When was flat glass invented?

Although the Romans and Greeks used glass to make vessels, the use of glass for window panes did not start until the middle ages. Bede states that the first use of glass in England was in 684. Until about 1200 window glass was very expensive and only found in cathedrals or large monasteries or other similar major buildings. From about 1200-1600 the Venetians of Murano dominated the glass industry and glazing tended to be found only in state buildings or commercial buildings. During the Tudor period glass started to spread to the homes of the wealthy. Glass was usually blown as a disk and then cut into small squares or diamonds.

Before the 19th century glazing was almost always made from small squares about 3-6 inches on a side. Before the 20th century, the glass was blown as a cylinder, which was then cut and laid flat, then sectioned into small squares. By 1800 plates as large as 12 feet by 4 feet could be be made, but they were very expensive, and therefore usually cut into small squares for sale.

During the 19th century this process was developed and automated to the point where full size windows became possible. The glass blower would stand on a rafter and blow a 6 foot cylinder hanging down, which was then opened and flattened. Lower fuel costs and greater purity of materials allowed window pane to become cheap enough that 16 inch or 20 inch window became common in ordinary houses.

Modern plate glass is made from the float process, first developed by Henry Bessemer around 1850, but not truly perfected until around 1950. Floating glass on molten tin allows it to be continuously produced relatively cheaply because it automates the process and requires no blowing.

In the United States the majority of people had glazed windows by about 1850.

I saw a television show "The Naked Archaeologist" that had on the show a large (several foot square) slab of glass in Israel with some graves around it. The slab was intended for the Temple in Jerusalem but never left for some reason, and later rabbis liked to be buried near it after the fall of the Temple to partake in its leftover spiritual closeness to said temple.

This takes it to around 70 AD or earlier in Israel. I'm not sure if they have an exact date for the slab itself.

Looking at the list of shows, it is probably Season 3, Ep 2 "Ancient Glass".

Technically speaking "flat glass" was introduced by the Romans as glass for construction of window panes. These were produced as early as the first century CE, and is commonly found in Roman sites in Britain as well. Pieces as large as 30 by 40 inches have also been uncovered at Pompeii.

Such early windows were usually quite small, of irregular thickness, and not clear or transparent. These flat panes may have been produced using one of several different processes.

One suggested method is "cast glass". This produced glass of uneven thickness, with one side having a polished or "glossy" surface and the other side a matt finish. Although the exact technique is lost, it is suggested that they were produced by pouring molten glass into a mold.

Another method suggested is casting of soft, hot glass on a flat surface and then pressing it into flatness with a moist, wooden mallet. Contemporary glassmakers have been able to re-produce glass panes in this method, and the results closely match the original Roman panes.

Another method used by the Romans for the production of window glass was called the "crown method". First a hollow sphere was blown at the end of a blowpipe. Then the end opposite the blowpipe was opened. The opened, soft sphere was then vigorously rotated. The centrifugal force would flatten out the glass into a disk. The disc was then cooled and cut into small sheets. Every disk had a lump of glass at the center, known as the "bull’s eye" or "crown".

Although another source clearly mentions that crown glass method was invented in the eighth century in Syria:

The method of spinning window crowns was probably first discovered in Syria in the eighth century. The glass workers blowpipe, used for this process, was invented at the beginning of the Christian era. - source: Flat Glass Technology.

The well-known Venetian glass industry dates back to the tenth century. Venetian glass makers did not specialize in flat glass products but a certain amount of window glass was manufactured and part of it was exported. In the fourteenth century mirrors were made by coating plates of glass with an amalgam of tin and mercury.

English settlers introduced glass making into America. The first manufacturing establishment in America was a glass factory. This was erected at the beginning of the seventeenth century at James Towne, Virginia. The crown method of manufacturing flat glass was replaced by the cylinder process.

Larger sheets of glass could be made in this way and it was the dominating method of making flat glass in the nineteenth century. At the beginning of the twentieth century the machine-cylinder method was introduced. A circular metal bait about 25 cm (10 in) in diameter situated at the end of a blow-pipe, was lowered to the surface of the molten glass. Using compressed air for blowing, it was possible to draw a cylinder of glass, approximately 1500 cm (50 ft) high. This cylinder was subsequently split, flattened and annealed.

At the end of the nineteenth century attempts were on to draw a flat sheet of glass directly, to avoid the second step of flattening the cylinder. The first successful method was invented by Emile Fourcault in Belgium, who took a patent in 1904. Around the same time, two methods for sheet glass drawing were developed in America. These were the Colburn, or Libbey-Owens, process and the Pittsburgh process. These processes are still in use, the most successful being the Pittsburgh process.

Several refinements and processes have been made since the seventeenth century in France, Germany, and the United States.