I added zinc sulfide to a dry mixture porcelain enamel frits to solve some problems related to weldings seam. However, after firing the samples I've got a surface with lot of small dimples (foam aspect).
I'm looking for an inorganic additif who can help to get a smooth surface without defects.
Zinc sulfide is oxidized during sintering in the air with formation of gaseous SO2, which get foam. During sintering in protective gas no SO2 is formed - the problem solved.
Thanks Mr Verlotski, It's very interesting idea. However, practically we couldn't suggest this solution to costumers.
What about a reduction fire? It could prevent oxidation... A flux that allows the bubbles output before the end of the fire can also help.
You have to manage the redox of your glass to avoid, as mention before, the gas formation. The reducing fire mentioned can be a solution. Adding element to reduce the lass also.
The following reactions may seemingly occur as the ZnS containing frit melts in oxidizing atmosphere: (i) 2 ZnS + 3 O2 → 2 ZnO + 2 SO2, (ii) 2 SO2 + O2 ↔ 2 SO3; (iii) ZnO + SiO2 → ZnSiO3; (iv) ZnO + SO3 → ZnSO4; (v) ZnSO4 + 3 ≡Si―O―Si≡ + 2 H2O → SO3 + 4 ≡Si―OH + ≡Si―O―Zn―O―Si≡. Phase separation of a melted ZnSO4 rich phase seems also a possibility, for high enough ZnS content.
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