The author knew that the sectioned slab pictured below could not be iron-furnace slag because of the large metallic-iron inclusions, and jumped to the conclusion that this material was the chemically-reduced igneous crust of a hot-classical Kuiper belt object that impacted Earth as the YD-impact comet on the Laurentide ice sheet, 12,800 years ago, and was delivered to Central Pennsylvania, embedded in a ballistic ice-sheet fragment.  Instead, the author now understands that the pictured material is merely industrial lead-silver slag, where metallic-iron is part of the waste stream of lead-silver smelting.

Sectioned slab of presumed industrial lead-silver slag, containing metallic-iron inclusions