Modern glass blocks, also known as glass brick, are manufactured by bonding together two molded pieces of glass under high temperatures (around 800 degrees Celsius), leaving an insulating air cavity of partial vacuum in the middle. After being fused, the blocks are annealed in a cooling oven for strength.
This type of glass block dates back to the 1930’s, when Pilkington Brothers Ltd. patented a technique for mass-producing these. Various forms of prismatic glass had been in use long before this. They were used in shipbuilding and cellars as far back as the early 1800’s.