I read some more and consulted with some people in my area and it seems that I made at least one big mistake AND that I need to tweak my recipe.
The mistake: I did not sterilize my containers and tools.
As for the possible tweaks:
Firstly, I think I can use more of my alcaline solution to raise the pH of my paint.
Secondly, I was recommended a preservative to use in my paint. I had read about sodium otrhophenylphenate but the only place I found to order it sold it in pallets of 500 kg... I had to go for another option...
This mix's recipe:
Casein milk protein — 10 g.
Boiled water — 100 cc.
NaOH — 2 cc. (or about 40 drops)
Grapefruit seed extract (glycerin vehicle) — 1 cc.
Dry pigment (titanium white) — 60 g.
Total cost: I don't care anymore...
Above: First observation, more NaOH made my binder
more homogeneous faster and a darker yellow.
Above: Mixing away. I used rubbing alcohol to
sterilize my surfaces, containers and tools.
Above: A 37 cc. tube filled to the brim.
(I actually put too much paint and ended up wasting
about 15 cc. Most went on my fingers but I also used
some as a ground on an old canvas I was planning to paint over.)
Above: Messily filling up the tube.
As you see on the last picture, I have a lot of unused binder left. I plan to leave it on my shelf and see how much time it takes to rot. The last batch started smelling ater 7 days (the tube took about 18 to 20 days — I thought I was okay since the oxygen input was practically cut off but I was wrong), we'll see how long this one lasts... if it rots at all.
Also, the canvas I prepped with the leftover paint (it was a store-bought gessoed canvas already covered with a layer of acrylics) ended up being glossy. This is strange since casein is supposed to be super-matte. It might be an indication that my recipe is wrong. We'll see how the paint handles and how it interacts with Richeson caseins. I'll keep you posted.