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Preserving Tomorrow

The preservation of 20th-century buildings must balance original design intent with contemporary knowledge of material performance.
By Elizabeth Corbin Murphy, FAIA

The conscientious preservation architect is faced with many challenges when dealing with 20th-century building materials. The century was one of great experimentation in design and significant innovation in manufacturing. Often these two tendencies were mutually beneficial, but sometimes they collided. This piece will investigate the interrelation between design and material during the pre- and post-war periods and concentrate on particular materials, namely aluminum and glass, as examples.

Early Modernism
At the beginning of the 20th century, architects continued to build with traditional materials. The industrial revolution had influenced design, but it took a while for everyone to understand the properties of new materials and it was easier to try to stretch the capacities of the traditional ones.

If we were to follow the architects "in the news" at the time, we would see not altogether successful incorporations of new materials: We would see Frank Lloyd Wright cantilever concrete beyond that which is normal or acceptable or cut the corner out of a building and replace it with glass; We would see Eliel Saarinen use crisp, clean lines by placing a stone cap flush with the brick wall below (no overhang, no drip edge). Knowing what we now know, we argue over restoring these buildings for better maintenance or for design intent. Wright did not like downspouts: Do you add them when you restore his buildings so they do not deteriorate quite so rapidly?

Having not yet totally abandoned traditional form, the clients were, in the first half of the 20th century, still requesting traditional styles. Albert Kahn built with traditional styles in mind and modern architecture motivating him to better buildings. At the request of his clients, the Edsel & Eleanor Ford House at Gaukler Point, MI, was influenced by the Cotswold cottages in England, but the construction of the house was an innovative combination of traditional masonry construction and steel-and-concrete framing sure to weather the strongest nor’wester! Programmatically, the complex was designed not necessarily for seclusion, but for security: the Lindbergh kidnapping had made everyone very nervous.

While buildings were reaching longer spans with steel frames and taller heights with elevators, in the first quarter of the 20th century design was still not stretching too far. It was limited to traditional materials and perhaps restricted by the breadth and speed of communication. Advancing communication and periodicals in particular altered the face and the pace of change in architecture and allied arts.

In perusing Architectural Forum (October 1924, Volume XLI, No. 4), the advertisements remind one of the new, cool (or hot!) materials available to the "thoughtful architects" to be used on buildings "of beauty, dignity, refinement, convenience and security." Again, most of the materials are not new, but are used in new ways, marketed differently or now have a wider audience and wider availability ("when California looks to Southern Indiana for..."). (In 1923, by the way, Architectural Forum cost $2 per issue – very, very valuable.)

The 1930s brought a little more experimentation with form and modern materials. The Art Deco and Art Moderne/streamline began to truly bend the rules and the materials. The mainstream architectural community was really just initiating the exploration into metal and glass skins and still exploring available materials. Timidity and ignorance often determined the best use of materials. Until the behaviors were witnessed, changes were less likely. The Chrysler Building is an example of such indecision, designed in aluminum, but completed in stainless steel. Perhaps the designers were a bit uneasy. Perhaps they actually recognized the wild coefficient of expansion inherent in the aluminum.

Completed in 1931, the Empire State Building was the first example of large-scale use of aluminum in construction. Until 1886, when scientists from the United States and France discovered a method to extract it from its natural state (bauxite compound), aluminum had been considered a precious metal. For the next decade, the process was not widely used, but the value of aluminum was still about five times that of copper. However, once aluminum was used for military aircraft in World War I – because it was lightweight – use of the metal slowly increased. By the 1920s it was popular as a decorative element.

The architects who undertake the preservation of these materials have the advantage of years of observing their behavior and, therefore, cannot claim timidity and ignorance. The preservationist must be fully cognizant of the properties of the materials and accept responsibility for their selection and specification, whether in a full or partial replacement, or a restoration. Manufacturers’ recommendations and suitable placement of materials are essentials to a successful project. This being the case, homework, field work, research and consultation with the manufacturers of the materials is no longer "helpful" but obligatory.

Aluminum
Aluminum is wonderfully lightweight and corrosion resistant. It is half of the weight of copper or brass. As soon as it hits the air bare aluminum will form an oxide layer that protects the metal until the layer is breached in some manner or the metal is exposed for a length of time to corrosive materials.

The list of corrosive materials is similar to the list for copper (and actually includes copper). With salt at the top of the list, environmental corrosives include treated wood, airborne dirt, chlorides, sulphides and acid rain. Some materials that are a bit more surprising that can corrode aluminum are damp or unseasoned oak, cedar or redwood (in fact any wood can be a problem if it is wet); wet lime mortar; Portland cement; plaster; and concrete. Cured masonry is not usually a problem; however, constant exposure to stone and brickwork that hold moisture can corrode the aluminum. Galvanic action (dissimilar metals in contact with each other) will also corrode it. Copper is one of the biggest offenders. Aluminum is fairly stable with zinc and with non-magnetic stainless steel, but even stainless steel with aluminum should be monitored if the two are in an industrial setting. In preserving aluminum, it is essential to observe the behavior of the material and ascertain the most likely cause or causes of the deterioration.

Environmental factors that didn’t exist at the time of construction or installation but occurred throughout the years are as likely as the possibility of human error in the deterioration of aluminum. Was the correct alloy of aluminum specified? The correct finish? Did the original detail place the aluminum in a position that hastened its deterioration? Was the material correctly specified and incorrectly installed?

One other cause of deterioration must be considered in preserving or restoring aluminum – fatigue. The high coefficient of expansion and the softness of the metal itself are often factors in failure by fatigue. Thermal expansion and contraction will actually cause mechanical failure even when corrosion is not present. Aluminum roofing and wall cladding are especially susceptible to this type of mechanical failure. The inherent softness of aluminum components often results in erosion by wind and rain and damage by abrasives.

Several finishes are available for aluminum that reduce the risk of the corrosive and mechanical failures. Coatings must be identified in order to specify the proper treatment for rehabilitation or even for cleaning. The Arlington, VA-based Aluminum Association suggests simple tests for determining the finish prior to cleaning, repair or refinishing.

• Non-finished/bare – Aluminum’s own self-produced oxide layer is thin but tough. The appearance of the "bare" finish varies with the fabrication technique. The unfinished surface will discolor throughout time, from gray to brown or black depending on the environmental factors. A steel needle can easily mark the metal surface and a rubber eraser may actually scratch the surface while the eraser itself darkens.

• Anodized finishes – The thick oxide coating produced by electro-chemical treatment can be transparent or have integral color. Red, green and blue dyes used in the 1950s are fading unevenly. The gold, brown, gray and black pigments have a better survival rate and still hold their color. Anodizing can improve the resistance to weather and can be further enhanced (depending upon the chemical formula) to resist wear and abrasion. An eraser will not scratch an anodized coating, nor will the eraser darken. It is more difficult to pierce the coating with a needle and one would hear the scratch. The scratch in the surface would actually be wider than the point of the needle. Sometimes anodized aluminum is coated with an organic coating (lacquer). When testing, be sure to recognize the organic coating as well.

• Chemical-conversion finishes – Created by chemical methods only, the conversion coatings are thinner and are generally used as a base for painting. Scratch marks will be a bit broader than the needle and there would be no scratch or color change with an eraser.

• Painted finishes – Painted finishes should be easily detected with a steel needle as paints are much softer than any metal. Paint is applied over a conversion coating in a controlled environment or in the field to any weathered aluminum that is clean and free of grease and has had the surface abraded.

• Porcelain-enameled finishes – Porcelain finishes are very hard and can be smooth or textured. These finishes go through a ceramic process that requires firing. A steel needle can break before it penetrates this surface.

• Plated finishes – Nickel, chromium, copper, tin, silver and gold can be electroplated to aluminum either directly or on an intermediate metal. Usually, the plated metal is very thin and can be scratched, revealing the dull gray color of the aluminum below. Chromium plating can be identified by its silver color. Chromium is harder than gold, silver and nickel, but is more porous, leaving it susceptible to pitting if it is not maintained. If the plating cannot be visually identified, inorganic spot tests can be performed.

• Laminate finishes – Thin layers of fabric, plastics, wood or any other material that one may imagine can be laminated to aluminum. Testing is generally not necessary as observation should be sufficient.

To clean the aluminum, one would follow all of the common-sense rules for preservation treatment. Start with the mildest cleaner: water or mild soap and water. Be sure to test the methods first. Avoid splashing adjacent surfaces. Do not mix cleaners. Be consistent; do not change concentration levels in the field. Follow manufacturer’s instructions. Be sure to rinse and wipe dry.

The human tendency is to use the fastest method and to believe that using an aggressive method less frequently will save money. Generally the opposite is true, as the aggressive methods can be destructive and remediation is costly. Cleaning more often with milder methods is recommended, especially in areas that are easily viewable. Aluminum that is higher on a building needs to be cleaned only rarely. Remember also, as with cleaning other materials and painting or applying coatings, do not apply solutions in the bright sunlight. Work on a cloudy day or keep to the shady side.

The Aluminum Association categorizes five different types of cleaners. Mild soap and detergents and non-etching cleaners are best for oil- and grease-based soil. Even this mild category can bleach the metal or irritate skin. Solvent and emulsion cleaners may be used on bare, anodized, conversion-coated and porcelain-coated aluminum. They will remove dirt and grime, but not heavily crusted soils. Be aware that many solvent-based cleaners are flammable or give off fumes. They also will not restore the appearance of the aluminum.

Abrasive cleaners include waxes, soaps or even water that has had an abrasive added to it. Traces of wax, oil or silicone are remaining on the aluminum upon completion, adding to the luster and protective finish. Abrasive cleaners will remove most stains and grime and will restore weathered aluminum. Power-driven polishers and steel wool (use stainless for aluminum) fit into this category. Testing, however, is essential. Generally some type of polish, wax or organic coating is preferred after an abrasive method. Often a combination of abrasive and solvent cleaning is necessary to prepare the surface for the organic coating after an abrasive.

Etching methods are not used on painted, plated, anodized or conversion-coated aluminum. Usually, the etching cleaners are added to water and used on bare aluminum. An area like a storefront is best cleaned from the bottom up. Drips from the etching solution will not affect the cleaned aluminum as much as they will the uncleaned areas. With a tall building, it may be best to start at the top. The etching cleaners will remain in place for a tested period of time, and then be thoroughly rinsed and removed. The final appearance may be cloudy like a surface that has been etched.

Special cleaners include abrasive blasting, steam cleaning and rotary-wire brushing. Extra equipment places these methods into the "special" category. Steam can be very effective, but one must be cautious as steam can destroy a painted finish or craze an anodized finish. Steam can also distort the aluminum if improperly used. Aluminum is soft and subject to heat and abrasive blasting that would remove some of the metal and leave a surface texture. This aluminum should be coated. Power-driven wire brushes are a last resort and require great skill and experience.

Surface protectants or clear coatings (such as lacquers) may preserve the good appearance after cleaning. Microcrystalline wax is one such coating. Wax will necessarily be applied more often, however it is a forgiving surface and a breach can be easily addressed. Wax may be the best solution for areas that are in high traffic. Walker Johnson of Chicago, IL-based Johnson-Laskey Architects warns that if one cleans and restores exterior aluminum, it is essential to seal the cut edges of the aluminum as well as the exposed surfaces. Black streaks (especially from bare aluminum) can be avoided in this manner.

In order to avoid repeat deterioration, aluminum architectural elements must be protected from contact with the causal elements. If the aluminum is in contact with another metal, the contact surface of the adjacent metal should be painted. If the aluminum is in contact with a masonry surface, the aluminum should be painted with a heavy bituminous paint and then coated with an aluminum metal and masonry paint. Aluminum flashings and drip should be fastened only with aluminum nails and rivets and should be protected from acidic run-off (rain on wood shingles, for instance). Aluminum should not be painted with red lead primers or copper-containing paints. Zinc chromate primers are better.

Aluminum architectural elements should be designed to accommodate movement due to thermal expansion. If a section fails due to fatigue, it generally cannot be repaired, and must be replaced. Likewise, severe erosion or abrasion may render the aluminum element inviable and require its replacement. (Minor erosion can be stopped and coated with appropriate lacquer, varnish or paint.) Welding can be performed in the field, but replacing an entire section of aluminum architectural elements is usually recommended. Soldering should be avoided, as often galvanic action takes place between the solder and the aluminum.

With access to scientific conservation technologies, restoration of the deteriorating building parts becomes more accessible. There are fewer and fewer reasons to demolish a re-usable building.

Post War
After World War II, when Modernism moved from cutting edge to mainstream, new products followed. Moving a bit faster now, industry scurried to keep up with design and construction. Design and construction scurried to use every new product. Retrospect allows one to safely address the fact that not all new products were adequately tested prior to their incorporation into our cities’ largest structures. Materials failures were not always the "fault" of the materials, but perhaps the system designers had not the time to fully realize and accommodate the behaviors of the modern materials.

When one attempts to restore or retrofit a glass-skinned structure for example, or even a structure with large expanses of windows, one is confronted with the behaviors that may have been overlooked in the past. One is confronted, also, with the contemporary pressures to meet energy efficiency, ease of maintenance and lowest possible initial costs.

Glass
The Solarcool (trademark of PPG Industries) glass on a building built in 1972 was intended to reduce the solar gain through the glass by use of a metallic oxide coating that was deposited on the float glass during production. These bronze- and gray-colored glasses were intended to make their mark on the urban context, also, by reflecting their surroundings or adding a specific color to the context, the degrees of which were determined by the placement of the film on one of four faces of the glass system. (Two sheets of glass, both with an inside and an outside face, are generally referenced by numbers one through four. The one face is inside the building and the four face is outside of the building. The air space, for insulating purposes, is between the two and three faces.)

Systems with the metallic oxide coatings have failed because of any number of factors, but even if they were properly designed and installed during the initial construction, the deep shadows or patterns of shadows cast by newer adjacent buildings (subsequent construction) can alter the thermal expansion enough to cause the glass to fail. The metallic coating expands at a rate different than the glass or, because of deep shadows across the individual sheets of glass, the sunny pattern (hot) and the shadow pattern (cold) cause such a thermal stress in any one sheet that the glass fails.

Factors that should have been incorporated into the original design of the glass system to avoid the problems caused by thermal stresses must be included in the restoration of the same, including the type of glass, the type of coating or tinting on the glass and the size and the configuration within the frame. Was it heat strengthened? The influences on such failures go much beyond these few factors and include other factors in the system itself such as the type of frame, frame material, gasket fabric, gasket color and outdoor glazing stop color.

Because these are thermal stresses that are being considered in this list, the design temperatures provide an expected range of temperatures that the system must handle. But one must think about the swing during any one day, or even hour, and the differential that can be caused by the shade and shadow in any one sheet at any one moment.

PPG’s "Thermal Stress Update #TD-109" states that "it is the design professional’s responsibility to ensure compliance with all of these requirements" at the beginning of a 27-page document on thermal factors alone! Switching to the PPG’s "Glass Design Guidelines," one would note that the company offers software to simulate the multitudinous combinations of glass and then warns again that "it is the design professional’s responsibility…" PPG offers a software program for computing preliminary thermal design. In it, there is a caution that states that the software package is not a substitute for a thorough thermal analysis and "it is the design professional’s responsibility…"

Restoring the Discarded
As restoration architects are only beginning to address the latter half of the 20th century, restoration of newer materials is still a bit of a mystery. Conscientious architects and conservators are arousing their courage to attempt restoration of the materials that every other architect has simply discarded for fear or disdain. The late-20th-century materials are being replaced with even newer ones – titanium, stainless steel, plastic, carbon fiber, aluminum, blast-resistant glass, resins, biodegradables, nylon and polymers. Since one may now attain a Ph.D. in plastic injection molding, one must also learn how to conserve it!  

 

 

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