[M.D. Holloway]

External temp has everything to do with it! The sink rate is meaningless until the keys are a few cms from the ground, until them the ice will provide adequate support.

As for external temps - your friend should live in a Florida orange grove in the winter. They will spray the fruit with water if there is a frost coming and projected temps get below freezing. Why? becuase ice doensn't get colder than 0C and the fruit will tolerate 0C but not lower - the thinn film of ice will actually insulate the fruit from getting any colder.

As kids, we could make igloos and it would actually get rather warm inside them when the outside temps were -10C or -15C.

Those keys are going to sink in porporation to the shinking of the ice which is absolutley dependent on the ambiant temp.