They are oak apples (aka oak galls) KK - the home of the larvae of gall wasps.
Used in the past as a source of tannin for home made ink.
And in answer to Eric, no, I never have, if I could figure a way to make suphuric acid at home I would - but having to buy ingredients goes against the grain for me these days. If anyone knows a recipe where I can make all the constituents for the ink, I'd be very grateful
Red