Clay, iron or boxwood - all were used and none were necessary once the US Military thinned the skirt a bit. As far as I know, the plug were inserted after the bullets were cast, but maybe not in this case.

The problem with any of the plugs, is they were being blown through the bullet leaving a ring of lead in the bore, once the bore was fouled a bit.

A mere 60gr. musket powder charge (weak powder) was enough to expand the skirts into the grooves.