I confess- I took the easier road by purchasing a 100lb box of pre-mulled "Jupiter Blend" molding sand. This is fine sand that has been mulled with an oil based binding agent called Petrobond. Because it has no water, there is no steam- and while some gases are liberated from the oil, I haven't found venting to be necessary. The sand has a somewhat oily texture and smells bad after it has been burnt by the molten metal- but it works well and eliminates some trickiness from the casting process. The sand closest to the molten metal will be burnt to black dry sand after casting. This used sand can be blended back into the mix or can be scraped away (as best as you are able) and discarded. Unburnt sand can be used again.
https://www.youtube.com/watch?v=HHk-zvJ_jr4 DIY: Easy to make greensand for casting metal - Aluminum sand casting Simple step by step instructions and sources for ingredients. Your goal for good greensand is a clumping function so that it will hold it's shape easily. This is accomplished with bentonite clay powder which is the same ingredient that clumping cat litter uses. It has many industrial and health uses but you want to make sure your bentonite is a find powder. I picked up the Bentonite from http://www.foundry101.com at a reasonable price. I used about half of it to make this batch of greensand. I store my greensand in a plastic tote it a sealed lid.
The other dry ingredient is silica sand which I ended up getting from Carquest Auto Parts. It is used for sandblasting. It was not as fine as I hoped but the results were very good none the less. I have found playsand to be unacceptable coarse and don't suggest using it.
The final ingredient... the water! It came from my sink. I am using a simple hand pump pressure spray bottle to mist the water for even distribution then folding and mixing using various methods. Mixing well is absolutely critical. After I made this video I learned that covering the sand with a tarp and stomping it often during the process will help improve the quality of the final product.
The better quality the greensand the better your molds and final casting will turn out. Once I made this actual greensand my metal aluminum casting projects escalated to a whole new level. I also built a set of reasonable quality casting flasks which I will make a video about soon.
Sodium silicate sand Edit
https://www.youtube.com/watch?v=qPnSb3yzytE (see propane, used motor oil vacuum pump 24min)
SA suppliers Edit
http://ceramicartsdaily.org/pottery-making-techniques/making-ceramic-molds/plaster-mixing-101-how-to-mix-plaster-for-ceramic-molds/ plaster of paris mixing 73:100 , water : plaster