This was from an ESL (English as a Second Language...) site on casting...."Permeability is the size of the sand grains. Higher permeability can reduce the porosity of the mold, but a lower permeability would (get) to have a good surface finish." Looks like that Ford sand is post treated pretty good to maintain a .003 core tolerance. That 50's Ford plant video reminds me of that Bugs Bunny cartoon where he is in awe of the automation of the 'modern' plant.
