Ive never made clay with cat litter but i do use the same type of rock for background and subtrates. If you live near a napa autoparts in your area check out their oil absorbent clean up. Its only a few bucks for a huge bag. http://m.napaonline.com/parts/PartsD...305_0375270349
If not look for other oil cleanup absorbents containing only Diatomaceous*earth.

I just rinse the earth rocks really good and they change color when moist they will have earth tones with reds browns and greys. When dried they turn white. This makes a nice moisture indicator as well. I know not to rinse it with hot water or else i get mush. Maybe thats the clay your looking for lol. It sounds like a great idea for landscaping if the mush hardens. Id like to see what you come up with.

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