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