A kiln heats from 20 to 1,200 degrees in 6 hours. What is the average heating rate?
Answer: C
The rise of 1,180 degrees over 6 hours is about 197 degrees an hour. Real schedules vary the rate considerably, so the average conceals holds and slow passages through critical ranges. An average rate conceals almost everything that matters in a schedule.
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Waste heat from the cooling zone of a kiln is most usefully directed to
Answer: D
Correct answer: the dryer handling green ware
Ware leaving the firing zone must be cooled, and the air used to cool it comes off hot and clean - exactly what a dryer needs, and at the modest temperature drying requires. Ducting the cooling zone to the dryer is the largest single heat recovery available in a ceramic works, which is why kiln and dryer are normally designed and sited together.
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Consider the following statements about firing.
1. Pyrometric cones measure heat work rather than temperature alone.
2. Dunting is caused by too rapid cooling through a phase inversion.
3. The firing curve is a single straight ramp to the peak.
Answer: A
The first two are correct. The third ignores the holds and rate changes that every practical schedule contains, each matched to something happening within the body. Each hold and each rate change has a reason behind it.
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State whether the following is true or false.
The cooling part of a firing schedule needs as much attention as the heating part.
Answer: B
Correct answer: True
More ceramic ware is lost in cooling than in heating, because the cooling body is rigid and brittle and cannot relieve stress by deforming. The quartz inversion at 573 degrees and any large thermal gradient across a thick section both do their damage on the way down. A schedule that is carefully graded upward and then simply allows the kiln to cool at its own rate is only half a schedule.
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Electric kilns are preferred to gas kilns where
Answer: C
Resistance heating gives close control and a clean oxidising atmosphere, which suits technical ceramics and many glazes. Producing a genuine reducing atmosphere in an electric kiln is awkward and shortens the element life.
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Assertion (A): Reduction firing is more easily achieved in a fuel-fired kiln than in an electric one.
Reason (R): Restricting the air to a burner produces a reducing atmosphere directly.
Answer: D
Both are true and the reason gives the mechanism: combustion provides the reducing species as a by-product of running rich, whereas an electric kiln must have a gas introduced deliberately and its elements suffer for it.
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