Glazing and Decoration Questions and Answers

Answer: A

It is melted onto the surface during firing and solidifies as a glass, sealing the porosity and giving the finish. Because it is a glass rather than a coating, it must be matched to the body it sits on. Expansion mismatch is the source of most glaze faults.

Answer: B

Correct answer: glass formers, fluxes and stabilisers

A glaze is a glass, so its oxides are classified by the role they play in the network. Glass formers - chiefly silica, with boric oxide - build the network. Fluxes such as the alkalis, lead and boron break it up and bring the melting temperature down to something a kiln can reach. Stabilisers, principally alumina, stiffen the melt so it does not run off the ware and improve the durability of the fired glass.

Answer: C

Without it the glaze would flow off vertical surfaces and pool at the foot. Too much makes the glaze stiff, matt and prone to crawling, so the quantity is a balance rather than a maximum. Alumina and silica are adjusted together rather than independently.

Answer: D

Both are true and the reason gives the mechanism: the melt must be fluid enough to smooth out and viscous enough to stay where it was applied, and alumina is the principal control on that balance. Silica and the fluxes are adjusted alongside it.

Answer: A

Correct answer: render soluble or toxic materials insoluble and safe

A frit is made by melting the raw materials to a glass, quenching and grinding it, so that soluble or toxic constituents are locked into the glass structure. Lead and boron are the usual reasons - raw lead compounds are toxic to handle and borax is soluble in the glaze slop, which would let it migrate into the body and change the recipe.

Answer: B

It is done to render soluble and toxic constituents insoluble and to bring the melting behaviour under control. If anything the frit melts more readily than the raw materials from which it was made. Control rather than temperature is the object of fritting.

Answer: C

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

Both are true and the reason is the cause. Raw lead compounds are soluble and readily absorbed, so they present a real hazard to the people preparing and applying the glaze. Fritting binds the lead into a glass in which it is far less available, and in many jurisdictions raw lead in a glaze is prohibited outright - the frit is the condition on which lead may still be used at all.

Answer: D

Correct answer: left in tension as the piece cools down

Crazing occurs when the glaze contracts more than the body on cooling, so it ends up stretched over a body that will not shrink with it and relieves the stress by cracking. The remedy is to lower the glaze’s thermal expansion or raise the body’s until the glaze finishes in slight compression - which is also stronger, since a glaze in compression resists impact better.

Answer: A

Excessive compression makes the glaze flake away, particularly at edges and rims. It is the opposite fault to crazing and points to an expansion mismatch in the other direction. Rims and edges show the fault first because the stress concentrates there.

Answer: B

Both are true and the reason gives the design principle: an exact match leaves no margin for variation, whereas a little compression is tolerated comfortably and even strengthens the ware. A glazed article is stronger than the same body unglazed.