nash, how big is yer pond you want to seal?

my 2c:
there are several types of clays that are distinguished mineralogically and can be distinguished based on the "swelling" properties. i am guessing here, but i would think typical pottery clay is a montmorillonite or kaolinite type clay which does not swell as much as bentonite, but still would work fantastic tilled into yer existing soils at a good percentage...if you already have 25% clay fraction in native soils, i would try and boost that to near 50% along pond bottom and think it would be excellent for pond sealing. i would want a tilled (AND COMPACTED) depth of at least 2 feet.

lets see if you get any other opinions, and good luck.

GSF are people too!