Float glass is made by pouring molten glass onto a bath of molten tin, where it spreads out into a perfectly flat, uniform sheet. This single innovation, conceived by Alastair Pilkington in 1952, replaced older methods that required extensive grinding and polishing. Today, virtually all flat glass in windows, mirrors, and screens is produced this way, with modern plants turning out up to 1,000 tonnes per day.
The Raw Materials
Float glass starts as a carefully measured mix of minerals. The dominant ingredient is silica sand, which makes up about 73% of the final glass by weight. Sand provides the glass-forming network, the basic structure that makes glass transparent and rigid. On its own, though, silica would need extremely high temperatures to melt, so other materials are added to bring that melting point down and improve the finished product.
Sodium carbonate (soda ash) supplies about 12 to 14% of the mix and acts as a flux, making the sand melt at a more practical temperature. Limestone and dolomite contribute calcium and magnesium oxides, together around 11 to 15%, which make the glass more durable and resistant to water. A small amount of aluminum hydroxide, under 2%, improves chemical resistance. These raw materials are weighed, blended, and fed into the furnace as a powder called “batch.”
Recycled glass, known as cullet, is also mixed into the batch. Factories typically add between 10 and 60% cullet, and occasionally use up to 100%. Cullet melts at a lower temperature than raw batch, so using more of it cuts energy consumption. A batch with 30% recycled glass can reduce carbon emissions from the melting stage by roughly 23%, and pushing that to 90% cullet can cut emissions by as much as 71%.
Melting the Batch
The blended raw materials are fed continuously into a large furnace heated to between 1,000 and 1,450°C. Inside, the sand grains dissolve, gases escape from the decomposing carbonates, and the mixture gradually becomes a homogeneous molten liquid. The furnace is designed to keep the glass at high temperature long enough for bubbles and impurities to rise to the surface and dissipate. By the time the molten glass exits the furnace, it’s a clear, bubble-free liquid ready to be shaped.
Floating on Molten Tin
This is the step that gives float glass its name. The molten glass flows from the furnace onto a shallow bath of molten tin at around 1,000°C. Tin works for this because of a fortunate combination of physical properties: it melts at just 232°C, doesn’t boil until above 2,000°C, and is far denser than glass. Molten tin has a density of about 6.5 grams per cubic centimeter compared to roughly 2.3 for soda-lime glass. The glass literally floats on top, the way oil floats on water.
As the glass spreads across the tin, gravity pulls it flat while surface tension keeps it smooth. The result is two nearly perfect surfaces formed without any mechanical contact on the top side and shaped by the atomically smooth liquid tin on the bottom. This natural flattening is why float glass doesn’t need the grinding and polishing that older sheet glass required.
The atmosphere inside the tin bath enclosure is tightly controlled with a mix of nitrogen and hydrogen. Without this protective gas blanket, the tin would rapidly oxidize at such high temperatures. Any tin oxide that does form gets collected in dross containers along the edges of the bath.
Controlling Thickness and Width
Left to its own devices, molten glass on tin would settle to an equilibrium thickness of about 6 to 7 millimeters. To make glass thinner or thicker than that, manufacturers use pairs of grooved steel wheels called top rollers positioned along the edges of the ribbon. These rollers grip the edges of the still-soft glass and either stretch it outward to make it thinner or compress it inward to make it thicker. Each roller pair has an independently adjustable motor, allowing precise control.
The number of roller pairs depends on the target thickness and how fast the glass is being pulled through the bath. Pulling the ribbon faster stretches it thinner, while slowing the line produces thicker sheets. Modern float lines can produce glass ranging from under 1 millimeter to over 20 millimeters thick, all by adjusting roller angles, speeds, and pull rate.
Annealing: Cooling Without Cracking
By the time the glass ribbon leaves the tin bath, it has cooled enough to be solid but is still around 600°C (roughly 1,050°F). At this point, the outer surfaces are cooler than the interior, which creates internal stresses. If the glass cooled quickly from here, those stresses would make it brittle and prone to shattering during cutting.
To prevent this, the ribbon passes through a long, temperature-controlled tunnel called a lehr. Inside the lehr, the glass is cooled gradually over a carefully managed gradient. As the temperature drops through a critical zone (between what glassmakers call the annealing temperature and the strain temperature), the internal stresses slowly relax. The glass transitions from a state where it can still flow and release tension into a true solid with minimal locked-in stress. By the time it exits the lehr, the ribbon is cool enough to handle and cut.
Cutting and Inspection
The continuous ribbon emerging from the lehr is inspected by automated scanning systems that detect flaws like bubbles, inclusions, or surface distortions. Sections that pass inspection are scored and snapped into standard sheets, typically large stock sizes that downstream manufacturers cut further for windows, facades, or display panels. Defective sections are broken up and recycled back into the batch as cullet.
What Makes Float Glass Distinctive
The float process produces glass with consistent optical quality. A standard 6-millimeter sheet of clear float glass transmits about 88% of visible light and has a refractive index of 1.5. That high clarity is a direct result of forming on molten tin: both surfaces are smooth enough to need no further finishing for most applications. The tin side and the air side of float glass do have subtly different surface chemistries, which matters for certain coatings and treatments, but for everyday use the difference is invisible.
Since Pilkington’s first commercial production in 1959, the float process has become so dominant that it has largely replaced all other methods of making flat glass. The combination of high throughput, optical quality, and the ability to produce a wide range of thicknesses from a single line made older techniques like drawn sheet and plate glass economically obsolete.