This is the Finishing Flux that Gillie Hoyte Byrom recommends.

This Soyer finishing flux, 619, will not be manufactured again until late 2023. Soyer highly recommended 518 as a substitute as it is very clear.

Go to the 518: https://www.enamelartsupply.com/soyer-enamel/ng52xlf52uouxbnc4fcac781082x88-95z7c

Merry-Lee found some interesting idiosyncrasies while testing these two fluxes. They are both silver sensitive! On the test tiles the top row is the flux over opaque black to see how clear it is, below that is direct on silver (Surprise surprise!) and below that is over G110 flux over silver. Both Soyer Finishing fluxes are very clear over the opaque black #36. For Merry-Lee. this is the acid test.