See also "Duroplast", made from cotton and phenol resin (carbonic acid), used in the production of the (in)famous Trabant in the GDR.

[1] https://en.wikipedia.org/wiki/Duroplast

There's an interesting passage in this Wikipedia article, regarding its limitations in industrial use:

> Because it can be made in a press similar to shaping steel, it is more suitable for volume car production than fiberglass. However, in comparison to shaping steel, duroplast needs much more time for hardening in the press. This was one of the reasons why it was not possible to scale-up production of Trabant to the volume that was demanded.

(I guess, the same may be true for other organic filler materials, e.g., hemp.)

Edit: While the German edition of Wikipedia doesn't mention the Trabant in the context of duroplast at all, the entry on the Trabant cites a curing time of about 20 hours.