Screens for Melts

You can buy various stainless steel screens such as barbecue grids for supporting glass melts. The grids need to be of stainless steel. Type 304 is the most common, but there are other grades which work at high temperatures too [link to stainless steels]

You can make your own grid as Cynthia Morgan does. This provides a more flexible arrangement for various effects. 

Instead of imbedding the rods into the brick, you could also place them on top. Place a kiln brick or other kiln furniture on the ends of the rods to secure the metal from moving. Then you can put the glass on top of the rods without them shifting as the glass is placed.

Stainless Steel for Kiln Uses

The reason for using stainless steel is that it differs from carbon steel by the amount of chromium present and reduces the spalling. Unprotected carbon steel rusts readily when exposed to air and moisture. This iron oxide film (the rust) is active and accelerates corrosion by forming more iron oxide, and due to the greater volume of the iron oxide this tends to flake and fall away (spall).

Stainless steels contain sufficient chromium to form a passive film of chromium oxide, which prevents further surface corrosion by blocking oxygen diffusion to the steel surface and blocks corrosion from spreading into the metal's internal structure, and due to the similar size of the steel and oxide ions they bond very strongly and remain attached to the surface.

There are a number of grades of stainless steel. Some of the ones that perform better in hot conditions are:

300 Series—austenitic chromium-nickel alloys. Austenitic steels have a cubic crystal structure. Austenite steels make up over 70% of total stainless steel production. They contain a maximum of 0.15% carbon, a minimum of 16% chromium and sufficient nickel and/or manganese to retain an austenitic structure at all temperatures from the extremely cold to the melting point of the alloy.

Type 304—the most common grade; the classic 18/8 (18% chromium, 8% nickel) stainless steel. Outside of the US it is commonly known as "A2 stainless steel", in accordance with ISO 3506 (not to be confused with A2 tool steel).

Type 304L—same as the 304 grade but lower carbon content to increase weldability. Is slightly weaker than 304.

Type 304LN—same as 304L, but also nitrogen is added to obtain a much higher yield and tensile strength than 304L.

Type 309—better temperature resistance than 304, also sometimes used as filler metal when welding dissimilar steels, along with inconel.

Type 316—the second most common grade (after 304); for food and surgical uses; alloy addition of molybdenum prevents specific forms of corrosion. It is also known as marine grade stainless steel due to its increased resistance to chloride corrosion compared to type 304.

Type 316L—is an extra low carbon grade of 316, generally used in stainless steel watches and marine applications, as well exclusively in the fabrication of reactor pressure vessels for boiling water reactors, due to its high resistance to corrosion. Also referred to as "A4" in accordance with ISO 3506.

Type 316Ti—variant of type 316 that includes titanium for heat resistance. It is used in flexible chimney liners.

Type 321—similar to 304 but lower risk of weld decay due to addition of titanium.

400 Series—ferritic and martensitic chromium alloys

Type 439—ferritic grade, used for catalytic converter exhaust sections. Increased chromium for improved high temperature corrosion/oxidation resistance.

Type 446—For elevated temperature service

500 Series—heat-resisting chromium alloys

Based on Wikipedia 

First Firing

Even if this is not your first kiln there are a number of things to do when starting.

The first is to read the manual. Obvious, but in our enthusiasm to get started, reading seems boring. It is essential to understand what the manufacturer wants you to do and to understand how the kiln and controller work.

Then, you can prepare kiln. Test fire the kiln empty to make sure it works and burn out any binders remaining in the kiln materials. You can do this firing at about 400C/hour as there is no glass to damage. Fire to around 800C. Then you can shut off, or programme a dummy anneal. This also ensures you know how to work the controller.

While waiting for the kiln to complete the fast test firing, read the manual again.

When the kiln is cool, apply kiln wash to the bottom (if it is brick) and lower sides, below any side elements. If the bottom or sides are fibre, no kiln wash is required. Kiln wash the shelf and any kiln furniture too.

Now is the time to test for how even the heat is in your kiln. Arrange the kiln furniture around the shelf as described and put glass over. The kiln furniture can be any refactory material, even folded 3 mm fibre paper will be strong enough to hold the small pieces of glass above the shelf. Fire the kiln as described in the Tech Note 1.

Now try out the suggestions in the manual, especially the programming of the controller, even if it has pre-programmed schedules. Look at Bullseye and Spectrum sites to get sample schedules. Enter these as trial schedules.

Then you should be ready to fire the first piece of glass. Place the glass in the kiln, programme it, and record the information about the firing. Now turn the kiln on for the first real fuse.

Cleaning Glass Before Painting

If your glass is not really clean, you can get gaps in the paint line. You may also have areas where the paint beads up rather than flows evenly.

You need to clean as best as possible first. Then just before painting you can use a dilute solution of the paint to scrub all over the glass with your finger or other firm material. Wipe any residue off with a paper towel and you will find that the prepared paint will flow evenly onto the glass.

Fiber Paper Safety

There are often concerns raised about safety relating to the dust from Thinfire, a trade marked product from Bullseye. These concerns also apply to the Spectrum product called Papyros. The main constituents of these are cellulose, aluminium hydroxide, fibre glass and organic binders. It therefore is mainly a nuisance dust and irritant. Greg Rawles, a certified industrial hygenist with a scientific and common sense approach, says that the toxicology of fibre glass is not yet determined. So the best idea is to wear a P95 respirator while dealing with the dust – mainly the cleaning phase of dealing with the shelf. He also recommends keeping the kiln closed during the burn-out phase of the binders, although others recommend venting to avoid anything settling on the glass during the burn-out.

Some people seem to have skin reactions to dust including that from Thinfire and Papyros and tend to wear gloves. If you are wearing rubber or latex gloves for extended periods, it may actually promote a reaction to the gloves rather than a primary reaction to the dusts of the paper. So checking on alergic reactions to the gloves is advisable in addition to the dust. 

If in fact, you are reacting to fibre paper dust, consider using kiln wash instead. If this is not practical or desirable there are several things you should do.

• Use gloves (for those with sensitive skin),
• wear long sleeved high necked shirts and pull the gloves over the ends of the sleeves to prevent dust getting to your skin,
• vacuum the kiln with HEPA filters,
• dampen the powder with water spray before brushing,
• take precautions against taking the dust home by changing in the studio and placing clothing in plastic bags to take to the wash.

Black Specks in Mesh Melts

The first time you use a mesh for a melt, it doesn't spall until it cools. By that time, the glass has hardened enough that any black specks of metallic oxidisation just land on the top of the melt and can be brushed away.

But, once a mesh has been fired previously, it can spall and drop little bits at any time during the firing process, so some of the bits get embedded in the glass.

The only way I have found to prevent this is to sandblast the mesh between firings to remove any loose flakes of metal. This is time consuming enough that you may wish to use a new piece of mesh for each melt. The alternative is to ensure you are using stainless steel as the grid.

There are several options for grids.

Applying Kiln Wash

Kiln wash, or batt wash as used in the ceramics field, is largely made up of alumina hydrate, kaolin (china clay), and often some colouring to indicate an unfired shelf.

These solids are heavy and settle to the bottom of the container quickly. So, you have to agitate the contents with each dip of the brush onto the liquid. To provide adequate - and even – coverage of the shelf, mould or other refractory material, you should paint in four directions. Up, down and the diagonals. You need to apply just enough that you do not see the shelf surface.

Alternatively you can spray the solution onto the surface. This is an easier way to get an even covering, but it sometimes is overly stippled.

A tip I was given for the smoothest kiln shelf is to level the damp prepared shelf and spray a layer of warm water over the wash to form a very shallow puddle. As the water is absorbed into the shelf, the only limitation to the smoothness of the surface is the granular nature of the kiln wash.

Layering Glass Textures

When using textured glass there is a decision to be made on whether the smooth or textured side is up.

Oddly, the largest, but thin bubbles occur when putting the smooth sides together. it seems that as the glass is not perfectly flat, it holds air within the fused piece.

The fewest bubbles seem to be promoted by placing the rough side down on all pieces. This is easy as cutting is done on the smooth side anyway, and so no reverse cutting is required. It seems that the rough side of the glass provides ways for the air to escape during the bubble squeeze although it does promote micro bubbles within the glass.

If more bubbles are desired, you can place the textured sides together. That seems to allow the majority of the air out, but still leaves the micro bubbles from both sheets.

I have had good results following the Bullseye recommendation to keep the smooth side up on all layers. 

Re-firing Poorly Annealed Items

Sometimes you suspect a piece has not been adequately annealed and want to re-fire it to make it sound. The question arises as to how quickly it can be re-rfired.

These pieces are very easy to heat shock, so the initial rate of advance needs to be much slower than for any piece of the same size, possibly less than half the usual rate. This slow rate should be steady without pauses until about 540ºC, which is above the annealing point of most fusing glasses. At this point you can speed up the rate of advance to whatever your normal one is.

Of course, it is best to anneal each piece on each firing to the extent that there is no question  that the piece is properly annealed. Looking at the Bullseye project notes and the annealing of thick slabs can help for evenly thick items. For tack fused and items of uneven thickness, you could review this posting.

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.

Annealing High Temperature Items

Every time you go above the annealing temperature, you must anneal again. You cannot skip or skimp on the annealing. You cannot rely on the annealing in the final firing to make your piece durable. Each time you fire a piece you are putting a lot heat stress into the piece.  If it has not been adequately annealed in the previous firing, it is much more likely to break on the heat up phase of the firing than if you annealed well on the previous firing.

The annealing at each stage in multiple firings is just as important as the previous one. In addition, pot melts and other high temperature items are inherently more delicate than those fired at their designed temperatures, so more careful annealing (including the annealing cool) is advisable. This is because the compatibility of glass alters a little at high temperatures. For example, you will observe that hot transparent colours opalise in the 900C range. This opalisation in itself will have altered the compatibility a little, because the opalescence alters the viscosity from what it was as a transparent. Other factors are at play too, such as some minor burning off of the colouring metals. So, careful annealing is required to ensure the maximum amount of stress is relieved. You also need to have a slower than usual initial rate of advance for any fire polish or slump firing after any high temperature process.

Even when firing at fusing temperatures, but beyond the tested number of firings, more careful annealing is required. In the case of Bullseye they have tested for three firings, although people get many more firings than that without difficulties. When taking glass beyond the design limits, more care is required in all phases of the firing to get durable results.

Writing Your Own Schedules, Part 2

Time Versus Rate

Schedules can be expressed as a rate per hour, or a time to get to the target temperature. What you feel most comfortable with relates largely to your background and teaching. Most ceramics based people use the time to get from one temperature to another. Most kiln formers without a background in ceramics tend to use rates per hour when writing schedules.

The rate of 100/hour to 100 degrees is the same as 1 hour to 100. 2.5 hours to 200 is the same as 80/hour to 200. So the conversion to a time to get to a target temperature is a simple one of dividing the temperature by the rate per hour to give the number of hours to achieve the target temperature. Some controllers will allow hours and minutes to be programmed; others allow only minutes – in which case multiply by 60 to give 150 minutes.

This is the same thing you do to find out how long a firing will take. If you see a schedule expressed as time e.g.,

3 hours to 677 for 0.5 hour,

1.25 hour to 800,

asap to 482 for 1 hour,

2.5 hours to 370

you already know approximately how long this firing will take – a bit more than 8.25 hours (3+0.5+1.25+1+2.5) plus cool down.

It can also be expressed as

225/hr to 677 for 30 mins,

102/hr (800-677=123/1.25) to 800,

afap to 482 for 30 mins,

45/hr (482-370=112/2.5) to 370.

The time to target temperature method of writing a schedule comes into its own when dealing with thick castings that require very slow cool downs. For example, a 60mm thick casting calls for an initial annealing cool of 2.4 degrees per hour over the range 482 to 428. I don't know of a programmer than can deal with decimals. So the alternative is to programme in time to target. In this case it would be a time of 22.5 hours.

The reason for avoiding the choice of 2 or 3 degrees per hour is accuracy. If you had put in 2 degrees per hour you would have spent 27 hours, possibly excessively long. If you had put in 3/hour it would have taken 18 hours, possibly not enough time for the glass to adequately anneal. So, for very slow rates of advance, time to target is much the most accurate method of writing the schedule.

Part 1

Making Billets

One of the uses of cullet (small pieces of glass) is in casting. However, simply placing the glass into a mould and firing, leaves many bubbles and often shows the edges of the original pieces of glass. Billets (ingots of glass) are more useful because they have fewer of the small bubbles and fewer edges than cullet.

It is possible to make your own billets. This can be done in a fashion similar to pot melts, although the temperature does not have to be so high. And the results are easy to store, if the dimensions are kept regular.

You need to have a mould for the melting glass to be contained within. These moulds can be made from plaster. A simple way is to use old margarine tubs placed upside down and fastened to the base within a dammed area. Pour the plaster of paris over the tubs to make the moulds. An alternative is to use strips of refractory material (fibre board or cut up kiln shelves) surrounded by heavy bricks to stop any movement due to the weight of the glass.

The glass to be formed is put into ceramic flower pots and can be directly onto the plaster of paris or dammed areas. You should put at least one piece of glass to cover the hole at the bottom of the pot. All this glass must be clean. Calculate the amount of glass required by determining the volume of the containment area (in cubic centimetres) and multiply by the specific gravity to give the number of grams required.

Don't get too ambitious about size, as these billets need to be fitted into the mould reservoir for filling the mould. A small margarine tub is approximately 12 cm wide, 7 cm deep and 7 cm high. This is as large as required, and smaller may be better. If you are making your own from dams, something like 4 cm by 8cm by 2cm may be better. This size is convenient for filling a reservoir, and has the advantage of being able to compare the intensity of colour the different thicknesses will give to the casting.

Remember that the thicker you make the billets, the longer you have to anneal. So the annealing time of the billet may be the factor that determines time. A 2 cm billet will take at least 9 hours of annealing time; one of 4 cm will take 28 hours of annealing.

When setting up the kiln for making the billets, remember that in general the higher the reservoir above the billet mould, the fewer bubbles you will get in the billet, although you are confined by the height of the kiln. Although there still will be some bubbles, these will further reduce by the second flow of the glass during the casting process.

To fire the set up, you can advance the temperature rapidly to 650/670ºC with a long soak there (possibly 3 hours). The final temperature can be below pot melt temperatures, so a casting temperature of 830ºC with a long soak (possibly 6 hours) will be sufficient. Take note of your final thickness – including any containment material – to determine the annealing soak and schedule.

Writing Your Own Schedules

Most introductory kilns are now being supplied with pre-set schedules. This can make moving on to the schedules you need for the new work you are doing appear to be difficult.

The first thing is to get the print-out of the pre-programmed schedules and determine what each stage of the programme is designed to achieve. If you compare the programme temperatures with a description of what is happening with the glass at that temperature, you will be going a significant distance to making your own schedule with an understanding of what you will be achieving with each stage of your purpose made schedule. A very good guide to what is happening to glass at various temperatures is this note from Bullseye. This also has the advantage of telling you what happens with different thicknesses of glass.

Next compare the pre-programmed schedules with those printed on the manufacturer's website, for example:

System 96

Uroboros

So, now you know what temperatures you are trying to achieve, how fast should you go to get to that temperature? I have developed a guideline that the initial rate of advance should be no more than twice the rate of your initial cooling rate for the final piece. This means that you start planning the schedule from the annealing portion of the full schedule. If you will have a final flat thickness of 6mm, the annealing rate will be around 80ºC, so the initial heat up rate could be about 160ºC. This is a conservative rate, and experience will guide you to how much quicker you can heat up the glass. This initial heating phase can be all the way up to the bubble squeeze/ slumping temperature, but must be to a temperature at least 40ºC above the annealing point.

There are at least three elements that will reduce this initial rate to less than this general guidance: Thicker pieces need more care. The more layers, the more difficult it is to get the heat to the bottom layer, so slower rates of advance are needed. The greater the unevenness in thickness, the slower the rate of advance.

There are, of course many other variables relating to the kiln, some of which are:

Side or top elements

Distance to the elements – side or top

Distance to the sides of the kiln

Placement in the kiln – e.g.,floor or shelf and how high

Nature of the firing surface – e.g., ceramic, fibre board, fibre paper

Placing in relation to the hot and cool spots in the kiln

How the glass is supported - especially on a slump or drape

At the initial stages of learning about fusing schedules, you need to make notes of all these things (and the results) on your firing records so that you can refer back to get guidance on what rates of advance are acceptable for any given firing.

Part 2

Glue Placement

Many people use glue to hold their arrangements of glass together to get it to the kiln. There are many kinds of glue that can be used. It is best to avoid resin based adhesives, but most other kinds of glue can be used – including hair spray, lacquer, super glue, CMC and PVA in addition to the proprietary fusing glues. The cheapest with the fewest additives seem to get good results.

Remember the glue burns away long before the glass becomes sticky, so if the glass won't stay in place while you are assembling it, it won't in the kiln either. The glue is only to keep things together while being transported to the kiln.

But this note is about were to apply the glue you choose to use.

The glue should always be used in minimum amounts. If it is a strong water based glue, such as PVA, it can be diluted with water and still provide sufficient adhesion. The glue should be runny, not thick or a gel. Unless the adhesive is a spray, a small dot at the edge of the piece to be glued will be sufficient. Capillary action will draw enough glue under the piece to stick it to the base glass.

If you are spraying the adhesive, that should be done at the end of assembly, to avoid flooding the base glass with adhesive. It is often best when using these lacquer based adhesives to spray a small amount of liquid into a container and use tooth picks or other pointed implement to dot the lacquer at the edge of the pieces to be attached. This way you can glue as you assemble rather than waiting to the end.

Adhesive under the middle of a piece of glass is likely to give black marks and even large bubbles, as the combustion gasses cannot get out from under the glass. So always confine your glueing to the edges of the pieces. A dot at each end is all that is required.