Here is a quick video on making your own investment mix compound. It can be used for molding metal, but I use it for making glass slumping molds. It's easy to do and inexpensive. I have not tried it on large molds yet, but I will. If you like this video please hit the "Like" button and subscribe to my channel.
Watch Susan's video about investment mix and other stuff here: https://www.youtube.com/user/SusanMcGarry/videos
Start at these two sources for silicon sand/flour: http://www.davensceramiccenter.com/
https://www.axner.com/
Dental plaster here: http://www.atlanticdentalsupply.com/Plaster_c_36.html