Grog and calcined alumina used as fillers share the property that they
Answer: A
Having been through a firing they contribute no further shrinkage, which is why they are used to control dimensions in large or thick pieces. Their inertness during firing is exactly what makes them fillers rather than fluxes.
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Assertion (A): Refractory bodies contain a high proportion of pre-fired filler.
Reason (R): Material that has already been fired contributes no further shrinkage of its own.
Answer: B
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason gives the purpose. Grog, chamotte and calcined alumina have already undergone their shrinkage, so they occupy their final volume and only the bonding clay around them shrinks. That keeps the total shrinkage low and even, which is what allows large refractory shapes to be made without cracking, and it opens the texture so that drying and firing gases can escape.
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A body is 40 per cent grog by mass. If grog shrinks not at all and the remainder shrinks 12 per cent linearly, what is the approximate overall linear shrinkage?
Answer: C
Roughly sixty per cent of the body contributes a twelve per cent shrinkage, giving about seven per cent overall. The estimate is approximate because shrinkage is not a simple mass-weighted average, but it shows the effect clearly.
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State whether the following is true or false.
Adding a non-shrinking filler reduces the overall firing shrinkage of a body.
Answer: D
The filler contributes nothing to the contraction, so the average falls in proportion to how much of the body it makes up. This is the principal reason grog is used in heavy clay and refractory work. Section thickness largely dictates how much filler is required.
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Vitrification of a ceramic body means
Answer: A
Enough glass forms to draw the remaining particles together and seal the pores, without the piece losing its shape. Distinguishing vitrification from melting is essential, since the two are only a few tens of degrees apart.
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Assertion (A): A body that is fully vitrified is close to the temperature at which it will deform.
Reason (R): The glassy phase that closes the porosity also softens the body as its quantity increases.
Answer: B
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason is the same fact seen from two sides. Porosity closes because the liquid phase fills it, and as the temperature rises further that liquid increases in quantity and falls in viscosity until the piece can no longer support itself. Full vitrification is therefore near the deformation point by construction, and the firing range is the gap between them.
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State whether the following is true or false.
Vitrification and melting are the same process at different temperatures.
Answer: C
Vitrification forms enough liquid to close porosity while the body holds its shape, whereas melting means the piece flows. The interval between them is what a firing range actually measures. One is wanted and the other is a failure of the firing.
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A body intended for fast firing should have
Answer: D
Correct answer: few gas-evolving constituents and a wide firing range
A fast cycle gives little time for gases to escape or for the temperature to equalise, so carbonates, sulphates and organic matter must be minimised and the body must tolerate the temperature spread across the kiln. A wide firing range is what supplies that tolerance, which argues for alkaline earth fluxes rather than a heavy alkali content.
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Consider the following statements about fillers.
1. Quartz remains largely unmelted at whiteware temperatures.
2. Grog contributes no further shrinkage on firing.
3. Fillers act by lowering the temperature at which melt forms.
Answer: A
The first two are correct. The third describes a flux rather than a filler, and confusing the two functions is the commonest error in reading a body recipe. Function rather than name is what identifies a constituent's role.
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State whether the following is true or false.
A single material can act as a flux in one body and a filler in another.
Answer: B
Correct answer: True
The role depends on the temperature reached and on what surrounds it, not on the material itself. Quartz is a filler in an earthenware body and takes part in the melt in a porcelain fired higher. Alumina fluxes nothing in a refractory and stiffens the melt in a glaze. The classification describes what a material does in a particular body at a particular temperature.
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