How glass is made

About 90% of the glass made today is one recipe, and it is mostly melted sand.

stations: 5
things upstream: 16
raw sources: 10
steps from the ground: 5

The short version

Glass is sand melted with soda ash and limestone in a gas-fired furnace, then cooled fast enough that its atoms never line up into crystals. The usual soda-lime recipe is around 70% silica. Furnaces run up to 1,575 °C. The syrupy melt is shaped, then slowly cooled in an oven so internal stress does not crack it.

Weigh out the batch

Silos hold 1–5 days of sand, soda ash and limestone. Scales, chutes and conveyors measure the recipe and blend it. Crushed scrap glass goes in too, because it melts sooner than raw sand does.

1–5 days of material in the silos

Goes in
Silica sand Soda ash Limestone
Melt it in the furnace

The batch slides into a furnace fired by natural gas or fuel oil at a slow, steady rate. The hottest furnaces reach 1,575 °C, and the limit is set by how well the furnace roof and walls survive.

up to 1,575 °C
Tune the color

Ordinary soda-lime glass looks clear in a thin pane, but iron oxide impurities tint thick pieces green. Glassmakers can add a little manganese dioxide to cancel the green, or add other metal oxides to color the glass on purpose.
Shape it while syrupy

Flat glass is floated out as a ribbon. Bottles start as a gob of glass sheared off a stream and blown in a mold. Either way the glass is shaped while it is still thick enough to hold a form and soft enough to move.
Cool it in a lehr

Glass shrinks as it cools. If the surface sets first and the middle contracts later, the middle ends up pulling against the skin and the piece can fracture. An annealing oven warms the glass to about 580 °C and then lowers the temperature gradually.

20–60 minutes of gradual cooling