Architects man defenses with technology in place or armies. The component material of glass is silica; sand, but not the sand of most coastal beaches. Glass is fine quartz sand with other elements (about 30% by weight) including sodium carbonate, lime, and aluminum oxide. It seems impossible that heating a handful of rock grains transform it into liquid, clear and cold. Holding a fragment of glass does inspire care for the fragile and intemperate matter, as if it may continue to melt (which it does – a promise evidenced over the long life span of many a Victorian glass lite.) The more likely demise will be to shatter into dangerous and splintered shards. Glass was first devised in Mesopotamia over 5,000 years ago for pottery glazes, tools and jewelry. Egyptians created glass drinking vessels, leading to the Romans’ experimentation with cast glass to fill wall openings.
The term “curtain wall” evokes a plane of insufficient material to form a solid, a wall made from glass. Mullions began shrinking to imperceptible dimensions since Mies’ Barcelona Pavilion of 1929, culminating in the current infatuation for butt glazing, the joint between glass panes limited to a bead of silicone. Curtain walls are irrevocably associated with the skyscraper, the city, and the panoramic views through the expanse of glass.
There is an archaic definition, an almost directly opposite meaning. In the Middle Ages, a curtain wall was the “the plain wall of a fortified place; the part of the wall which connects two bastions, towers, gates, or similar structures” (first referenced in 1569, according to the Oxford English Dictionary). Associated with curtain fire, or the rapid shelling to prevent enemy advance, the curtain wall was the outermost cover, the first layer of a castle’s defense.
The towne was well manned… and the curten of suche height and thicknes that the besieged with great ease became victors.
(Thomas Stocker, Diodorus Siculus, 1569)
Modern construction uses glass less discriminately. Reliant on materials such as polycarbonate, frames can approach impossible thinness with windows of great spans. Although it still allows views, glass may now do much more: generate electricity through thin-film solar collectors, reduce heat gain and ultraviolet rays, create patterns through shape or sandblasting, and eliminate views into private spaces. Glass with plastic interlayers can bounce back after impact with a 2×4, even launched at 100 miles per hour.
Although the UAE has just announced limitations on the use of unshaded glass for new structures in Dubai, glass is one of the most sustainable products available. Fabricated widely and requiring very little of scarce mineral resources, glass is endlessly recyclable without loss of quality. But glass must be treated with respect. Glazed openings must provide views and daylighting, promote thermal comfort and enhanced ventilation, and provide protection from exterior threats. Double-wall facades may guide innovative design for all-glass cladding – harkening back to the original, protective definition of a curtain wall – the enemy now transformed into high energy use and emissions.