Thanks Benjamin,
So to simplify, a cast is made only against a mold. Right? A mold is only made by direct contact with the original fossil. Correct? Therefor my steinkern is an internal mold since it formed against the original shell… right?
I am a little confused by your use of “positive” and “negative” in reference to impressions. Does “positive” and “negative” refer to concave and convex? If “negative = cast” then the definition of a “mold” would be a: mold impression of the original formed by contact with the original. That is a confusing definition.
If “negative” = mold = convex, then some molds break that rule. A steinkern which can be an interior mold is convex in most bivalves. Exterior molds are usually concave but are partly convex when the original bivalve has a concave valve (some pectens and brachiopods.)
Thanks for you help.