Drying of Ceramic Bodies Questions and Answers

Answer: A

Correct answer: from a continuous film at the surface

In the constant rate period the surface is kept wet by water drawn from within faster than it can evaporate, so the piece behaves like a free water surface and the rate depends on the air conditions rather than on the ceramic. The particles are drawn closer together as that water leaves, which is why this is also the shrinkage period and the dangerous one.

Answer: B

Correct answer: constant rate period

Shrinkage happens while the water being removed is the water separating the particles: as it goes, the particles pull together and the body contracts. That is the constant rate period. Once the particles are in contact the shrinkage stops even though a good deal of water remains, and the falling rate period that follows empties pores without further contraction.

Answer: C

Both are true and the reason gives the reason for the emphasis: differential shrinkage between a dry surface and a wet interior generates the stresses, and once shrinkage is complete the remaining drying is comparatively safe.

Answer: D

Correct answer: the critical moisture content

The critical moisture content is the point at which the surface can no longer be kept saturated by migration from within, so the evaporation front retreats into the body and the rate begins to fall. Practically it is the point at which shrinkage ends, and it is why a drying schedule is cautious up to it and can be aggressive after.

Answer: A

Correct answer: False

The rate is constant only for the first stage. Once the critical moisture content is passed the evaporating surface withdraws into the body and the vapour must diffuse out through an increasing thickness of dry material, so the rate falls continuously towards zero as the piece approaches equilibrium with the surrounding air.

Answer: B

Correct answer: shrinkage against water content during drying

Bourry’s diagram plots the shrinkage against the water removed and shows the two regimes as two straight portions with a knee between them. The first portion has shrinkage tracking water loss almost one for one; the second is nearly flat. The knee identifies the critical moisture content, so the diagram tells the operator exactly where the drying schedule may be accelerated.

Answer: C

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

Both are true and the reason gives the justification. Cracking in drying comes from differential shrinkage, so once shrinkage has ceased there is nothing to set up the stress and the remaining water can be driven off as fast as heat can be supplied. A good schedule is therefore slow and humid at first and hot and dry afterwards - and running it at one uniform rate wastes either time or product.

Answer: D

The slow start controls shrinkage while the body is still contracting, and the faster finish removes pore water when there is no longer any shrinkage stress to fear. Reversing the order would be the worst possible arrangement.

Answer: A

A surface that dries and shrinks while the interior is still wet is put into tension by the restraint. Slower drying, higher humidity and better shape design all reduce the gradient. Humidity control is the principal tool in the early stage.

Answer: B

Correct answer: about 224 grams

The dry solids are what stay constant. One kilogram at 24 per cent water holds 240 g of water and 760 g of solids; at 2 per cent those same solids are 98 per cent of the piece, so the piece weighs \( 760/0.98 = 776 \) g and holds 16 g of water. The water removed is \[ 240 - 16 = 224\ \mathrm{g}. \] Subtracting the percentages directly gives 220 g, and forgetting the residual moisture gives the 240 g offered.