by Elio — Grime on old build­ings is a mi­cro­bi­o­log­i­cal con­cern – not be­cause mi­crobes cause it, but be­cause they can help get rid of it. The gray pall shroud­ing old build­ings comes from the con­ver­sion of chalky stone such as mar­ble (cal­cium car­bon­ate) to gyp­sum (cal­cium sul­fate), a re­sult of at­mos­pheric pol­lu­tion. Not only is gyp­sum...