It's "Volara", and it's a type of Polyethylene foam- archivally stable.
It's also very resilient and smooth as a baby's bottom.
McMaster-Carr carries it in a variety of thicknesses and with the option
of pressure-sensitive adhesive on the back. I don't know about the
archival stability of the adhesive, but it sticks to just about anything

On Wed, 6 Dec 1995, Suzanne Quigley wrote:

> Diane,
>
> We use Volare (sp?) a soft, dense inert foam at teh Guggenheim.  Detroit
> Institute of Arts uses it too.  Great stuff, but pricey.  Sorry I don't
> have a source, but someone on the list must!
>
> Suzanne Quigley
>
> >     Our registrar is searching for information on storage shelf
> >linings.  Information from personal experience or suggestions for
> >resource material regarding properties of various foams would be
> >welcome.  She is specifically interested in nalgene and its archival
> >qualities.
>