Porosity and little holes which are in the mold
It oftens happens when you cast two or three molds one after the other using the same dish, because the last casting comes from the bottom of the dish and is thicker, and then often the last cast which comes from the bottom and which is the thickest of the dish, may easily become porous. Cast thinner and neater, as long as you blow very strongly on the soaked sand while casting in order to clear the little air bubbles.
at left top margin of folio 133v
When the sand is casted thick, it often becomes porous