Hi Adrian,
I see your point. I have scratched hard again with a stiletto like, sharp pointed blade and again only residual contents came out. So probably only the chiseling opearation would result.
It is rather hard to measure or estimate the shell wall thickness, as part of the whatever contents is stuck to the walls right up to most of the hole aperture. Although easy to discern that, the small part that is not continued by the shell contents, measures some 5 m/m, so much thinner than the British ratio you mention, this is no reliable evidence, as the hole entrance looks completely uncharacterized by age elements. But again judging with no scientific basis, the (heavy) weight of this shell seems to denounce a thick wall.
My stone balls specs;
The limestone one measures 19 cms and weighs 7,8 Ks (17 pfund ). Apparently from Germany, as announced by the German auctionner.
The granite one mesaures 18 cms and weighs 8 Kgs. (18 arratels). Apparently Portuguese, as acquired on the Lisbon area.
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