It depends on how and where you are drying and the clay you are using. Is "earth clay" a type of clay? I'm afraid I'm not familiar with the term and see it more generally as "clay I got out of the earth myself". like the yellow clay sculptures I made when I was a kid from an exposed deposit nearby my childhood home. With respect to the dimensions, if you mean 15x15x2 Inches, it's significantly longer dry time than for 15x15x2 centimeters.Kiln or natural dry time?anyway, I'm going to make this as simple as I can. Without details to narrow the scope, it's a big question, too big for my morning coffeehttp://www.bigceramicstore.com/info/ceramics/tips/...http://www.americanclay.com/#!product-application/...there were literally hundreds of valid links simply by googling the subject.