You are at the mercy of your gold supplier. Preparations of gold emulsions used in ceramic decorating are prepared in secret. Also, the emulsion suppliers often adjust emulsions to resolve problems not caused by regularly used emulsion formulations.
On firing, the gold must sink to the proper extent. It is called soft if it easily abrades off with a pencil eraser. Hard gold can be caused by soft glaze. The decorating supervisor tells the lab that the glaze is soft, lowers his decorating kiln temperature, and hopes the next lot of glazed ware will not cause him more problems.
Soft gold from hard glaze causes him to raise his decorating kiln temperature. This may cause blistering of the gold or other problems. The gold emulsion supplier can make the emulsion more suitable for higher firing.
So the problem is glaze control.
Most fine china glazes are lead based. If the calculated lead content is too low, it must be increased.
The best control for the glaze itself is the FIRED specific gravity. Fire a cup of glaze, break out a chunk, and measure the specific gravity. To maintain better control, blend frit batches before glaze formulation to even out the composition. Blend glaze batches to get the right glaze hardness.
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