Yes, the stuff in windows that lets the light in. So what? Well, next time you walk round the town see how many different types of glass you can spot. Look for the reflections, flat or wobbly. Generally, the older the glass the more distorted and broken will the reflections be and the more exciting, dynamic and attractive will the window appear. 'Progress' in glass making has been from the interesting to the boring.
Modern window glass is now all float glass. Invented in about 1955, molten glass is floated on a bed of molten tin. It cools in sheets that are very flat and of very even thickness but the resulting optical perfection, while fine for looking out of, offers no intrinsic interest in itself. Float glass is dull.
Older glass was drawn out of the melt in sheets and then were passed between rollers. Slight imperfections in the process cause wobbles in the view and distortions in the reflection. Most glass of the first half of the 20th century, at least from 1916 when Emile Fourcault perfected a viable production system, is rolled. You can still buy rolled glass, sold as 'horticultural' because it's used for greenhouses. Most comes from Poland and Russia, is more interesting than float and cheaper.
Earlier 19th century glass (and going right back to medieval times) was mostly 'cylinder' glass. And this is where things get really interesting. It was made by gathering some molten glass at the end of a blowpipe, blowing it into a sphere, swinging the pipe and sphere around until it extended into a cylinder, cutting off the ends, cutting along its length, opening it out and reheating till it formed a rectangular sheet. There was, at first, a limit to the size of sheet that could be produced easily, maybe 2 feet by 3 feet. This limited the size of window, or at least the spacing of glazing bars, determining the character of much Victorian and Georgian architecture. By the mid-19th century however, techniques had improved sufficiently to produce 52 by 36 inch sheets routinely. Sash windows, undivided by glazing bars, were now possible. Thicknesses ranged from less than 2 mm to almost 4 mm but glass was still sold by weight, the heaviest, 32 oz per square foot, being about 4mm thick.
And the really exciting part is that the glass surfaces were far from flat, resulting in complex patterns of distortion that give these lovely old windows their lively character. There are three factories, in France, Germany and Poland, where hand made, mouth blown, cylinder glass is still made. It is sold as 'antique' glass even though the glass is new. The adjective refers to the method of production rather than the actual product.
Towards the end of the 19th century machine drawn cylinder glass was invented. A cylinder could be drawn up from a crucible of melt, forty feet long and four feet in diameter. Cut into manhandleable pieces it could be flattened into sheets about 12 x 5 feet. It was made for the first thirty years of the 20th century but was usually regarded as of poorer quality that mouth blown cylinder glass. The quarter round cylindrical panes used at corners of shop fronts originate from this process.
There were some large sheets of very flat 'plate' glass made in the early 19th century. Molten glass was cast onto a flat stone or metal plate and the the surfaces had to be laboriously polished. The result was fabulously expensive so used more for mirrors than windows.
Much of the 18th century glass was spun or 'crown' glass. This was made by collecting molten glass on an iron rod, rotating the rod until the glass formed a flat disc, detaching the rod and, when cooled, cutting the resulting circular sheet of glass into small rectangles for windows. Distortions and traces of bubbles formed concentric rings and these curving patterns in a window pane identify crown glass. There was a serious size limit to glass that could be made by this method so 18th century houses just don't have large pieces of glass in their windows. Techniques gradually improved and by the mid-19th century, discs of about four feet were being made but with the 'bull's eye in the middle one still could not obtain a sheet of more than about 18 inches. Leaded lights made good use of the very small pieces of glass. The piece of glass at the centre of the disc to which the iron was attached was so distorted one could not see through it. These bullions or 'bulls eyes' were either recycled into the melt or sold very cheaply as squares about six inches across and used in places such as basement lights where the view didn't matter. Crown glass was made very thin, perhaps around 2 mm, as the excise duty levied on glass manufacture was based on weight rather than area of sheets. That tax was removed in 1845 so very thin window glass is evidence of an earlier date.
There is no longer any commercial production of crown glass so if you find some, treasure it for it is an irreplaceable treasure. Crown glass was sometimes of variable thickness and glaziers tended to put the thicker end of the glass downwards, perhaps giving rise to the myth (yes, it is a myth) that glass is a liquid that flows so that old windows are now thicker at the bottom. The name originates from the maker John Bowles, of Southwark, London, who patented an improved method in 1691 and stamped a crown mark on his sheet glass.
Historic glass, that's anything before the 1950s when float took over, is a finite asset that should be valued and conserved. Remember that window frames can usually be mended and should only be replaced as a last resort. If frames with old glass really have to be scrapped, keep the glass for reuse. Extracting old glass from old putty can be tricky. The putty can be softened with an infra red putty lamp that heats the putty without heating the glass. These lamps are expensive, around £500, so it's a bit of a specialist's job.
The glass industry of the 18th and 19th centuries gave rise to the curious cone glasshouses. Here are some old pictures and here's a picture of one of the only four surviving cones at Lemington, on Tyneside.
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Contact: biff@biffvernon.freeserve.co.uk
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©Biff Vernon 2004