Drying of Ceramic Bodies Questions and Answers

Answer: C

Wet air near the surface reduces the evaporation rate, so the interior has time to keep up. It is the standard remedy where a shape is prone to cracking and cannot be redesigned. Humidity is reduced progressively as the piece becomes safer.

Answer: D

Correct answer: False

Fast drying dries the surface long before the interior, so the outside tries to shrink around a core that has not yet contracted. The resulting tensile stress at the surface cracks or warps the piece. The schedule must be slow enough through the shrinkage stage for moisture to migrate outward and keep the gradient shallow - and only then may it be accelerated.

Answer: A

The rate in the falling period is governed by transport within the body, so doubling the thickness more than doubles the time. Designers avoid abrupt changes in section for exactly this reason. Drying time rises faster than in proportion to thickness.

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 mechanism. A thin section reaches the critical moisture content and finishes shrinking while the adjoining thick section is still contracting, so the two pull against each other and the piece cracks at the junction. Designing with gradual transitions, coring out thick regions and keeping wall thickness even are all responses to that single fact.

Answer: C

Uniform wall thickness and generous radii remove the gradients that a schedule can only manage. Designing for drying is far cheaper than compensating for a difficult shape in every cycle. A better shape saves time on every piece thereafter.

Answer: D

Correct answer: shrinkage water

Shrinkage water, sometimes called interparticle water, is the film separating the particles, and its removal lets them approach one another - hence the contraction. Pore water occupies the voids that remain once the particles are touching and leaves without shrinkage, and chemically combined water is bound in the clay lattice and comes off only on firing.

Answer: A

Correct answer: during firing at several hundred degrees

The chemically combined water is hydroxyl bound within the clay mineral structure, and it leaves as dehydroxylation between roughly 400 and 600 degrees - kaolinite becoming metakaolin at around 550. Drying at a hundred degrees removes only physical water and leaves the lattice intact, which is exactly why a dried piece can still be reclaimed and a fired one cannot.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason explains the difference. Drying is a physical change: the clay mineral is unaltered, so adding water restores the plasticity and the piece can be slaked back into slip. Firing destroys the crystal structure by driving off its hydroxyl groups, and that change cannot be undone by any amount of water - which is why greenware is recycled routinely and fired ware never is.

Answer: C

Twenty parts of water to a hundred of dry solids is 20 in 120, which is about 16.7 per cent of the total. Confusing the two bases is among the commonest errors in drying calculations. Stating the basis explicitly removes the ambiguity entirely.

Answer: D

The dry basis divides by the solids alone and the wet basis by the total, so the dry basis figure is always the larger. Every drying calculation should state which basis is being used. The dry basis figure exceeds the wet basis figure for the same body.