the float glass process invented in the 1950s by Sir Alastair Pilkington of Pilkington Glass, in which molten glass is poured onto one end of a molten tin bath. The glass floats on the tin, and levels out as it spreads along the bath, giving a smooth face to both sides. The glass cools and slowly solidifies as it travels over the molten tin and leaves the tin bath in a continuous ribbon. The glass is then annealed by cooling in an oven called a lehr. The finished product has near-perfect parallel surfaces. The side of the glass that has been in contact with the tin has a very small amount of the tin embedded in its surface. This quality makes that side of the glass easier to be coated in order to turn it into a mirror, however that side is also softer and easier to scratch. Glass is produced in standard metric thicknesses of 2, 3, 4, 5, 6, 8, 10, 12, 15, 19 and 25 mm, with 10mm being the most popular sizing in the architectural industry. Molten glass floating on tin in a nitrogen/hydrogen atmosphere will spread out to a thickness of about 6 mm and stop due to surface tension. Thinner glass is made by stretching the glass while it floats on the tin and cools. Similarly, thicker glass is pushed back and not permitted to expand as it cools on the tin. Prism glass Prism glass is architectural glass which bends light. It was frequently used around the turn of the 20th century to provide natural light to underground spaces and areas far from windows. Prism glass can be found on sidewalks, where it is known as vault lighting, in windows, partitions, and canopies, where it is known as prism tiles, and as deck prisms, which were used to light spaces below deck on sailing ships. It could be highly ornamented; Frank Lloyd Wright created over forty different designs for prism tiles. Modern architectural prism lighting is generally done with a plastic film applied to ordinary window glass. Glass block Glass block, also known as glass brick, is an architectural element made from glass used in areas where privacy or visual obscuration is desired while admitting light, such as underground parking garages, washrooms, and municipal swimming baths. Glass block was originally developed in the early 1900s to provide natural light in industrial factories. Annealed