Wednesday, 21 March 2018

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.