Upon the wall of a structure is written its years of history, from the forest or mine through the process of building, to the marks the inhabitants of the city have left behind, whether intentional or irrevocable. The patina formed upon the material marks the passage of time. Some are fashionable – the oxidation of copper, the thick buildup at the edge of lead sheets – but these are outnumbered by unfortunate material juxtapositions that leave a streak across the whiteness of plaster, and awkward forms retaining rainwater sludge along a stone cornice line.
Hurricane Katrina left scars in its wake, and because of the easy rhyming slang, the waterlines and muspatters in the wake of the debris were called the “Katrina patina.” It wasn’t earned by longh-term exposure to phase-changing elements. It was just dirt. But it was comprised of the worst substances imaginable: oil from overturned tanks, sewage from broken pipelines, effluvia from dead animals, hazardous chemicals from sheds and storerooms, and much more. An unbearable cocktail with a memorable odor of sickly sweet mud. Still, this patina could be erased with a sharp stream of water and a scrub brush. A real patina becomes intrinsic to the material.
Architects understand the basics of galvanic action, corrosion, and oxidation. We separate steel and aluminum with neoprene gaskets, place Cor-Ten below finer finishes, and explore self-cleaning finishes for tall towers. Materials scientists have discovered many solutions for wrapping buildings in unsuitable materials, chemistries which can transform wood into insect- and water-resistant timbers, paragons of dimensional and compositional stability. Companies including DuPont are busy making glass bulletproof, rocks impervious, and paper waterproof.
These heroic modifications expand the palette of materials that can be used onto new planes. We are accustomed to clay tile roofs in Mediterranean climates, but not brick ones. The standard of using wood shakes is long-established, but not wood rainscreen roofs. What makes these installations possible (if still improbable) is chemistry’s magic: polymers and resins, impregnations of aldehydes, vinyls, fibers, graphite, borate, arsenic, and various minerals in a cocktail of toxic ingredients. Only then can the designer wrap the entire structure in one homogenous material so that form reads more clearly than any feature.
The other current fashion is to use as many materials as possible to break down the scale and display our design affinity for collages of disparate things. But perhaps architects should stick to the materials we know well, harvested from our surrounding natural resources, which can be detailed with confidence.
Architects acknowledge that buildings must resist the elements of the atmosphere, including air with all its suspended acids and atoms, and water sluicing the exposed surfaces with particles and salts. Pollution deposits upon a façade are a lesser concern than other effects, although a soot-blackened building is never aesthetically pleasing; abrasion, direct and indirect chemical attack, and electrochemical damage are more critical to the ultimate life of the material. “Weathering” is the life-sized demonstration of the effect of these attackers upon buildings, over time. Invisible particles, especially Sulfur dioxide (SO2) and Nitrogen oxide (NOx) have laid waste to stone, steel, and zinc in cities, materials expected to have a long life before the advent of burning fossil fuels. Sulfur trioxide (SO3) reacts with marble and limestone to form gypsum… which then crumbles, as has happened at the Caryatids of the Erechthion at the Acropolis.
So where man has caused the problem, modern man will clean it up. With protective sealants and coatings, underlayments and interlayers, stone walls may be stabilized, the fermenting pockmarks filled with sulfuric compounds neutralized, and the legacy of the industrial revolution eradicated. This is the history we regret: one which damaged the icons of antiquity and jeopardized the noble and sacred temples of our ancestors. Without a constant reminder of the imprint of the last 150 years, will we forget the degradation, as well as the cause?