Architectural Dermatology

According to the BBC, Indian officials have concluded that a "deep-cleansing mud pack may be the only way to return a yellowing Taj Mahal to the cultural landmark's former white marble glory."
By "caking the building in clay," we read, those same officials can actually "draw out surface impurities" – but how do they know whether or not they've purchased the right mud mask...?

[Earlier on BLDGBLOG: the oddly popular Hurling Taj Mahals into the Sky].