Shmuel wrote: >>Maybe I am misunderstanding. I assumed equal quantities meant by volume. Water retained was by weight (which is great cause 1ml = ig). Just checking because obviously same weight of perlite and sand is a huge difference in volume. Thanks for the question, and I apologize for the confusion. I tested equal volumes of each ingredient (in this case, half a cup of each one). I weighed it dry, wetted it thoroughly, waited six hours, drained the water, and then weighed it again. The difference in weight was assumed to be the quantity of water retained. The chart I posted was the relative amount of water retained, compared to #2 sand from my local rock yard. So 1=retained the same amount of water as the sand. 2=retained double the amount of water. In a fortunate coincidence that made the math simpler, 1/2 cup of #2 sand retained one ounce of water. (Metric: 118 ml of sand retained 28 grams of water, or .24g of water per ml of sand.) By the way, I ran the perlite test again with perlite that had been sifted to remove fines and dust. The water retention of the sifted perlite dropped to about the same level as the pumice I tested. So as several of you have said, sifting the perlite makes a huge difference. Unfortunately, it's impractical for me to sift the big bags of perlite you get from the home centers, so I think I'll move toward pumice, which I can get in big bags pre-sifted at about the same price. Mike San Jose, CA