Factory Installed Firing Schedules

Factory installed schedules are a quick starting point for the novice kilnformer.  

Many kiln manufacturers install schedules in the controllers of entry level kilns.  Some install them in larger kilns too.  They will work for for gaining basic experience of kiln operations.

However, these schedules are not universal.  Each maker programmes schedules according to their understanding of a mid-range firing schedule for various processes. 

An example of some installed programmes from Scutt

This means that when referring to an installed programm on your controller, you need to give the full schedule so others can understand.

Why?

Not only because a tack fuse schedule may be to a different temperature, but also a "fast" schedule as programmed into one kiln might be quite different to one in another.

This matters, because how fast you get to the top temperature affects what temperature you need to use. You will probably experience the difference in final effect between a fast and a slow fuse to the same temperature.  If you haven’t seen it yet, try both schedules on the same layup of glass.

You will see that a fast rate of advance to a tack fuse will give a much more angular appearance, while a slow rate of advance will give a much more rounded appearance.  This is the effect of heat work, which is essentially the combination of temperature and time.

The longer it takes the glass to reach a given temperature, the greater the heat work.  Longer times to the top allow the use of lower temperatures. 

The consequence of accounting for heat work is that a simple top temperature cannot be given.  It is not just that kilns are different, but that the amount of heat work put into the glass will change the top temperature required for a given look.

Revisd1.1.25

Heat Work is Cumulative

“…. the first fuse (contour) I brought it up to 1385°F and held for 5 minutes - it did not contour as much as I would like - do I re-fire at same temp and hold longer or go up in temp and hold same amount of time or something else?”

Observe

Of course, the smart answer is “Observe to get it right first time”.   Observation will enable you to determine when the piece is fully fired.  To observe you need only peek at 5-minute intervals to determine if the piece is as wanted. 

Know your Controller

In combination with this you will need to know your controller well enough to be able to advance to the next segment if the piece is done before the segment finishes; or how to stay on the same segment until it is finished and then advance to the next segment.

Of course, there are circumstance when you cannot or do not want to be present at the top temperature of the firing.  Then consider using the delay function to enable you to be present. This gives a countdown until the kiln starts.  The practice is fully described in this blog entry.

Time or Temperature

If you are experiencing an under-fired piece and want to re-fire it to get a better finish, the usual question is whether to fire for longer or at a higher temperature.

The response is – “Neither”.

Re-fire to the same temperature and time as before, unless you are looking for a radically different appearance.  Heat work is cumulative.  You have put heat into the glass to get the (under fired) result.  By firing it again, the heat will begin to work on the glass as it rises in temperature.  The piece, in this instance, is already a slight contour.  The additional heat of this second firing will begin to work just where the first firing did, and will additionally change the existing surface just as the first firing did.  The degree of contour achieved by the first firing will be added to equally in the second firing.  It is of course, a good idea to peek in near the top temperature to be sure you are getting what you want. More information on heat work is available here with its links. 

Rate of Advance

It is important to remember that on the second firing the glass is thicker, and you need to schedule a slower rate of advance until you get past the strain point – about 540°C for fusing glasses, higher for float and bottle glasses.

Future firings

At the finish of the second firing you will have soaked at the top temperature for twice the scheduled time.  You can use this extra time for the next similar firing, or increase the temperature slightly and keep the original firing’s length of soak. 

As pointed out earlier, observation for new layups, sizes, thicknesses, etc., is important to getting the effect you want the first time.

Slumping Breaks

“Why does my full fused disc break when I slump it?”

There are several possibilities. The two main ones are annealing and ramp speeds.

Inadequate annealing in the fusing stage can lead to a very fragile piece when being re-heated.  If there is significant residual stress in the fused piece, it is much more sensitive to heat changes during subsequent firings whether full, tack, or slumping/draping. It is important to thoroughly anneal any piece at every firing.  If you are firing a different layup or contrasting colours and styles, you should check for stress using polarising filters.  

The slump – or drape – firing needs to be much slower in temperature rise than the fuse firing.  You now have a thicker piece which takes longer to absorb the heat evenly. 

If your piece is tack fused, it needs an even more slow rate of advance.  Sometimes this needs to be as though the piece were two to four times the actual thickness of the piece.  The more angular and pointed the tack fused elements, the greater the reduction in firing speed.  This post gives guidance on how the piece is designed and its thickness affects the rates and soaks in tack fusing. 

Further information is available in the ebook Low Temperature Kiln Forming.

Broken Base Layers

Sometimes in fusing, the base layer can exhibit a crack or break without the upper layers being affected.  This kind of break almost always occurs on the heat up.  In a tack fuse, the top layers may still be horizontal and unaffected by the break beneath them.  If a full fuse, the upper layers will slump into the gap, or apparently seal a crack that is apparent on either side.

An example of tack fused elements on top of a previously fused base

Causes

This is more likely to be seen where there is a large difference between thicknesses.  If the base is a single or double layer and there are several layers of glass – especially opalescent – on top, there is a greater chance for this kind of break to occur.

The reason for this kind of break is that the upper layers insulate the part of the lower layers they are resting upon.  Glass is an insulator, and so a poor conductor of heat.  This means that the glass under the stack is cooler than the part(s) not covered.  A break occurs when the stress of this temperature differential is too great to be contained.

An example of  stacked glass in a tack fusing

This kind of break can also occur when there are strongly contrasting colours or glasses that absorb the heat of fusing at different rates.  One case would be where the dark lower layer(s) were insulated by a stack of white or pale opalescent glass.  The opalescent glass will absorb the heat more slowly than the dark base.  This increases the risk of too great a temperature differential in the base.

Reducing the risk of these breaks.

Even thicknesses

One way to reduce the risk of base layer breaks is to keep the glass nearly the same thickness over the whole of the piece.  Sometimes this will not give you the effect you wish to obtain.

Slow the firing rate

Another way is to slow down the temperature rise.  Some would add in soaks at intervals to allow the glass under the stack to catch up in temperature.  As we know from annealing, glass performs best when the temperature changes are gradual and steady.  Rapid or even moderate rates of advance with soaks, do not provide the steady input of heat.

This prompts the question of how fast the rate of advance should be, and to what temperature. 

Rate of advance

The rate of advance needs to take account of the thickness differential and the total thickness together.  A safe, but conservative, approach is to add the difference in thickness between the thinner and the thickest parts of the piece to the thickest.  Then program a rate of advance to accommodate that thickness.  E.g., a 6mm base with a 9mm stack has a total height of 15mm.  The difference is 9mm which added to 15mm means you want a rate of advance that will accommodate a 24mm piece.

The rate of advance can be estimated from the final annealing cool rate required for that thickness.  In the example above, the rate would be about 100°C per hour.  The more layers there are, the more you need to slow the heat up of the glass. The Bullseye table for Annealing Thick Slabs is the most useful guide here.

Firing already fused elements

If you were adding an already full fused piece of 9mm thick to a 6mm base, you could have a slightly more rapid heat up, bu not by a lot. This is because the heat will be transmitted more quickly through a single solid piece to the base glass.  It is safer to maintain the initial calculation. If your stack is tack fused, this will not apply, as the heat will move more slowly through the layers of the tack fusing much the same way as independent layers on the initial firing.

Conclusion

The general point is that you need to dramatically slow the speed of firing when you have stacked elements on a relatively thin base.  Even a two layer base can exhibit this kind of break when there is a lot of glass stacked on it.

Rounded Bottom on Drapes

Sometimes drapes, such as the handkerchief drape over a cocktail shaker, finish with a rounded base.

The base is rounded because not enough time or heat was allowed to get it flat. The glass will benefit from a moderate, but steady advance in temperature all the way to the top temperature.  This rate will be around 100°C to 150°C per hour.  There is no need to speed the rate of advance at any time during the process of the drape.  Too rapid an increase in temperature may even give uneven drapes if there are differences in thickness or colour.  There is no need for a soak at the strain point on the way to the top temperature. Any thermal stress from the rate of advance - that some suggest may occur - will already have taken place by this temperature.

This slower rate of advance will mean that the glass will not dome so much on the drape.  It will have time both to conform to the top (which will become the bottom of the piece) of the mould support during the drape stage. 

You need to visualise what the glass is doing during the forming process. As the glass begins to drape, the glass on the support rises because it is not yet soft enough to stay flat on the supporting mould. It is only later at higher temperatures, that the glass on top of the support can conform to it.

If you watch the process – a really good practice - you will be able to tell when you have a good drape. And with this reduced rate of advance, you should have a flat bottom. And all of this may happen at a lower temperature than you expected.

Firing Rates

Top temperature is, to a small extent, variable between kilns, even from the same manufacturer.  But it is a small part of variations in top temperature required to get the same results in differing kilns.

An example of a firing schedule

It is, more importantly, a function of how the heat is put into the glass. Firing as fast as possible to the top temperature does not allow all the glass to be at the same temperature. This is because glass is a good insulator and the transfer of heat from the top or the sides is relatively slow.  For small things, you can fire very fast, as there is a small mass of glass to absorb the heat.  But a speed of 250°C is fast enough for anything more than 100mm square and at least two 3mm layers thick.  (Thicker glass requires slower rates of advance as surprisingly do single layer projects).  The slower rate of advance allows the glass to be all of a similar temperature from top to bottom, allowing the desired effect to be achieved at lower temperatures or shorter soak times. 

For example, a slower rate of advance will give rounded edges at shorter soak times than a rapid rate of advance will require.  Alternatively, it might require a lower temperature with the same soak time.  Keep in mind that, in general, lower temperatures with slower rates of advance, give better results.

The faster your rate of advance, the more the glass lags behind the air temperature (which is what pyrometers are measuring). Therefore, a reasonable pace will give better results than the as fast as possible rate of advance. 

In short, the variations in top temperature required and length of soak is not about the kiln firing cooler or hotter as much as it is about the firing rate.

Shape of Aperture Drops

The shape of an aperture drop can be controlled by the speed of the slump. The speed at which the glass drops is a combination of heat and size of the hole. Patience is required.

Rapid drops result from high temperatures. Rapid slumps cause a thinning of the glass at the shoulder where the glass turns over the inner rim of the aperture. The pattern is distorted and the colours are also diluted. And a relatively large rim is left around the fired piece.

A much slower rate of drop spreads the strain of the slump over the whole of the unsupported area of glass. This tends toward a bowl with a gentle slope toward the bottom, reduced distortion of the pattern, maintenance of the colour densities, and a more even wall thickness all over the piece.

The slumping temperature for a shallow angled slump is less than that used for normal slumps, and takes a lot longer – up to five hours typically. This means that observation is required at intervals, say every half hour.

A starting point for the slumping is around 100ºC above the annealing temperature for the glass. So for Bullseye and System 96 the temperature is about 615ºC. If after the first half hour, there is no movement, increase the temperature by 10ºC. Check again in another half hour and if the slump has begun, leave the temperature at that level and observe at the half hourly intervals until the desired slump is achieved. Otherwise, increase the temperature by another 10ºC with the check after half an hour, and repeat until the slump has begun. After you have done the first one of these with a particular size of aperture, you will know the temperature to start the slump.

The temperature you need to use is affected by the size of the hole. The smaller the aperture, the higher the temperature will be needed. But be patient. If the temperature is increased too much, you will get the thinning of the sides that you are trying to avoid.

Additional information on aperture drops can be found in this series.

First Ramp Rates

There is a lot of literature about annealing and cooling rates, as they are the most critical elements in producing a piece with minimum stresses within it.  But there is not so much information on initial ramp rates.

It is possible to break the glass by heating it up too fast during the initial temperature rise.  How fast you can increase the temperature is dependent on how even the heat is within your kiln and the profile of the glass.  Any suggestions have to be tested within your own kiln and setup rather than relying exclusively on others' experience.  Some of the considerations relating to the kiln are given in this blog about initial rates of advance.

So with those precautions, I put forward a suggestion based on my experience and information gleaned from the Bullseye site, education section and from Graham Stone's work. These lead me to suggest that the initial rate of advance can safely be the same as the second cooling segment as listed in the Bullseye chart Annealing Thick Slabs (Celsius and Fahrenheit). This ramp rate applies up to the softening point of the glass.

Experiments have shown that an evenly thick piece of glass 6mm thick cooled at 150ºC/270ºF per hour during the second cooling segment - can also be fired up at the same rate. And by extension:

• A 12mm thick piece could be taken up at 99ºC/178ºF per hour
• A 19mm piece could have an initial rate of advance of 45ºC/81ºF per hour
• A 25mm thick piece of glass could be taken up at 27ºC/49ºF per hour.

These rates depend on a number of factors:

• how the glass is supported,
• the nature of the shelf,
• the composition of the mould, and
• the kiln characteristics as well as
• the colour combinations and
• whether the piece is tack fused or full fused.

Slower rates of advance are indicated if  

• the kiln is side fired or has cool spots.
• the shelf has not supported on 25mm/1" kiln posts.
• the piece is tack fused, you need to slow the ramp rate by half.
• there are strongly contrasting colours next to each other   

Remember that these numbers can only be used as a guide in conducting you own experiments.

More information is given in the eBook Low Temperature Kilnforming available from

Bullseye 

and Etsy