Re: Chemical equations of some common glasses

G'day Miranda!

Your question requesting information on the chemical equations of common 
glasses is both easy and hard to answer.

The easy part of the answer is that most of the glasses that are around and in 
common use are not chemical compounds of fixed compositions at all -- they're 
not "stoichiometric."  Essentially all of the glasses are just mixtures of 
oxides of a number of the oxides of various materials.

Many of the oxides which are used are metal oxides of the alkali and alkaline 
metals.  Alkali metals are the ones in the first row of the Periodic Table of 
the Elements, and have valence +1 -- things like lithium, sodium, potassium, 
and so on.  Alkaline metals are the ones in the second row of the Periodic 
Table, and have valence +2 -- things like magnesium, calcium, and barium.

A lot of not-so-expensive glasses incorporate sodium oxide and calcium oxide as 
major constituents -- making "soda lime glass" which was used for a long time 
as inexpensive windows or glass ornamentation.

If you look up the melting points of all of these oxides in the Handbook of 
Chemistry and Physics, you'll find out that they require pretty high 
temperatures to melt.  Early glass makers -- like the guys three or so thousand 
years ago who "discovered" glass in hot campfires -- learned that there are 
certain materials that melt at relatively low temperatures and which will cause 
other oxides that would ordinarily not melt  to dissolve.  It's sort of like 
the way that salt will dissolve in water at room temperature, even though you 
have to heat it up to pretty high temperatures -- above 500 C, I think.  
Materials with this neat property to dissolve others well below their melting 
points are called "fluxes" and are often used in glass making.  Boron oxide is 
often used in this role.

Probably the most common constituent is silicon dioxide -- which is just sand, 
although super-pure stuff is used for really good glasses.  And if you want 
really chemically resistant glass, melting pure silicon dioxide and cooling it 
so that it forms a glass instead of a crystal makes material that's useful in 
many applications.  But the temperature required to melt it is pretty high, 
because no flux is used in this "fused silica."

Almost any other oxide can be tossed into glass to yield special properties.  
Lead oxide produces glass which is heavy, has a high index of refraction, and -
- if it's used in enough concentration -- is prized for crystal tableware and 
sometimes jewelry.  Phosphorous oxide is often used to make glass that is 
capable of incorporating other oxides -- like erbium or neodymium oxide -- 
which make it into pretty good laser materials.  And still other oxides allow 
the glass to take on characteristic colors -- cobalt oxide generally makes the 
glass blue.

Because glass is a solid solution, rather than a chemical compound, it's 
possible to incorporate the different oxides in different concentrations 
depending on the material you.  Interestingly, it's not just the stuff that you 
put into a glass that gives it it's properties -- it's also how you produce it.

Glass is made in a process of several steps.  

The first is call "melting," but the term is only a small indication of what 
goes on.  Powders of the constituent oxides are added together, stirred up 
(actually, they're usually put in a closed container which is rolled end-over-
end to mix), put in a crucible of metal or ceramic that has an ultra-high 
melting point and doesn't dissolve in glass, and heated to melt/dissolve all 
the powders into a liquid state.  During this time, chemical reactions are 
going on among the constituents which subtly change the valence states or the 
interatomic distances in a way that modifies the properties of the material.  
Depending on how hot and how long the melt is allowed to stay liquid, and 
depending on whether gases are bubbled through the melt and/or whether it's 
stirred, the material "cooks" in unique ways.  Perhaps the best way to think 
about it is like a person cooking a sauce or a cookie -- depending on how you 
cook it, the results may look different and almost always will taste 
different... even though the ingredients are the same.  After glass is melted 
(which really forms its final chemical composition), it's generally allowed to 
cool pretty quickly -- which results in it cracking into fractured chunks.

The fractured chunks then go through a process known as "refining" -- which is 
a remelting, stirring process at a temperature just a little above the melting 
temperature.  During refining, the non-uniformities and bubbles in the glass 
are removed, after which it's allowed to cool a little more slowly than during 
the melting process -- so it solidifies in a big gob... that's what it's often 
called, a gob or sometimes a blank, even though it's generally poured into a 
hot metal or graphite mold to give it a rough general shape which won't 
fracture when it's finally cool.

Last, but not least, the glass is heated to just a little bit below the melting 
temperature and held there for many, many hours.  During this time, the very 
tiny areas where the glass solidified at different times get hot enough so that 
atoms can move around and remove any mechanical stresses that exist to make 
nearly perfect glass.

This glass can then be blown or fabricated into lenses, laser materials, or 
whatever.

Just in case you're of a mind to try to make some glass -- let me add a word to 
the wise.  You almost certainly don't have the material on hand around the 
house that you'd need to melt the materials -- stoves aren't hot enough, and 
most houses don't have kilns or high temperature furnaces.  And the potentially 
bubbling, boiling, offgassing liquid media that form during melting are awfully 
dangerous -- they can cause fires and if the material touches your skin, it 
isn't skin any more... it's on fire, turning to carbon.  If you do want to 
learn to make glass, the best bet might be to go to a local jeweler and ask if 
he could recommend someone who works in glass or enamels (which are different, 
but kind of close).  There are many artists who make and fabricate their own 
glass, and they're usually very happy to talk with others about their work -- 
and teach it as well.

Hope this helps!


Steve Guch
Mount Dora, Florida, USA