About: http://webisa.webdatacommons.org/375746734

Property	Value
http://www.w3.org/1999/02/22-rdf-syntax-ns#type	http://www.w3.org/ns/prov#Entity
http://www.w3.org/ns/prov#value	This can be due to oils, compounds, sealers or other materials reacting with the metal and forming a permanent stain or bond to the metal surface.
http://www.w3.org/ns/prov#wasQuotedFrom	pfonline.com