John,

Sure, you could weigh the whole thing before and after glassing. But, there are industry standards that are more accurate, especially on small test specimens cut from a larger test sample. My measurements were done in accordance with ASTM standard D 2584 – 94 “Standard Test Method for Ignition Loss of Cured Reinforced Resins”. The standard is used throughout the composite industry, and the composite research community, to determine the fiber content of glass composites. If one is interested in some of the organic fibers, there is ASTM standard D 3171 – 76, which covers matrix digestion, usually with strong acids (so you don’t burn off the fibers too!).

I would encourage everyone to “try this at home”, but it requires an analytical balance accurate to 1.0 mg, and a furnace controlled at 1050 + or –50 degrees F.

Sam