this afternoon I happened across a very small exhibition of Leonardo da Vinci drawings in Exeter(UK). Being the geek I had to look to see how the drawings were mounted.
There was a double overmat and then a 1/4 inch space and then a rear mat. The drawings were suspended by 4 thin diagonal strips, one in each corner which were attached to the rear of the overmat and the rear of the drawing so that the drawing was not touching either the rear mat or either of the two overmats. I don't know what the hanging strips were made of.
For those of you who want the ultimate in museum archival mounting then this is probably it.
The exhibition was from the collection of HRH QE2