Perlite is a unique filter aid made from volcanic rock that is expanded and milled. The result is a product that is chemically inert with a neutral pH. Perlite, being lighter than D.E., is also easier to mix and keep in solution. Vertical leaf screens are now easily coated. The 23S comes in 34# bags and has a Mean Pore Diameter of 8-10µm. The 27M comes in 40# bags and has a Mean Pore Diameter of 3-5µm.