Eight dry quarts of grade 2 medium grain vermiculite. Works well for casing mixes. Popular as a substrate ingredient in the well known Brown Rice Flour and Vermiculite "PF Tek" jar cultivation method.
For PF Tek Jars use 2:1:1 recipe
measure 2 units Vermiculite
add a little less than 1 unit water (be careful adding water, continue stirring while adding the water only add enough until a small puddle forms under the verm)
continue stirring while adding 1 unit brown rice flour to evenly coat the wet vermiculite.