A body recipe is normally written as
Answer: A
Percentages scale to any batch size and make two recipes directly comparable. A recipe written in kilograms is tied to one mixer and has to be recalculated whenever the batch size changes. Percentages travel between plants; kilograms do not.
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A recipe is 45 per cent clay, 25 per cent feldspar, 25 per cent quartz and 5 per cent talc. How much feldspar for a 600 kg batch?
Answer: B
Twenty-five per cent of 600 kilograms is 150. Scaling a percentage recipe to a batch is the most routine calculation in a body shop and is normally done to the nearest kilogram. Weighing accuracy rarely justifies carrying decimals here.
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A batch is weighed out as 270 kg clay, 150 kg feldspar, 150 kg quartz and 30 kg talc. What percentage is clay?
Answer: C
The batch totals 600 kilograms and 270 of those are clay, which is 45 per cent. Converting a weighed batch back to percentages is how a recipe is recovered from a works record. Percentages also let two recipes be compared directly.
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State whether the following is true or false.
A recipe stated in percentages and the same recipe stated in kilograms describe different bodies.
Answer: D
They describe the same proportions expressed in two ways, and either converts into the other. The percentage form is preferred only because it is independent of the batch size chosen. Either form converts into the other without loss.
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Raw materials are weighed on a dry basis, which means the recipe assumes
Answer: A
Since delivered materials do carry moisture, the weighed quantity must be increased to supply the intended dry mass. Ignoring this puts every proportion in the recipe out by a different amount. The correction differs for every material in the recipe.
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A recipe calls for 100 kg of dry clay. The delivered clay contains 8 per cent moisture. How much wet clay must be weighed out?
Answer: B
The wet mass needed is 100 divided by 0.92, which is 108.7 kilograms. Adding eight per cent to 100 gives 108, which is the wrong answer and the one this calculation most often produces. Dividing rather than adding is the rule to remember.
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Assertion (A): Adding the moisture percentage to the dry mass gives the wrong wet mass.
Reason (R): Moisture is quoted as a proportion of the wet material, not of the dry solids.
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 cause. Moisture is quoted as a fraction of the wet mass, so to deliver \( m \) kilograms of dry solids one must weigh \[ \frac{m}{1-w} \] rather than \( m(1+w) \). At ten per cent moisture the correct figure is \( m/0.9 = 1.111m \) while adding gives \( 1.10m \) - a shortfall that puts the whole recipe out of proportion.
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State whether the following is true or false.
The error from adding rather than dividing the moisture correction grows as the moisture content rises.
Answer: D
Correct answer: True
The two expressions are \( m/(1-w) \) and \( m(1+w) \), and their ratio is \( 1/(1-w^2) \), which departs from unity faster as \( w \) grows. At five per cent moisture the error is a quarter of one per cent; at twenty per cent it is over four per cent, which is more than enough to shift a body’s fired properties. The shortcut is tolerable for a dry powder and not for a wet clay.
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A recipe needs 200 kg of dry ball clay and the delivered clay is 20 per cent moisture. What wet mass is required?
Answer: A
Two hundred divided by 0.8 gives 250 kilograms, of which 50 is water. The correction is substantial at this moisture level and cannot be approximated by adding twenty per cent. Adding twenty per cent would give 240 and leave the batch short.
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Loss on ignition must be allowed for when
Answer: B
Correct answer: calculating the fired mass from a raw batch
Loss on ignition is material that leaves during firing - combined water, carbon dioxide, organic matter - so it affects what comes out of the kiln and not what goes on the scales. Recipes are written and weighed on the raw materials as received, so the loss enters only when the fired yield or the fired composition is wanted.
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