I found something that worked. Water, and lots of it. It didn't stop it from sticking completely, but sure made a big difference. The trench I've been working on has been slowly filling with water. If I spread it around and got the bucket good and wet before every scoop, most of the clay would not stick. Today it was misting out all day. Not enough to accumulate, or even run down the gutters, but enough to keep everything wet. I had to stop using the dump truck, because it just slid and sank with a load. The loader was useless with the old 2wd case, and the mini-excavator couldn't even make a dent in the clay. So I used the backhoe to dig out the clay and create a big old pile of clay, but I couldn't get straight on to get the trench cleaned out. So the excavator was used to finish clearing out the loose stuff.