Photo by Anna Carlgren
How Is Glass Made?
In its most basic form, glass is made from quartz sand (silica), which melts at a temperature of about 3100° F (about 1700° C). The addition of a flux such as soda ash (sodium carbonate) lowers this extremely high melting point to a more feasible 2400° F (about 1316° C). However, it also makes the glass water soluble. Lime is therefore added to the mixture of dry ingredients (the batch) in the form of crushed stone as a stabilizer to help the glass resist moisture.
The usual proportion of these three basic ingredients is 72% silica, 15% soda, and 10% lime. The remaining 3% is made up of impurities found in the other ingredients (such as iron or magnesium). This formula is a basic type of soda-lime glass, still the most common form of glass—it represents 90% of the glass made today.
In order to work with molten glass, a glassworker must control its viscosity. For ancient soda-lime glass, the temperature necessary to melt the glass batch to the viscosity of a semi-hard cheese (viscous enough to poke a hole through it with a pointed tool) was about 1275°F (about 690°C). At a temperature of about 1830°F (about 1,000°C), ancient soda-lime glass melted to the consistency of honey—the viscosity suitable for blowing glass. 
To control viscosity, a glassworker needs to continually reheat the piece he or she is working on, as well as frequently cooling it. However, cooling glass too quickly can cause strain that could crack or shatter it. An annealing oven is used to slowly lower the temperature of the finished piece.