No building material has undergone, in its history, a production transformation comparable to that of flat glass. For seven hundred years, from the first Gothic stained-glass windows until well into the 19th century, producing a transparent, flat pane of glass was a slow, artisanal process, with results that were inherently irregular. Float glass, today the absolute industrial standard, has only existed since 1959. Understanding this history means understanding why medieval glass has a look that no modern glass truly replicates, and why the curtain wall as we know it only became possible once someone solved a purely production-side problem: how to obtain a flat, thin, uniform pane over a large surface.
Gothic glass: cylinder and crown, never perfectly flat
The stained-glass windows of Gothic cathedrals — Chartres (1194–1220), the Sainte-Chapelle in Paris (1248) — are made with two techniques, both based on hand-blown glass. In the cylinder technique (or "muff glass"), the glassblower blows an elongated, cylinder-shaped bubble, cuts off its ends, scores it lengthwise and flattens it out, still hot, onto a flat surface: the result is a pane with slightly irregular thickness and a surface that's never perfectly smooth. In the crown-glass technique, the glassblower blows a spherical bubble, detaches it from the blowpipe with a rapid spinning motion that, through centrifugal force, flattens it into a disc — with a thickness that increases from the centre (where the mark of the pipe, the "bullion," remained) toward the edges. Neither technique produces a large, flat, uniform pane: Gothic stained-glass windows are mosaics of small pieces of coloured glass, held together by a network of lead (the leaded-glass technique), not because lead was a primary aesthetic choice but because it was impossible to produce large panels in a single piece. The irregular thickness and the bubbles and streaks in medieval glass — today perceived as precious, almost painterly aesthetic qualities — were, quite simply, the physical limits of the technology available.
Industrial flat glass: from cast to rolled
The first leap toward larger, more regular panes comes with cast plate glass, developed in France in the 17th century: molten glass is poured onto a cast-iron table and spread out with a roller, then ground and polished mechanically to eliminate surface irregularities — a long, costly process that makes plate glass a luxury material for most of the 19th century, reserved for mirrors and the shop windows of prestige stores. It's only with the industrialisation of rolling, in the second half of the 19th century, that pane sizes grow large enough to make a fully glazed building envelope conceivable: Joseph Paxton's Crystal Palace (London, 1851), with its 300,000 prefabricated panes of blown glass, still remains a mosaic of relatively small pieces — not a single continuous skin.
The float process: the perfect pane, industrialised
The real revolution comes in 1959, when Alastair Pilkington (Pilkington Brothers, UK) perfects the float process after years of experimentation: molten glass is poured onto a bath of molten tin, on which it floats and spreads under gravity into a perfectly flat layer of uniform thickness, with no need for any subsequent mechanical polishing. It's the process — with incremental refinements but no change in principle — still used today in every industrial glassworks in the world to produce flat glass for construction. Float glass makes possible, for the first time in history, large panes (up to more than three by six metres), perfectly flat and optically uniform, produced continuously and at relatively contained cost: it's the technical condition that makes conceivable the curtain wall as Mies van der Rohe means it in the Seagram Building (1958) and in all the modernist architecture that follows — a continuous skin of glass, not a mosaic of small, irregular panels.
What hasn't changed: transparency as a cultural ideal
While production technology has changed radically, the cultural idea that glass carries with it — light as a value, the dissolving of the boundary between interior and exterior — runs almost unchanged through the seven centuries between Chartres and the curtain wall. The Gothic cathedral uses glass to bring a divine light, filtered and coloured, into an otherwise dark space. The modern movement, an era of total secularisation, uses glass for the apparent opposite — neutral, unfiltered light, absolute transparency — but the underlying principle, that the wall can stop being an obstacle to light and become itself a means of conducting it, is the same. The physicist Paul Scheerbart, in his essay "Glasarchitektur" (1914), explicitly theorised this continuity: an architecture of glass as an instrument of spiritual elevation, not simply as a construction technique — an idea that directly influenced Bruno Taut in his Glass Pavilion for the Werkbund-Ausstellung in Cologne (1914), the theoretical link between the Gothic cathedral and the 20th-century glass skyscraper.