Effect of AFAP Rates

 

 

This graph illustrates the effect of a rapid increase (500C/hr) in temperature on the glass.  The blue line represents the air temperature measured in the kiln.  The orange line represents the temperature between the glass and the shelf.  At an air temperature of 815°C, the temperature of the glass at its bottom is around 750°C.  This is a large difference, even though the glass is in the plastic range.  It means that the potential for stress induced by the firing rate is large.  The graph shows the temperature difference evens out during the annealing soak.

 The fast rise in temperature at the initial part of the firing where the glass is still brittle risks breakage.  The difference in temperature between the top and bottom of a 6mm piece of glass is shown to be 100°C plus throughout this initial phase up to 500°C.  Most breaks due to thermal shock occur before 300°C. This large temperature difference that occurs with rapid rates of advance risks breakage early in the firing.

 As an example, I took a piece out at 68°C to put another in.  During the time the kiln was open, the air temperature dropped to 21°C.  I filled the kiln and closed the lid and idly watched the temperature climb before switching the kiln on for another firing.  It took a bit more than two minutes for the thermocouple to reach 54°C with the eventual stable temperature being 58°C.  I had not been aware how long it takes the thermocouple to react to the change in temperature.  Yes, it takes a little time for the air temperature in the kiln to equalise with the mass of the kiln, but not two minutes.

 With a two-minute delay the recorded temperature can be significantly behind the actual air temperature.  For example, a rate of 500°C per hour is equal to 8.3°C (15°F) per minute or 16.6°C (30°F) overshoot of the programmed temperature. Even at 300°C it is a 10°C (18°F) overshoot.  This effect, added to the way the controller samples the temperatures, means the actual overshoot can be significant for the resulting glass appearance.

 This is just another small element in why moderate ramp rates can be helpful in providing consistent results for the glass.

 More importantly at top temperature, the surface will be fully formed while the bottom is only at a tack fuse temperature. This does have implications for the strength of the piece.  There will be an only tack fused structure through much of the piece, but a full fused structure at the surface.  The potential for breaking in further kilnforming or during use is high.

 In addition to the effects on the glass, there will be effects on the operation of the controller.  Controllers operate by comparing the instructions on firing rate with the air temperature recorded by the pyrometer.  In doing this the variances become smaller with time.  An AFAP firing does not give a lot of time for the controller to “learn” the firing curve.  So, the controller tends to overshoot the top temperature by some (variable) degree.  This makes it difficult to precisely control the outcome of the firing.

 There is some concern that the structure of the kiln will be affected by AFAP firings. This is a small risk.  The expansion and contraction of the kiln materials will occur whether quickly or more slowly.  It is not a major concern.  It is a concern for the glass, though.

 AFAP firings have potentially harmful effects on the structure of the fired glass leading to thermal shock and fragile completed pieces.

High Fast Slumps

What are the possible effects of fast rises to a high temperature for a slump?

Some of the possible effects of fast rises to a relatively high temperature slump are these:

Uneven slumps can occur. 

·         This largely due to differential heating of thicker/thinner parts. 

·         It can also emphasise anything off level.

·         Any unevenness in the heat across the kiln can also be emphasised by the rapid rise in temperature.

Uneven slumps can be promoted by contrasting colours. Dark and light colours heat at different rates, leading to one area of the glass slumping before another.

A dark/light contrast can lead to stress fractures in fast firings.

In a fast firing the top heats faster than bottom leading to the possibility of splits on the bottom of the piece

The edges of the piece heat faster than centre, increasing the possibility of spikes at the edge.

Fast slumps require higher temperatures to achieve the slump.  This means there will be more marking of the bottom surface.  It often includes stretch marks especially at the rim.

The Alternative to Fast High Temperature Slumps

Slow and Low

Slow rises in temperature means the slumps can be done at lower temperatures. Lower temperatures usually mean more control and fewer marks from the mould.  It does mean that you will need to observe at intervals to get the soak time you need, but this is required for all variations in rates and layups, as well as new moulds.

AFAP firings

As Fast As Possible (AFAP), sometimes referred to “as soon as possible” (ASAP) firings need caution.  Usually, this AFAP rate is applied only above ca. 540⁰C or higher.

This is possible for small pieces in smaller kilns.  It is often desirable for pieces under 100mm.  In the case of smaller items, the heat can be distributed across and through the pieces easily.  There is no need for the same caution as for larger or thicker pieces.

But

There are effects on glass and kiln that AFAP rates have and need to be considered when setting the schedule for the firing.

Effects on glass

An AFAP rate softens the upper surface of the glass early and before bottom can catch up. This leads to greater possibilities of creating bubbles, as the surface is more easily moved by the air underneath.  So, the air can push upwards rather than be pushed by the weight of the glass from under and escape out the sides. 

The characteristic dog boning of thinner glass is increased, as the temperature overshoots, allowing the glass to become much less viscous, so the surface tension of glass can take over to draw the glass in to create a greater thickness.  This “robbing” of glass occurs both from the interior and edge.  The interior glass becomes thinner and so less able to resist bubble formation.

Effects on the Kiln Control

The controller is learning the relationship between the energy input and the temperature achieved all the way through each firing, even though you fired the same piece yesterday.  The controller is constantly (well, about once a second) comparing the actual rate of temperature increase or decrease with the programmed one.  When there is a difference, power is applied. On the way down there is no input of energy unless the cooling is too fast, so there are no concerns about the controller having to catch up.

If you programme AFAP, especially in a small kiln, you will get overshoots in temperature.  This is because considerable time is required for the controller to determine the continuing energy requirements for the rate set.  In small kilns, the upper temperature can rise quickly as there is less kiln mass to heat than in a larger kiln.

Also, the amount of energy required at the higher temperatures is greater than at the lower ones. This means the controller must constantly adjust the rate of energy input at different temperatures.

Both the above factors combine to give overshoots of the top temperature, sometimes by as much as 20⁰C.  During the soak time at top temperature, the kiln will attempt to adjust the energy input to maintain an even temperature. The result of this constant comparison is that the temperature drops considerably below the one set. The controller then overcompensates and goes over the set point again.  It continues bouncing above and below with less and less variation as the soak proceeds, because the controller is “learning” the heat input required.

This bouncing of the temperature gives you less control over the results of the firing.  This is especially so when there are voltage variations in the electricity supply.