CIRCLE WITH A DOT

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If you don't have a cat then a reference to your favourite niche interest will do.

@Maker_of_Things

That sounds alarming! I can't imagine seeing a ceiling collapse.

That dust may also have been Victorian insulation, if that ceiling was also made of plaster and lathe.

@Maker_of_Things

Yes, the dirt is a deliberate layer.

Victorian era properties are full of this. It often also provides some sound insulation and fire retardation between floors, so you may need to put some other material in its place if that's something you want to restore.

@Maker_of_Things

You know that plaster and lathe ceilings used to be covered with a layer of dirt for insulation?

So that layer might not be soot. Also may contain arsenic, so wearing a mask is definitely a good move.

I could be wrong but that's what I've found out from watching several renovation videos on older properties.