# How Glass Is Made

How flat window glass is made, from melting sand, soda ash and limestone at about 1500 °C to floating the ribbon on molten tin and cutting it\.

By PETE\!

Topic: Engineering

[Open the interactive Jig](https://ask.jig.com/j/XuHaCA6pwppu)

## Transcript

### 1. From sand to window

Almost every window starts as a heap of sand\. This model of a float\-glass line follows it from raw minerals to a clear, flat sheet\. Drag the empty space around the model to turn it\.

A real line stretches for hundreds of metres and can run without stopping for 10 to 15 years\.

### 2. The recipe

The batch is mostly silica sand\. Soda ash is the flux that lets sand melt at a lower temperature, and limestone is the stabiliser that stops the glass dissolving in water\. Crushed recycled glass, called cullet, melts first and helps the rest along\.

Window glass ends up about 73 percent silica, 14 percent soda and 8 percent lime\.

### 3. Melting at 1500 °C

Gas flames heat the batch to about 1500 °C\. The soda ash breaks down and gives off carbon dioxide, which rises as bubbles until the melt runs clear\. In this cutaway the molten glass glows like a lake of fire\.

Pure sand would need more than 1700 °C\. A real furnace holds about 1200 tonnes of molten glass\.

### 4. Cooled, then poured

The glass flows along a canal and cools to about 1100 °C, thick as honey\. A gate called the tweel sets how much spills over the spout lip onto the next stage\.

### 5. Floating on tin

Here is the clever part\. Molten tin is denser than glass, so the glass floats on it and spreads into a ribbon with flat, parallel faces\. A nitrogen and hydrogen atmosphere stops the tin oxidising\. By the far end the ribbon has cooled to about 600 °C\.

Tin melts at only 232 °C, so it stays liquid the whole way\.

### 6. Speed sets the thickness

Thickness is not pressed in\. Top rollers grip the edges while the line pulls the ribbon away: pull faster and it stretches thinner, pull slower and it stays thicker\. Try the pull speed\.

Float glass is made from 0\.4 to 25 mm thick\. Left alone, it settles at about 6 mm\.

### 7. Slow cooling in the lehr

Rollers carry the ribbon through a long oven called the annealing lehr, cooling it slowly from about 600 °C to 200 °C\. Cooling slowly lets the whole sheet shrink evenly, so no hidden stress is locked inside\.

Glass is not a slow liquid\. Old windows are thicker at the bottom because of how they were made, not because they flowed\.

### 8. Score, snap and stack

At the cold end a hard cutting wheel scores a fine line across the ribbon, and the sheet snaps cleanly along it\. Press the button to cut a sheet\.

Look at the edge of a thick pane: the green tint comes from a trace of iron in the sand\.

### 9. From furnace to your window

Sand, soda ash and limestone, a lake of molten glass, a bath of liquid tin and a long slow cool: that is how most flat glass is made today, for windows, mirrors and screens\. Turn the model to see the whole line again\.