The visitor to my glue handout on my website has comes back with more questions. As this is turning into a deeper look at the casein and borax "glue" I am posting it up here.

And yes, I'm still saying that the mix will make a weak glue far inferior to traditional cheese glue. Read on for details:

Hi Cat, thanks for the quick reply. I am referring to this ( - casein and borax glue, which seems to contain no lime. and no soda bicarbonate. Borax is an emulsifier so it is possible to incorporate linseed oil and pigments as well. What is the chemical reaction here? acidic / basic conditions? They claim that the final pH is neutral. To the best of my knowledge, borax in water is alkaline. That should help the hydration of the casein. Anyways, this whole milk paint is pretty old and seems to have been abandoned. I like stuff like that... Hope we can kick this pebble down the road, Likewise i will purchase some sodium (bi)carbonate to try your idea too. I was also thinking of using potassium-carbonate...

Dear ---,

Sorry on the delay. I've been a bit busy lately.

I'm making an educated guess here as to what's going on with a little help from a glue chemistry text - given my environmental geochemistry background, protein chemistry is not my best discipline. I'm really best when it comes to the inorganic stuff. Regardless, looking at what Sinopia has on their web site, what they are selling is dried powered casein protein, probably produced by the classic curd making process of adding a weak acid to a low-fat milk. When you hydrate dried casein using water and no additives, you get a weakly acidic solution with a pH of approx. 6.6. When you hydrate borax, (Na2B4O7·nH2O) you get some Na+ ions, half as many (B4O5(OH)4)2- molecules [the 2- here is a valence of minus 2 and not a doubled B4O5(OH)4 molecule] and the release of the compositional water of the borax crystals. But there's an additional hydrolysis reaction with the borate ion where boron is hydrolyzed, to wit: B(OH)3 + H2O ---> (B(OH)4)− + H+. It's this reaction that makes borax so useful as a pH buffer. The boron hydrolysis will buffer any excess alkalinity back toward neutrality but will always be weakly alkaline.

Mind you, in the world of art supplies, near-neutral alkalinity is passed off as neutral pH (pH 7 to 8) and for all intents and purposes, that's okay. Most of the world's drinking water is slightly alkaline as is all mammalian blood chemistry. Most of the biochemistry of living things happens in a slightly alkaline environment. It's normal - but when you're selling art supplies, it's not that easy to explain to people that their world is really slightly alkaline so things get sold and "acid free" or "neutral pH."

There is no real glue reaction here at this point in the traditional sense of forming a caseinate: in making a solution of water, casein and borax, the sodium ions will not react with the casein to make sodium caseinate because the casein will not dissolve and so the sodium can't get to it to react with it. You could say that keeping the casein in suspension as a colloid is like keeping it in a chastity belt, to protect it from being raped by those rogue sodiums. Casein will only dissolve in decidedly alkaline conditions, which you won't get in a borate-buffered solution. The casein particles will stay suspended in the solution.

Thus far, I'm pretty confident my description of the chemistry is correct since my glue book (A. L. Lambuth, Protein Adhesives for Wood, p. 26, in: A. Pizzi, Ed., Wood Adhesives, CRC Press, 1989) tells me that a mixture of casein and borax is a simple dispersion of casein particles as the colloid with borax as the emulsifier. The addition of heat here is not to drive a chemical reaction but to mechanically disperse the casein throughout the solution. The boron hydrolysis will buffer the solution to near neutral pH. The creation of a translucent mass by hydrolysis that the Sinopia instructions mention leads me to believe there may be an additional reaction here where the kappa-casein is hydrolyzed, leaving the alpha- and beta-caseins behind. The swelling would be due to the unkinking of the casein micelles with the destruction of the kappa-casein wrapper around the alpha- and beta-caseins by the hydrolysis process. I have no reference for that reaction - I'm just going here on what I know about the behavior of casein micelles.

Using it as a binder for pigment involves driving off the water in the solution through drying. As water exits the solution, there will be a limited opportunity to form sodium caseinate at the solution/casein-surface interface, which would be the only true aglutenant reaction of this stuff. Heating the mixture will mechanically break and disperse the solid casein granules, thus increasing the available specific surface for such a reaction to occur.

Given that I've found a discussion of casein borate emulsions as a paper paste and pigment binder in an industrial glue science book implies this is well-known chemistry. But if you give it a little thought, this casein in borate buffer emulsion is going to be a weak glue. Weak glues are okay as pigment aglutenants. That's because you don't need strong adhesion for pigments - what you need is chemical stability and protection for attack from atmospheric humidity and airborne pollutants. But if you want to use this stuff as a true glue to stick one solid onto another, I'd go for a calcium caseinate glue made with pickling lime or a collagen-based glue.

If you're thinking of using this as a pigment binder, go for the Sinopia recipe. I'm a huge fan of Sinopia and have ordered certain hard-to-find art materials from them for years and years now. If the folks at Sinopia say it works for pigments, it works.

If you're going for true glue as opposed to making an aglutenant for pigments, try the potassium carbonate. Use it like you would use the pickling lime in the Cennini cheese glue recipe and see how it works. If it doesn't work, try it in the same proportion as the baking soda in the high school recipe I put in my glue handout.

Mind you, potassium carbonate is not as bad as the lime compounds in powder form but it is a skin irritant and it does make a strong alkaline solution that you don't want in your eyes. Just because it isn't as strong a caustic as lime doesn't mean it's benign as a chemical. Seriously, always protect your eyes when mixing any of this stuff.

ttfn Cate

Frankly, I have too much work to do at the moment to spend a lot of time on doing more glue chemistry - but I really like that this guy is interested in the chemistry of glue. People don't do amateur science like they use to and I don't want to discourage anyone from having some fun with this sort of stuff...and so, now I have to work most of the evening to catch up with what I didn't do in order to reply to the glue questions. There really is no rest for the weary...