Tom Buck on sat 10 aug 02
Ivor:
Perhaps this quote from a chemical dictionary will help explain
why sometimes calcining ZnO is helpful, other times may be not:
Quote:
zinc oxide (Chinese white, zinc white) ZnO
Properties: Coarse white or grayish powder; odourless, bitter taste;
absorbs carbon dioxide from the air; density 5.5 g/mL; m.p. 1975 oC;
soluble in acids and alkalies; insoluble in water and alcohol.
Non-combustible, non-toxic as powder.
Derivation: a) oxidation of vapourized pure zinc [metal] (French
process);
b) roasting of of zinc oxide ore (franklinite) with coal and subsequent
oxidation with air;
c) similar treatment starting with other ores;
d) oxidation of vapour-fractionated die castings.
Grades: American process, lead-free; French process, lead-free, green
seal, red seal, white seal (according to fineness); leaded (with lead
sulfate), USP, single crystals.
--end Quote
----------
So,you can have different problems with ZnO depending which process it
comes from, and what particle size you get. some ZnO is quite fluffy
(relatively) and this alone means you must adopt care in dispering it in
the glaze liquid/slurry (aka manure in Ireland, says Paul Taylor).
If the ZnO fluffy stuff sits around it will change its weight by becoming
Zinc Carbonate (to some unknown amount, that is) and this then needs
calcining to bring it back to pure ZnO, an important step for potters
making high ZnO crystalline glazed pots.
til later. Peace. Tom B.
Tom Buck ) -- primary address.
"alias" or secondary address.
tel: 905-389-2339 (westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street, Hamilton ON L8T 3E1 Canada
On Fri, 9 Aug 2002, iandol wrote:
> Dear Geoff Walker,
>
> I understand the reason for calcining Kaolin or other clays is to reduce the shrinkage potential due to plasticity or water adsorption and prevent shrinkage problems with high clay glazes.
>
> I understand that many raw materials, especially natural minerals are calcined to cause them to disintegrate.
>
> I understand that bones are calcined to eliminate the organic fractions, that some salts are calcined to remove water of crystallisation.
>
> But I fail to understand why a refractory oxide such as Zinc Oxide must be calcined. Zinc Carbonate yes, to discharge the carbon dioxide. So why calcine Zinc Oxide?
>
> Best regards,
>
> Ivor Lewis
>
> ______________________________________________________________________________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
>
| |
|
