Glazing and Decoration Questions and Answers

Answer: C

Crazing arises when the glaze contracts more than the body and shivering when it contracts less, so the mismatch runs opposite ways. Diagnosing which fault is present tells you which way to adjust the expansion. The two faults are diagnostic rather than merely descriptive.

Answer: D

The glaze contracts more than the body on cooling and is therefore left in tension, which is the condition for crazing. Lowering the glaze expansion or raising the body's would bring the pair back into a safe relationship. A difference of one unit is more than enough to craze.

Answer: A

Dust, oil from handling or a dried glaze layer that has cracked before firing all prevent the melt from wetting the body. Careful cleaning of biscuit ware before glazing is the usual prevention. The fault appears where the glaze has pulled away entirely.

Answer: B

Bubbles from decomposing body constituents or from the glaze itself burst at the surface and the melt does not always close behind them. A slower approach to peak temperature and a short soak usually let the surface heal.

Answer: C

Both are true and the reason gives the mechanism: the fault is one of timing rather than of composition, so holding the glaze fluid a little longer allows the surface to smooth itself out. Too long a soak, however, may dissolve an opacifier.

Answer: D

Fine particles suspended in the glass scatter light and hide the body beneath. Zircon has largely replaced tin on grounds of cost, though tin gives a softer white that some producers still prefer. Opacifier content is set against both cost and the whiteness wanted.

Answer: A

It must remain as discrete particles to scatter light, so a compound that dissolves gives a clear glaze instead. Firing too high can dissolve the opacifier and lose the whiteness entirely. The particles must survive the firing to do their work.

Answer: B

Correct answer: transition metal oxides dissolved in the glass

Colour comes from transition metal ions dissolved in the glass: cobalt for blue, copper for green or red, iron for amber and celadon, manganese for purple. Because the ion is in solution the colour depends not only on which metal is present but on its oxidation state and its surroundings - which is why the kiln atmosphere changes copper from green to red.

Answer: C

Both are true and the reason gives the advantage: raw oxides interact with the glaze chemistry and the atmosphere in ways that shift the colour from batch to batch, whereas a stain has already been fired to its final form.

Answer: D

Being covered by the glaze protects the decoration from wear and from attack, which is why underglaze is used for heavily used tableware. The range of colours is narrower because they must survive the full glost temperature.