Diagnosis of Breaks in Kiln Formed Glass

Often more can be learned from failures than a number of successes. A common failure in kiln forming is broken glass. The appearance of the break will tell you a lot about the problem so that you know where to look for the solution.

Cracks and breaks can occur at various times in the kiln. These will have occurred by the time you open the kiln:

• Curved cracks and breaks are usually caused by inadequate annealing. Often the break will have a hook or sharp curve near the edge of the glass. The edges will be sharp.
• Cracks and breaks occurring where two pieces of glass meet is usually an indication of incompatibility between the two glasses. This means that you need to perform a compatibility test with the two glasses. Sometimes it is caused by a large difference in the thickness of the glass, especially when light and dark glasses are side by side. This is normally an annealing problem.
• Breaks in the piece (often more than one) with rounded edges indicate a thermal shock break caused by raising the temperature too quickly for the size or thickness of the piece.
• Breaks that cross the piece in a reasonably straight line, going across and through pieces of glass are an indication of thermal shock.  The line will be rounded or the pieces even formed together again if it was shocked on the rise in temperature.  If the piece was cooled too quickly, the edges will be sharp.
• Multiple breaks into small pieces - normally sharp - are an indication that the glass has stuck to the shelf or kiln furniture. This is caused by inadequate batt wash on the shelf and kiln furniture. It tends to happen with high temperature firings more than lower temperature firings.

Other cracks and breaks occur after the piece has cooled.
Breakage occurring long after a piece has been completed are an indication that the stress within the glass has overcome the strength of the piece. There are several possible individual and combined factors:
· improper annealing,
· thermal shock,
· incompatible glass,
· wear and tear.

But the most likely problem is inadequate annealing. Unless you have access to your firing records and can determine how the piece was fired and the materials used, you will need to accept it as experience and extend future annealing times.

The best cure for these is prevention.

First is to do a compatibility test to determine if the glasses fit together in the combination you plan for your piece.
Second, if you check the stresses of the flat piece between polarizing filters, you will be able to see if there are stresses within the piece before you do any further kiln forming with this glass or setup. If the stress is from incompatibility - where you see the stress halos around specific pieces of glass - you will need to destroy the piece. If the stress is more generalized, you can put the piece back in the kiln, reheat slowly and soak at the annealing point for a longer time and use a slower annealing cool.

Organic Burnout Marks

Occasionally there is a haze at the centre of the back of large pieces of fired glass. This seems to happen when a large piece of glass is placed over fibre paper (of whatever thickness) that has not been pre-fired. 

 This is based on my experience of doing large pieces on thinfire or other fibre paper with a relatively fast rate of advance. What seems to happen is that the edges of the glass soften enough and early enough that not all the binder in the fibre papers can burn out and the combustion gasses escape from under the glass. The resulting haze is the remnants of the combustion product fired to the surface of the glass.

I have found that flipping the piece over and taking the glass to a low temperature fire polish is enough to return the glass to its usual appearance. You can, for extra insurance, apply a devitrification spray, although I have not found it necessary.

You could, of course, work the back of the glass with pumice and cerium oxide to bring back the original shine without firing. But my impression is that the areas with haze are fractionally depressed into the back surface. This means that a lot of glass has to be removed to reach and polish the hazy areas.

Effects of Multiple Layers

Stacking layers of glass fully or partially over the base layer has significant effects on the firing of the whole piece.

Glass is a poor conductor of heat, so you need to be careful to allow the heat to penetrate to the base layer to avoid thermal shock. There also is the effect of the (very small) insulating space between each sheet. The effects of multiple, even layers can be seen from this table based on Graham Stone's* work:

3mm layers

1 sheet – Initial Rate of Advance =1000ºC to 475ºC (less than half an hour)

2 to 3 layers – IRA = 240ºC to 475ºC (ca. 2 hours)

4 layers – IRA = 100ºC to 475ºC (4.75 hours)

6 layers – IRA = 25ºC to 125ºC, then 30ºC to 250ºC, then 40ºC to 375, then 50ºC to 475 before 150C to top temperature (ca. 15.5 hours)

This shows the dramatic effect increasing the number of layers has on the firing schedule to make sure the heat gets to the bottom sheet evenly. If you compare the initial rates of advance (IRA) with the same thickness, but fewer sheets you can see the space between layers is important.

6mm layers

1 sheet – IRA = 320ºC to 475ºC (ca. 1.5 hrs)

2 layers – IRA = 240ºC to 475ºC (ca. 2 hrs compared to 4.75 hrs for 4 layers of 3mm)

3 layers – IRA = 200ºC to 475ºC (ca.2.5 hrs compared to 15.5 hrs for 6 layers of 3mm)

These are the fastest safe firing speeds for evenly covered sheets. 

This difference in firing times for stacks of thicker glass, shows how important it is to fire sections of the stack before the final firing of all the layers together.  It also reduces the risk of bubbles developing within the stack. 

If you are thinking of tack fusing with thicker and thinner areas, you need to take account of the differences in thickness in the various areas of the piece when preparing your schedule. You will need to decrease your IRA by quite a bit. So you might want to be thinking of firing some of your pieces to be added to the base layers before tacking them in an additional firing to reduce the risk of thermal shock to the base layer.

* Firing Schedules for Glass; the Kiln Companion, by Graham Stone, ISBN 0646 39733 8

Cleaning

A lot of devitrification resembles dirty smears over the glass that will not clean away. This kind of devitrification results from inadequate cleaning.

The glass needs to be made “squeaky clean”. The glass needs to be free of dust, oils and minerals before firing. An initial wash of the glass with a minimum amount of liquid soap will dispose of the dust and oils. However it may leave behind minerals and additives from the soap and water, so a rinse in clean water followed by a polishing with unprinted paper towels or lint free cloths washed without softeners. As the glass dries you may very well hear the squeak of glass that is well polished to dry.

If there are still residues of labels or markers, use of a spirit may be required to remove these marks. Then the glass will have to be cleaned again in the normal way to remove the residues from the spirits.

If you are fortunate to be in an area with very few minerals in the water, you will not have to take as many precautions as those in areas with hard water. If you have hard water, you may need to think about using distilled water for the final rinse if you have streaks of devitrification after the standard cleaning process. The use of spirits is not necessary. The glass still needs to be polished dry with unprinted paper or dedicated towels.

An alternative (that I use most often) is to use a window cleaner without additives, such as supplied by glaziers. This avoids the local water supply, and most often is sufficient to remove dust and oils.

Slowing the Rate of Advance

The question is sometimes asked whether the rate of advance in a firing schedule should be slowed when re-firing; for a fire polish for example.

Cynthia Morgan contributes four circumstances where you would want to slow the rate of advance:

“1) On the previous firing you were fusing a whole bunch of little pieces into a much thicker piece, so you need to reduce your ramp to avoid thermal shocking the thicker glass

“2) You think you might not have annealed the piece well enough on the previous firing, so you're playing it safe

“3) You suspect there's a crack somewhere in the piece (from cold working or whatever) so you're reducing the chance it will expand quickly and open the crack

“4) You've got to do something to the glass/kiln at a certain point in the firing cycle, and if you go at your normal rate you'll wind up doing it at 3AM...so you slow down the firing and get more sleep.

“Otherwise, well-annealed is well-annealed. If none of those four conditions obtain, I don't see why you'd need to slow down”.

Cleaning Frit and Powder

If you make your own fine frit and powder, make sure it is clean to avoid black specks, or a grey appearance caused by metal dust and fragments.

Clean the glass you are going to break up before you start the process.

Use mild steel or other magnetic metal to break up the glass, or protect the glass from the breaking tools with layers of paper, plastic, cloth or combinations of these materials.

Then with a powerful magnet remove any metal residue from the frit and powder. The magnet will need to be passed over and through the glass particles a number of times, cleaning the magnet after each pass. To ease the cleaning you may wish to put the magnet in a plastic bag. Then move the bag over the waste bin and remove the magnet. The particles fall into the bin.

Do not use stainless steel to break up the glass as it will not be attracted to the magnet. Stainless steel particles will result in the same discolouration as if you left the glass uncleaned.

Super Glue Safety

Super glue is frequently used as a temporary fixative in assembly of kiln forming projects. There is some concern about safety, as it is known that super glue is made from cyanoacrylate, which it is feared will break down in the kiln into cyanide gas.

Greg Rawls, a certified industrial hygienist says "I looked at the MSDSs for several forms of super glue. The main component is Ethyl 2-cyanoacrylate, which has a TLV of 0.2 ppm which is relatively toxic. [However,] the thermal decomposition products are carbon monoxide and carbon dioxide. I did not see a reference to cyanide gas. However, as I recall cyanide gas dissociates into elemental carbon and nitrogen at about 800 F. Since you use it in such small quantities, I would not worry about it. In my opinion the worst thing that could happen is you glue your fingers to the glass."

Safety issues

To treat the safety issues seriously and determine if you feel Greg Rawls' view is justified, you need to look at the issues of toxicity, reactions, adhesion of tissue, ventilation, first aid and decomposition products in the whole context.

Toxicity

The fumes from cyanoacrylate are a vaporized form of the cyanoacrylate monomer that irritate sensitive membranes in the eyes, nose, and throat. They are immediately polymerized by the moisture in the membranes and become inert. These risks can be minimized by using cyanoacrylate in well ventilated areas. About 5% of the population can become sensitized to cyanoacrylate fumes after repeated exposure, resulting in flu-like symptoms. It may also act as a skin irritant and may cause an allergic skin reaction. On rare occasions, inhalation may trigger asthma. There is no single measurement of toxicity for all cyanoacrylate adhesives as there is a wide variety of adhesives that contain various cyanoacrylate formulations.

The United States National Toxicology Program and the United Kingdom Health and Safety Executive have concluded that the use of ethyl cyanoacrylate is safe and that additional study is unnecessary. 2-octyl cyanoacrylate degrades much more slowly due to its longer organic backbone that slows the degradation of the adhesive enough to remain below the threshold of tissue toxicity, so the use of 2-octyl cyanoacrylate for sutures is preferred.

Reaction with cotton

Applying cyanoacrylate to some materials made of cotton or wool results in a powerful, rapid exothermic reaction. The heat released may cause serious burns, ignite the cotton product, or release irritating white smoke. Users should not to wear cotton or wool clothing, especially cotton gloves, when applying or handling cyanoacrylates.

Adhesion of the Skin

Various solvents and de-bonders can be used. These include:

Acetone commonly found in nail polish remover, is a widely available solvent capable of softening cured cyanoacrylate

Nitromethane

Dimethyl sulfoxide

Methylene chloride

Commercial de-bonders are also available.

Warnings include:

• It is a mild irritant to the skin.
• It is an eye irritant.
• It bonds skin in seconds.
• Any skin or eye contact should be copiously flushed with water and medical attention be sought immediately.
• Do not attempt to separate eye tissues – the bond will separate naturally within a few days.

Precautions

• Use goggles.
• Do not wear cotton or wool clothing while using super glue
• Ventilate the area well. Since cyanoacrylate vapours are heavier than air, place exhaust intake below work area. Activated charcoal filters using an acidic charcoal have been found effective in removing vapours from effluent air so the bench top air filters are suitable for use while using super glue.
• Avoid use of excess adhesive. Excess adhesive outside of bond area will increase level of vapours.
• Assemble parts as quickly as possible. Long open times will increase level of vapours.

Evaporation Effects

• The effects of heating cyanoacrylate are not completely known. The flash point is known to be greater than 85ºC. As a precaution do not remain in the area of the kiln after that temperature has been reached.
• The decomposition products are carbon monoxide and carbon dioxide. There is no reference in the literature to cyanide gas. It is highly unlikely that heat will cause the release of cyanide gas at any time during the heating. To be certain, you should make sure the evaporation of the glue is be complete before firing the kiln.

See this tip for the use of super glue in kiln forming.

Pre-Set Schedules

Moving on from pre-set schedules

If your kiln has come with pre-set schedules, the first thing to find out is what rates, temperatures and times are set for the fast medium and slow fuse, tack and slump schedules.

Then, rather than just pressing the appropriate button, enter the numbers into the controller for each firing. This will give you confidence in programming the firings. Alter one element (such as the rate of advance, or the soak length) each time you enter the schedule and record the results. This will enable you to see what different rates, temperatures and soaks will do to your glass.

Make quick observations for fusing from about 750C every quarter of an hour to see how the glass is reacting. For slumping the observations should start about 600F. If the glass has reached the state you want before that segment of the schedule has completed, just advance the programme to the next segment (read your manual to find out how to do that on your controller).

It is only by making alterations and observing the results that you will gain the confidence to do your own programming when you do something the manufacturer didn't think about. There are so many factors, the programmes work for a limited range of possibilities.

Temperature conversions

The internet is dominated by North America which continues to use the traditional imperial measurements, although the rest of the world uses the metric system with its length, volume and weight units inter-related. Until North America catches up with the rest of the world, we will continue to need to convert temperatures from one system to another.

The conversion factors relate to the reference points of water's freezing and boiling points.

The Fahrenheit system has these at 32 and 212 – 180 degrees apart.

The Celsius system has these at 0 and 100 – 100 degrees apart.

This means the conversion rate is 9/5 to go from C to F or 5/9 to go from F to C.

Instead of dealing with the fractions, it is easiest to multiply or divide by 0.555 which is accurate enough for kiln forming purposes. Multiply the Fahrenheit by 0.555 to get the Celsius equivalent. From Celsius divide by 0.555. So a rate of advance of 200F/hr becomes 111C/hr( 20*0.555) and a rate of 80C/hr becomes 144F/hr (80/0.555). This works fine for calculating the rate of advance.

It does not work for temperatures. The complicating factor is the water freezing point in the Fahrenheit system which is 32F. To calculate the Fahrenheit temperature in Celsius, you first have to subtract 32 from the Fahrenheit temperature. So to convert 212F to C, you first have to subtract 32, giving 180 which is converted by multiplying 180 by 0.555 which results in 99.9 which is close enough to 100C.

To convert from C to F you divide the C temperature by 0.555 and add 32 to the result, e.g., 515C becomes 960F (515/0.555=927.9+32=959.9)

Alternatively you can bookmark one of the conversion sites and go to it for the calculation, but make sure that you distinguish rate from temperature when this calculation is done.

Some of the common (approximate) equivalents are:

515C =   960F a common annealing temperature

650C = 1200F low temperature slump

677C = 1250F standard slump temperature

750C = 1380F angular tack/ lamination

770C = 1420F rounded tack

800C = 1470F full fuse

830C = 1525F casting temperature

900C = 1650F low temperature pot or wire melt

925C = 1700F higher temperature pot or wire melt

Capping

This term most often refers to placing a single piece of glass over the whole of the project. The decisions relate to whether to do it at all, in what circumstances and in what order. Whatever you place on top of the project is what the eye will first see. A tinted top layer will give that tint to all the pieces making up the object. So most often the top is a piece of clear glass.

Many times the purpose of capping is to give the volume of glass required to keep the piece contracting as a result of the surface tension of the glass trying to pull itself up to 6mm thickness.

When using opalescent glass as the main component in the work, you should consider capping with clear. Opalescent glass is slightly more prone to devitrification than transparent glasses, so any work to be fired a number of times might be best fired with a clear cap. It also protects against any bubble formed between the other glass and the cap showing as a clear spot within the opalescent as it pushes the colour aside and reveals the clear below.

There are some times when you should consider placing the clear on the bottom. If your design layer is made up of lots of pieces where air might be trapped, but is uneven enough to be the likely cause of bubbles, then the clear should go on the bottom to ensure there is sufficient volume. An alternative is to do a high tack or full fuse of the whole upside down on fibre paper, then clean up and fire right side up with the capping glass.