Susan,
I don't really see the point of using acid-free foamcore on a drymount because most of the time a drymount is used for what I call "disposable" framing, i.e., posters, prints, etc. When someone wants a poster of Britney Spears dry-mounted, I fail to see the point of archivally framing it unless the customer requests it; they're going to be tired of Britney long before the poster becomes chemically damaged. It's cheaper for them to drymount it on regular foamcore, and just as effective.
And honestly I don't have a whole lot of call for dry-mounting something archivally, anyway; if you want to preserve it, why would you drymount it to begin with? I never recommend a drymount for anything of real value. The only difference between archival and non-archival drymounting is that one is reversible; if you eventually want to reverse it, why would you bother at all?
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I don't care what color your sofa is.