marble

Tiny impurities in the starting glass can cause incredibly fine fiberglass filaments to break. One solution, used since 1938, is to melt the glass from marbles that can be inspected for purity. Each marble produces up to 1,529 miles (2,462 kilometers) of filament. The finest filaments made in this way were used for the Apollo program's space suits. Today, buckets of marbles have been largely replaced by sophisticated tanks that produce high-purity glass.