Last week I used NurdRages's YouTube sodium silicate recipe that involves silica gel kitty litter and caustic soda. He gives the correct proportions of the two ingredients with water as well as the process of just heating up the water and adding the caustic soda to the silica gel (silicon dioxide/quartz). I found that there was a fair bit of heat released during the mixing as well as lots of bubbling, so do it outdoors as it's highly alkaline and will irritate your skin...gloves are a must and some eye protection. I made three litres of thick syrupy liquid which weighed 5 kilograms to give a specific gravity of 1.666. I looked up the density of concentrated liquid sodium silicate and found it was 1.7, so I was pretty close. After a few days the stuff crystallized into a solid lump that had to be smashed with a hammer and I plan to add some water and heat to dissolve again. So far I've tested it with aluminium castings but not iron or copper alloys. The end of the video describes making it with higher silicon content that may be more heat resistant. It works perfectly fine for making cores at 6% by weight as well as 15% water and 12% sugar relative to the sodium silicate from the Tom Cobett notes. After a day the CO2 hardened test cores could be thrown at the concrete floor and only suffered damage at the point of impact.