Although the English language can be an incredibly difficult language to use correctly, one of the great advantages of this language is that it is extremely flexible, and there are many forms and levels of expression within the language that can be acceptable . Most professions and trades have their own use for English words, and these trade usages very often are incorrect when measured by the meanings accepted in Standard English, which is the form of the English Language regarded as the norm for an educated person.
I think we can accept that within the silversmithing craft, Tim's use of repousse, and repoussage is inarguably accurate.
However, that accuracy of usage does not carry over into Standard English.
Yes, it is true that a living language develops and changes structure and meaning over time. Equally it is true that the various colloquial forms of the English Language vary from the Standard form of the language. However, since we are not all members of the silversmithing craft, and since there are defined and accurate terms available to describe this work under discussion, perhaps it might be advisable to use these Standard English terms in order to avoid confusion.
(Standard English:-
"Standard English is that set of grammatical and lexical forms which is typically used in speech and writing by educated native speakers. It includes the use of colloquial and slang vocabulary, as well as swear words and taboo expressions."---Trudgill)
Within Standard English, the meanings of repousse (repoussage) and chasing are still current, and as I have given them.It not archaic usage, nor is it even slightly dated usage. It is correct usage for any educated person.
http://en.wikipedia.org/wiki/Repouss%C3%A9_and_chasing