a hard, brittle substance made by melting together under intense heat, sand and either potash, soda, lime or oxide of lead. When the substances from which it is made are pure, glass is usually transparent. HISTORY. Glass has been in use for so many centuries that we do not know when it was first manufactured. Tombs of the ancient Egyptians which were built more than 3000 years B. C. show glassblowers at work, and specimens of glass that were made more than 2000 B.C. have been found in other tombs of these ancient people. The Chinese and other peoples of the East also became very skilful in the manufacture of glass before the Christian era. Vases, pitchers and other ornamental vessels found among the ruins of the buried cities in Greece and in Asia Minor, show that the Greeks and other ancient peoples of this part of the world were well acquainted with the manufacture of glass. The earliest use of glass was for ornamental purposes, and all of the articles which have been discovered in the ruins of ancient cities are of colored glass. The use of glass for windows is of more recent origin. As far as known, it was first used for this purpose in England in the latter part of the seventh century, but its use was restricted to colored glass which appeared in cathedral windows. The manufacture of glass was introduced into the United States at an early date, but it did not become successful until after the Revolutionary War. The leading countries in the manufacture of glass now are Austria-Hungary, England, France and the United States. In this country the center of the industry is around Pittsburg, Pa., and in the natural gas and coal regions of Indiana, Illinois and some other states. KINDS OF GLASS. Owing to the different substances used in its manufacture, the various proportions in which these may be combined, and the different methods of manufacture, there are many different varieties of glass. Colored Glass. However strange it may seem to us, the glass made by the ancients was colored. Originally this coloring was probably due to the impurity of the materials used. Later it was discovered that different colors could be produced by mixing certain substances with those ordinarily used in the manufacture of glass, and all colored glass made at the present time is produced in this way, the oxides of metals being used for most of the colors. Chloride of silver and gold produce yellow; compounds of iron, copper and gold produce red; blue is produced by cobalt, and orange by peroxide of iron and chloride of silver, while the oxide of tin gives a white color. By mingling these coloring matters in various proportions, a great variety of tints can be produced. Ornamental Ware. There are many methods of making ornamental glassware. Sometimes the glass is colored by dipping a transparent glass into colored glass when the latter is in a molten state. Figures worked into vases and other articles, as shown in Figs. 6, 7, 10 and 11 in the accompanying color plate, are usually made by working the colored glass into the glass forming the body of the object. This is usually done by placing the figures upon the outside of the vessel and then subjecting the glass to such heat as will partially fuse it. The figure then sinks into the body of the glass and blends with it. Other beautiful effects, such as are shown in Figs. 4 and 9,are produced by blending glasses of different colors, either in the manufacture of the article or in the material from which it is to be made. For the method of making colored glass windows, see article on STAINED GLASS. Crown Glass is the English window glass and differs from that made in the United States by being blown into spheres, instead of cylinders. When these are flattened they make circular plates. Crown glass is considered by some persons to be of better quality than cylinder glass, but it is not so desirable for windows, because it is of uneven thickness. Bottles and most small articles are made by blowing the glass into molds (See BOTTLE). Bottles and fruit cans are also made by machinery. Pressed Glass. Much of the table ware and many other small articles are made by pressing the glass in molds. The mold is of iron or steel and contains any ornamental designs which the article is to take. A sufficient quantity of melted glass to make the article is dropped into the mold, and then a plunger, which forms the inside of the article, is forced down upon it. This presses the glass into every part of the mold and impresses upon it the desired design. Plate Glass. Plate glass is made by casting on an iron table. The melted glass is poured onto the table, which has a steel rim rising above the surface to the height of the desired thickness of the glass. As the melted glass is poured, a heavy roller is passed over it, forming the plate. The peculiar softness and brilliancy of plate glass are due to the purity of the material used and the polishing which the glass receives. Flint Glass. This is a variety in which potash and pure sand are used, making it particularly clear and transparent. It is used for the best articles of table ware and in the manufacture of small bottles, vases and other smaller articles. Cut Glass. This is made by grinding the best quality of blown glass on wheels, over which flow streams of water and sand. This glass is prized for its brilliancy and is very expensive. MANUFACTURE. Glass is made in factories especially constructed for the purpose. The most important part of the factory is the furnace. This may be circular, with a shelf running around next to the wall, or it may be rectangular, with the bottom sloping towards one end. In either case, the furnace is the base of a huge chimney. The fuel used is gas, and the furnace must be supplied with a strong draft, in order to insure the intense heat necessary to melt the materials. These are placed in pots, when circular furnaces are used, and the pots rest upon the shelf around the wall of the furnace. These pots are made of fire clay and are very carefully constructed. Each holds from 1500 to 2000 pounds of material. For the best quality of glass the pot is hooded and has an opening on the side of the hood, through which the material can be put in and the melted glass taken out. For cheaper varieties the pots are open at the top. In the tank furnace the material is thrown into the tank, where as fast as it melts, it runs down the sloping bottom to the lower end of the tank, from which it is taken out, while the raw material is put in at the opposite end. The kind and quality of glass depend upon the substances used. For a good quality, all of the raw material must be pure. Sand forms the basis of all varieties and is the most difficult substance from which to separate the impurities. Animal and vegetable impurities are removed by burning, but mineral impurities cannot be separated. Iron is the most objectionable of these, since it discolors the glass and, except for the poorest qualities, makes the sand worthless. Most sand is obtained from banks, but that for the highest grade of glass is prepared by crushing pure quartz rock. The other materials determine the kind of glass. Lime makes a hard, brilliant glass, but too much of it makes the glass brittle. Potash makes a clear, transparent glass, while soda imparts the green tint usually seen in cheap bottles. Oxide of lead imparts a brilliant luster to glass and also makes it soft. Window glass is usually made of sand, lime and potash or soda, and plate glass of sand, soda and lime. Preparation of Material. The ingredients are ground to a fine powder and thoroughly mixed in proper proportions, forming what is called the batch. To this a small quantity of broken glass, or cullet, is added to assist in melting. In pot furnaces it requires about twenty-four hours for the glass to melt and become clear. When this has been done the fire is lowered, and the glass is taken out and worked. In tank furnaces the material is constantly being added and the glass being withdrawn. Methods Used. There are three general methods of shaping glass, blowing, pressing and casting. Bottles, window glass, vases and the most expensive tableware are blown. The glass blower uses only a few tools of the simplest patterns and depends almost entirely upon his skill to obtain the desired results. He gathers on the end of his blowpipe, which is a straight iron pipe about four feet long, a sufficient quantity of melted glass to make the article desired, then by blowing into this, rolling and swinging the pipe and using such tools as calipers and burnishers, he proceeds to fashion the object. In blowing window glass, the workmen stand upon a bridge over a trench, which is several feet deep, and they use larger and longer blow-pipes than those used in making small articles. The workman gathers upon his blowpipe from twenty to twenty-five pounds of glass. By blowing into this and swinging his pipe, he causes this mass at first to take on a shape resembling a pear; then by holding his pipe upright and blowing into the glass and rotating the pipe, he changes the pear into the form of a cylinder. When the cylinder has been perfected its ends are cut off; it is then laid upon a table and scratched lengthwise with a diamond; it is then cut in two on the side by laying a cold iron over the mark. The open cylinder is placed in the flattening furnace, where, as it softens, a workman flattens it by pressing it down upon a table with a piece of charred wood attached to a long handle. This makes a pane of glass about forty-five inches long and thirty-six inches wide.