Gillian Welch seems preserved in amber. Her music is so far out of time it's a little startling to stumble across a photo of her even wearing jeans. Since Welch arrived in the mid-'90s, her songs have belonged to another era, full of laborers and orphans, brushing up against ancient spirituals and wry humor; "that's the way the cornbread crumbles" is a key line from her most recent album, last year's The Harrow & the Harvest. One of the most respected and revered singers in the alt-country/Americana world, Welch is actually from a big-city showbiz family — her parents were writers on The Carol Burnett Show — and studied at Boston's Berklee College of Music, where she met longtime partner David Rawlings. His spare stringwork, often finger-picked guitar and banjo, is usually the only sound on Welch's albums apart from her cryptic, contemplative vocals. Each record, from Harrow back to 1996 debut Revival, sounds as smooth and aged as the whiskey Welch occasionally sings about ("Tear My Stillhouse Down" was a Revival highlight). But if Wednesday will be less than a hot time in the old town tonight, it should be an audience with a singer so entrancing she can slow your breathing to a crawl without your even noticing.