Symposium Materials of Construction in the Building Industry (Continued from October Issue) Papera presented before the Division of Indutrial and Engineering Chemistry at the 89th Meeting of the -4merican Chemical Society,
New
York, N Y., April 22
t o 26, 1935
LASS as we know it is about 6000 years old, so far as archaeologically dated relics go; its discovery as a “synthetic” material is hidden in the mists of mythological folk lore. One small, ill-shapen, opaque relic, much like a bead, and assigned a date of approximately 4000 H . C., is the oldest known example of the glassmaker’s art. From that opaque bead t o the limpid transparence of modern plate glass is a tremendous step which has required centuries to encompass ; howeyer, from a manufacturing standpoint virtually all of the distance has been covered since 1688, and the most startling advances have come wibhin these first few decades of the twentieth century. Flat glass, essentially opaque, was known and used in ancient Rome as a translucent closure for openings in walls. Chiefly, however, glass was used for decorative articles or containers of one sort or another. It attained great importance in Alexandria along with the famous library. S t about the time that the Alexandrian Library mas destroyed, the seat of the art was moved to Rome. Seemingly, Greece had little interest in glass and it vas not important there. RINGROLLOF G L a s s - C m r I x
A\PPARATUS
Glass in Building R. A. MILLER Pittsburgh Plate Glass Company, Pittsburgh, Pa.
In Rome the glassmaker attained considerable importance, and one entire section of the city was given over t o glassmaking. These Roman shops were all individual, personal ventures, similar to establishments to be found even today in the Black Forest and the more heavily wooded portions of Czechoslovakia and other European countries. From Rome the center of the art moved to Venice where the Doge considered the glassmaker one of his chief citizens and the social-political power of the craft probably attained its highest pinnacle. The art was surrounded with the strongest safeguards and every precaution was exercised to prevent the transmission of the secrets of glass t o other parts of the world. It was a capital offense for a glassmaker to disclose his secrets to a foreigner or to leave Venice for the purpose of instructing others. Sonetheless, the art was disclosed inevitably, and France, Germany, and England became supreme in the field of artistic tableware. Perhaps the most n o t a b l e c o n t r i b u t i o n t o t h e knowledge of glass composition during the Middle Ages was the discovery of lead glass in London about 1554. The great brilliance of the lead glass far outshone the previous products and spelled the doom of the Venetian art. Lead glass is the basis of the cut crystal and many of the optical glasses of today.
Manufacture of Window Glass Approximately a t the end of the seventeenth century the manufacture of window glass began to take on some of the aspects of modern technic, and from this time until the present, and particularly in the last few decades, progress has been rapid. Formerly, window glass was produced by blowing a 1291
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VOL. 27, NO. 11
INDUSTRIAL AND ENGINEERIKG CHEMISTRY
the draw and the temperature of the molten metal. Even with this imjxoved method, varying thickness and waviness of surface is inevitable, and objects viewed through window glass almost never appear true to line, but nearly always seem crooked and out of shape.
Plate Glass
SECTIONOF GRINDERS cylinder of glass, splitting it the long way, and flattening it out into a sheet. Originally, the air for blowingwas furnished by the lungs of the glass blower; he took advantage of the weight of the ball of glass on the end of this cylinder to help him enlarge it. There was a “glory hole” in which he swung the lengthening cylinder in the intense heat of a very bright lire. It was hard, hot work, and the largest cylinder a glass blower could produce from the forty odd pounds of glass, which was all that he could handle a t one time, was about 15 inches in diameter and 100 inches in length. Later, by use of compressed air t o expand the cylinder and machinery to raise the formed cylinder above the main body of glass from which it was being formed, it was possible t o produce cylinders about 3 feet in diameter and 30 or 40 feet in length. These cylinders were drawn from a crucible of molten glass and not blown from a “gob” of glass. The glass in the melting crucible was kept a t the proper temperature to form a sheet until it was all used. By this method, glass of much more uniform thickness could be produced than was possible by hand. Flattening the cylinder was accomplished by splitting it lengthwise and placing it in an oven which was just hot enough to soften the glass. When soft enough it was ironed relatively flat by means of a thoroughly water-soaked and charred block of wood on the end of a long rod. Even after flattening, cylinder-process sheets were more or less bowed and had to be set in the window with care to face the bowed side outward. Much more recently window glass has been produced in sheets flat from the start. The question as to which side shall be glazed is eliminated, and one is no longer bothered by “burned” spots where the glass stuck partially to the floor of the flattening oven. The three methods by which this flat drawn window glass is produced involve the same general procedure, and all produce good glass. Melting is accomplished in large tanks similar in construction to the open-hearth furnace. The sheet is formed a t the working end of the tank. A bait is fed into the molten mass and, when the glass has gathered and stiffened, is drawn slowly upward from the bath. The sheet follows and its width is maintained by various types of edgeholding device. The thickness is determined by the speed of
Plate glass, although having the same fundamental composition as window glass, differs in other respects. Plate glass has true plane, parallel faces while window glass surfaces are wavy and uneven; plate glass is nearly uniform in thickness while window glass may vary considerably; plate glass will not cause distortion of vision while window glass may do so. Plate glass is rolled out in sheets of almost exactly uniform thickness. The older method of rolling produced individual sheets but the newer, continuous method produces a sheet of glass 400 feet or more in length which is then cut into convenient lengths for grinding and polishing. For grinding and polishing the glass is bedded in a highgrade wall plaster on perfectly true cast-iron decks. These decks are moved under huge cast-iron grinder heads to which a continuous stream of sand and water is fed. As the surface of the glass approaches a true plane surface, the size of the sand grains fed to the grinders is reduced until finally very fine emery is used. The plates then pass under polisher "spiders" where the surfaces are polished to a brilliant, clear finish by the w e of fine rouge, similar to that used in cosmetics, on heavy felt pads. Inspection and cutting to size completes the process.
Structural Glass The heavier thicknesses of plate glass become truly structural glasses and are used in many different ways in building construction. Ordinarily, however, the term “structural glass” is applied to the obscure and nontransparent glasses of the Carrara glass type. These glasses are produced in different colors including white, black, green, cream, and others; thicknesses from 11,’32 of an inch up are available. Structural glasses have found their chief uses, to date, in the larger monumental buildings, particularly for the wainscoting of hallways, lavatorieq, etc. Today, the field of structural SECTIOU OF POLISHERS
NOVEMBER. 1935
INDUSTRIAL AND ENGINEERING CHEMISTRY
glasses has been widened t o include many domestic uses; kitchens and bathrooms wainscoted with structural glass are becoming quite common and are exceedingly satisfactory. Glass is particularly adapted to these purposes because of its impervious, nonabsorptive character and the ease with which such surfaces may be kept clean. Structural glasses are fabricated in the same way as other plate glasses, and their surfaces are true plate glass surfaces. This enables the architect and designer to secure a decorative effect unobtainable with the uneven surfaces of glasses which have not been ground and polished.
Decorative Mirrors Glass is finding favor in the growing use of decorative mirrors. One of the outstanding recent installations is in the Rockefeller Center a t New York, where many different types of decorative mirrors have been used with success. Several special types of glass have been developed for the decorative mirror field, including such types as flesh-tinted glass, blue glass, a green glass which gives a highly attractive aquamarine effect, and an especially transparent glass, crystalex, which renders almost perfect reflection of all visible colors. In this field the development of the electrocopper plating of the mirrors has added immeasurably to the permanence of the reflecting film and has rendered it far more impervious to damage from atmospheric causes.
Thermal Insulation with Glass A recently developed window glass of the heat-absorbing type transmits a maximum of visible light while excluding a maximum amount of solar heat energy as such. Such glass is particularly useful in school room fenestration, office buildings, on southern exposures, and in many different types of loft buildings and warerooms. In certain industries the use of heat absorbing glass in the windows is especially desirable from a process standpoint. Other glasses are designed t o transmit a maximum of solar radiation and some of them are especially good filters for selective transmission of heat, ultraviolet, or visible light, As heat insulators, practically all types of glass are equivalent, when equal thicknesses are considered. Such differences as exist are due to differences in radiant energy transmission rather than t o differences in sensible heat conduction. Excellent insulation can be obtained by the proper utilization of glass in specific application. Multiple glazing of window openings offers a means of heat economy which is just beginning to be appreciated. In air-conditioning clf buildings, multiple-glazed windows are virtually essential. The use of storm sash has long been practiced. Permanent Jouble glazing in the same sash offers the same or better insulation a t materially less maintenance cost and a t reasonable capital charges in new structures. Glass brick, recently developed by several g1a.s manufac-
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turers, incorporate the principle of multiple glazing in units which are essentially bottles. Glass bricks, while highly translucent, are in general not transparent. Glass wool, spun glass, or glass silk has been found an excellent insulator. In most, instances where glass serves as an insulator its insulating value is derived from the so-called dead air spaces. Multiple glazing ordinarily involves the use of a gas a t atmospheric pressures, and glass wool insulates by breaking up the continuity of air spaces. Glass brick may be made with a certain amount of vacuum within them, although it is perhaps problematical how great a value may be assigned t o this partial vacuum.
Protective Glasses Glasses of the protective type are finding utility in the building trade for different purposes. Laminated safety glass, fabricated by laminating together two lights of glass with a light of a plastic material sandwiched between them, lends itself well for use in partitions and other structures within the building. Adherence of the glass faces and the plastic membrane is established by heat and pressure, resulting in a product in which the tendency of the glass to fly, when broken, is resisted by its adhesion to the plastic medium so that even when badly smashed the light remains in place. The application of laminated safety glass to automobile and aeroplane glazing is well known. Bullet-resisting glasses are fabricated of numerous lamellas of glass cemented together to form a succession of component plates which must be broken before penetration is possible. Certain bullet-resisting glass, approximately 2l/2 inches thick, will stop several rounds of armor-piercing ammunition fired from regular service rifles. This glass is adapted for glazing partitions and windows in banks and other places subject to attack. The use of glass as a fire retardant and for the glazing of fire walls, until very recently, has been confined entirely t o wire glass, a special type of glass with an insert of either chicken wire or parallel wire. .When vire glass breaks, the wires tend to hold the glass rather well in place, a t least until the heat has started to melt the glass. Recently there has become available a heat-strengthened glass, called “Herculite,” a type of plate glass which will resist temperatures up t o 650” F. for an indefinite time without danger of breakage and materially higher temperatures for moderate lengths of time. When this new type of fire-resisting glass breaks, it disrupts into granular fragments which tend to fly in the plane of the glass and thus tend t o bind the light firmly in place so lone as it is not subjected to considerable pressure. Glass with its unique property of being transparent, supplemented by the many other special properties which modern scientific research has developed for it, is evidently destined to continue its role as an indispensable material for the building industry. RECEIVED . I p r ~ l27, 1935.
TEKNESSEE EASTMAN CORPORATION, KINGSPORT, TENN.
