Wednesday, 14 November 2018

CoE Useage

Does anyone know what CoE means?

·         First the proper abbreviation is CoLE.
·         This means Coefficient of Linear Expansion.
·         A coefficient is an average.  This number may be exact at a given temperature, or an average over a range.
·         Linear is the length.  
·         Expansion is measured in fractions of a metre e.g., 0.0000096 metre.
·         The coefficient is given as the average amount of expansion per each degree Celsius.
T     The temperature range used is 0C to 300C.  Expansion characteristics vary greatly at higher temperatures.

So CoLE is the average amount (in metres) that glass expands for each degree (Celsius) increase in temperature from 0C to 300C. 

Whether you call it CoE or CoLE is immaterial, as it still does not equal compatibility.

It does not measure viscosity. Viscosity is a (possibly the major) element in making a range of compatible fusing glasses.

It does measure expansion rates, but up to 300C only.  It does not tell you how glass expands above that temperature.  Note: it does not behave in a linear pattern as crystalline materials do.

The CoE must be adjusted to match the viscosity to achieve compatible glass.  Spectrum has stated that their glass has a range of CoE of at least 94 to 98 to make compatible fusing glass.  Bullseye have not stated the range, but have indicated their base glass is nearer to 91 than 90.  It is to be presumed that their CoE range is approximately 89 to 93.

The only constant in fusing glass is compatibility

CoE varies within each manufacturer’s range of fusing compatible glass to match the viscosity. And remember the CoE of glass at the critical annealing point is  higher than the low temperature expansion rate. See this post for details.

Viscosity varies according to the materials used in the colouration of the glass, requiring the glass manufacturer to make adjustments in CoE to get compatible fusing glass.  More information here.

CoE does not mean what you think.  It does not mean compatibility.  It does not measure volume expansion at the glass transition point.  It does not measure the most important element – viscosity.  It is not even the correct term for the measure – CoLE is.

Since CoE does not equal a fusing compatible glass, its continued use can lead people (especially novices) to believe the simple number means any glass (mistakenly) labelled with that number will be compatible with others so labelled.  This leads to unexpected incompatibilities for newcomers to the field.


What can you use instead? It is easy – use the manufacturer’s name.  Where the manufacturer is making more than one range of fusing compatible glass use the manufacturer’s nomenclature.


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.

Wednesday, 31 October 2018

Lubrication for cutters

You can cut glass without oil.  It has been done for a long time.  But it has been found that there are advantages to using oil on a score line.

The purpose of oil:
·        A minor element is to oil the cutter wheel.

·        A major element is to oil the score line.  An oiled score line stays open longer than a dry one.  

·        An oiled score line reduces the amount of visible chipping from the score line.

The kind of oil
·        Mineral oil does not oxidise to gum up the scoring wheel.

·        Any light mineral oil from sewing machine oil to WD40 is acceptable. Some use very light oils such as turpentine or white spirits.

·        There are cutting oils that are synthetic and easier to clean than the standard oils and spirits, in that less residue is left when the oil is wiped off.

·        Vegetable oils might appear to be a good substitute.  But they oxidise and become sticky, attracting dust and other particles which soon block the turning of the scoring wheel.  This requires frequent checking and cleaning.  Avoid vegtable oils.

Methods of applying
·        The oil can be put into the cutters that have a reservoir.

·        The cutter can be dipped into a container of oil with or without an oil-soaked material.

·        The oil can be painted onto the glass before scoring.

  Any combination of the above will work.

Wednesday, 24 October 2018

Frit by thermal shock

Frit can be created by thermal shock.  You will still need to do some manual breaking up. The principle is that you heat the glass and then cool it rapidly, causing the glass to break into pieces.

Place the glass in a stainless steel bowl and heat as fast as possible to 300C – 400C. Turn the kiln off and pull out the bowl, using heat resistant gloves and dump the hot glass into a large bucket of water. Once the glass is cool, pour off the water and dry the glass.  When dry, you can break the crazed glass into smaller bits just as you would with other glass.  Note that pouring water over the glass has two disadvantages – one, it does not completely thermal shock the glass, and two, the large amount of steam released is very dangerous.

The advantages of this quenching method of obtaining frit are that you can create frit with less effort.  You also get less fines and powder with this method. And less effort is required to smash up the glass.

Some indicate that ice cold water to quench the glass is a good idea.  This is because warm water will not provide enough of a shock to the glass to craze it throughout.  But if you have a large bucket of water, there is no necessity, as the volume of water will cool the glass quickly enough.  Of course, if you are planning another quenching, you need to renew the water, as it will not be cold enough to thoroughly craze the glass.

You can, in part, control the size of the resulting frit.  Firing at 300C results in larger frit than firing at 400C.  However, firing at 500C does not provide even smaller frit.  The best results are between 300-400C, although frit can be made at 200C as well.  Experiment with temperatures to get the frit you want.

Once you have dried the frit, you can begin to break it up. Some can be done by hand, but the pieces are often sharp, so gloves are essential.  The other standard methods of breaking up glass to make frit are applicable. But it does not take as much effort as breaking from cullett.

Wednesday, 17 October 2018

Annealing vs toughening

The statement “annealing stained glass makes it stronger” appeared on the internet some time ago.  Of course, without annealing there is no glass, it would simply crumble.  Annealing is the process of allowing the glaseous state to be achieved.

I think the statement is more about the difference between annealed and toughened/tempered glass.  In summary, it relates to the amount of stress within the glass.  Well annealed glass has less stress than inadequately annealed glass and so is more stable.  Toughening is a process that balances stress and tension in the glass.

The processes are for different purposes and follow different processes. 

Annealing of glass is a process of slowly cooling hot glass to relieve residual internal stresses introduced during manufacture. Annealing of glass is critical to its durability. Glass that has not been properly annealed retains thermal stresses caused by rapid cooling, which decreases the strength and reliability of the product. Inadequately annealed glass is likely to crack or shatter when subjected to relatively small temperature changes or to minor mechanical shock. It even may fail spontaneously from its internal stresses.
To anneal glass, it is necessary to soak it at its annealing temperature. This is determined mathematically as a viscosity of 1013 Poise (Poise is a measure of viscosity). For most soda lime glass, this annealing temperature is in the range of 450–540°C, and is the so-called annealing point or temperature equalisation point of the glass. At such a viscosity, the glass is too stiff for significant change of shape without breaking, but it is soft enough to relax internal strains by microscopic flow. The piece then heat-soaks until its temperature is even throughout and the stress relaxation is adequate. The time necessary for annealing depends on its maximum thickness. The glass then is cooled at a predetermined rate until its temperature passes the strain point (viscosity = 1014.5 Poise), below which even microscopic internal flow effectively stops and annealing stops with it. It then is safe to cool the product to room temperature at a rate limited by the thickness of the glass.
At the annealing point (viscosity = 1013 Poise), stresses relax within minutes, while at the strain point (viscosity = 1014.5 Poise) stresses relax within hours.  Stresses acquired at temperatures above the strain point, and not relaxed by annealing, remain in the glass indefinitely and may cause either immediate or delayed failure. Stresses resulting from cooling too rapidly below the strain point are considered temporary, although they may be adequate to promote immediate failure.

But annealed glass, with almost no internal stress, is subject to microscopic surface cracks, and any tension gets magnified at the surface, reducing the applied tension needed to propagate the crack. Once it starts propagating, tension gets magnified even more easily, causing it at breaking point, to propagate at the speed of sound in the material.

In short, the aim of annealing is to relieve the stress to create a stable piece of glass. The above describes when and how that occurs.

Toughened/Tempered Glass

Toughening or tempering glass starts with annealed glass to form one type of safety glass.  This done through a process of controlled thermal or chemical treatments to increase its strength compared with normal glass. Tempering puts the outer surfaces into compression and the interior into tension. Such stresses cause the glass, when broken, to crumble into small granular chunks instead of splintering into jagged shards as annealed glass does. The granular chunks are less likely to cause injury – thus safety glass.

Toughened glass is stronger than normal glass.  The greater contraction of the inner layer during manufacturing induces compressive stresses in the surface of the glass balanced by tensile stresses internally. For glass to be considered toughened, the compressive stress on the surface of the glass should be a minimum of 69 megapascals (10,000 psi). For it to be considered safety glass, the surface compressive stress should exceed 100 megapascals (15,000 psi).
It is the compressive stress that gives the toughened glass increased strength. Any cutting or grinding must be done prior to tempering. Cutting, grinding, and sharp impacts after tempering will cause the glass to fracture.
Toughened glass is normally made from annealed sheet glass via a thermal tempering process. The glass is placed onto a roller table, taking it through a furnace that heats it well above its transition temperature of ca. 540°C (depending on the glass concerned) to around 620°C. The glass is then rapidly cooled with forced air drafts while the inner portion remains free to flow for a short time.
An alternative chemical toughening process involves forcing a surface layer of glass at least 0.1 mm thick into compression by ion exchange of the sodium ions in the glass surface with potassium ions (which are 30% larger), by immersion of the glass into a bath of molten potassium nitrate. Chemical toughening results in increased toughness compared with thermal toughening and can be applied to glass objects of complex shapes. 

This blog entry is largely based on Wikipedia
and other sources.

Wednesday, 10 October 2018

Slumping Different Glasses in the Same Firing

The question has arisen as to whether it is possible to slump Bullseye and Spectrum in same slump firing.

Yes, it is possible.

But precautions are necessary.

Different temperatures are generally recommended for Spectrum and Bullseye.  Spectrum is generally expected to do the same slump as Bullseye at 25C less.

This implies that Bullseye should be put in larger or easier slump moulds than Spectrum and fired to the lower temperature required by Spectrum. The thinking behind this is that smaller spans require longer or more heat to slump.  Steeper moulds require more time and heat than less steep ones.

In general, shallow slumps will work better for both glasses together than more steep or textured ones.

To be certain of a good result, you should fire as low as practical for an extended soak.  Follow this with an extended annealing and a slower cooling rate than normal for Spectrum.

This applies to almost all the glass that is being produced with the aim of being compatible with these two glasses.  It is not possible to get a good result for float glass if it is put into the same firing as for Bullseye or Spectrum.

Wednesday, 3 October 2018

Tack Fuse vs Fire polish

Are tack fuse and fire polish the same thing?


They both occur in the same temperature same range, depending on the degree of tack fuse you want.

What you are doing in the fire polish process is heating the top surface enough to appear polished. Very little time is needed in a fire polish at top temperature as opposed to a tack fuse.

In a tack fuse, you want the bottom of the upper pieces to be hot enough to stick to the bottom layer. This requires a higher temperature or longer soak than a fire polish.

At around 730C, depending on your kiln, you will be softening the upper surface of the glass enough to give a polished appearance.  To determine whether the polished surface has been achieved, you can peek into your kiln at the chosen temperature to see if the polish is complete.

This is also the temperature at which sintering, or a lamination of the glass pieces occurs.  The edges will still be sharp, but cannot be pulled apart.  This kind of fusing needs careful annealing – long soaks and slow cools.

Tack fusing of various degrees occurs in the temperature range from 730C to 770C.  To determine which temperature and soak time will give you the result you desire will require experimentation and observation.  Generally, you can achieve the desired level of fuse with lower temperatures and longer soaks, as you can at higher temperatures and longer soaks. 

It is also possible to give a fire polish to your glass at a really low temperature, such as 550C, with a very long soak. This will avoid significantly flatening the surface of your piece.  This is the effect of heat work.

Wednesday, 26 September 2018

The relative order of kiln forming events

When preparing for multiple firings of elements onto a prepared piece, you need to consider the order and temperatures of events so that you do not harm an earlier stage of the project.  This blog entry will not give definitive temperatures, as that varies by glass and by kiln.  Instead, it indicates what happens in progression from highest to lowest temperatures in approximate Celsius degrees.  

ca. 1300C  -  Approximate liquid temperature 

ca. 850 – 1000C  -  Glass blowing working temperature

ca. 950C  -  Raking and combing

ca. 850C  -  Casting

ca. 810C  -  Full fuse

ca. 790C  -  Large bubble formation

ca. 770C  -  High tack, low contour fuse

ca. 760C  -  Tack fuse

ca. 750C  -  Fire polish

ca. 700C – 760C  -  Devitrification range

ca. 700C  -  Lamination tack

ca. 600C – 680C  -  Slump and drape

ca. 650C  -  Vitreous paint curing temperature

ca. 600C  -  No risk of thermal shock above this temperature 

ca. 540 – 580C  -  Glass stainers enamel curing temperature

ca. 520 – 550C  -  Silver stain firing temperature

ca. 550C  -  Glass surface beginning to soften

Slow rates of advance needed from room temperature to ca. 500C

These temperatures are of course, affected by the soak times. The longer the soak time, the lower temperature required. The rate at which you achieve the temperature also affects the effective temperature.  Slower rates of advance require lower temperatures, than fast rises in temperature.  These illustrate the effect of heat work.

The table shows for example you need to do all the flat operations and firings before slumping or draping.  It also shows you can use vitreous glass paints at the same time as slumping and draping.  This emphasises that the standard practice is to plan the kind of firings you will need for the piece and do them in the order of highest temperature first, lowest last.

In general, you do need to do the highest temperature operation first and lowest last.  But there are some things you can do with heat work.  For example, if you needed to sandblast a tack fused piece, but did not want to risk reducing the differences in height there things you can do.  From the list above, you can see the glass surface begins to soften around 500C.  It is possible to soak the glass for a long time around 500C to give it a fire polish, instead of going to a much higher temperature.  You will need to experiment to find the right combination of temperature and soak length, but it can be done.

This article is to show that knowledge of what is happening to the glass at different temperatures, can help in “fooling” the glass into giving you the results you want without always following the “rules”.  This may also be what it is to be a maverick glass worker.  Use the behaviour of glass to your advantage.

Wednesday, 19 September 2018

Repairs to a Vermiculite Mould

Occasionally, during the demoulding of a form, the mould will break.  Not all is lost.  It can be repaired. 

In this example, the mould is not yet fully cured and is damp.  But this can be applied to fully cured and dried moulds too. Notes will be included where the practice varies for the dried mould.

The first stage is to make up a paste of the ciment fondue for the edge to edge repair.  This should be the consistency of pancake batter or slightly wetter.  The mixed cement is shown at the top of the picture in a small plastic tub.

Wet the edges of the mould pieces thoroughly.  This is to prevent the mould from sucking too much water from the cement, which would give a weak adhesion.  On dried moulds, you may have to do this several times to thoroughly wet the mould and the broken piece.

Then begin applying the wet cement thinly to all the edges.  Do not put it on thickly, as you want the pieces to fit back together smoothly. 

Place the pieces together with gentle pressure. 

Then begin to smooth the wet ciment fondue into the cracks between the broken pieces and the main body.  Be careful to smooth the ciment fondu immediately, as it is very difficult to change once cured.

Continue to work the ciment fondue into any cracks that appear as the mould is wetted.

Make sure the cement is smoothed into the cracks so there are no proud areas above or around the cracks.

This photo shows the smoothed ciment fondu on the interior.

Continue smoothing the cement into the cracks at the edges.

Fill the cracks from the outside also

When the application of the cement is completed, make up a mixture of 1:4 ciment fondue to vermiculite. 

The purpose of this is to strengthen the mould in the weak area.  It is not wise to rely entirely on the strength of the edge bonding of the ciment fondue.

You will need to estimate the total volume required, but it is better to mix too much rather than too little.  Make this mix a little wetter than for the original mould.  Water should not be standing in the mix, but you will be able to squeeze water from the ball of mix easily. 

This is especially important for moulds which have already been cured.  You should also put water on the surface that you are going to back up.

It is important to put a water proof material on the workbench to avoid the mould sticking to the bench, or water dripping over other things.

Having wetted the mould exterior again, begin applying the mix to the outside of the mould.

Continue building up the mixture in thin layers.  This allows the best adhesion of the material to the mould and to each layer.  It is easier to compact a small amount of material than a large amount all at one time.

In this photo, you see some of the water being forced out of the mixture by the compaction of the mix onto the mould.

Continue building around the broken area until you have applied sufficient material to the mould to strengthen it.

When you have finished, one area of the mould may be a little larger than the rest.  This is not a problem in its use, as it does not thermal shock, and it does not keep one part of the glass hotter than the glass touching the rest of the mould.

You can now loosely wrap the water proof material around the mould.  Do not seal it completely.  Place the mould in a plastic bag to cure for a day or more, just as for the original mould.

You can then unwrap the mould and fire it to cure it just as the original. The method for curing vermiculite moulds is given here.

Wednesday, 12 September 2018

Schedules for Steep Drapes

I have been asked for a schedule for draping in the context of a tip on steep straight sided drapes.

What you are trying to do with a steep drape is two things. One is to compensate for the heat sink that the glass is supported by, and the second is to compensate for the relative lack of weight at the outer edge of the glass.

The supported glass transmits its heat to the support, leaving it colder than the unsupported glass. This often leads to breakage due to heat shock at much lower temperatures and slower rates of increase than glass supported at its edges. My experience has shown that - contrary to what I recommend for other kinds of firings - a slow rise with short soaks at intervals up to the working temperature works best. The reason for these slow rises and soaks is to try to get the support and the glass to be as nearly as possible at the same temperature throughout the rise in temperature. The soaks help ensure the mould is gaining heat without taking it from the glass.

The other problem with steep drapes is that the edges of the glass begin to drop more quickly than the area between the support and the edge. This leads to the development of an arc that touches the mould side near the bottom before the glass between the edge and the and the support. Extended soak times are required to allow the glass to stretch out and flatten. If this is done at high temperatures, the glass will thin - possibly to the extent of separating.

So the requirements for a firing schedule on this kind of drape are slow increases in temperature with soaks to avoid thermal shock, and an extended soak at the forming temperature.

Whether using steel or ceramic moulds, I use a slow rise in temperature to 100C with a soak of 15 minutes. I then increase the rate of rise by 50% for the next 100C and give a 15 minute soak there. For the next 200C I raise the temperature at twice the original temperature rise, again with a 15 minute soak. The glass and mould should now be at 400C. This is still at the point where the glass could be heat shocked, so I only increase to 2.5 times the original rise rate but this time all the way to forming temperature.

Each kiln has its own characteristics, so giving schedules is problematic. A side fired kiln will need slower heat rises than a top fired one. The closer the glass is to the elements, the slower the rate of increase needs to be. The kind of energy input - electric or gas - has an effect. The thickness of the glass is also a factor in considering what rate to use. The size of the glass in relation to the size of the support is important - the greater the differential, the slower the heat rise should be. So in making a suggestion on heat rises, it is only a starting point to think about what you are doing and why you are doing in this way.

I have usually done this kind of draping in top fired electric kilns where the elements are about 250mm above the shelf, and about 120mm apart. In the case of a 6mm thick piece about three times the size of the support area, I use 50C/hr as my starting point. This is one third of my usual rate of temperature rise. However you must watch to see what is happening, so that you can make adjustments. You should observe at each of the soaks, so you know how the glass is behaving. It will also help you to pinpoint the temperature range or rate of advance that may be leading to any breakages.

On steep slumps, the temptation is to use a high temperature to complete the drape. This is a mistake as the glass will be more heavily marked and tends toward excessive stretching and thinning. What you really need is a slow rate of advance to a relatively low temperature. If you normally slump at about 677C, then you want to do this steep, straight sided drape at 630C or less. It will need a long soak - maybe up to an hour. It will also need frequent observation to determine how the drape is progressing. So plan the time to make yourself available during this forming soak.

Annealing is done as normal, since the mould and glass are more closely together and will cool at the same rate.

The original tip on the set up of a steep straight sided slump is here.