Hi,
I recently (last 5 years or so) found out the one of the best products from removing soot from walls was a brown colored clay like substance. Sorry I can not recall the name but it cam out sometime around 1900-1920. It was a great seller.
As heating home moved away from coal to natural gas or electricity, the demand for the product slopped. A school teacher who was somehow (5 degrees of separation think) had gotten some of the soot removal clay and used it in her class for the students to mold and learn using their hands made a suggestion that ended up going over the grape vine to the company. They started making the clay in four or five different colors for kindergarten and 1st and second graders.
The soot removal clay was renamed Play-Doh and the company eventually got out of the soot removal business.
The did not die being the best buggy whip makes, they found an alternate market and were dominant in it.
So if you want to get coal soot off the walls, just use Play-Doh.
Doug vV