I am sure our slabs are made of nearly inert plastics. Stored indoors, out of light, not in a humid cellar, I would think they would be fine for a very long time.
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Net 54-- the discussion board where people resent discussions. 
My avatar is a sketch by my son who is an art school graduate. Some of his sketches and paintings are at
https://www.jamesspaethartwork.com/
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