How glass jars are made

A glass jar is pressed into a rough shape by a plunger and then blown to its final form in a second mold.

stations: 5
things upstream: 17
raw sources: 10
steps from the ground: 6

The short version

A blob of molten glass, called a gob, drops into a blank mold, where a metal plunger presses it into a thick preform called a parison. The parison flips into a second mold and compressed air blows it out to the jar's final shape. The jar then goes through an annealing oven, called a lehr, which relieves stress.

Shear off a gob

Molten glass runs from the furnace in a stream. Shears snip it into glowing lumps called gobs, each carrying the weight of one jar.

Goes in
Glass
Press the preform

A gob drops into a blank mold, and a metal plunger presses it out to fill the cavity. This forms the neck and a thick-walled blob called the parison.
Flip and blow it out

The parison swings upside down into the final mold. Compressed air or vacuum pushes the glass out against the walls until it takes the shape of the jar.
Anneal in the lehr

Forming leaves stress in the glass. The jar passes through a long oven that reheats it to relieve the stress and then cools it slowly under control.

reheated, then cooled slowly
Inspect every jar

Machines look for cracks, thin spots and bad finishes. Rejects are crushed and go back into the furnace as cullet.

End of the lineGlass jar Next it goes into mustard, jarred salsa, pickles and 3 more.

Worth repeating

Press-and-blow is mainly used for jars, while blow-and-blow is the usual method for bottles.
A modern glass-forming machine can make more than 600 containers a minute.
Pressing the first stage controls how the glass spreads through the wall better than blowing it does.