The simplest explanation is usually the right one. I finally confirmed my suspicion that my supplier had sent me the wrong grade of Perlite and after talking to their supplier I checked the code on the bag and sure enough they had sent me grade 3 instead of 7. I have a bag of 7 coming and hopefully that will work. Thanks for all the advice. They should label the stuff clearly, the bag has the number 23 somewhere in the code which means grade 3 and grade 7 has 27 somewhere in the code. At least I get a free bag of Perlite for my trouble. And I was just starting to suspect you all were shills for the megaperlite corporations.....