GLASS

What would our world be like without glass? Think about it:… walls without windows, bland stone and concrete buildings, drinking from pottery, no Hubble telescope, and on and on.

It is doubtful any man-made material is as abundant, or so widely used in so many forms as glass. Equally remarkable is the availability and low cost of the basic raw material used to produce it, silica, a compound found in quartz, sand, and similar substances. Glass is an amorphous material produced by fusing silica and additives, at very high temperatures. It cools to a hard, brittle state without grain structure and can be re-melted to a viscous liquid state. Glass can be found in nature as obsidian, probably created from volcanic action or the result of lightning striking sand, and tektites found in meteors.

Glass is thought to have first been made as glass beads before 2000 B.C. in the middle-east. Hollow containers were molded in Egypt around 1500 B.C. and glass blowing did not occur until the 1st century B.C.. Glass production flourished throughout the Mediterranean region. The Roman era saw the evolution of more decorative glass with techniques combining glass blowing, molding and the introduction of colored glass threads in the process.

Stained glass windows were developed for churches in the 11th century with the finest installed in France and England in the 13th and 14th centuries. The glass was colored or flashed, then cut into desired shapes and painted with enamels. The pieces were fitted into lead strips and held in iron frames.

Flat glass is produced in a variety of ways. One of the earliest techniques was to pour molten glass onto flat steel plates to cool. Thickness was controlled by rolling steel rollers on ridges at the edges of the plates. Cooling plates are used today to produce beautifully colored stained glass, both for professional construction of windows and doors as well as for the amateur craftsman. Roller designs can create patterns or ripples in the molten glass. The stained glass industry has grown steadily in recent years and has produced works not only of utilitarian value but has combined a very high level of artistic blending of shape and color.

Clear flat glass is required in millions of tons each year to serve the window, architectural, and structural needs of the world. We take for granted the wonders of windows. In his book, Walden, Thoreau said,

"The animal merely makes a bed, which he warms with his body, in a sheltered place, but man, having discovered fire, boxes up some air in a spacious apartment, and warms that, instead of robbing himself, makes that his bed, in which he can move about divested of more cumbrous clothing, maintain a kind of summer in the midst of winter, and by means of windows even admit the light, and with a lamp lengthen out the day."

The evolution of flat glass production has followed many steps over time, including but not limited to molding, decorative art glass, glass blowing, vertical drawing from a furnace and today's float process.

An early technique was to blow a large sphere, then transfer it from the blowing iron to a pontil, a metal shaft. While still in a pliable state, the pontil was spun rapidly. Centrifugal force flattened the globe into a large disk. When removed from the pontil it was cut into squares and rectangles and became windows. This technique produces so-called bullet glass where the circular patterned, thicker glass from the center of the spun disk is included in the window imparting a quaint antique effect if not causing major distortion.

An improvement over the sphere was to blow a large cylinder, cut one side and while pliable, let it lay down flat. Over 300,000 pieces of glass were produced this way and incorporated in the construction of the crystal palace for the 1851 exposition in London.

The development of continuous processing involved passing the hot glass along and between steel rollers controlling the thickness, but unfortunately marring the surface. In order to produce flat, uniform, non-distorting glass, extensive and costly grinding was performed after the glass cooled. This process was used to produce what was called plate glass, thicker than today's window glass.

The next step was to draw molten glass vertically from the furnace, thus avoiding contact with rollers. Thickness was controlled by initially passing the glass through a constricting space then controlling the speed of the draw. Of course, this procedure required enough vertical space to permit the glass to cool slowly. Such controlled cooling is required to "anneal" the glass, which reduces the physical stresses in the finished product. In addition, more vertical space is required to permit workers to score and break the glass into manageable, marketable sizes. Since it is a continuous process, the pieces must be removed to make room for what follows. Although the quality of the glass surfaces was much improved over previous methods, some grinding and polishing was still required to produce what the market wanted.

A major breakthrough in the production of flat glass occurred in England with the development of the so-called "Float" method. Sir Alastair Pilkington of St. Helens, patented the process in the 1952 and licensed its use to most of the manufacturers of glass throughout the world. As with most pivotal engineering developments, the concept is fairly simple. Carrying it out was not.

In the float method, the process is completely carried out on a very long horizontal assembly of furnace, float tank, annealing lehrs, and cutting stations. A ribbon of glass is started from the furnace and moved along by rollers that contact only the "selvage" edges of the ribbon. It passes into a tank of molten tin and a controlled atmosphere above the glass. Tin melts at about 1500 º F at which temperature glass is still in a malleable state. As the glass floats through the chamber it is polished. After leaving the tin bath, it is cooled under the control of ovens (lehrs) which anneal by slowly bringing the temperature down preventing most of the internal stress induced by rapid cooling.

After the annealing lehrs, the ribbon continues along the rollers and passes under cutting wheels which score the moving glass with cross-cut bridges and longitudinal scoring, creating the individual sizes required by the current orders for flat glass. Break-out of the glass occurs when rollers with projections bump the glass upward directly under the score as it passes. The separated pieces are removed from the production line by robotic arms with suction cups to grip the glass. If additional cutting and sizing is required, computer controlled cutting tables are located adjacent to the line for this purpose.

It is awesome to observe a float line. It is approximately 1/4 mile from furnace to shipping and it never (almost never) stops. The only reason to stop the line is to rebuild the furnace which may be required after 10 to 12 years. Laser inspection identifies inclusion blemishes, and other defects as the glass ribbon passes inspection stations. This information is transmitted to a computer system which permits the defect to be cut from the glass and conveyed back to the furnace as "cullet".

The selvage edges are scored and broken from the ribbon and also conveyed back to the furnace. Cross cutting is particularly interesting to watch. The cutting wheel pillar post rapidly travels across the ribbon of glass at a computer controlled speed based on the speed of the glass. The result is a perfectly square end.

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