18th September 2005, 08:02 PM
Not entirely sure, but as you've got no other takers I'll have a go. Surely a clamp produces a reducing, oxygen low environment where carbon (from organic inclusions) is not burnt out leaving vessels with a grey or black core. Oxidized vessels conversely (which might be expected in an oxygen rich bonfire) are more uniform in colour and sometimes reddish or brown when produced from iron rich clays.
PS Apologies to proper specialists out there, I'm a field type not a pot man.
PS Apologies to proper specialists out there, I'm a field type not a pot man.