E-Z Soil Analysis for Cob, Adobe, and Rammed Earth Construction.
Take a small ball of the mixture and toss it between your hands for about a minute. Then hold the small ball of sand and clay at waist height. Let the ball drop from your hand. You want to have the ball keep shape and not go past the equator. If the ball flatters past the equator then you have too much clay and should add sand. If the ball cracks then you need to add clay and possible some more water.
Use the "burrito" technique. After your burrito is created use both of your hands and dig your fingers into the burrito and pull toward your self. The straw is the tensile strength for the cob and so it should be difficult to pull the cob apart. If it is not difficult add some more straw.
Make a ball of cob. Hold onto the ball with your finger tips. Now without pinching the ball pull your hands apart. This should be difficult to do.
