Nigel,
I agree that filtration marks do vary. I have often found that, after removal of the black, a yellow marking remained. I used to assume that this was due to the smoke suphates but, under doors, this could sometimes be phenolic yelowing.
I recall one particular 2-day job, a three-story victorian house in SE London. On the upper floors all the carpets were pale-coloured polypropolene and the floorboards had shrunk after installation of central heating. The parallel black lines only responded to a rotary shampoo with Ultrapack, a 20 minute dwell, an extraction rinse and finally bonnet buffing with Prochem Bonnet Buff.
I think that the reason for the added dificulty in removeing these marks is due to the fact that they are sorced beneat the carpet and pad, so innitial attempts at removal cause the carbon ect. to be drawn from beneath both by vacuum and wicking.
John.