I'm looking for a chemical hardener that I can add to breastmilk to turn it into a stone. I have seen others use one but they will not share what they are using. In their video they add about 1ml of secret solvent to 15ml of breastmilk and heat it on low heat for a couple minutes. It hardens completely within a few days. Any help would be greatly appreciated.
Referring to real milk (bovine, human breast milk; but not coconut milk) contains casein which may be polymerized. You may end up with edible cheese, or green adhesives, or gemstone-like galalith, for example.
Frankly, I have no hands-on experience to experiment with breast milk. And I recommend to refrain from experimenting with formaline at home. However, the procedure "Sculpted Science: Turn Milk into Plastic!" in the open access section Bring Science to Home by the Scientific American, which triggers the process just by vinegar and gentle heat, sounds much safer and may be worth to follow.