I found this thread because I'm renovating an old schoolhouse interior and I think I may have milk paint on the walls and I'm looking for a way to confirm it, or not.
Intanjir is spot on correct and the question is basically answered but I wanted to know more about the sodium hydroxide that was mentioned and whether it was important. It turns out to be very important so I wanted to share what I'd found.
Previous posters have already identified these but just to list them again:
Sodium Carbonate = Washing Soda
Calcium Hydroxide = Hydrated Lime (slaked lime; builders lime; lime putty)
I Googled "dissolving casein" and found this comment from Marcia Moss at https://www.researchgate.net/post/What_is_the_best_method_for_dissolving_casein_in_water
"Casein is soluble in alkaline pH as it forms sodium caseinate. Some people have used a solution of sodium or ammonium hydroxide. If you use a buffer at pH 8, it may help. But you may need to readjust your pH once you add the protein to your solution."
Further searching uncovered this document http://www2.ohlone.edu/people/jklent/labs/101b_labs/casein.pdf
that contains lots of chemical reactions but a good plain english description that someone like myself can understand:
"Casein has been used as a glue since the days of ancient
Egypt, Greece, Rome, and China. It is mixed with sodium
carbonate and calcium hydroxide, and then dissolved
in water. The function of the sodium carbonate is to react
with calcium hydroxide to produce sodium hydroxide:
The sodium hydroxide reacts with casein, producing
sodium caseinate, which is sufficiently water soluble to
form a well dispersed sol, a solid dispersed in a liquid:
When the sol is spread on the surface of objects to be
glued, the sol wets the surfaces and adheres to them.
When the water either evaporates, or soaks into the
objects, the solid protein molecule remains, adhering to
This also explains why it's important to keep the paste wet after application.