History of Glass and its Influence on Conservatory Development
Around 7,000 years ago an amazing new material was discovered that would eventually change the way plants would be raised and the architecture of the buildings that would allow this. We don’t now how the discovery was made, but according to the ancient-Roman historian Pliny the Elder , Phoenician merchants transporting stone in Syria, discovered glass accidentally. Pliny relates the story of how these merchants rested cooking pots on blocks of nitrate placed by their fire. The heat from the pots melted the nitrate that then mixed with the sand on the beach, forming an opaque liquid which when cooled was glass.
Modern scholars dismiss this as a fable, since the temperature required for the combination of carbonate of sodium and silica is considerably higher than that needed for cooking. But be this as it may, around this time an accident of nature or by-product of some man made venture would have produced the first beads of opaque and coloured glass.
It probably took years of experimentation to find ways of replicating what happened by accident into the craft of making glass beads and from this to wrapping threads of molten glass around a core to make pots. But by 1,500 BC there was a glass industry in Egypt, manufacturing jars, pots and beautiful decorative products.
The earliest surviving examples of Egyptian glassware are three vases bearing the name of the Pharaoh Thoutmosis III (1504-1450 BC), who brought glassmakers to Egypt as prisoners following a successful military campaign in Asia. Glass was also used to make sculptures such as the one of Amenhotep II exhibited at the Corning Museum of Glass.
By around 750 BC, glass was being blown using a pipe and for the first time this allowed glass to be made that was both thin and with a more even thickness. At last, the basic technique for making a window had been discovered and with it a material to keep the weather out of buildings and away from plants, whilst admitting light.
The Romans & Glass
The Romans did much to spread glassmaking technology and it was they who started to use glass for architectural purposes. They discovered that by adding manganese oxide to their glass ingredients a clear glass could be produced. This was around 100 A.D. in Alexandria and subsequently, they made cast glass windows, still more opaque than clear, that were used in important buildings in Rome and in Herculaneum and Pompeii where window glass fragments have been discovered in the bath-house.
We know that the Romans used cold frames (rudimentary conservatories) in the first century A.D. to grow out of season cucumbers for the Roman Emperor Tiberius, but these were probably glazed using mica, a “transparent stone”. You might be familiar with the use of mica for the windows in wood-burning and other stoves, but not all mica is produced in tiny panels and there is a 1-metre sheet of mica on display in the Smithsonian Institution.
Early Sheet Glass Making
Church architecture and the building of cathedrals spurred the development of the use of glass in architecture. Once architects had perfected the use of spanning a space in stone, Gothic architecture needed something to fill these arched spaces, leading to the use of enormous expanses of stained and painted glass – the east window of York Minster is the size of a tennis court.
The 11th century saw the development by German glass craftsmen of a technique, further developed by Venetian craftsmen in the 13th century, for the manufacture of glass sheets. By blowing a hollow glass sphere and swinging it vertically, gravity pulled the glass into a cylindrical tube up to 3 metres in length and 45 cm wide. Working quickly, while the glass remained hot, the ends of the tube were cut off leaving a cylinder that was then cut lengthways and flattened on an iron plate. This is now known as Broad Sheet glass.
Around 1680, the French discovered a method of creating plate glass. Molten glass was poured onto a special table and rolled flat. After cooling, the glass was first ground with rotating cast iron discs and then increasingly fine abrasive sands before being polished with felt disks. It was very expensive but provided the material for the stunning mirrors in the Palace of Versailles, completed in 1685 and is known today as Polished Plate glass. Thermally, these windows were not effective and it is said that wine and water froze on the dining table in the cold winter of 1695.
Other types of sheet glass included Crown Glass, also known as Bullion Glass and believed to have been first produced in Britain at Crutched Friars in London in 1557. The glass was made by blowing a glass ball and spinning it so it flattened into a disc with the outer portion beyond the central knob cut into small panes. The central knob of glass was not wasted however and you often see it as a pane of glass in a larger window and is popular to this day to add a period flavour to modern glass doors.
With both Crown and Broad Sheet glass, the glass would not be of a uniform thickness due to the centrifugal force of the spinning glass causing the centre of the glass plate to be thicker than the edges. So the glass would be cut into smaller pieces and matched with those of similar pieces before being joined with lead strips to create windows.
Perfecting the making of glass and making it strong pre-occupied glassmakers. Blown glass was often thin and weak and as described above could only be made in small pieces. Glass was often deliberately made thin, and therefore light, as Window Tax was levied on the weight and not the size of the glass panes. In the 17th century the largest panes of glass would be approximately 15cm x 10cm (6 inches x 4 inches); in the 18th century this had increased to about 40cm x 25cm (16 inches x 10 inches).
It was not just the size of the panes that influenced the design of conservatories and orangeries. Windows made up of panes joined together with lead were not completely draught proof and heating systems had to be accommodated to keep plants alive in the winter months. Ostentation also played a part; the Window Tax encouraged the rich to flaunt their wealth by putting ever-larger windows in their homes and building conservatories and orangries to show off to their neighbours.
William III introduced the tax; it was assessed on the rental value of properties with more than six windows and worth more than five pounds per annum. The tax hit the middle classes particularly badly and many of them avoided the tax by bricking up a few of the windows in their homes until they had reduced the number of openings to six. You still see bricked-up windows in old houses today. The tax was eventually abolished in 1851.
As the size of the available sheets of glass increased, architects found new ways of using glass. Georgian windows of the 18th century are epitomised by the use of multiple panes of glass, most typically made up of six panes, three across and two down. These were joined together with glazing bars, but it wasn’t until the invention of glazing putty in The Netherlands in 1737 that a weather and draught proof window could be made.
Towards An Industrial Process
The above methods of manufacture lasted into the 19th century and variations on these methods are used today to make glass for listed buildings. It was not until the Industrial Revolution that technology allowed the move from hand-blown to machine manufactured glass and larger windowpanes. The great innovator was Robert Lucas Chance . In the early 19th century a new method of sheet glass production, the cylinder process, was developed in Europe. Chance was the first to adopt this method in Britain in 1832 and by combining this with his own design of machinery to grind and polish the glass; he produced a finished product that exceeded all others in brilliance and transparency. By 1841 the company was turning out more than 4,000 feet of glass per week to meet the enormous demand.
The Influence of Chatsworth and the Crystal Palace
There were two further influences in this development. In 1851 the Window Tax was repealed in Great Britain allowing making glass more affordable for the lower classes. Simultaneously, the Crystal Palace, home of the Great Exhibition, had caught the popular imagination of 6 million visitors who could see the construction possibilities available with glass.
It was a visit by Sir Joseph Paxton, (1801-65), to Rohault de Fleury’s Jardin des Plantes in Paris in 1834 and his Mexican Hothouse which was to influence the design of the Crystal Palace. Paxton, was the Duke of Devonshire’s gardens superintendent, his iron-framed conservatory at Chatsworth House was built between 1836 and 1841 and covered three-quarters of an acre – at the time it was the largest glass building in the world – shaped like a tent and 277ft long and 67ft high. The largest sheet glass available generally available then was three feet long but Robert Chance managed to produce four-foot sheets for Paxton’s benefit.
The Great Conservatory at Chatsworth became Paxton’s model for the Crystal Palace. Erected in 22 weeks, it covered 19 acres, was five times as long as the Palm House in Kew (no doubt by design 1,851ft long), higher than Westminster Abbey and contained 293,635 panes of glass.
With over 6 million visitors, it was a major advertisement for glazed structures and thus exerted an influence on the popularity of conservatories during the latter part of Victoria’s reign and into the Edwardian era. Paxton received his knighthood from Queen Victoria for his design of the building.
The Use of Antique Glass
As described earlier, when making hand-blown glass it is impossible to achieve a perfectly flat surface. No matter how skilled the blower, the glass will contain blemishes such as “ream” – small waves or folds in the glass, and “seeds” – small air bubbles. What’s more, unless making coloured glass, the glassmaker’s natural product will have a green to bluish green tinge. This is caused by naturally occurring iron impurities in the sand – one of the raw materials from which glass is made.
Thus, if you examine the facade of old buildings, you will see how different original panes of glass are compared to those produced today. Not being completely flat or completely clear, old glass distorts the reflections of trees, buildings and the surrounding scenery and reflects light in different ways to produce an effect that many believe add greatly to the personality of the buildings.
Further Advances In Industrial Glass Making
Another contributor in the evolution towards mass production was Friedrich Siemens, who invented the tank furnace. This replaced the pot furnace allowing the continuous production of greater quantities of molten glass. In 1903 Machine Drawn Cylinder Sheet was invented in the USA and was a mechanical method of making Broad Sheet glass but in 40 ft (12 meters) high cylinders drawn vertically from a circular tank, reheated and flattened. This process was used in the UK up to the end of the 1920’s.
Modern Flat Glass Technology
Float glass was first used invented by Sir Alastair Pilkington and first used in 1959. In the process, a layer of molten glass is floated on a bath of molten tin. The glass, which is highly viscous and the tin, which is very fluid, do not mix so that the contact surface between these two materials is perfectly flat. Today, this accounts for 90% of glass manufactured. It is completely clear and transparent and without any impurities in the ingredients used for its manufacture has none of the bluish green tinge described earlier.
This coloured glass used in a the conservatories and orangeries of the 18th and 19th centuries protected plants from the some of the effects of direct sunlight and it wasn’t until these panes of glass were replaced with modern clear glass that it was discovered that the discoloured glass had a beneficial effect.
Glass in Modern Conservatories
Unless you live in a listed building or a in a conservation area controlled by strict inspectors, you will be able to use state of art glass that will help keep the interior space cool in summer and warm in winter, whilst offering other benefits such as minimal maintenance, ease of cleaning and much more besides.
A subtle blue tint in the glass helps ensure that unwanted glare is kept to a minimum allowing for all year round enjoyment of the conservatory, with excessive solar heat deflected away, providing year-round comfort.
We also understand the importance of thermal efficiency, so the glass we use has been chosen because it outperforms standard glass units by 50%, thereby reducing potentially expensive heating bills.
It’s been a 7,000-year process, but finally we have a glass that would astonish the Egyptians and make Paxton envious.
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