Hmm I've been thinking- if the 'grease' did fill a die, how come it always seems to fill the die perfectly flat so an area has no detail?
there is no time during a strike for much to happen, and surely some grease will be left on the coin, so it leaves the scene very quickly.
If this is the case, considering the numbers minted, a grease spot would change in volume, and the effect on the coins would be very varied,
and the coins having a flat blank area would be almost unheard of.....
And why is it only the sea that gets 'greasy'?????