Coe and Annealing

If you have changed CoE (i.e., the manufacturer), then the annealing temperature is different. If you don't correct that, it's never going to work quite right.

 

I have several problems with this statement.

CoE does not determine the manufacturer. There are several manufacturers who claim to manufacture fusing glass to the same CoE.

No manufacturer makes to one CoE. All manufacturers have to vary the CoE of a particular glass to balance its viscosity. The CoE is a dependent variable. It depends on what the viscosity of the colour is. Spectrum at one point stated their System96 glass had a 10-point variation in CoE number. Oceanside will be no different. Bullseye have stated a 5-point difference. Other manufacturers have not stated their variations.

No manufacturer can guarantee compatibility with another’s. This is because the ingredients to make a fusing range of glass varies from one manufacturer to another. These variations can make the glass incompatible. To determine if you can combine two glasses from different manufacturers you need to do the compatibility testing yourself. The CoE number does not determine the temperature characteristics of the glass either. 

Annealing 

Having got my disagreements with the statement out of the way, I can go on to looking at differing annealing temperatures. There is a difference between annealing point and annealing temperature.

Annealing Range

Annealing occurs over a relatively small range between the softening point at the higher end to the strain point at the lower end of the range. The softening point is the temperature, above which the glass is so plastic that it cannot be annealed. The strain point is the temperature at which the glass becomes so solid than no annealing can occur below it.

Annealing Point

The annealing point is mathematically determined as the point at which the glass most quickly relieves the stresses within it. That temperature is determined by the viscosity of the glass. It is known as the glass transition point, and is expressed as Tg. In practice there are advantages in annealing at or below the published annealing point.

A soak above the annealing point is of no effect. Any equalisation of temperature that occurs on that soak is negated by the drop to the annealing point. It is better to spend the cumulative soak/hold time at the (lower) annealing temperature.

Annealing Temperature

The average annealing point for Bullseye is 516°C/962°F. Different formulations of their fusing compatible glass have different Tg temperatures. Research showed the best results for their thick glass is 482°C/900°F. Other research in academic institutions has shown that annealing at the lower part of the range provides a denser and stronger finished glass piece. This applies to thick as well as thin glass.

Bullseye has chosen to use a temperature 34°C/61°F below the average annealing point, based on their research. This is still about 7°C/13°F above the strain point. This approach can be applied to any fusing glass.

The strain point is approximately 43°C/78°F below the mathematically determined annealing point. If you know the annealing point you can choose to anneal – i.e., equalise the temperature of your glass – up to 30°C/54°F below that. 

This has a practical demonstration. Wissmach for some years designated 510°C/950°F as the annealing point for W96. A few years ago, they changed their recommended annealing temperature to be 482°C/900°F. The annealing results are good at both temperatures. The difference is that the annealing soak is for a in longer time at the lower than at the higher temperature. But it still provides a shorter annealing cool.

Firing with different anneal points

This apparent diversion - into annealing ranges - shows that it is possible to anneal glass with slightly different glass transition points at the same temperature. You may compromise a little for one glass or the other. You will also use longer times at the annealing temperature.

The annealing soak of Oceanside and Wissmach96 could both be at 482°C/900°F. Or, if it felt safer, it can be an average of the two. The average of the difference would make the annealing soak at 496°C/926°F. You would use a longer soak at this temperature than at the higher one. The safest would be to hold for an hour instead of 30 minutes for 6mm/0.25” of glass.

However, if the annealing point differs greatly, it is much more difficult. For example, float glass with an annealing point of 540°C/1005°F would be difficult to fit in the same firing with most fusing glass because of the wide range of official annealing points.

 

It is possible to anneal different glass at the same time if the annealing points are not widely different. Compromises need to be made.

 

Ceramic Drape Moulds

Characteristics of materials

One of elements you need to consider in selecting a mould for draping is the characteristics of the ceramic material in relation to the glass being draped.  

Ceramic

Ceramic materials have what are called inversions. These are  points at which the ceramic has a quick change in expansion both on the heat up and cool down.  The two major ones are cristobalite inversion temperature at around 225°C and the quartz inversion at about 570°C.  The Crystobalite inversion is a sudden change of about 2.5% and the quartz is a sudden change of 1%.  These are very sudden and dramatic changes in comparison to the average of around 0.1% over the temperature range of 570°C to 800°C.  The crystobalite inversion does not occur until ca. 225°C.  This means that the whole structure of the ceramic is contracting less than the glass – exhibiting a CoE of ca. 66 rather 90 to 96.

Ceramic drape mould from Creative Glass Guild

Glass

We are used to saying glass expands and contracts at a standard rate, depending on the glass this may be a CoE of 83 to one of 104.  This is not the case.  The coefficient is an average calculated between 20°C and 300°C.  If you change the temperature range, the coefficient will also change.  And if you look at the range 570°C to 580°C you find the CoE is around 500.  This means that as the glass cools into the annealing range, it is contracting about 7 times faster than the ceramic. 

This dramatic difference in contraction means that the glass is attempting to crush the ceramic by enclosing it tightly.  Sometimes it does it so strongly that the strength of the glass is exceeded, and it breaks.

Possibilities

It is possible to drape over ceramic in certain conditions.

Influence of draft

The term “draft” indicates the slope of the sides of the form.  The steeper the sides, the more likely the glass is to trap the ceramic mould.  To be useful, the draft of the mould needs to be sufficient for the glass to slide upwards on the mould as it cools. This means the mould needs smooth sides and be well covered with a separator.

Compensations

You can compensate for steep drafts by wrapping the ceramic form in 3mm refractory fibre paper.  You can bind this with high temperature wire to ensure it stays throughout the firing. The fibre paper can be compressed and so provides a cushion between the rapidly contracting glass and the slowly contracting ceramic.

These need a circle of 3mm fibre paper over the open top of the kiln posts that have no draft at all before use. Of course, they need to have a circular piece of fibre paper over the hole in the post.

The use of ceramic forms to drape over requires care about the draft of the ceramic or addition of a cushion to avoid the greater contraction of the glass than the ceramic grabbing the mould so tightly it cannot be removed.

Characteristics of Some Glasses

This information has been taken from various sources. Some manufacturers may change the composition of their glasses or the published information about them from time to time. Therefore, this information can only be used as a guide. If the information about strain, annealing, and softening points is important, contact the manufacturer for the most accurate information.

The temperature information is given in Celsius.

Strain point – the temperature below which no annealing can be done.

Annealing point – the temperature at which the equalisation soak should be done before the annealing cool.

Softening point – the temperature at which slumping can most quickly occur.

Armstrong – Now made by Kokomo

Typical Borosilicate – nominal CoE 32

Strain point – 510 - 535C / 951 - 996F

Annealing point – ca. 560C/1041F
Softening point - ca. 820C/1509F

Blackwood OZ Lead – nominal CoE 92

Annealing point - 440C/825F

Blenko – nominal CoE 110

Annealing point – 495C/924F

Bullseye – nominal CoE 90

Transparents

Strain point - 493C/920F

Annealing point - (532C)  Note that Bullseye has changed this to 482C/900F for thick items

Softening point - 677C/1252F

Opalescents

Strain point - 463C/866F

Annealing point – (501C)  Note that Bullseye has changed this to 482C900F for thick items

Softening point - 688C/1272F

Gold Bearing

Strain point - 438C/821F

Annealing point - (472)   Note that Bullseye has changed this to 482C/900F for thick items

Softening point - 638C/1182F

Chicago – nominal CoE 92

Desag  Note that this glass is no longer produced

Artista – nominal CoE 94

Strain point – 480 - 510C / 897 - 951F

Annealing point – 515 - 535C / 960 - 996F

Softening point – 705 – 735C / 1302 - 1356F

Fusing range – 805 – 835C / 1482 - 1537

Float Glass (Pilkington UK)

Optiwhite

Strain point – 525 - 530C / 978 - 987F

Annealing point – 559C/1039F

Softening point – 725C/1338F

Optifloat

Strain point – 525 - 530C / 978 - 987F

Annealing point – 548C/1019F

Softening point – 725C/1338F

Float Glass (typical for USA) nominal CoE 83

Strain point - 511C/953F

Annealing point - 548C/1019F

Softening point – 715C/1320F

Float Glass (typical for Australia) nominal CoE 84

Strain point - 505-525C / 942 - 978F 

Annealing point – 540 -560C / 1005 - 1041F

HiGlass “GIN” range – nominal CoE 90

Annealing point - 535C/996F

Gaffer colour rod – nominal CoE 88

Gaffer NZ Lead – nominal CoE 92

Annealing point - 440C/825F

HiGlass

Annealing point - 495C/924F

Kokomo – nominal CoE 92 - 94

Cathedrals

Strain point - 467C/873F

Annealing point - 507C/946F

Softening point - ca. 565C/ca.1050F

Opal Dense

Strain point - 445C/834F

Annealing point - 477C/891F

Softening point – ca. 565C/1050F

Opal Medium

Strain point - 455C/834F

Annealing point - 490C/915F

Softening point – ca.565C/1050F

Opal Medium Light

Strain point - 461C/863F

Annealing point - 499C/931F

Softening point – ca.565C/1050F

Opal Light

Strain point - 464C868F

Annealing point - 502C/937F

Softening point – ca.565C/1050F

Kugler – nominal CoE

Annealing point - 470C/879F

Typical lead glass – nominal CoE 91

Lenox Lead – nominal CoE 94

Annealing point – 440C/825F

Merry Go Round – nominal CoE 92

Moretti/Effetre – nominal CoE 104

Strain Point: 448C/839F

Annealing Range: 493 – 498C / 920 - 929F

Softening Point: 565C/1050F

Pemco Pb83 – nominal CoE 108

Annealing point – 415C/780F

Schott Borosilicate (8330) nominal CoE 32

Annealing point - 530C/987F

Schott “F2” Lead – nominal CoE 92

Annealing point - 440C/825F

Schott “H” & “R6” rods - nominal CoE 90

Annealing point – 530C/987F

Schott “W” colour rod – nominal CoE 98

St Just

MNA

Strain point - ca.450C/843F

Annealing point – ca. 532C/ca. 991F

Spectrum

System 96 – nominal CoE 96

Transparents

Strain point – 476C  +/- 6C  /  890F +/- 11F

Annealing point – 513 +/- 6C  /  956C +/- 11F

Softening point – 680 +/- 6C  /  1257F +/- 11F

Opalescents

Annealing point – 505 -515C  /  942 - 960F

Spruce Pine 87 – nominal CoE 96

Annealing point – 480C/897F

Uroboros system 96 – nominal CoE 96

Transparents

Strain point - 481C/899F

Annealing point - 517C/964F

Opalescents
Strain point - 457C/855F
Annealing point - 501C/935F

Uroboros - nominal CoE 90

Transparents

Strain point - 488C/911F

Annealing point - 525C/978F

Opalescents

Strain point - 468C/875F

Annealing point - 512C/955C

Wasser - nominal CoE 89

Annealing point – 490C/915F

Wissmach
Wissmach 90
Annealing point - 483C/900F
Softening point - 688C/1272F
Full Fuse - 777+

Wissmach 96
Annealing point - 483C/900F

Softening point - 688C/1272F

Full Fuse - 777+ / 1432+

• 
  

Observations on Some Suggestions about Annealing

There are writings from a teacher attempting to make glass fusing simple.  Unfortunately, glass physics and chemistry are very complicated.  Attempting to avoid these complications leads to failures and other difficulties as the practitioner progresses. 

Proper annealing is one of the fundamentals to achieving sound kilnforming results.  Some suggestions have been made by a widely followed person to “simplify” the understanding of the annealing process.  Discussion of the meaning and importance of annealing can be found in many places, including here.  

Annealing temperatures

It has been suggested that the annealing temperatures can be inferred from the CoE of the glass that is being used. Discussion of what CoE is and is not can be found here and here.

Annealing temperatures are not directly related to the expansion coefficient (CoE) of the glass.  This can be shown from the published annealing temperatures for different glasses organised by presumed CoE:

·        “CoE96”: Wisssmach 96 - anneal at 482°C;  Oceanside - anneal at 515°C

·        “COE94”: Artista - anneal at 535°C

·        “CoE 93”: Kokomo - anneal between 507°C and 477°C – average 492°C

·        “CoE 90”: Bullseye - anneal at 482°C; Wissmach90 - anneal at 482°C; Uroboros FX90 - anneal at 525°C

·        “CoE 83”:

o   Pilkington (UK) float - anneal at 540°C;

o   typical USA float - anneal at 548°C;

o   Typical Australian float - anneal between 505°C and 525°C, average 515°C

This shows there is no direct relationship between CoE and annealing temperature.  Do not be tempted to use a CoE number to indicate an annealing temperature.  Go to the manufacturer’s web site to get the correct information.

Temperature equalisation soak

Annealing for any glass can occur over a range of temperatures.  The annealing point is the temperature at which the glass can most quickly be annealed.  However, the glass cannot be annealed if it is not all at the same temperature throughout the substance of the glass.  It has been shown through research done at the Bullseye Glass Company that a temperature difference of more than 5°C will leave stress within the glass piece. To ensure good annealing, adequate time must be given to the temperature equalisation process (annealing). 

From the Bullseye research the following times are required for an adequate anneal soak:

6mm /   1/4"            60 minutes

[9mm /  3/8"           90 minutes]

12mm  / 1/2"          120 minutes

[15mm  /   5/8"       150 minutes]

19mm   / 3/4"         180 minutes

[ ] = interpolated from the Bullseye chart for annealing thick slabs

Anneal Cooling

There are suggestions that a “second anneal” can be used on important pieces.  Other than observing that all pieces are important to the maker, the suggestion should be investigated.  On looking into the idea, it is essentially a second soak at 425°C, which is slightly below the strain point, rather than controlled cool from the anneal soak temperature.

It is reported that the Corning Museum of Glass considers 450°C as the lower strain point – the temperature below which no further relief of strain is possible.  This means that any secondary soak must occur above 450°C rather than the suggested 425°C. Such a soak is unnecessary if the appropriate cooling rates are used. 

Cooling Rate

Except in special circumstances, the cooling rate needs to be controlled as part of the annealing process.  Soaking the glass at the anneal is not the completion of the annealing.  Most practitioners follow the practice of choosing a slow rate of cooling from the annealing soak to some point below the strain point rather than a rapid one with a soak at the strain point temperature.

Annealing is not just the soak time (which is there to equalise the temperature), it is about the rate of the annealing cool too. The rate at which you cool is dependent on the thickness of the glass piece and whether it is all of one thickness or of variable thicknesses.

Even thickness

                                         Cooling rate

Dimension      time (mins)     1^st 55°C   2^nd 55°C

6mm              60                 83°C       150°C

9mm              90                 69°C       125°C

12mm            120                55°C       99°C

15mm            150                37°C       63°C

19mm            180                25°C       27°C

The “first 55°C” and the “second 55°C” refer to the temperature range below the chosen annealing temperature.  So, if you choose to anneal at 515°C, the “first 55°C” is from 515°C to 460°C and the “second 55°C is from 460°C to 405°C.  If you choose 482°C as the annealing temperature, the “first 55°C” is from 482°C to 427°C and the “second 55°C from 427°C to 372°C.

Tack fused/ uneven thickness

If your piece is tack fused, you need to treat the annealing rate and soak as though it were twice the actual total thickness. This gives the following times and rates:

Tack fused

Dimension (mm)                                Cooling rate

Actual     Calculated       time (mins)     1^st 55°C   2^nd 55°C

6            12                 120                55°C       99°C

9            18                 150                37°C       63°C

12          25                 180                25°C       27°C

15          30                 300                37°C       63°C

18          38                 360                7°C         12°C

Contour fusing required firing as though the piece were 1.5 times thicker.  Sharp tack or laminating requires 2.5 times the the actual thickness.

Fusing on the floor of the kiln

There is a further possible complication if you are doing your fusing on the kiln floor, or a shelf resting on the floor of the kiln.  In this case you need to use the times and rates for glass that is at least 3mm thicker than the piece actually is. 

Thus, a flat 6mm piece on a shelf on the floor would use the times and rates for 9mm: anneal soak for 90 minutes, anneal cool at 69°C to 427°C and then at 124°C to 371°C.  It would be safest if you continued to control the cooling to room temperature at no more than 400°C per hour.

But if it were a tack fused piece of a total of 6mm you would use the times and rates for 18mm.  This is using the rates for twice the total thickness plus the additional 3mm for being on the base of the kiln.  This gives the times and rates as being an anneal soak of 360 minutes and cooling rates of 7°C to 427°C and 12°C to 370, followed by 40°C per hour to room temperature.  Any quicker rates should be tested for residual stress before use.

Source for the annealing and cooling of fused glass

These times and rates are based on the table derived from Bullseye research, which is published and available on the Bullseye site.   It is applicable to all fusing glass with adjustments for differing annealing soak temperatures.

Annealing over multiple firings

It has been recommended by this widely followed person that the annealing soak should be extended each time subsequent to the first firing.  I am uncertain about the reasoning behind this suggestion. But the reasons for discounting it are related to adequate annealing and what is done between firings.

If the annealing is adequate for the first firing, it will be adequate for subsequent firings unless you have made significant alterations to the piece.  If you have added another layer to a full fused piece, for example and are using a tack fuse, you will need to anneal for longer, because the style and thickness have been altered.  Not because it is a second firing.  If you are slumping a fired piece, the annealing does not need to be any different than the original firing.

The only time the annealing needs to be altered is when you have significantly changed the thickness of the piece, or the style of fusing (mainly tacking additional items to the full fused piece).  This is when you need to look at the schedules you are planning to use to ensure your heat up is slow enough, that your annealing soak is long enough, and the cool slow enough for the altered conditions.

Determining the annealing point of unknown glass

You don’t have to guess at the annealing temperature for an unknown glass.  You can test for it.  It is known as the slump point test.

This test gives the softening point of the glass and from that the annealing point can be calculated.  This test removes the guess work from choosing a temperature at which to perform the anneal soak. The anneal temperature is important to the result of the firing.  This alone makes this test to give certainty about the annealing temperature worthwhile.

You can anneal soak at the calculated temperature, or at 30°C below it to reduce the anneal cool time.  This is because the annealing can occur over a range of temperatures.  The annealing occurs slowly at the top and bottom of the range. But is at least risk of "fixing in" the stress of an uneven distribution of temperature during the cool when the annealing is done at the lower end of the range.

Do not be fooled into thinking that CoE determines annealing temperatures.  Use published tables, especially the Bullseye table Annealing for Thick Slabs to determine soak times and cooling rates.  Use the standard test for determining the softening and annealing points of unknown glasses.

Further information is available in the ebook Low Temperature Kiln Forming.

Approximate Temperature Characteristics of Various Glasses

Various glasses have different temperature characteristics. This listing is an attempt to indicate the differences between a variety of popular glasses used in kiln forming. They are not necessarily exact, but do give an indication of differences.

Bullseye Transparents
Full fusing 832C
Tack fusing 777C
Softening 677C
Annealing 532C
Strain point 493C

Bullseye Opalescents
Full fusing 843C
Tack fusing 788C
Softening 688C
Annealing 502C
Strain point 463C

Bullseye Gold Bearing Glasses
Full fusing 788C
Tack fusing 732C
Softening 632C
Annealing 472C
Strain point 438C

Desag GNA
Full fusing 857C
Tack fusing 802C
Softening 718C
Annealing 530C
Strain point 454C

Float Glass
Full fusing 835C
Tack fusing 760C
Softening 720C
Annealing 530C
Strain point 454C

Oceanside
Full fusing 788C
Tack fusing 718C
Softening 677C
Annealing 510C
Strain point 371C

Wasser
Full fusing 816C
Tack fusing 760C
Softening 670C
Annealing 510C
Strain point 343C

Wissmach 90
full fusing  777C
Tack fusing
Softening  688C
Annealing  510C
Strain point

Wissmach 96
Full fusing  777C
Tack fusing
Softening  688C
Annealing  510C
Strain point

Youghiogheny 96
Full fusing  773C
Tack fusing  725C
Softening  662C
Annealing  510C
Strain point

Viscosity Changes with Temperature

This image is taken from Pate de Verre and Kiln Casting of Glass, by Jim Kervin and Dan Fenton, Glass Wear Studios, 2002, p.27.

It shows in graphic form how the viscosity of glass decreases with increases in temperature. The temperatures are given in Fahrenheit.  

The coefficient of expansion also changes with temperature. 

This graph is also from Kervin and Fenton

 It is these two forces of viscosity and expansion that must be balanced around the annealing point to give a stable and compatible range of fusing glass.

Float Annealing Temperatures

Float glass annealing temperatures vary quite a bit from one manufacturer to another; and even within one manufacturer’s product line.

Comparisons of various float glasses

Some companies are more informative that others.  Pilkington are one of the more open of European glass manufacturers on various bits of information.

Pilkington Float

CoLE 83 *^10-5

Softening point:  715°C

annealing point:  548°C

strain point: 511C

Pilkington Optiwhite ™

Softening point:  ca. 732°C

annealing point:  ca. 559°C

strain point:  ca. 526°C

There is a difference of 11C between two of the Pilkington product lines for the annealing points.  The softening and strain points are slightly wider.

Glaverbel, a Belgian company, restricts their information to CoLE and the softening point.

CoLE 91 * 10^-5

Softening point: 600°C

Saint-Gobain, a French company, shows some more of the variation in the product lines, although they do not give specific annealing points for the different products.

CoLE 90 * 10^-5

annealing range:  520 - 550°C

Low E glass

softening – 840°C

strain - 617°C

R glass (sound reducing)

softening – 986°C

strain - 736°C

D glass (decorative)

softening point – 769°C

Compatibility

Even this small sample of float glasses shows there is a significant difference between manufacturers for the softening, annealing and strain points.  This means that, unless you are sure of the glass merchant’s source of glass, you will need to test each batch of glass for compatibility with previous batches, if you are combining from different suppliers.

I included the CoLE numbers (which all the manufacturers specified as an average change in length for each degree C increase in temperature from 0 to 300°C) to show the variation and to challenge anyone to find Bullseye and Saint-Gobain or Glaverbel compatible with each other.  My experience has shown that the Optul coloured frit and confetti is more likely to be compatible with Pilkington than the other two.

Annealing

I have been beginning my annealing of float glass at 525°C.  This little bit of literature research shows that my annealing soak should be starting higher, possibly at 540°C, certainly no lower than 530°C.  Other areas of the world may find their float glass has significantly different annealing ranges.

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.