Wednesday, 7 November 2018

Specific Gravity, CoLE, and Colourants of Glass

I’ve been asked the question “is there is differential in specific gravity as related to COE or colorant used in the glass (white opal v clear)”? 

Using the typical compositions of soda lime glass (the stuff we use in fusing), both transparent and opalescent and combining the specific gravity of the elements that go to make up the glass, I have attempted to answer question - the last part of the question first.

Difference in specific gravity between transparent and opalescent glass

Transparent glass

Typical transparent soda glass composition % by weight (with specific gravity)

Material                         Weight        S.G.
Silicon dioxide (SiO2)           73%         2.648
Sodium oxide (Na2O)            14%         2.27
Calcium oxide (CaO)               9%         3.34
Magnesium oxide (MgO)          4%         2.32
Aluminium oxide (Al2O3)          0.15%    3.987
Ferrous oxide (Fe2O3)               0.1        5.43
Potassium oxide (K2O)             0.03       2.32
Titanium dioxide (TiO2)            0.02        4.23

There are, of course minor amounts of flux and metals for colour in addition to these basic materials.

The specific gravity of typical soda lime glass is 2.45.

Opalescent glass

Initially opalescent glass was made using bone ash, but these tended to develop a rough surface due to crystal formation on the surface.  The incorporation of calcium phosphate (bone ash) and Flouride compounds and/or arsenic became the major method of producing opalescent glass for a time.

The current typical composition by weight (with specific gravities) is:

Silicon Dioxide (SiO2) –             66.2%,     2.648 SG
Sodium Oxide (Na2O) –            12%,        2.270
Boric Oxide (B2O3) –                10%,        2.550
Phosphorus pentoxide (P2O5) –  5%,         2.390
Aluminum Oxide (Al2O3) –         4.5%,      3.987
Calcium oxide (CaO) –              1.5%,      3.340
Magnesium oxide (MgO) -         0.8%,      2.320

The combined specific gravities are within 0.03% of each other -  a negligible amount.  So, the specific gravity of both opalescent and transparent glass can be considered to the same. For practical purposes, we take this to be 2.5 rather than the more accurate 2.45.

Other glasses exhibit different specific gravities due to the materials used, for example:

Lead Crystal Glass
Lead Crystal glass contains similar proportions of the above materials with the addition of between 2% and 38% lead by weight.  Due to this variation the specific gravity of lead crystal is generally between 2.9 and 3.1, but can be as high as 5.9.

Borosilicate glass
Non-alkaline-earth borosilicate glass (borosilicate glass 3.3)
The boric oxide (B2O3) content for borosilicate glass is typically 12–13% and the Silicon dioxide (SiO2) content over 80%. CoLE 33


Alkaline-earth-containing borosilicate glasses

In addition to about 75% SiO2 and 8–12% B2O3, these glasses contain up to 5% alkaline earths and alumina (Al2O3).  CoLE 40 – 50


High-borate borosilicate glasses

Glasses containing 15–25% B2O3, 65–70% SiO2, and smaller amounts of alkalis and Al2O3

All these borosilicate glasses have a specific gravity of ca. 2.23

Correlation between CoLE and and specific gravity?

This comparison of different glasses shows that the materials used in making the glass have a strong influence on the specific gravity.  However, there does not appear to be a correlation between CoLE and specific gravity in the case of borosilicate glass.  If this can be applied to other glasses, there is no correlation between specific gravity and CoLE.

Correlation between specific gravity and colourisation minerals and CoLE?

The minerals that colour glass are a very small proportion of the glass composition (except copper where up to 3% may be used for turquoise).  The metals are held in suspension by the silica and glass formers.  That means the glass is moving largely independently of the colourants which are held in suspension rather than bring part of the glass structure. There is unlikely to be any significant effect of the metals on the Coefficient of Linear Expansion.  The small amounts of minerals are unlikely to have an effect on the specific gravity.  So, the conclusion is that there is no correlation between CoLE, specific gravity, and colouring minerals.

The short answer

This has been the long answer to the question.  The short answers are:
·         The specific gravity of soda lime transparent glass and opalescent glass is the same – no significant difference is in evidence.
·         There appears to be no correlation between specific gravity and CoLE.
·         There is unlikely to be any correlation between colourant minerals and CoLE or specific gravity.

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