Way back when I was in university (2 years ago), one of my soils teachers was telling a story about a farmer he knew that would filter the dye out of farm (or off-road) fuel. I remember he ran it through a clay, but I don't remember what type of clay it was. I want to say montmorillonite, but I don't remember for sure. Now I'm in Houston in mud school, and the other day we learned how to do a test to determine how much of the solids content of a drilling fluid was due to the clay we added. It's a long drawn out process, but part of it is adding a chemical (methylene blue) and testing to see how much of that chemical is absorbed by the clay, and how much remains in the solution. Where I'm going with this is the clay that the farmer used might be the same stuff we sell to oil companies to make drilling fluid with. It would be the same idea as the methylene blue test we do in the lab. Colored liquid goes in, clear liquid comes out. Has anybody heard of such a thing? Does anybody do it? Would it be more trouble than it was worth? What kind of filter do you need to catch those clays?