I am an English goldsmith and have been in the trade for 44 years.
When I was first starting in the trade fire gilding was legal, but it
is not now illegal for health reasons in this country. "Partial Fire
Gilding"as it is called over here was a process of dissolving fine
gold in mercury, making a paste which was painted on the surface of
sections of a silver article. Heating the whole painted article over
a heat source evaporated the mercury leaving a coating of fine gold
on the sections painted, these sections were then burnished bright,
this is where the term Partial Gilding comes from. (parts are gilt) A
modern version is available that gives a similar effect via electro
plating / gilding. This method requires the use of a barrier chemical
painted onto the surfaces that you do not want gilt, then the whole
item is electro gilt, leaving the painted areas silver, and when the
barrier solution is cleaned off the final appearance is partial
I hope this makes sense to you.
Regards James Miller