Richard
Your operation may be a couple of hours per day but how many years have they been running a couple of hours a day for? When did they last clean up? You are probably the first Manager they have had that cared! The chances are that its tyre dust, if you get an analysis done it will come back as carbon whether its tyres or exhaust. From the tests you have done your engines are running lean so it wont be an exhaust problem. Black tyres are made from a mixture of natural rubber, synthetic rubber and carbon black, the reason carbon is used is because its very hard and the tyres last longer (thats why they make diamonds with it too!). White tyres are made from chalk which is soft, they last about 5 minutes and cost quite a bit more - you end uo with white dust - prettier but much more of it! You may however be using cheap black tyres which contain a lot of carbon - they are too hard and dont last that long. Good tyres to try if you can get them are Gumasol or Marangoni. If you can't get these ask your supplier what the rubber compound used in any replacement tyres you buy is - whats the chemical composition? Ask about the characteristics - grip, hardness, softness. There are rating factors for all these characteristics - a long time ago I used to know the names for them! A simpler rule, however, is dont buy the cheapest.