Natural Glass

There are two forms of glass created by nature. Obsidian is formed by volcanic action and is found in quite large lumps. It is a shiny black mineral and there is a large example of it in the Earth Sciences Museum in Kensington, London.

Tektite is formed when meteorites hit the earth's surface, generating a short but intense heat. The tektite glass is usually found as small spherical beads.

Many artifacts have been found made from carved Obsidian and Tektite.

Man Made Glass

The commonly told story, as recorded by Pliny (AD 23-79), relates that Phoenician sailors camping on the shores of the river Belus (now known as the river Naaman in modern Israel) cooked their meal over open fires on the sandy shore. They rested their cauldrons on lumps of soda. The combination of the heat, the sand, wood ash and soda, resulted in small rivulets of molten glass running from the fire. While this is theoretically possible, they would need to be using very hot fires - approaching 1000 C.

A more likely story for the accidental discovery of glass would be that it was made by chance when firing pottery. Perhaps here the wood ash, soda and sand happened to combine within the temperature of the kiln.

We do know that the Egyptians were experienced and talented glass workers. Items have been found from the time of the Pharaoh Thotmes (~1370BC). These articles were formed by molding molten glass. The use of a hollow pipe for glass blowing is thought to have been developed by 100 BC and in use in Israel and Syria. Mouth blown vessels could be mass produced and therefore be made more cheaply and be widely available. Many glass artifacts have been found buried in the ruins of Pompeii (79 AD). Much of this glass was coloured but was quite opaque and therefore nothing like modern window glass.

The Romans

The Romans were skilled glass workers and glass was available and used throughout the empire. Some believe that it was the Romans who developed the technique of using lead to join flat glass in order to make larger windows. The Romans could cast small slabs of flat glass. However, with the fall of the Roman empire, glass usage dramatically declined in western Europe.

All but the very rich would not be able to afford glass at all. Windows would either be open to the elements with shutters to keep out the weather, or they could be glazed with oiled paper, waxed linen, shell or even alabaster. Therefore houses would have small window openings with dark rooms.

The widespread use of glass was very slow to return to Northern Europe. The technology slowly spread with the building of churches and particularly the building of the great cathedrals. In Britain glass workers were imported from France. However, glass was still very expensive. There were only two methods of creating thin flat glass.

Mouth Blown glass.

A large blob of molten glass is held on the end of a long hollow metal tube. The worker then blows air into the blob to form a small bubble. The glass is repeatedly re-heated and more air blown in until the glass has formed a bubble perhaps two feet long. The worker then cuts off the ends of the bubble leaving a cylinder. The cylinder is then cut length ways and the glass is unfolded to form a sheet. This is all done while the glass is very hot. It is a slow and highly skilled job. The sheet of glass must then be cooled in a controlled way so as not to introduce cooling stresses into the glass. The final sheet can be a few square feet in size.

Spun Glass

An alternative method is to put molten glass on the end of a solid metal rod and then spin this while the glass is still hot enough for the centrifugal force to cause the glass to form into a small disk. The glass is re-heated and spun again to make the disk thinner but increasing the diameter. The worker repeats this until the disk is sufficiently thin. A skilled worker can make disks up to a meter in diameter. Again the glass is cooled in a controlled way. Spun glass always has a raised knob in the centre. When the sheet is cut, the centre was traditionally either re melted with the next batch of glass or sold off as cheap off cuts. Hence the tradition of shops and Inns having roundels or bullions in the small glass paneled doors.

Float Glass

Glass remained and expensive product until late in the 19th century when a glass worker called Pilkington developed a technique of floating molten glass on liquid tin. The glass would not stick to the tin but would form a perfectly flat sheet. Sheets could now be very large, made by machines, and the price of sheet glass fell dramatically. The size of windows in even working class properties steadily increased through the 20th century, reaching its peak in the 1960's. There has since been a reaction to the 60's and a wish to reclaim the quaintness of Georgian windows in modern housing.

Leaded Glass

Traditional leaded glass used lead came (pre-formed lead with a 'H' cross section) to join glass quarries. The pattern is drawn full size (known as a cartoon). The glass is cut and placed on the cartoon. The lead came is cut and placed between the glass pieces. The lead is soldered together and then lead and light cement, a black putty like goo, is rubbed into the gap between the lead and the glass. This stiffens the panel and makes it weather proof. The finished panel contains no iron or steel so there is nothing to rust.

A leaded panel should last at least a century without maintenance providing it is not abused. Over time the cement will slowly dry out and begin to fall out. It is a very easy job to recement a panel if it is caught in time. Some soldered joints may crack as the solder is more brittle than the lead. Joints can be re-soldered, ideally to do a good job the panel may be laid flat. Lead slowly crystallizes but this should only be noticeable a century or more after the panel was made. If this becomes a serious problem, the whole panel can be re-leaded

Thus a leaded panel can last many centuries with only occasional routine maintenance. A life time guarantee by a maker should be a risk free offer. The wreck of the Titanic has been at the bottom of the Atlantic for approaching a century. Some of the leaded panels are still intact.

Copper foiling.

Louis Comfort Tiffany pioneered this new technique late in the 19th century. A thin copper strip is wrapped along the edge of each piece of glass. They are then soldered together. This forms a similar bond to leaded glass but much finer work can be created and it is easy to create three dimensional work.