A polyester product with embedded glass. Note the left edge, which is a broken edge of the putty. You can see glass fibers sticking out.
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Zoomed in area of previous picture. One strand of glass is visible.
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Woven Roving
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Threaded fiberglass. Has a checkerboard appearance. The dirt specks here may be from the glue used, or other dust. Sample should've been sprayed with air before putting under microscope.