The chemistry of Al-Fl-Si glass

Silicate cement was probably the very first tooth colored filling material (if you discount whalebone). Glass Ionomer restoratives came later.  However, in order to understand silicate cement, and, indeed, in order to understand the characteristics of most modern composites, it is very important to understand the composition and chemistry of the glass powder that gives them their special characteristics.  

Glass is composed of three major components.  The first, silica (silicon dioxide),  is a naturally occurring mineral known as quartz..  Silica is the chief component in ordinary sand.   Silica is the major glass former in most glass formulations (but it is not the only glass former).  The melting temperature of quartz is very high, but it was discovered early in human civilization that the addition of certain metallic oxides could serve to lower the melting point of the glass quite a bit.  These additional metalic oxides, when added to sand in order to lower the melting temperature are called "fluxes".  Fluxes are the second major constituent in glass.

Fluxes are oxides of alkaline metals such as sodium, potassium, lithium, boron and lead.  They serve to dissolve the silica, a bit like water dissolves sugar.  This is important, since glass is composed of silicon dioxide which has a very high melting temperature. ( Pure quartz melts at 1713 degrees centigrade.  The addition of 25 % sodium oxide can lower the melting temperature to 793 degrees centigrade.)  The most common fluxes used in ceramics are sodium and potassium oxides, but there is a long list of fluxes, each one with its own set of characteristics and uses.

The third component of glass is a stabilizer.  Stabilizers make the glass strong and water resistant. Calcium carbonate, (limestone) is a stabilizer. Without a stabilizer, water and humidity attack and dissolve glass. Glass lacking a stabilizer is often called "waterglass" since it can dissolve in water.

• When lead is used as the stabilizer, the resulting glass has superior clarity and durability, and will ring like a bell when tapped.  Historically, lead "crystal" has been used for years in the manufacture of fine tableware including drinking glasses and wine canisters (Reference Waterford crystal).  It is also fairly insoluble, even in acidic solutions, and has never been implicated in any cases of lead toxicity in humans.  However, the FDA (US food and drug administration) has recommended that lead stabilized glass not be used to store liquids as small amounts of lead have been known to leach out of the glass and into the liquid.  Lead is not used to flux or stabilize any dental glass manufactured in North America or Europe.

• Boron oxide is, like silica, a glass former.  When added to silicon based glass at a minimum of 5% by weight, the glass becomes a borosilicate.  Glass fortified in this way is resistant to mechanical and thermal shock and is used to make baking pans (Pyrex), laboratory ware and sealed beam headlights.

• Alumina (aluminum oxide) is found combined with silicon in naturally occurring glasses called feldspars.  It is used as a glass stabilizer in molecular form to toughen the glass and and is also used as a crystalline structure dispersed throughout the  glass that acts as a sort of framework or skeleton.  This "framework" stiffens the glass during firing and makes it less likely to slump.  The inclusion of crystalline structures transforms the glass into porcelain which is much tougher and less prone to fracture than the same glass without such a matrix.  Alumina is a major component in ordinary clay and is present in nearly all the ceramic products you buy such as the plates and cups in your dinnerware and your mother's bone china.  It is generally added to dental porcelain in the form of aluminum oxide.

• The addition of trace metals can give color to the glass. Cobalt imparts a blue color, while gold imparts red and copper a green color.  (These metals are added as oxides, and they generally have fluxing qualities, but they are not alkaline metals, and are added in such small amounts that they are not considered fluxes for purposes of calculating glass formulas.)

• The addition of zirconium and titanium oxides add opacity to the glass.  These oxides form a crystalline structure within the otherwise translucent glass, and this diffuses light as it penetrates, creating a milky or pure white appearance depending on the amount of zirconium or titanium oxides used.

Melting temperature vs. Solidus

When a glass melts, it becomes a liquid with the consistency of syrup on a cold day.  Glass does not have a specific melting temperature, and when it cools, it remains a "supercooled" liquid (think of a hard candy, like a lollipop), however contrary to mythology, it does not continue to flow at normal temperatures.  While any given glass does not have a definite melting temperature, it does have a definite solidus temperature.  The solidus is the lowest temperature at which a non crystalline material shows any characteristics of melting, including a tendency to flow. 

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