Black stain on interior woodwork in 1930’s house

[Teenz]

Hello,
I’m currently stripping old gloss off of my woodwork in my early 1930’s Council house, and the last layer is some sort of black wood stain that just doesn’t want to lift.
From what I can tell it is on all the interior wood under many layers of paint, it’s not sticky, and will minimally scrape away if warmed with a heat gun but not enough to make any real difference. Does anyone know what stain/paint it is ?
Also is it the same as the brown stain I keep seeing on undipped but gloss stripped door.
Ive tried paint stripper and some sanding on a barn find (literally) door that was in pretty bad shape paint wise for two days and like the black on my woodwork just isn’t really shifting.
Don’t really want to go down the route of dipping as doing this on a virtually non existent budget.
TIA

 

[Cubist]

If you're finding this on floorboards it's possibly a bituminous paint rather than a stain. On an unobtrusive area, try a little white spirit or petrol on a cloth to check whether it dissolves and lifts off.

If the doors you've seen are hardwood, typically oak, theres a possibility these were stained using the 'fuming' technique. If so you will not be able to remove the stain as in this approach it can penetrate to the full depth of the wood.

 

[Teenz]

The reclaimed doors are pine, as are the floorboards. The black stain on the floor boards is actually quite light, but the interior woodwork (frames, bannisters etc) now I’ve taken the gloss off is deep black.