Black lumps in old internal house walls

[TripleC]

Hi,

Noticed a small protrusion on a bedroom wall that was painted last summer. I assumed it was maybe where I had filled in an old screw hole with filler and the filler hadn't quite set and and had bulged out. Sanded it back and removed the cracked plaster which revealed this large black lump in the wall, which looks as if it has moved and caused the protrusion in the wall. I sanded back another small bulge in the same wall and found the same thing. It's an original internal wall, not plasterboard.

It's a terraced house, built around 1900 so very old and I'm used to hairline cracks appearing where there is movement. I'm sure it's nothing to worry about it's more that I'm curious as to what they are. They're hard to the touch, don't easily break away but leave a black smudge when wiped. Very old screws/nails?

Thanks for your thoughts!

 

[footprints]

I would go with old nails, try a magnet on them. If so treat with rust killer and a touch of oil based primer. I keep a car touch up stick of primer handy for jobs like this.

 

[TripleC]

I think you're right, the dust smells quite metallic. I've got some zinsser primer so I'll spot over the area before repainting. Thanks for your help.