Fluxes and Fillers Questions and Answers

Answer: C

Correct answer: False

Both supply lime, but by quite different routes. Limestone must first decompose, absorbing heat and releasing carbon dioxide, and the resulting lime then reacts. Wollastonite is calcium silicate already, so it contributes its lime directly with no decomposition, no gas and no associated shrinkage step - which is exactly why it is preferred in fast firing.

Answer: D

It is refractory at whiteware temperatures and holds the shape while the glass forms around it. Some of it dissolves into the melt at the edges, but the bulk survives as discrete grains. Those surviving grains are why quartz inversion matters on cooling.

Answer: A

Only the outer edges of the grains dissolve, and the bulk remains as discrete quartz. Its survival is what makes the quartz inversion a concern during cooling of the finished ware. A body free of quartz would need no pause near 573 degrees.

Answer: B

Less flux means less melt at a given temperature, so more heat is needed to reach the same degree of vitrification. The trade is against the risk of deformation, which more flux always increases. Refractoriness and ease of firing pull against each other.

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 conflict. Fluxes work by forming a liquid phase at temperature, and it is that liquid which fills the pores and vitrifies the body - and equally that liquid which lets it sag. Lowering the firing temperature therefore lowers the deformation temperature with it, and the two goals cannot be pursued independently.

Answer: D

Correct answer: fall, since there is more flux

Feldspar rises from 25 to 30 per cent and quartz falls from 25 to 20, so the change substitutes flux for filler. More liquid forms at a given temperature, so vitrification is reached lower down. The firing range will narrow at the same time, which is the price of the change and the reason the substitution is not made without limit.

Answer: A

Calcium phosphate forms a glassy phase and a crystalline phase that together give the characteristic whiteness and translucency. Its proportion is high, typically around half the body, so it is a major constituent rather than an addition.

Answer: B

Correct answer: act at higher temperatures and give a stiffer melt

The alkalis, sodium and potassium, form a melt at comparatively low temperature but one that is fluid and continues to thin as the temperature rises - hence a narrow firing range. Lime and magnesia begin later but produce a more viscous melt whose viscosity changes less steeply, which widens the firing range and improves resistance to deformation.

Answer: C

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: alkalis act at lower temperatures than the alkaline earths, and nepheline syenite is the stronger flux of the two feldspathic materials. Statement 3 is the reverse of the truth - more flux means more liquid forming faster with temperature, so the window between vitrification and deformation narrows rather than widens.

Answer: D

More flux brings vitrification and deformation closer together, so the usable window narrows. This is why highly fluxed bodies demand tighter kiln control than more refractory ones. Fast-fired tile bodies are among the most demanding in this respect.