I meant to post on this thread last week but i just didn't have the time...I wanted to point out that during most of the 20th Century London was a really dirty city with respect to air pollution. This had a profound affect on a lot of londons bulidings covering them in a layer of a soot type material.
http://en.wikivisual.com/images/b/b3...edralSouth.jpg
this is just one example of St Paul's (1970's in colour) to illustrate the point its difficult to do in b/w photos. London suffered from "smog" during the first half of the 20th Century due to sulphur gas being realised from coal. I personally remember seeing historic buildings being cleaned up during the 1980's and they were black with soot. hope it helps.
