Initial Rates of Advance

Almost everyone has fired the glass from cold too fast at least once. This provokes the question of how fast it is possible to fire glass. Of course, there are lots of variables which relate to the evenness of heat across the firing surface of the kiln. A number of factors will affect this. Among them are:

• The size and shape of the chamber
• Composition of the chamber – brick, fibre or a combination of both
• Location of the elements - lid, sides or elsewhere
• Distance of elements from the work
• Spacing between the elements

This means that only general guidance can be given for you to use in gaining experience with your kiln, as your kiln is unique, even if it is one of a production series.

Now with all those cautions, some general guidance can be given. This table assumes that you have a top fired kiln and that the glass is about 300mm below the elements and that it is at least 50mm from the sides of the kiln. It also assumes that the glass has not already been fused.

First Segment Heating Times for Top Firing Kilns
by Number of even 3mm Layers and Size

100mm dia: 1 layer - 999C/hr || 2-3 layers - 999 || 4 layers - 750 || 5 layers - 600

200mm dia: 1 layer - 999C/hr || 2-3 layers - 500 || 4 layers - 430 || 5 layers - 330
300mm dia: 1 layer - 999C/hr || 2-3 layers - 335 || 4 layers - 275 || 5 layers - 215
400mm dia: 1 layer - 750C/hr || 2-3 layers - 250 || 4 layers - 200 || 5 layers - 155
500mm dia: 1 layer - 600C/hr || 2-3 layers - 200 || 4 layers - 165 || 5 layers - 120
600mm dia: 1 layer - 500C/hr || 2-3 layers - 165 || 4 layers - 135 || 5 layers - 105

First Segment Heating Times for Top Firing Kilns
by Number of Uneven 3mm Partial Layers and Size

100mm dia: 1+ - 850C/hr || 2+ - 750 || 3+ - 500
200mm dia: 1+ - 500C/hr || 2+ - 430 || 3+ - 335
300mm dia: 1+ - 300C/hr || 2+ - 250 || 3+ - 215
400mm dia: 1+ - 215C/hr || 2+ - 188 || 3+ - 158
500mm dia: 1+ - 177C/hr || 2+ - 162 || 3+ - 137
600mm dia: 1+ - 150C/hr || 2+ - 143 || 3+ - 120

If you are adding decorative elements to the basic disk, you need to consider the effect that the additional pieces have on the whole. Each piece of glass shades the pieces below it from the heat of the elements, so to keep the glass heating at the same rate throughout, you need to slow the rate of heating by using the suggestions for the next higher thickness. So, for example, a two layer piece of 300mm can be fired at ca. 330C/hr, but if you have decorative pieces spread across the base, you need to slow the rate of advance to 250C/hr.

Note that these rates should be taken as the fastest possible rates. They may be too fast for your kiln.

Resists for Large Areas

Avoiding bubbles appearing under the vinyl resist on large sheets of glass when preparing etching or other resist based processes is often difficult.

A trick I learned from the firms that apply advertising vinyl to vehicles is to use a spray bottle filled with water that is just soapy. A few drops per pint will be sufficient.

Their process uses pre-cut vinyl with an adhesive backing. So the first thing to do is to pick out the unwanted pieces. That is the pieces covering the areas that will be etched. Then you need to put a backing onto the picked out vinyl – usually really wide masking tape.

Lay out the vinyl on the glass. Tape one end of the vinyl securely to the glass. This ensures that you get the vinyl correctly aligned over the whole area. Fold the whole piece of combined vinyl and backing back to the taped edge.

Carefully peal back the covering for the adhesive side making sure you do not pull off any of the isolated vinyl pieces. Spray the glass with a mist of soapy water to ensure all the glass is covered, do not have the glass running with liquid, but be generous.

Start the application process by folding the vinyl onto the glass. Use a credit card or better, a large squeegee such as used for grouting mosaics. The tool you use must be smooth to avoid scratching the vinyl. Push the soapy water forward and to the sides as you move along the piece of glass. Keep pulling the protective layer evenly off the adhesive side as you work forward.

When completely attached, remove the backing from the vinyl. This will enable you to see any bubbles you may have left. Work out any bubbles by further pressing the soapy water out from under the vinyl to the edges. Where any remaining bubbles are in the way of the design, puncture them and work out the bubble of moisture through the hole. Cover the puncture with a small piece of vinyl.

Leave for a day for the vinyl to become firmly attached to the glass and then you are ready to do the etching.

Where you are going to cut the vinyl by hand, you do not need the backing. All the rest of the process is the same.

Flat Bottoms for Bowls

There are at least three ways to achieve flat bottoms to bowls without the use of external supports.


Using drop out rings will enable you to get a flat bottom of whatever diameter you wish depending on how long you let the aperture drop run.

You can put some dry kiln wash into the bottom of the mould, then firmly press it flat with a round piece of glass. You will need to make sure it is horizontal, so the use of a small round levelling bubble can make this easier.

Grind a flat spot on the bottom of the otherwise finished bowl. It is a good idea to use a two way leveling bubble while grinding. The round bubble is easier to use, while the two way bubbles – two leveling bubbles placed at right angles – are more accurate.

Getting Water to the Mini Work Surface of a Glastar G8

Sometimes the water does not rise to the mini work surface. There are a number of things to check. These, in order, are usually the reasons the water does not get to the Mini Work Surface.

• Ensure there is enough water in reservoir, right up to the overflow

• Ensure channel from impeller to the up tube is clear

• Ensure the up tube is clear

• Ensure tap at the top is clear

• Flush the feed lines with a syringe or bulb instrument

• look at the position of the impeller on the shaft. It can move up or down. Repositioning it can improve the flow of water to the top story.

Supports for round bottomed bowls

A number of useful moulds for slumping do not have flat bottoms. There are a number of possibilities to have the bowl sit firmly without grinding the bottom flat. Remember that you do not need to surround the whole bottom to give the bowl stability.

Some of these include things like:

• A rubber “O” ring, although they usually come in black only.

• Thin slices of wide-diameter tubing.

• Wok support rings.

• Plastic tubing with a small joining dowel allows you to make any size. You can then paint it with the appropriate colour.

• Macramé, embroidery and curtain rings can be suitable.

• You can make them using hole saws. Cut out the big ring first so you can use the pilot hole to line up the smaller hole. Then bevel the inside to fit the bowl.

• Use three bumpons on the bottom. Be sure that the bottom of the bowl is perfectly clean, dry and free from oils. Then use some weight pressing on the bumpons for a day or more so that they stick permanently. You can do this by turning the bowl upright and fill it with some heavy objects.

Firing schedules – what are they for?

Firing schedules or programs are the means of controlling the temperature rises, soaks and falls to accommodate the needs of the glass. They consist of a number of segments –or steps - each of which includes: rate of temperature rise, target temperature, and soak time. They vary according to the thickness of the glass and the forming and annealing needs of the glass. Read and understand the Bullseye Technical Note on the way glass behaves at different temperatures. This will give you a good understanding of what happens to the glass at the different temperature ranges and will help you design a suitable schedule for what you want to achieve.


To assist in visualising what the numbers in a kiln programmer do, you can graph the temperature changes indicated by the numbers in the controller. Visualised from the start of the schedule, it appears as a mountain with a steep cliff on the left rising to a ledge. There is then a steeper rise to the top where there is a small plateau. The mountain then has a very steep face on the right, falling to a broad ledge a bit lower than the one on the left. There is a long shallow slope to the right of the ledge that leads to a much steeper drop to the level again. This is the shape – with variations - that you are attempting to achieve in each program/schedule.

The variations have to do with the type of glass being used and thickness of the glass. These variations determine the amount of heat and the speed with which it is put into the glass. It sets the points at which any soaks are introduced to allow the glass and associated moulds or kiln furniture to equalise in heat or to allow air to ease from between sheets of glass. It sets the top temperature and determines the length of soak at that temperature. It controls the temperature fall to the annealing soak - to equalize the temperature throughout the glass. It then controls the rate of fall to anneal the glass – removing the stress and follows up with the fall to room temperature.

A description of each of these stages includes the heat rises and any soaks required, the temperature fall, annealing soak and cool, and the cool to room temperature.

Initial heating rise

In the simplest form, the initial heating is a relatively slow rise to a point about 50C above the annealing point. This allows the glass to gain heat without thermal shock. The initial heating may be achieved in several segments, depending on what you are doing. A thick piece, or one fired many times, might be taken up in a number of stages - initially very slowly (with or without soaks - also known as holds), and then at more rapid increases. A 6mm piece being slumped into a simple curve mould would need only one segment to the top temperature.

Another example of variations required would be a 6mm piece suspended over a cylindrical mould for a drape. My experience has shown that there is a requirement for multiple segments. This starts with an initial rise of 50C/hr to 100C with a 10min soak, then 100C/hr to 250C, 10 mins, then 150C/hr to 500C, with 10mins and finally 200C/hr to forming temperature - in the region of 630C - 677C with an appropriate soak to achieve the effect desired - peeking is required to determine the length of this soak. The point being that some circumstances require much more complicated arrangements. Here it is because the mould drains the heat away from the centre of the glass while the edges heat up.

Final heating rise

Above the annealing plus 50C temperature is when the rise can be much faster up to the working/top temperature. This speed should not be as fast as possible, because it has a number of drawbacks. The speed of this rise is influenced by the amount of heat work you wish to put into the glass. This in turn will influence the top temperature and length of soak at that point.

You most often want to insert a bubble squeeze in this rise to avoid large bubbles due to trapped air.

Cooling phases

The cooling phases are several: fast drop to annealing soak, annealing cool, cool to room temperature.

Fast drop

Once the soak at top temperature is finished the requirement is to cool the glass and kiln as fast as the kiln will allow. This is to avoid the devitrification that can occur in the range of 650C to 760C.

Annealing soak

This soak at the annealing point is to allow the glass to reach the same temperature throughout from side to side and top to bottom. The length of this soak will depend on the thickness of the glass. More information on annealing is here.

Annealing phase

The slow steady cool from the annealing point to about 55C below the annealing point is where the annealing of the glass is done. What is required is a gradual, but steady decline in temperature to allow the glass to reduce in temperature evenly throughout its thickness. This even reduction in temperature should continue to the strain point and slightly below. So this phase must not be done quickly. For a 6mm piece 80C/hour is usually adequate. More on the annealing phase is available here.

Cooling to room temperature

Cooling to room temperature should be done at an even rate, although faster than the annealing cool. Too fast a cool below the strain point can cause thermal shock and therefore breakage. Typically the cool to room temperature from the strain point can be two to three times faster than the annealing cool. It is a good idea to control this cool to at least 100C. If your kiln cools more slowly than this, it will not be using any electricity, but it does protect against too rapid cooling if you open the lid or door.