Spandrel glass

 Spandrel glass originally referred to ceramic-coated plate glass. Today, the term is used to include many types of transparent flat glasses used for spandrels. Spandrel glass is meant to cover the space above and below horizontal strip windows where the knee walls and spandrel beams are located.

History
At the beginning of 1880 until the mid 1950s, a variety of translucent and opaque glasses were being manufactured using mainly a pot plate process for glazing and façades. The expense and limited color selection made these glasses rare for use in spandrels. Glass was capable of taking the place for masonry and other materials because it was more durable, lighter, larger, and less costly. Tempered structural glass was used for wall spandrels for a limited time even though it had a variety of colors and opacities. After World War II, innovative construction methods and materials were being invented. The production of enamel frits, which were fine powders used for bottle decoration and labels for porcelain enamel panels, was found to be appropriate for spandrel glass. Before frits could be used for spandrel glass though, they had to undergo significant changes in the ingredient mix. The Pittsburgh Plate Glass company later introduced Spandrelite, the first ceramic-coated glass. New colors and large sizes made this material popular in new architecture. By 1958, the glass could come in sizes measuring up to 5 by 7 feet. Soon, manufacturers found that the more variety of colors and patterns they offered, the more product they could sell. Pittsburgh Plate Glass and Libbey-Owens-Ford (which called its ceramic-coated spandrel glass Vitrolux) soon launched a national campaign to promote their product. Spandrel glass and tempered transparent glass needs seemed to grow simultaneously. Small manufacturers invested in heat-strengthening and tempering furnaces to meet the tempered glass patio door demand and found it more convenient to offer ceramic-coated spandrel glass while buying substrate glass from Pittsburgh Plate Glass and Libbey-Owens-Ford. Substrates and frits came from a limited number of sources, so competition for competing spandrel glass products differed due to furnace design and firing practices.

Manufacturing Process
The first step in creating spandrel glass is the production of plate glass or float glass. Plain tempered glass can be used for spandrel glass but spandrel glass is usually ceramic coated. Glass and a variety of colorants are used to create ceramic frits. The glass is cooled quickly to cause it to break into small pieces that will later be ground with pigments for a particular color. The frits will adhere to the substrate light when fired. The frit is mixed with a liquid vehicle which allows a thin coat to be applied to the substrate glass with a roller, spray, or silk screen after the substrate light is cut to the desired size and is ready for heat strengthening or tempering. After one side has been coated with the frit, the glass is fired in a furnace. Vertical furnaces have sharp metal tongs that suspend each light. In horizontal furnaces, the glass is held up by rollers. The furnace should have a temperature of 1200 to 1500 degrees Fahrenheit to melt and bind the frit coating to the glass substrate. The glass can be tempered by quickly cooling each light with a blast of air.

Uses and Installation
The term glass box is used to describe buildings that had been erected using spandrel glass with transparent substrates. These were popular after World War II; the best example of a glass box building is the Lever House in New York City. The substrate of this building had an opaque coating of a green hue. The Museum of Modern Art annex used some of the first ceramic-coated spandrels produced by the Pittsburgh Plate Glass Company. The installation was so successful on this building that the company further developed its ceramic-coated glass, which was later introduced in 1955 as Spandrelite. Spandrel glass has one characteristic that it benefits from, which is its design flexibility. Spandrel is capable of being matched to adjacent doors and window and indoor shading, lighting and color. The use of silk screens and frit patterns techniques were used for the production line, making it less expensive and timely to reproduce patterns. Spandrel glass saw success in large offices, storefronts, schools, and hotels. Spandrel glass was usually ¼ inch with a glazed in glazing rabbets ½ to 1 inch wide and ¾ deep. Blocks were set at sills and centering blocks indoors and at jambs provided some structural support.

Conservation
If spandrel glass is properly designed and maintained, it is a strong reliable material. Spandrel glass is likely to experience some types of deterioration if not properly cared for. Broken lights may require custom replacement to match unbroken lights.

Deterioration
Solar ultraviolet radiation can cause the transparent substrate sheets to change in color. This change is subtle and may not be noted unless replacement of the window is done for other reasons. Dirt buildup is likely and the amount is usually based on the direction the wind blows as well as the amount of wind blowing. Nearly neutral pH dirt usually will not damage spandrel glass. Alkaline solutions and hydrofluoric acid are likely to permanently etch the exposed surface. Concrete mortar and cement are usually the source of these problems.

Using masking tape, chalk, or crayons to mark spandrel glass openings during construction can cause visible variations when removed. Ceramic enameled spandrel glass will not deteriorate biologically. It is unlikely that spandrel glass will change in size or shape after it has been fired. Drilling, sandblasting, or grinding surfaces and edges can weaken a window enough to cause it to break by small stresses. Breakage can also be caused by bullets, hot metal spatter from welding, and vandalism. Spandrel glass can be scratched by local sandblasting or grinding, and pitting is likely caused by wind-driven sand or broken roof gravel.

Spandrel glass will absorb and release heat by convection, conduction, and radiation. Heat intake can cause a severe difference in the spandrel and its interface and can cause thermal stresses in excess of 5,000 pounds per square inch. This pressure is likely to cause damage. Severe damage that infiltrates into the central tension core of either heat-strengthened or tempered spandrel lights can cause breakage.

Visual examination of broken glass is likely to indicate areas requiring maintenance and even the cause of breakage. Broken pieces can be taped back together and may permit examination to distinguish the cause of the break. A survey system is required before inspection of spandrel glass can begin. All lights should be examined, including windows at the highest point (which may be inspected with the use of binoculars or a spotting scope). It is possible to inspect higher glass by standing on adjacent buildings or rooftops and is less costly than forming stages or scaffolding. Investigating weeps holes, rabbet sealants, and gaskets should be done to ensure they are still functioning properly. All cracks should be investigated. A crack pattern into lots of other cracks (measuring ½ by ½ or 2 by 2 inches) indicates the light has been tempered. Cracks that originate at a cat’s-eye with no displacement are usually caused by a nickel sulfide stone. If a black speck is found between the two sides of the cat’s eye, the piece should be saved for laboratory analysis. A similar break pattern may be caused by a bullet of a hard-point missile, which may cause dislocation. If the crack defines between five to fifteen large, irregular pieces of glass and most are retained in the opening without displacement, the window has been heat strengthened.

Conservation Techniques
Spandrel glass should be regularly inspected and maintenance should be done when needed. Glazing, framing, and weepy systems will require regular maintenance to ensure they stay working properly. It is suggested that more thorough inspections be given every five years, but more frequent inspections and cleanings may be required in industrial settings. The cost for rehabilitating spandrel glass is expensive, making planning and execution of great importance. Shop drawings and maintenance records should be consulted during any repairs. Repair proposals should be detailed and should be created only after a condition survey has been executed. Proposals should include repairing identified deterioration, covering up identified deterioration with new layers or replacing severely deteriorated walls with new pieces that match as closely as possible to the historical piece. If covering up an old layer, new spandrel glass glazed off-site may be possible to attach in structural gaskets over the old with screw-applied retaining clips, but only if the existing wall can carry 4 more pounds per square foot.

It is possible to clean spandrel glass with high-pressure water. Fingerprints, glass-cutting oils, and interleaving paper scum are easily removed with a variety of detergents. Tape and label adhesives, grease, and tar deposits are capable of being removed with mineral spirits or isopropyl alcohol. An acidic chemical may be required to undercut hard water salts. Manual cleaning can be done with a low-foaming detergent, but glass should be cleaned thoroughly afterwards with clean water to remove all contaminants. Surface corrosion can appear like deposited dirt and may initially be hard to see. Corrosion is removable with a careful application of fine cerium oxide abrasive and distilled water. Leaking walls can be resealed once rabbet surfaces are cleaned. New sealants and gaskets are important and should be selected based on their durability and their similarity to original sealants. Structural glazing gaskets are easy to manage compared to sealants and require less cleanliness for proper performance. Silicone sealants, when applied properly, can last longer than any other type of sealant. Darker colors, like black or dark gray, are more physically stable.

Replacement
Finding identical spandrel glass is difficult and can become costly. Professional photographs should be provided to the manufacturer to ensure that the two pieces will look identical. A sample should be supplied for examination. Manufacturers often supply and glaze a piece of the existing wall for comparison. Each opening should be measured because of settling and adjustment of expansion joints that may cause a difference in size and shape. Before reglazing, glazing rabbets must be cleaned thoroughly. Flashing joints must be resealed.