Framers do all sorts of things.  But many framers have only a vague
idea of what's really necessary to meet archival requirements, and
even less interest in doing any extra work to carry it out.

People have been mounting directly to foam core for years, and "acid
free" foam core has existed for at least a decade and a half.  But it
is considerably harder to know that a sheet of foam with a paper surface
stuck to both sides is really archival than it is to know that a piece of
pure rag board (buffered for monochrome work, unbuffered for color) from
a reputable paper company is.  How do you really know if you've even got
"acid free" foam core or not?

And in my experience framers are averse to putting foam core into a
mounting press and so often lie and claim they dry mounted when in
fact they used spray adhesive.

I sometimes mount color work to "acid free" foam core.  I seem to recall
that there's a foam board in the Artcare product line, and I'd have few
qualms about using that.  However, I mount black and white work to 100%
rag board or, if I must use a cheaper material, to Light Impressions'
archival corrugated board.  Acid-free foam core is an acceptable material
to back a sheet of relatively thin board with the photograph mounted to
it, but I don't mount black and white photographs to it directly.

  By undermat do mean backer-board?  So a foam
backer-board? If so what has foam over some other
material upon which to mount directly? Dan