Brick and stone buildings have mostly been kept clean the last 15 years or so, and they don't darken as quickly as they did in the 20th C when both transportati

Brick and stone buildings have mostly been kept clean the last 15 years or so, and they don't darken as quickly as they did in the 20th C when both transportation and industry produced far more soot. That said, I've seen buildings that were nearly black with 100 years of accumulated soot at the end of the 1980s undergo deep cleaning (sandblasting) in the 90's and come out off-white. It's pretty remarkable. 25 years later, those same buildings are now almost exactly the same color they were immediately after the cleaning, without any further maintenance.