Safe Cooling Speeds

Almost everywhere you read that you should not open your kiln to cool it. But we know that the kiln cools progressively slower as it nears room temperatures. So how do we safely cool the kiln at rates the glass can cope with.

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Some advocate directing a fan at the closed kiln on the grounds that air movement will cool the kiln faster than still air will. Yes, but at a negligible rate due to the insulation properties of the the kiln.

It is much safer to program a sensible cooling rate all the way to room temperature from 370C/700F. This allows you to open any viewing ports knowing that any too rapid cooling will be counteracted by the controller switching on the heat. Upon hearing that power input, you can close the ports partially until the kiln cools a bit more.

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At lower temperatures, you can open the lid or door. The controller will operate as before warning you of too rapid cooling, so you can reduce the opening until more air is required.

Clay Moulds

Making your own clay moulds is not as difficult as often imagined. You do need to have access to a kiln that can fire to bisque temperatures, as they are above the usual limitations of glass kilns.

There are a variety of clays you can use – paper clay, school clay or any cheap clay from a ceramics supplier. Make sure any “clay” you buy from craft shops is in fact ceramic clay and not one of the no-fire or oven fire “clays”.

The method for slab moulds is fairly simple. You need to roll out a sheet of even thickness. To do this there are a few items required.

• Board to support the rolling of the clay
• Rolling pin of sufficient length
• Cloth for the bottom of the clay
• Battens of 8-10 mm thick

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Place the smoothed cloth on the board.

Place or fix the battens to the width needed.

Put the clay onto the cloth and work it out to the approximate thickness you require by hand.

Use the rolling pin to complete the process by pushing the clay in front of the rotating pin to get an even thickness. This will require a number of passes, both away from and toward you. You can patch any shallow places by simply putting a small piece of clay there and going over it with the rolling pin.

At this point you can impress any design you like on the wet clay, or you can incise the leather-hard clay later.

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If you are shaping the clay, you can use a variety of materials to hold the clay in the shape you want. You can use another form to support the whole of the clay if desired.

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Set the clay aside to dry, ideally where air can get to all sides/surfaces. The clay will shrink during this drying process. It will take longer to dry if the clay can dry from only one surface, and it is more likely to crack. So if only one surface can be exposed to the air, you should cover the whole piece with a damp cloth to slow the evaporation from the top surface.

When the clay is leather hard, you can sand or mark the clay. Then have the clay fired to bisque temperatures. When you get the fired form back, you should sand any roughness away, as this will be the bottom surface of your completed piece.

It is important that you kiln wash or provide other separators between the mould and the glass, as the clay and glass will stick together. Because the clay and glass expand and contract at different rates, the glass will be broken or crazed by the ceramic.

Straight Edges on Thick Pieces

As glass tends towards 7mm at full fuse, it is difficult to keep straight edges on thick pieces as the glass moves. If you want straight edges without dams, there are a few solutions:

1) Don't flat-fuse - apply less heat work so that the stack stays vertical instead of spreading. The degree of tack fuse required will be a subject of observation and experimentation.

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2) Plan on trimming the edges straight. You can use a saw or grinder and then either cold work the edges to polish, or fire polish.

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3) Add a couple of centimetres or so on each side of your base glass, so that a 20x20cm piece becomes 24x24cm, and flat-fuse as normal. The volume change will (mostly) be absorbed by the extra glass, so that you can simply trim it back to the right size and cold work the cut edges.

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Straight Lines on Thicker Pieces

When you use a partial layer on two or three layers, you will get wavy lines as the thicker glass spreads more than the thinner. E.g., if you have 3 layers, especially near the edges, the fully fused glass will spread out, while the two layer areas will keep their volume. If you have straight lines at the edge, they will no longer be straight at fusing temperatures.

To keep the lines straight on thick pieces, build the design upside down, so that the final top design is down to the shelf. The shelf side is the coolest part of the glass being fired, so it will distort less. In addition, the weight of the glass above will assist in keeping the design elements in place.

This is often referred to as “flip and fire” 

Flow Melts without Metal

“Flows” seem to be popular just now.  These are variations either on aperture drops using multiple holes or on screen melts depending on the number of apertures.  This is not a complete tutorial, just some notes on how to prepare a more sound and so more lasting project.  It is not complete, as these flows are essentially incomplete screen melts or aperture drops.  The techniques and methods that apply to them apply to melts also.

It is inevitable that steel of a thickness that can support the weight of inch thick glass when encased will break the glass sooner or later.  Why take the risk of incorporating a material which is not of a size or nature suitable for inclusion in glass?

Instead, one alternative is to use fibre board.  Take a piece of 10mm board (or thicker if desired) and cut holes on a grid pattern.  This is done so that the whole board can be supported on steel rods for safety.  If you are using thicker board, you can insert the steel rods into the fibre board, so ensuring they cannot come into contact with the glass, but still support the whole structure.  I have done 300mm square fibre board drops without support, although the top surface is significantly bowed at the conclusion.  Because you have quite a bit of weight on the supporting board, I would include at least a couple of 3mm stainless steel rods (kiln washed)  at 1/3 and 2/3 distance across the piece, so plan your holes with that in mind.

Another alternative is to kiln wash - or coat in bead release - stainless steel rods of 3 or 4mm diameter and make the grid from them.  This grid can be supported on a rectangular frame of dams or a drop out mould.  As you are going to relatively high temperatures, bead release may work best in this situation. Lay the glass on top of the rods.

Fire your glass with the appropriate rates.  Remember in annealing that you are dealing with a piece that has pretty large differences in thickness.  You therefore need to lengthen the annealing soak, and slow the annealing cool.

Once the piece is cool, you can take it out and put it upside down.  Slide the steel rods out and gently remove the fibre board from between the “legs” of the piece.  If you have used a grid of steel rods, they can removed from the glass, by gently pulling as you twist the rods.  You may have to soak the rods in water to help soften the bead release.

Now you have a flow with no inclusions and much more likely to last. 

Texture Moulds

Texture moulds are popular but expensive and with limited designs. You can make your own unique ones with only a few items of equipment. 

An example made by someone else

Clay

Various forms of clay can be used. Roll out a flat piece using boards of 8-10mm thick to support the rolling pin and give consistent thickness to the clay. The board underneath should be covered in paper or cloth to make an easy release. I have found that grease-proof paper as used in baking works very well.  It releases easily from the clay.  

Paper clay provides light weight moulds that do not hold a lot of heat, but any standard clay will do the job. There are two approaches to developing the pattern. You can stamp the pattern into the wet clay with any prepared design on a stamp or other textured material. The other is to dry the clay to leather hardness. You can then incise the pattern you desire directly into the clay. Fire to bisque temperatures, and sand to remove any rough areas or undercuts. Kiln washing the mould before use is essential.

Using a patterned roller to impress the design on the clay

Fibre board

Various fibre boards can be used. Ceramic fibre board, Kaiser Lee board, Vermiculite board, insulating ceiling tiles such as Armstrong, and calcium silicate boards can be incised and marked as desired. The advantage to the ceramic fibre, insulating ceiling tiles, and Kaiser Lee boards is that they allow air to pass through the material. Kaiser Lee board of these three provides the easiest surface for incising. Calcium silicate has no fibres, but requires a separator. Ceramic fibre and Armstrong ceiling tiles have fibres, requiring a bit more work to get a smooth surface. Armstrong tiles require a separator, but ceramic fibre boards do not unless you harden them for durability.

Fibre paper

Three millimetre fibre paper gives a easy material for cutting with craft knives or scissors to the design wanted. You can draw through an existing stencil or copy the design with carbon paper. It is not easy to produce designs with lots of detail.  It is quick, does not require separators, but is probably a single use material, unless you use mould hardener and then kiln wash, although it still will be delicate. For large projects, the paper should be fired first to ensure the combustion of the binders do not produce gasses to cause bubbles or fogging.

Loose Material

Sand, whiting, and kiln wash provide easy materials for one-off designs. You can quickly draw the design you want into the flat loose material with your fingers, or tools. You can also use found items to press into the loose material. Place the glass gently on top of the material and fire. If you use sand, you should dust it with kiln wash or aluminium hydrate to ensure the sand does not stick to the glass.

Unique Designs

All of these methods will provide unique designs which will distinguish your work from others.

Calcium Silicate for Moulds

I have recently discovered calcium silicate board. It is light weight, with no fibrous material. It is workable with wood carving tools and can be smoothed with sanding papers. It is manufactured in 25 mm thick boards, and so is suitable for shallow forms.

It requires no hardeners, but it does require kiln wash to ensure clean separation of the fired glass. It also is durable, but needs careful storage and handling.  It does not appear to warp with repeated heating.

High performance calcium silicate board will withstand temperatures of 870C for limited periods, according to the manufacturers.  It is best to support the shelf well and consider it a mould or shelf for temperatures at or below full fusing where it is known to perform very well.

An additonal factor in its favour is that it is significantly cheaper than either ceramic fibre board or vermiculite board.

Flattening stringer

Placing stringer is often difficult. Not simply to put it into place, but to keep it there. People tend to hold the pieces with glue. However, the glue burns off before the stringer is anywhere near even tack fusing temperature. This allows the stringer to roll. Also an excess of glue will boil off during the heating and so move the stringer even more than gravity will. Two methods are effective in reducing the ability of the stringer to roll, by flattening one side.

Grinding

One method used by Bob Letherbarrow is to hold a stringer that is much longer than needed against the glass grinder bit. Use your thumb to hold the stringer against the bit. Slowly pull the stringer between the bit and your thumb or finger, using light pressure, to hold the stringer against the spinning grinder bit. This will slightly flatten one side of the stringer.

Firing

Another method is to place the stringers on a prepared kiln shelf, making sure they do not touch. Take the temperature quickly up to a tack fuse, soak for a few minutes and turn off. This will take only an hour or so and enables you to prepare a number of stringers with a minimum of effort.

In both cases, cut the stringer to the length needed and place on the glass with the flat side against the glass. Glue it very lightly to hold it in place while moving the piece to the kiln if it is absolutely necessary. When the glue burns off the stringer should not roll around. It is best, of course and if at all possible to place the stringers in the kiln, so no glue is needed. 

This blog post indicates yet another way to keep stringers from moving, if contour or fuse firings are planned.

Equalisation temperature

In my view a schedule has the following stages.

• Initial rate of advance to bubble squeeze,
• 
  
• Rapid increase in temperature to target, or working temperature,
• 
  
• Quick fall to temperature equalisation (often called the annealing point),
• 
  
• Slow decrease in temperature - to keep internal stresses at a minimum - to 110C below that temperature equalisation point,
• 
  
• Faster cool to 100C or less.

Of course, some of my firings have up to 10 segments, so don't mistake the stages as equivalent to schedule segments. The following graph is a generalised version of these stages.  The times and temperatures are for illustration only.

http://glassmuseum.moc.gov.tw/web-en/unit03/modepage/3-5-1-20.html

The equalisation temperature is what is most often called the annealing point. This is a mathematically determined temperature at which the glass most quickly anneals - has stress relieved. However, the way kiln formers work, annealing does not occur at one temperature point on the controller output, because of the inherent inaccuracy of our kilns and controllers. The soak at the annealing point has the purpose to equalise the temperature throughout the glass before proceeding to the anneal cool.

There is little point in soaking above this temperature, only to have another, lower temperature soak at the published annealing point. The soak at the annealing temperature will negate any effect of a soak at a higher temperature. So, a soak above the annealing temperature will simply slow the whole cooling process.

Of course, the soak at the equalisation temperature must be long enough to get the whole substance of the glass to the same temperature. The thickness of the glass will determine the length of this equalisation soak. Fortunately Bullseye have published a table to help determine the time required.

The slow decrease in temperature is to keep all the substance of the glass to within 5C difference on the cooling. Thus, the rate of cooling is related to the thickness of the glass. It will be increasingly slower with increasing thickness. The cooling to around 110C below the equalisation temperature is all part of the annealing process. The more rapid cooling after that is to control the rate of temperature fall to avoid thermal shock.

Thermal Shock

Thermal shock is a term for a break caused by a too rapid change of temperature within a piece of glass.

"Glass tends to be 

1) very brittle, 

2) expand and contract quickly when subjected to temperature changes, and 

3) is an insulator (when solid) and therefore does not readily conduct heat. 
That is why glass is highly susceptible to thermal shock"

http://www.glassfacts.info/indexf286.html?fid=210

This can occur on both an increase or decrease in temperature. Glass conducts heat poorly.  The ideal is to keep the temperature differentials within the glass to 5C or less.  This is the purpose of the anneal cool.  The risk of thermal shock can be increased by different thicknesses across the piece. Greater care is required in cooling these pieces than those of uniform thickness.

A piece showing large differences in thickness and so at greater risk of shock

Identification

The break normally is straight through the glass without following the edges of the various pieces of glass.

This shows the break crossing multiple colours of glass

The line of the break will be rounded if it parted on the heat up. In some cases, the glass will have stuck back together if it was dammed or the break was gentle enough to avoid pushing the glass apart.

If the shock occurred on the cool down, the edges will be sharp.  

The edges will also be sharp in a slump whether the break occurred on the advance or the reduction in temperature.  If the pieces fit together perfectly the break is likely to be in the down phase.  If the pieces are slightly different shapes the break likely occurred in the rise in temperature phase.

Other kinds of breaks are possible and are described elsewhere.