sand casting sand i am always adjusting
1) regular grit sand ( often kid play box sand at Home Depot)
2) fine sand (also usually at Depot)
3) Bentenite clay (pottery supply places sell dry clay in bags)
4) Fire clay (pottery / craft places sell dry bags)
5) water content
too much clay and fine sand and it will definitely stick together and take a shape but sometimes the water / dampness is tricky and during casting if it cannot vent through the sand and it will go through the molten metal leaving a swiss cheese casting.
grab a handful of sand and squeeze. it should take an impression of you fingers and within reason if you shake one end it should hold together.
aluminum is easy but finding sand for cast iron is tough as sand melts from heat, sticks to casting and leaves a rough finish.
petro bond sand is a oil / chemical mixed with sand. red when new and sand turn black from use (probably carbon soot). petro bond burns and smokes when molten metal poured in it but i got to say in general a easier sand to mold with and leaves a better as cast finish
PetroBond Foundry Casting Sand
it is hard getting foundry supplies in small quantities try budgetcastingsupply. most foundry supply places don't want to deal with less than ton or pallet size quantities. i even had a place tell me it was $200 to fill out paperwork for an order this was not even counting the cost of buying anything.