Today, let's talk about window panes. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

Glass has been made in various forms for about 4500 years. But two features of glass-making are suprising. One is that it was slower and more difficult to develop than we might realize. The other is that artisans could make really fine glass tableware long before they could make a decent windowpane.

The ancient Egyptians and Greeks made crude glass decorations. But today's basic soda-lime glass -- made of sand, limestone, and sodium carbonate -- is only about 2000 years old. The first glass of any real quality was made by the Alexandrian Greeks in North Africa around 300 BC. Soda-lime glass came quickly on its heels, and both the Alexandrians and the Romans after them made it into very fine tableware. Fine tableware remained the main glass product for a very long time.

Of course, the stained glass art in Gothic cathedrals was highly developed, and we might think that glass handling had reached a high level of perfection by that point. Actually, the thing that had reached a high level of perfection -- and one that we may have lost today -- was coloring the glass. A medieval window admittted light, but it was seldom smooth enough to provide a clear view. In fact, the windows of cathedrals were a way to tell beautifully lit bible stories to the faithful, who generally couldn't read.

Medieval glassblowers made two kinds of flat glass sheets. One was made by blowing a large cylinder -- then splitting it open and flattening it out while it was still hot. The other kind -- called crown glass -- was made by pouring molten glass out on a turntable and letting centrifigal forces spread it out from a central point. Crown glass became the basic form of flat glass for a very long time. It underwent considerable refinement; but, even as late as 1800, most domestic windows still displayed the characteristic umbilical imperfection -- called a crown -- at their centers.

Actually, the French had developed the superior -- but pretty expensive -- plate glass process in the latter 1700s. But mechanized methods for making relatively inexpensive window glass weren't developed until the early 1800s -- a scant 150 or so years ago.

The lowly windowpane serves to remind us of how much we take yesterday's great acts of inventive genius for granted. A windowpane is the result of an enormously complex set of chemical and high-temperature mechanical-handling processes.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

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You'll find one of the best accounts of the history of glass and glass-making in the 1911 Encyclopaedia Britannica. For a revised version of this episode, see Episode 1282.

(From the 1832 Edinburgh Encyclopaedia)
Typical 19th-century crown glass making operation