A ball of wax and lamp black or similar material used to make rubbings of brasses and inscriptions.
Compound word from 'heel' (the part of the foot, used to apply pressure when making rubbings) plus 'ball' (the spherical shape of the tool).
Heelballs were the essential tool for brass rubbing, a Victorian hobby where people would make paper copies of grave inscriptions and decorative brasses by rubbing over them—it's like the ancestor of modern scanning technology.
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