Pilkington floated glass on molten tin to make it flat

By the 1950s, sheet glass could be mass-produced continuously by drawing or rolling it, but the resulting surface was never quite optically flat. Any application that needed true flatness -- mirrors, car windshields, shop windows -- still had to go through a separate grinding-and-polishing stage afterward, an expensive, slow, capital-heavy process that consumed a large share of the glass itself as waste. Decades of industry effort had gone into making that finishing stage faster and cheaper, not into eliminating it.

Alastair Pilkington, a young technical director at Pilkington Brothers, began experimenting in December 1952 with a different premise entirely: instead of shaping glass and then correcting its surface by force, let the glass shape itself. He proposed pouring molten glass onto a bath of molten tin and letting it float there, on the theory that a liquid surface is already perfectly flat and would pass that flatness on to the glass resting on it.

Poured molten glass onto molten tin, letting gravity and surface tension flatten both faces for free, so the ribbon cooled already flat with no grinding needed.

Molten tin is denser than glass, so glass floats on it without sinking or mixing.

A liquid surface is naturally flat, so the glass inherits flatness without any tool touching it.

Gravity and surface tension hold the glass at a stable thickness on both faces.

The glass cools on the tin bath and is lifted off already flat, eliminating the grinding stage.

Before optimizing a correction step, ask if the defect can be prevented by changing the process environment to one that is naturally self-correcting.

The float process was announced to the industry on 20 January 1959, after roughly seven years of development and a reported £28 million in R&D spending -- every sheet of glass made during that development period had still come out imperfect until a fully viable production run was achieved in July 1958. Rather than keep the process exclusive, Pilkington licensed it to competing glassmakers worldwide starting in the early 1960s.