Aha!

"... if you're willing to make your own body, it's pretty easy to make something better than anything commercial. Especially if you make it as liquid slip, and then de-water it.

However, if you then give your special recipe to a clay manufacturer, the clay they make will not be nearly as good. This is because they make the clay stiff, adding only what water is absolutely necessary. The result is that the individual clay particles do not get as thoroughly "wetted," and therefore the clay is less plastic."

posted by William Melstrom

I just spotted this on the wonderful Crystalline Glaze Forum [click on title to visit],a regular lurk of mine. I've always known that 20% or so recycled clay wedged into the straight-from-the-bag stuff makes it much more throwable: this bit of information seems to explain why. Also,I like the word wetted.
Slicing and wetting fresh clay before kneading is already an improvement; leaving it like this for a day or so is even better,but recycled scraps are the best. In Winter,I use hot water to soften up the clay.
Anything else you'd like to know?