Material from 1936 that produces black dust?

[ZipLock]

I drilled some holes in a wall today. It went perfectly to drill without the hammer function on the rotary hammer, so the material was soft. First, there was a layer of plaster, but then came some material that gave jet black dust. The dust looked almost like coal powder and turned completely black if you rubbed it between your fingers.

I thought it was aerated concrete in the walls, but doesn't that give gray dust? Does anyone know what mysterious material they used when building the house in 1936?

 

[Himmapularen]

Kolstybb is my guess. http://www.byggahus.se/forum/isolering/50452-suga-ur-kolstybb.html

/Ola

 

[xibeca]

Most likely slag walls.

 

[ZipLock]

Don't think it's kolstybb, as there's no insulation in the wall. Googled a bit about slaggvägg and it could certainly be that. Seems to be a really crappy material. Why did they use that?

 

[Nyfniken]

Is slag free?