Answer: Liquid Tin

Historically, the process of making glass was enormously cost and labor intensive. Even into the mid-20th century the process remained expensive because even the automated methods of glass production required extensive polishing. Glass was created by stretching a ribbon of molten glass out between rollers and then, upon cooling, sent through an extensive system of polishing devices that ground and polished the imperfect surface down to the expected degree of optical clarity and smoothness.

The entire process became significantly more cost effective thanks to the efforts of Sir Alastair Pilkington who, in the 1950s, pioneered what is now the most common method of sheet glass production: float glass. His method involved floating molten glass on a pool of molten tin such that surface tension and gravity smooth both sides of the glass simultaneously (cutting out the lengthy and intensive polishing process required for glass created using the cylinder method).

By the 1960s, his methods were refined enough to make float glass profitable and over half a century later, the “Pilkington process” is the manufacturing process that drives nearly all flat glass production worldwide.