Make your own Cold Process
Core-Molding Sand / Paste
Hardens with CO2.
NO High Temperature Firing is Required.
Cores for metal casting:
To produce cavities within a casting, negative forms are used to produce cores.
Usually sand / paste molded cores are inserted into the casting box after removal of the pattern.

This Formula for Core-Molding Sand / Paste  Hardens in the presence of carbon dioxide,
No High-Temperature Firing required.

You will need Very Fine Sand available at your local Home Depot or other Home improvements store, we use Quicrete's Commercial Fine Sand. Another is Handy Sand. Both are very fine and look like sugar.

You will need some Sodium Silicate Solution to use as a binder and which hardens upon contact with Carbon Dioxide Gas. 

You will need something organic in your mix so that you can burn out and release the core after pouring the metal.  If you use straight sand and sodium silicate it will be hard to get the core out of your mold as they don't burn out.
You will need a source of carbon dioxide such as a CO2 tank from your local distributor with a regulator, a mixture of baking soda and vinegar in a container, or even breathing on it might work.  You will put your filled core mold in a plastic bag and gas it for about a minute which will harden it enough to take out of the mold.

You will need your household oven, starting out at 200 degrees for a couple hours, then going up to 450 degrees for another couple of hours or so to fully cure and drive out moisture from your core. .

Sand comes in a 50lb bag so we'll base our recipe on one bag of sand.
Sodium Silicate Solution about 5% by weight of the sand.
Table Sugar about 15% by weight of the Sodium Silicate.  

50 lbs sand needs 5% by weight of Sodium Silicate = 50 * .05 = 2.5 lbs of sodium silicate.
2.5 lbs of Sodium Silicate Needs 15% by weight of sugar = 2.5 * .15 = .375 lbs or 6 oz sugar.

Safety: Sodium Silicate solution is Corrosive.
Wear protective gloves and eye protection. You are responsible for your own safety.
Proper PPE is essential.

In a container weigh out the sodium silicate and add 6 oz of sugar; mix well.
Add the container of Liquid to your sand (in a large plastic tub) and Mix well.
Use immediately or store in ziplock bags and it will keep for months without hardening.

Tightly pack the core mix into your core mold.  Make sure that there are holes for the CO2 to get to it. Place the core mold in a plastic bag and gas it with CO2 for at least a minute. This will harden the core enough for you to take it out of the mold.  Place the core in a 200 degree oven for a hour or two then turn the heat up to 450 degrees for final curing of an hour or two. If you do not use the core immediately you may want to put it back in a 200 degree for an hour to re-dry it before using it.   

Once it cools it should be hard enough to stand the pouring process and dry enough to not explode when it contacts the molten metal during pouring.

**** NOTE: CASTING METAL IS INHERENTLY DANGEROUS. YOU ARE RESPONSABLE FOR WHAT YOU DO WITH THIS PRODUCT. BY PURCHASING AND USING THIS MATERIAL YOU ARE AGREEING THAT YOU WILL NOT HOLD Frostic Incorporated or it's Agents LIABLE FOR ANYTHING THAT YOU DO IN CONNECTION WITH THIS PRODUCT. USE EXTREME CAUTION AROUND MOLTEN METALS AND HIGH TEMPERATURES. MAY CONTAIN FREE SILICA; Sodium Silicate may cause skin irritation, wear protective clothing when handling****