The glass pressing machine, invented in the 1920s, was the first method of mass producing glass panes, and for decades, glass interior doors were sure to have been made using one of these machines. To do this, molten glass was poured into iron molds before a plunger pressed the glass firmly into the mold. However, the contrast of temperatures between the cold iron and the hot molten glass would cause the cooled glass to wrinkle; this is why windows and glass interior doors in older homes have that old-fashioned wavy look. By the 1940s, however, glass makers learned to heat the molds before pouring in the molten substance, thereby eliminating the wrinkles.
Today, windows and glass interior doors are usually made of float glass. This manufacturing technique was invented in England in the 1940s but not released to the public until 1959. To make float glass, molten glass floats on a surface of molten tin. The result is a brilliantly shining pane of glass with incredible clarity, the perfect type of glass to add beauty and light to the windows and glass interior doors in your home.