To clean debris from raw clay get yourself a 5 gallon empty bucket and fill it half full with water. Take your dried raw clay (if it's not completely dry, leave it out in the sun for awhile until it is completely dry, breaking it up into small chunks as it dries) and pour it into the water. Do not stir. Let this sit for a day or two. Get another 5 gallon bucket and some type of a sieve such as window screening and attach the sieve to the top of the empty bucket. Stir up your raw clay and water and pour into the sieve. This will remove most of your debris. Enough to do your first test.