I am going to disagree with most on this but have been doing it now since 1961 as a main income and living being a sculptor with his own foundry. I cast pieces from several pounds and up to 300 lb per pour in bronze and also huge amounts of aluminum. I also spent 3 years twice while teaching on the college level and having the resources to run investment test to further refine the results.

Oddly my final verdict is one of the simplest. I use #1 molding plaster mixed in a rough ratio of 50% to 50% of brick sand. The brick sand is the same as that used for mortar in the cement industry and contains fines from being roughly about 1/16 inch dia. down to fine silt. It is washed, but not all that clean and the finner particles work well as a good surface detailer also. My molds range from about 100# each (seldom work smaller since many pieces will be put in each mold when done) and on up to about 2000 pounds (which is an investment mold about 6 ft high x up to 38 inch dia. max. ) The burn out kiln usually dictates that as a maximum size as I want a decent amount of air space around and over it anyhow.

Here is the part I differ with everyone it seems. I burn out the molds as wet as I can get them. If they have been sitting awhile, I re-wet them . The reason is this: A wet mold conducts heat into the core of it faster than a dry mold will. The wax will get to a soft quality faster, will melt and run out faster, and will NOT SOAK INTO THE PLASTER INVESTMENT AS MUCH if the mold is still saturated and wet. That means a faster burnout time and less cracking due to the wax expansion without a way to get out yet and also many hours at the end of the burnout to not have to be cooking out the wax residue that would have otherwise have soaked deeper into the mold walls.

Last point, I always have an outer walled wire around the sides and bottom of any mold. Chicken Wire also known as Poultry Netting with about a 1 inch opening is my favorite and on big molds it takes about 2 turns. I also use a black plastic instead of tar paper to pour them in since it can be used many times over and over. And as you can see by the picture added, two or more turns of a chain is used to do the lifting and turning over but I also use side boards around it when they are really going to be heavy and stressed by the chain digging in. Hope this helps.