what kind of materials keep the polarisation of light but can be used
as a silver screen? can any kind of varnish or fabric or film be used?

Dear xxzzzzzzzzzzzzzzz,

The following links give advice as to the properties of the screen.
The second one seems most useful as the author describes a successful
homemade screen. I would suggest that fabric would not work as well as
the very smooth surfaces that could be obtained via spraying a
metallic, reflective paint upon a very smooth surface.

The basic principle is that you want all the light which is directed
at the screen to be reflected (to your eyes) perfectly evenly. Any
"bumps" in the surface of the screen would cause some of the light to
be reflected at slightly different angles - hence creating blurring of
the intended image.

[In itself, varnish is not as reflective as say metallic paint and you
would want to avoid any interference from the "front" surface
reflection of the varnish with the underlying "back" surface of the
material.]

Depending on what film you were considering - it may be appropriate,
but film implies low rigidity...

D'Antiques.com
http://www.dantiques.com/23904/
"Because stereo projection makes use of polarized light which will
reflect only from a metallic surface, you must use a special screen
having an aluminum coated surface which can be stretched perfectly
flat and free of wrinkles."

Stephen Spicer at Calcite
http://www.calcite.rocky.edu/cgi-bin/mfs/usr/digests/stereo?link=/photo-3d/digests/p3d/d0000-0499/p3d0046.txt&file=/usr/digests/stereo/photo-3d/digests/p3d/d0000-0499/p3d0046.txt&line=117#mfs

Your project sounds very interesting. Regards,

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Clarification of Answer by ozguru-ga on 11 Jul 2002 05:25 PDT

I meant to add the search terms used:
characteristics "silver screen"
requirements "silver screen" 
with 2D or 3D or polarizer