed with common salt, and combining with carbonic acid. It is proper to purify it from both of these foreign bodies before using it. This, however, is seldom done.
The earths are silica (the basis of flints), lime, and sometimes a little alumina (the basis of clay). Silica constitutes the basis of glass. It is employed in the state of fine sand or flints; and sometimes for making very fine glass, rock crystal is employed. When sand is used, it ought, if possible, to be perfectly white, for when it is colored with metallic oxides, the transparency of the glass is injured. Such sand can only be employed for very coarse glasses. It is necessary to free the sand from all the loose earthy particles with which it may be mixed, which is done by washing it well with water.
Lime renders glass less brittle, and enables it to withstand better the action of the atmosphere. It ought in no case to exceed the 20th part of the silica employed, otherwise it corrodes the glass pots. This indeed may be prevented by throwing a little clay into the melted glass; but in that case a green glass only is obtained.
The metallic oxides employed are the red oxide of lead or litharge, and the white oxide of arsenic.
The red oxide of lead, when added in sufficient quantity, enters into fusion with silica, and forms a milky hue like the dial-plate of a watch. When any combustible body is present, it is usual, in some manufactories, to add a little white oxide of arsenic. This supplying oxygen, the combustible is burnt, and flies off, while the revived arsenic is at the same time volatized.
There are several kinds of glass adapted to different uses. The best and most beautiful are the flint and the plateglass. These, when well made, are perfectly transparent and colorless, heavy and brilliant. They are composed of fixed alkali, pure siliceous sand, calcined flints and litharge, in different proportions. The flint glass contains a large quantity of oxide of lead, which by certain processes is easily separated. The plate glass is poured in the melted state upon a table covered with copper. The plate is cast 1/2 an inch thick or more, and is ground down to a proper degree of thickness, and then polished.
Crown-glass, that used for windows, is made without lead, chiefly of fixed alkali fused with silicious sand, to which is added some black oxide of manganese, which is apt to give the glass a tinge of purple.
Bottle-glass is the coarsest and cheapest kind, in this little or no fixed alkali enters the composition. It consists of alkaline earth and oxide of iron combined with alumina and silica. In this country it is composed of sand and the refuse of the soap-boiler, which consists of the lime employed in rendering this alkali caustic, and of the earthy matters with which the alkali was contaminated. The most fusible is flint-glass, and the least fusible is bottleglass.
Glass Types and Forms
In addition to compatibility, glass artists also differentiate among different types of glass in many different ways. One of the major criteria for differentiation is the transparency of the glass. Opaque glasses that do not transmit light are generally referred to as "opaques", as "opalescent" glasses, or as "opals." See-through glasses of various colors are usually called "transparent" or "cathedral" glasses. Combining more than one different opalescent or cathedral glass or color in a single kiln-formed work is common.
Several different companies offer lines of tested compatible glass, with the largest and most popular being Bullseye and Spectrum. Other companies offering tested compatible glass include Uroboros, Effetre (Moretti), Wasser, and Gaffer.
Bullseye, which has produced tested compatible glass since the 1970's, is generally acknowledged as the market leader, with a broader product offering than Spectrum or other brands. Spectrum's tested compatible program, initially launched in Spring 2000, contained glasses made by both Spectrum and Uroboros, and is marketed under the "System 96" name. Although the two product lines behave similarly in the kiln, they are not compatible, so most glass artists and hobbyists choose one or the other brand as their primary glass for fusing and slumping.
It should be noted that Bullseye, Spectrum, Uroboros, and many other firms also manufacture glass that is not guaranteed compatible. (A complete list would also include companies such as Armstrong, Desag, Freemont, GNA, Kokomo, Wissmach, and Youghiogheny. Sometimes the glasses made by these companies tests compatible for fusing, but often it does not. If you wish to use any of these glasses for kiln-forming projects involving more than a single sheet of glass, you will need to test for compatibility.
Virtually any stained glass, whether tested compatible or not, can be treated with an iridescent coating that causes the treated side of the glass to take on a metallic sheen. Some liken this effect to a shimmering rainbow. The shimmer goes away when the piece is lit from behind, allowing the normal color of the glass to shine through.
Another popular kind of glass coating, called "dichroic", has the unusual property of reflecting one color while it transmits another. This means that the different colors can be viewed by examining the glass at different angles. This unique glass is manufactured by spraying a thin chemical film on the glass. This must be done in a controlled environment in a vacuum chamber, making dichroic glass one of the most expensive glasses made for kiln-forming. Because of this expense, dichroic glass is more commonly used in jewelry and similar items, or as an accent in larger scale fusing projects.
One final type of glass that is often used for kiln-forming is "float" glass. Made by "floating" molten glass on a bath of molten tin, float glass is better known as common window glass. It is inexpensive and widely available. It also works well in the kiln, but care should be taken to test for compatibility if different brands and types of float glass are mixed together. If at all possible, cut pieces to be fused together from the same glass sheet.
Although some colored varieties of float glass are available, it is most commonly found in a clear (often slightly greenish) formulation. It tends to slump and fuse at slightly higher temperatures than most art glass (about 75 to 100 degrees F higher), and can be prone to devitrification. Its COE depends on the specific formulation used and can be as low as 83 or as high as 90, but it generally ranges from 85 to 87.
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