Great article by David Mitchell of Cloud Atlas fame. He writes about the problems of  writing historical fiction and reconstructing dialogue that is archaic, yet sincere. He calls it “Bygonese.”

And then you have to worry about language. Unless you have an entire historical novel made out of reported speech (easier to digest bubble pack) the characters must open their mouths at some point, and when they do, how are they going to speak? This is the “lest” versus “in case” dilemma: the sentence-joint “in case” (as in “eat now in case we don’t have time later”) smells of modern English, but a “correct” translation into Smollett’s English (“Eat on the nonce, My Boy, lest no later opportunity presents itself”) smacks of phoniness and pastiche if written in 2010. It smacks, in fact, of Blackadder, and only a masochist could stomach 500 pages. To a degree, the historical novelist must create a sort of dialect – I call it “Bygonese” – which is inaccurate but plausible. Like a coat of antique-effect varnish on a pine new dresser, it is both synthetic and the least-worst solution.