What Is Float Glass? The Foundation of the Glazing Industry

Almost every pane of glass you handle — whether toughened, laminated, low-E coated, or part of a double-glazed unit — starts life as float glass. Understanding where glass comes from and how it's manufactured helps you understand the entire product chain and why glass behaves the way it does.

The Pilkington Float Process

Float glass gets its name from the manufacturing method. The process was invented by Sir Alastair Pilkington in 1952 and patented in 1959. It quickly became the universal method for producing flat glass worldwide and remains so today. Molten glass is poured onto a bath of molten tin and allowed to float on the surface. Because glass is less dense than tin, it floats, spreads under gravity, and forms a perfectly flat sheet with parallel surfaces. The result has no optical distortion — a significant improvement over the plate glass manufacturing it replaced, which required grinding and polishing both surfaces after production.

From Raw Materials to Glass Sheet

The manufacturing process has four main stages.

Melting: Raw materials — silica sand (about 72%), soda ash (about 14%), limestone (about 9%), and recycled cullet (broken glass) — are continuously fed into a furnace and melted at approximately 1,550°C. Cullet (typically 20–30% of the batch) improves energy efficiency and glass quality by lowering the melting temperature.

Float bath: Molten glass flows from the furnace onto the tin bath. The environment is carefully controlled — temperature gradients, atmosphere (nitrogen and hydrogen to prevent tin oxidation), and glass draw speed all determine the final thickness. Standard thicknesses from 2mm to 25mm are produced by adjusting these parameters.

Annealing: The glass ribbon exits the tin bath at around 600°C and passes through a long annealing lehr — a temperature-controlled tunnel conveyor. Temperature is gradually reduced over 100–200 metres of travel. This controlled cooling relieves the internal stresses that would otherwise make the glass fragile and prone to spontaneous fracture. The result is annealed glass — flat, stress-free, and ready for further processing or direct use.

Cutting and inspection: The cooled glass is automatically inspected for bubbles, inclusions, and surface defects using optical scanning, then cut into standard large sheet sizes for distribution.

Why Float Glass Weight Is So Predictable

Float glass is manufactured to tight tolerances, and its composition gives it a density of consistently 2,500 kg/m³. This means the weight formula is reliable: 2.5 kg per m² per mm of thickness. A 1.0m² panel of 6mm glass weighs exactly 15 kg; 10mm weighs 25 kg; 12mm weighs 30 kg. No estimation is needed. The Glazing Calculator uses this relationship to give instant weight calculations for any panel size and glass type, which is essential for safe manual handling decisions and hardware specification.

Float Glass vs. Annealed Glass

These terms are used interchangeably, correctly so. Float glass is annealed glass — the annealing process is what makes it stress-free. When glaziers refer to "annealed glass," they specifically mean float glass that has not been further processed by toughening or laminating. Raw annealed glass is the starting material for all other glass products.

Quality Grades

Float glass is produced to different optical grades depending on the application:

Standard building grade is used for the vast majority of windows, doors, and facades. It has excellent flatness and optical quality with minimal distortion or inclusion.

Ultra-clear (low-iron) glass uses higher-purity silica with reduced iron content. Standard glass has a noticeable green tint when viewed edge-on (most visible in thick panels) from iron oxide in the silica sand. Low-iron glass is nearly colourless, making it preferable for shopfronts, display cases, high-end residential facades, and solar panels where colour neutrality matters.

Patterned and obscure glass is produced using a textured roller applied to the glass ribbon during the float process, before the glass fully solidifies.

Further Processing — From Float to Finished Product

Raw annealed float glass is relatively fragile and produces dangerous shards when broken. For building applications, it's almost always further processed:

Toughening (tempering): The glass is reheated to approximately 620°C — just below the softening point — then rapidly quenched with cold air jets. This creates surface compression and central tension. The result is glass 4–5 times stronger than annealed, and when broken, the stored energy is released simultaneously, shattering the panel into thousands of small, roughly cubic fragments rather than large shards. See Toughened vs Laminated Glass for a detailed comparison.

Laminating: Two or more sheets are bonded with a plastic interlayer (PVB or SGP) under heat and pressure. The interlayer holds fragments together when broken.

Coating: Low-E, solar control, reflective, or anti-reflective coatings are applied either during manufacturing (hard coat) or in a dedicated post-processing step (soft coat).

IGU assembly: Float glass sheets — toughened, laminated, or standard — are combined with spacers and sealants to form double or triple-glazed units.

Frequently Asked Questions

Is all flat glass float glass?

Virtually all modern flat glass for building applications is float glass. Older types — plate glass (ground and polished) and drawn glass (pulled vertically from a molten bath) — are found in heritage buildings and are identifiable by slight waviness or distortion. For replacement work in heritage frames, standard float glass is used; the slight difference in optical character is accepted as necessary for practical replacement.

Why does annealed glass break into large dangerous pieces?

Annealed float glass has no internal pre-stress — it's the most stress-free state of glass. When it breaks, the crack propagates freely through the material along the path of least resistance, producing large, irregular shards with sharp edges. Toughened glass, by contrast, has stored energy from the toughening process. When it breaks, this energy releases simultaneously across the panel, producing thousands of small fragments.

Can float glass break spontaneously?

Standard float glass can break from thermal stress — uneven heating, such as direct sun on part of a pane while the rest is shaded — or from edge damage that weakens the panel over time. It does not break spontaneously in the way that toughened glass occasionally does from nickel sulphide inclusions. Spontaneous toughened glass breakage (where a panel shatters without apparent impact) is caused by tiny nickel sulphide particles in the glass that expand slowly after toughening; this is a known though rare failure mode, typically occurring within the first few years after installation.

What is the green tint in standard glass?

The green tint in standard float glass comes from iron oxide impurities in the silica sand — it's inherent to standard manufacturing and not a defect. You'll notice it most when looking at the edge of a thick piece or when viewing glass at a shallow angle. For most applications it's invisible in use. For projects where colour neutrality is important, specify ultra-clear or low-iron glass.

What's the difference between float glass and crystal glass?

Crystal glass refers to decorative glassware — bowls, vases, wine glasses — made from lead crystal or lead-free crystal formulations with high clarity and specific refractive properties. It has no connection to flat float glass for building purposes. When people describe a window as "crystal clear," they typically mean high-clarity glass, which in a building context means ultra-clear low-iron float glass.