I used the opposite word in my recent post, sorry.

Plastic should have been elastic.



When a material is strained it passes through rheological domains.
First is elastic.  If the strain is released the material returns to
normal: elastic behavior, e.g., rubber.  If a museum material is
restrained by tacks, staples or other mechanical attachment, and
moisture content decreases because % RH decreases, a stain is put on the
restrained material.

Plastic behavior follows elastic behavior in the straining process.
After the yield point, when the restrained material is strained, it is
deformed. The deformation is permanent.  The deformation could be a
crack, an increase in an existing crack, a failure or just an elongation
of the material, but it is permanent.

As a rule of thumb: small stains are usually in the elastic regime.
That is, unless degradation has rendered the material so brittle that it
no longer has an elastic component.

Generally, large strains pass into the plastic (deformation) domain.  An
exception is the straining of rubber in a rubber band, which is
generally all in the elastic domain until failure.

Tim Vitale
Preservation Associates
510-594-8277
440-775-7331 until 9/26/99

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