I use "old" to refer to a property from the late Georgian or early to mid Victorian era and "original" meaning that the glazing itself has not been replaced at any time.
I find it remarkable that some of this earliest window glass has survived given that it is so very thin and fragile.
It is as if a stiff gust of wind, a gale-propelled bit of vegetation of just a concentrated beam of high season sun could cause it fracture, splinter and evaporate into its constituent elements.
There is actually commercially available equipment by which to measure the thickness of glazing but I would challenge even the most accurate and calibrated tool to give a reading for these early examples of Period window glass.
The evolution of glass has been quite slow.
It is thought that it will have first been found by accident as its ingredients became fused together in some volcanic or seismic event.
The Mesopotamians of 3500 BC were known to produce very small quantities of glass only suitable for ornaments and chattels followed by the Egyptians and then taken to a new level by the Romans who were able to fashion the first glazing panes albeit of very small dimensions and likely to be opaque and cloudy.
It was not until the 13th and 14th Centuries that better techniques, although still on a trial and error basis, began to make it possible for the sizes of window glass to be gradually increased.
Crown glass was one of the two most common processes for making window glass until the 19th century. The other was blown plate. The process was first perfected by French glassmakers in the 1320s, notably around Rouen, and was a trade secret. As a result, crown glass was not made in London until 1678.
The process for Crown Glass making was labour intensive and very physically demanding.
The most familiar, and historically the oldest, types of glass are "silicate glasses" based on the chemical compound silica (silicon dioxide, or quartz), the primary constituent of sand.
To get these elements into a molten state took immense heat but when achieved it was gathered on a blow pipe and under the manual lung power of skilled workers formed into a bulbous shape. The blowpipe was then removed and replaced with a solid or "punty" rod allowing the glass to be spun rapidly until it transformed into a flattish disc.
The thinnest glass in a band at the edge of the disk was the better in quality and in order to fill large window openings many small diamond shapes would be cut from the edge of the disk and these would be mounted in a lead lattice work and fitted into the window frame.
The central part of the spun disk was in a distinctive hard and often dark swirl but under the name of a bullseye it became synonymous with, in particular the Georgian period.
I recall my parents buying a brand new house in the 1970's which was in the neo-Georgian style and featured characteristic multi paned softwood windows with randomly spaced bullseyes. We felt very posh indeed although in their heyday they were found in less expensive properties, almost as though a waste or bi-product of the process.
Crown Glass, because of its finer quality was the glazing of choice for well-to-do homes and the authentic manufacture carried on well into the 19th Century.
However, the demand for high volume production of window glass saw other techniques being brought to the market.
Handmade was still the main method but the emergence of steam powered grinders permitted larger cast sheets to be machine polished.
In 1834 a German method was adapted by a Smethwick, London company whereby an elongated balloon shape from molten glass was allowed to cool before cutting into a cylinder which was then re-heated in an oven, removed and flattened. The result was a much improved quality and this was no better demonstrated by its use on the great structure of The Crystal Palace of the Great Exhibition in 1851.
Industrialisation saw glass being rolled, wired cast, mechanically drawn, flat drawn and culminating by the pioneering Pilkington process in 1959 of float glass making which became the standard modern method of producing window glass.
In the quest for smoother and clearer glass we seem to have forgotten or indeed have been denied the pleasure of looking out through original glass in an older house.
Each pane varies a little from the next.
If you look closely you can make out small imperfections. That tight cheeked exhalation of a glass-blower left small air bubble imperfections.
There can be the trace of a fold mark or wave from the same handblown efforts.
Looking through such glass gives a varied view and if you squat down, move about and squint there is a perceptible distortion of the visual image of whatever lies beyond the four walls.
Everything is a bit in mottled effect.
Add a bit of stained colour and the effect is startling. That distinctive cobalt blue comes from adding cobalt. Copper oxides also make glass blue to bluish green. Sulphur and cadmium make yellow. Iron oxides produce greens and browns. Tin produces white. Chrome produces emerald greens. In early glass production, the rarest of colours was red. This is because red required the most costly of additives – gold.
I have enjoyed more than a few passing moments just standing and looking out of an old window.
It may, from the inherent imperfections in the glass give the appearance of a distorted and strange world but it is truly a thing of beauty to behold.
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