Who were the skilled iron workers, and from what countries were the slaves who were brought to this country to work in the furnaces?

The first governor of Maryland, Thomas Johnson, and his brothers Baker and Roger brought slaves to smelt and fashion iron at their
Catoctin Iron Furnace in Frederick Co. Md. in the 1770's.

It is believed that my 4th great grand father was one of these slaves.

Thank you,
Janice D. Laster