Entry Word: counterfeit
Function: noun
Text: Synonyms IMPOSTURE, deceit, deception, fake, fraud, gyp, hoax, humbug, phony, sell
Related Word copy, facsimile, reproduction; dummy, simulacrum
Entry Word: counterfeit
Function: adjective
Text: being an imitation intended to mislead or deceive <counterfeit money> <counterfeit sympathy>
Synonyms bogus, brummagem, fake, false, phony, pinchbeck, pseudo, sham, snide, spurious; SPURIOUS 3
Related Word feigned, pretended, simulated; deceptive, delusive, delusory, misleading; fraudulent
Contrasted Words authentic, veritable; actual, real, true; unquestionable, valid
Antonyms bona fide, genuine
Entry Word: counterfeit
Function: verb
Text: Synonyms ASSUME 4, act, affect, bluff, fake, feign, pretend, put on, sham, simulate
Related Word ape, copy, imitate, mimic
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Excerpted from the online Merriam Webster Dictionary site.
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Brummagem is the most esoteric of the lot.
It refers to a bastard form of pronounciation of the English city “Birmingham”. It specifically refers to sham coins or plated metals. (The Concise Oxford Dictionary)
If you make your art expert a Brit then the term will have more validity in its use. Pinchbeck was the only other word that was as good. It too, is of English origin and used to be a reference to a watchmaker. If your expert is also versed in metal jewelery it will help substantiate the use of such obscure terms.
Pinchbeck = Goldlike alloy of copper and zinc used in cheap jewelery.