I mix clay with top soil and peat moss for substrat.The obvious question is: what do you plan on using the clay for? If you are making something, like a vase or lamp base, you can tolerate having additives in the clay to make it work better for that, but if you want to make fertilizer balls you can't tolerate those additives.
Yes, red clay has more content of iron, it's not bad I guess (anyway it's cover by gravel and not gonna excess iron in water column), before I been use blue clay and plain modeling clay( which I got from local art store, but they out of business now ). I'll try to find tomorrow some pottery store around me. Thank you for idea.I looked up pottery supply stores in the yellow pages when I found mine. They sold me a 13 lb. block for $10.00 for so. Don't you want red clay though? It's my understanding that the redder the clay, the more iron content it has. Please correct me if I'm wrong in my understanding.
So, it's idea(anyway I got my top soil from forest), probably I need dig deeper (3-4'), if I can't get where to buy it I'll have some good exerciseWhere I live and have lived, there is a lot of clay available by digging down a foot or so almost anywhere. That is where I have obtained clay. And, now I use silt from the American River, which is virtually in my back yard, and which is largely coarse clay. That is a more fertile clay, plus it still has the high CEC of clay.