Your information about the "Bideford Paint Mines" was certainly correct.
A most interesting booklet by the Bideford & District Community
Archives, published in 1994, gives a comprehensive description of the
product and the industry that centred on East-the Water at Bideford. As
you say it was a carbon related substance similar to anthracite and was
used in the manufacture of paint, paper, cement, rubber reinforcing and
Graham J. H. Parnell