I actually did this six months ago, and the shape that I cast is still solid, virtually indestructible and mold-free, so I am speaking from experience here.
The instructions I wrote down here for
making milk paint are very similar, but with a few notable differences. It is very important that you work in a hygienic and sterile environment, which means really taking care not to introduce microorganisms and fungi.
Casein is a milk-based product that you can make by warming whole milk
in a copper saucepan and adding about 5 drops of lemon juice per liter. Whisk the milk vigorously, making sure to whisk the copper saucepan. You
don’t have to get it boiling hot, but it mustn’t be cold. You will see clumps
forming, and then remove it from the heat. Pour the entirety through a
fine-meshed strainer and reserve the watery part (the whey).
Because ultimately we want to remove all of the water, put the clumps in a clean nylon stocking and squeeze out all of the moisture that is left. Then remove the casein and put it on a baking sheet that is lined with waxed paper / baking paper. You will find that the casein is a little crumbly and tends to fall apart, but if you squeeze it between your fingers it will retain its shape.
Then you force the casein into a beadmaking form like this and bake it at about 150 degrees Celsius for three to four hours.
You may wonder why the copper saucepan. That is because by whisking the milk in it, you are adding copper ions to the casein, which act as a natural antibacterial and antifungal agent.
You can also add other materials like pigment or fillers to the casein - and although I haven’t tried it, I expect that using Marble Dust (available at better art supply stores) would strengthen the final product, as would adding mica flakes.