It is used in a dry mount press. The tissue turns to a glue like substance
that melts when heated in the press.

The tissue is tacked (glued on a few places with a tacking iron) to the
bottom of the print in the center. The edge of the tissue is tacked on to
the mounting board. Then they are placed in a dry mount press heated to
about 225 F for a few minutes. When removed, the print is attached to the
mount board.

Dry mount tissue is heat-activated adhesive impregnated into a thin porous
substrate.  It is used to mount photographs or other flat things onto some type
of backing -- mount board, wood, etc.  Heat from a dry mount press or iron
melts the adhesive, which re-solidifies when it cools.

Hope this helps.

mjs
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C program run.  C program crash.  C programmer quit.
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See: http://www.lightimpressionsdirect.com/  for more on the
tissue and dry mounting presses.
 Dry mounting tissue is a thermosetting adhesive. It is
placed between the print and the mount and heated with
either a special press or it can be done with a flat iron.
I've posted a set of instructions for dry mounting to this
group in the distant past, a Google search should find it.
 Dry mounting results in a perfectly flat and smooth
mounting which will not peel due to moisture or heat. The
adhesive seals the back of the print which can be an
advantage in protecting it from poor mounting materials.
 Dry mounting is not recommended conservationists and is
not desirable by gallery owners because it is difficult to
remove a print for treatment or if the mount becomes
damaged. It shuold be understood that the mounting itself
does not damage the prints but, as indicated above, actually
helps protect them. So, if your main consern is appearance
of displayed prints, there is no method superior to dry
mounting (IMHO). If you are making prints for sale you will
probably want to use a temporary mounting such as archival
photo-corners or archival hinge tape. Light Impressions
makes all. Even when dry mounting is not contimplated a dry
mounting press does an exceptionally good job of flattening
fiber base prints.
  There is also a cold mounting tissue. This is a pressure
sensitive adhesive used with a special roller press. It has
the advantage of not requiring heat but has pretty much the
same problems with difficult removal as dry mounting tissue
so is also not very acceptable to the collector's trade.