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Gold Overglaze/ Luster Question


Potpotpotter

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On 11/21/2023 at 3:10 AM, Pyewackette said:

ok.  Now I want to know how this works.  At first I thought it must be melting point related but gold melts at a relatively low 1948F, which is around cone 04.

Enquiring minds want to know ...

From https://community.ceramicartsdaily.org/topic/1557-gold-lustre-and-alkaline-glaze/?do=findComment&comment=10521
1. Lustre is a thin film of metal deposited on the surface of a fired glaze.

  • It is NOT a glaze in itself. It relies on the glaze interaction with the lustre for adherence and this can be anywhere between 720.C-820.C depending on the hardness of the glaze. Earthenware glazes are usually refired between 720-750.C while stoneware and porcelain usually refire between 780-820.C but that is not set in stone. Every glaze is different and you would have to test to find the common denominator for your glazes. Glass, depending on the type, starts to accept lustre from 600.c onwards. You will know if you haven't fired high enough as the lustre will rub off. If fired too high it will start to burn off and be patchy. Commercial porcelain blanks can be tricky to find the optimum temperature as a lot of these follow the procedure of high fired bisque with a low fired glaze. This appears to be the same with bone china.
  • It takes on the characteristics of the glaze it is on. If the the glaze is very glossy then the metallic layer is very glossy. If the glaze is matt then the metal layer is matt and this then follows on for all the permutations in between.
  • However I have noticed in my own work that copper glazes seem to be more problematic as my lustres always seem duller. This then brings me to the second point.

PS
These lustres (there are reduction lustres as well) are based on resinates, which are metallic soaps. When they are heated they decompose to metal atoms and various gases such as CO2. Why most of them (e.g. copper) don't decompose to the oxide is complex, and apparently relates to the electron distribution within the resinate ion during its decomposition. Which is why resinates are still used.

Danger of skin contact & fumes until they are fully fired, then you've got a metallic thin film (somewhat amalgamated with the surface of the underlying glaze). Some of the dangers are inherent to the resinate, others relate to the solvents used.

Johanna DeMaine
Lustre Ceramics Monthly
Health & Safety and Overglaze
Background Overglaze: Challenging the Myths

 

Edited by PeterH
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