Re: What are the bi-products of making glass?

Nick,

The glass association has a website that discusses many concerns of glass 
manufacturers:  http://www.gmic.org/rd.html  This 
gets to research topics.

Glass is a solidified melt of various oxides, mostly silicon dioxide[sand, 
quartz, silica]with oxides of other elements such as sodium, calcium, 
aluminum, boron, potassium, barium, even lead in certain crystal glasses.  
Different compositions give different properties. Sodium and calcium 
oxides make a lower melting[about 700 degrees Celsius] "soft" glass.  
Addition of boron and aluminum oxides make a borosilicate glass[Pyrex] 
which is higher melting[~900C] and more chemically inert.  Addition of 
lead oxide raises the refractive index and makes the glass sparkle
[crystal].  Pure silicon dioxide is found naturally as quartz and can be 
made into fused silica; these are high melting[~1600 C] and have very low 
thermal expansion and resist thermal shock very well[A quartz object can 
be heated red hot and plunged into water and it will not crack; regular 
glass will usually crack if you pour boiling water into a cold glass.]  
Other metal[iron, chrome, cobalt] oxides are added in small amounts to 
give colored glass.[Stained glass http://www.geocities.com/Paris/
1141/ ]

Glass manufacturing is essentially mixing the proper ingredients in the 
proper order at the appropriate temperatures.  There is a definite 
scientific art to this and the various manufacturers have their[usually 
secret] methods.  The molten glass must then be formed and cooled and 
annealed to relieve stresses.  If this is done properly there are no by 
products. All the ingredients go into the glass.

There are concerns, however.  The first is energy usage.  The glass 
industry uses a lot energy and is actively working on energy efficiency 
and conservation. The second is raw material acquisition.  The silica is 
usually mined from sand deposits and mining has its own concerns.  The 
other materials are usually manufactured and there are as many processes 
as there are chemicals. For example sodium oxide is possibly made by the 
reactions of sodium chloride with ammonia, water and carbon dioxide to 
give  ammonia and sodium bicarbonate [Solvay process].  The sodium 
bicarbonate is then heated to sodium oxide and water and carbon dioxide. 
The water, ammonia and carbon dioxide are recycled. Another concern is 
scrap material or mistakes and used glass.  These are usually recycleable, 
but as you can imagine as the chemical mix becomes more complex there is 
increased chance that the mixture will be not useable as glass.