Iron Microspheres and Lead in Slag Wool? (Mineral Wool)

Mick West

Staff member

After reading how most of the insulation in the world trade center was "Slag Wool" (a type of mineral wool insulation made from iron refining) I wondered if it might have some iron microspheres in it. So a little searching led me to the above image, from:

They are very interesting because they look like iron microspheres. But without checking with a magnet, it's impossible to tell. They might possibly be melted fibers with some carbon making them dark.

Another possibility is lead. Some slag wool was made from lead slag - although it's not clear when that practice stopped. Seems to have still being going on the 1970s though
Metabunk 2018-10-16 12-16-32.jpg

The "Fowler 1980" reference leads to:

Which describes how the process creates spheres ("shot") which is separated from the wool
Metabunk 2018-10-16 12-28-50.jpg

The shot here is slag though, not iron.
Last edited:

Mick West

Staff member
More on lead in slag wool:
Construction of the World Trade Center towers was in 1968 to 1970

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