The Glass Cycle
Though glass was originally mixed by hand and blown into shape, today it is
produced by automated machines. In principle, both processes are 
very
similar:
- The ingredients are measured, mixed and fed into a large tank at one end
of a furnace. The furnace heats the mixture to a temperature of about 1500
degrees Celsius. The molten glass is then removed through the other end of
the furnace.
- The molten glass is then slowly cooled to about 1100 degrees Celsius. This
process, called fining, allows gases to escape without trapping bubbles.
- The molten glass is then pushed through a hole and cut into globs. The
globs are then forced into a mold. Compressed air, or a solid insert, is then
injected into the mold. This spreads the glass against the walls to form the
desired container shape. Some machines can produce as many as 140 bottles
per minute!
- The container then undergoes a period of controlled cooling called annealing.
Annealing prevents cracking and is accomplished in a temperature chamber called
a lehr.
- The cooled bottles are then sent to a factory where they are filled and
sealed. Any broken or damaged bottles are crushed into small pieces called
cullet and reintroduced into the mixing chamber.
Recycling glass is simple. After crushing, the cullet is mixed in with the
"virgin" materials and melted in the furnace. The addition of cullet
actually decreases the melting point of the batch. On the average, recycling
one ton of glass saves 40 gallons of fuel oil.
The only type of glass accepted in most recycling programs is food and beverage
containers such as bottles and jars. Other types of glass, such as windows,
Pyrex, stemware and mirrors, are not accepted because they contain different
ingredients and therefore melt at different temperatures. Different colors of
glass also melt at different temperatures. Clear glass has the lowest melting
point and is therefore the most desirable for recycling.
"Small Planet" Monterey Regional Waste Management
District School Education Program.