iandol on sun 11 aug 02
Dear Stephani Stephenson,
Always a big problem when people put information in books without =
validating a procedure with a suitable reference or formal explanation.
First of all, heating an oxide in an oxygen rich atmosphere usually has =
no effect except that in some cases it increases the oxygen content of a =
molecule, thus changing one compound into another one, such as when we =
heat red copper oxide and it changes to black copper oxide. In the case =
of Zinc oxide, since we only seem to have one type, it remains the same, =
plain ZnO. Interestingly, Zinc oxide is one of those "Amphoteric" oxides =
and can behave as acid or base. With Cobalt oxide it behaves as an acid =
and forms Cobalt Zincate, a green compound, when the two are heated =
together strongly.
The only reasoning I can put to this problem comes from the way Zinc =
Oxide can be made. If Zinc is dissolved in acid and the solution =
reacted with Sodium Carbonate, Basic Zinc Carbonate is precipitated. =
This is a variable mixture of Zinc Carbonate and Zinc Hydroxide. When =
this mixture is heated strongly it looses both water and carbon dioxide =
and the residue should be pure Zinc Oxide. So if the stuff which was =
once sold to potters was contaminated with this basic carbonate due to =
poor quality control during manufacture and not one hundred percent ZnO, =
then to get accurate proportions it would have been advisable to calcine =
before weighing out a glaze recipe to ensure getting the right weight =
of Zinc Oxide.=20
The only other thought is that if heated to a high enough temperature, =
say above 1200=B0C, there may be some sintering, so that the particles =
coarsened. This might assist with adhesion.
But please treat these thoughts as speculation.
Good to hear from you again,
Best regards,
Ivor
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