[niftyc]

I have a household chemistry puzzle I'm hoping people on this forum might be interested in.

I live in a 1848 farm house that has had about 13 layers of paint put on it over the years. I've found that various strippers work OK for the top layers, but I'll then get down to the bottom layers and no normal paint remover will do anything. That tough layer is probably homemade milk paint.

I ordered a milk paint remover online even though it looked a little bit like an online scam. It looked very homemade (you be the judge: https://img0.etsystatic.com/121/0/5556274/il_875xN.878519080_s0dw.jpg).  But it worked like a dream!  The iron-hard milk paint that could only be painstakingly chipped off quickly became so easy to remove I could wipe all the milk paint off or spray it off with a garden hose. (Example FYI: https://www.youtube.com/watch?v=z18ZHjeFVsE)  Miraculous.

Here's the problem. My house has so much paint on it there's no way I can afford this stuff. It costs about $0.75 per ounce when shipping is included and shipping is a lot of the price. I'm wondering if there is some household or garden product that will produce this effect on milk paint. Alternately, I wonder if I can figure out what is in this mysterious online powder.

Here are some clues (?):

I see online that in the 1800s milk paint was usually cow/goat milk + hydrated lime (calcium hydroxide).

On the updated packaging for that online milk paint removing powder, it says "contains sodium carbonate and calcium hydroxide." (https://www.realmilkpaint.com/shop/strippers-removers/paint/). In household terms, that's "Arm & Hammer washing soda and Mrs. Wage's Pickling Lime" -- both are on sale at the Walmart near me.

On other Internet forums people recommend a variety of household concoctions to remove milk paint. Typically they recommend hot water and soap (this did not work at all for me) or ammonia (did not try).

In case it is not clear, I have NO CHEMISTRY KNOWLEDGE.

I'm hoping this post interests someone enough to help solve the puzzle! Happy to try things and report back.

[Intanjir]

Milk paint and glue are made up of casein. Casein doesn't dissolve in pure water but can be dissolved in a mildly alkaline solution. Both sodium carbonate and calcium hydroxide are good ways to make such a solution so their presence makes sense and they might be all there is to the remover.

[niftyc]

Thanks! I do have some Arm & Hammer Washing Soda (sodium carbonate) on hand, so I tried it on some old door hardware and... it works!  I can wipe the milk paint off with a sponge or a paper towel if I soak the hardware in a hot washing soda bath for a while.

However, my washing soda didn't dissolve easily in cold water. I had to use hot water.  The mystery powder *does* dissolve easily in cold water. Would adding pickling lime (calcium hydroxide) help?  Or something else? 

I was googling some more and I found the Material Safety Data Sheet for this online powder. I am not sure how to read it but it seems to be saying it is 30-40% calcium hydroxide.  Here it is if you are curious: https://www.realmilkpaint.com/wp-content/uploads/MSDS-sheet-for-remover.pdf

I'm sorry if this question is simple but why would you want both the calcium hydroxide and the sodium carbonate? I don't know anything about chemistry. I am hesitant to mix household chemicals I know nothing about but it sounds like mixing in the pickling lime is safe?  All advice welcome, I am a chemical idiot here.

[Intanjir]

Calcium hydroxide is less soluble than sodium carbonate. Why 30-40% of the powder is calcium hydroxide, I'm not sure, but presumably they wanted things to be a bit more alkaline than washing soda is able to achieve easily. However, since there is so much carbonate present I would have expected some precipitation of calcium carbonate, but I suppose this wouldn't lower the pH much because sodium hydroxide would be left in solution. Perhaps the pH would actually go up?

Anyways if the washing soda works you can just use that. It might be slower to dissolve because the powder has a larger grain size but this won't affect the final concentration you can achieve. Dissolving it in hot water and letting it cool is normal procedure.
You can mix in pickling lime if you want. It won't make anything poisonous, but it is a stronger base so it may be somewhat more caustic. I would just use the washing soda if that works well enough rather than handle a stronger base.

[niftyc]

Thanks so much. I guess when I said "it works" I mean that it dissolves the paint, but it is not very practical in this form. For the amount of milk paint I have, a gritty water (or liquid at all) isn't ideal.

The online powder forms a paste in cold water. A paste is ideal because it allows you to apply the stuff with some precision. I'm sorry to ask so many questions, but can you think of a household or garden filler that would make this into a paste and help it dissolve?

I was thinking cornstarch, which dissolves in cold water and forms a paste. I found this link that says the ph of cornstarch is "4.0-7.0" (https://foodsafety.wisc.edu/business_food/files/approximate_ph.pdf).  Maybe they also used cornstarch and the 40% calcium hydroxide is to offset any acidity in the filler that makes it a paste?

I can't emphasize enough how helpful this information is to me.  I really appreciate your time.

[Intanjir]

Ok, that actually makes more sense. I think the bulk of the calcium hydroxide does not dissolve and forms at least some of the solid portion of the paste. It might actually be all there is to the paste as slaked lime is known to make a paste when only modest quantities of water are used.

If they are only using the two ingredients then I guess their product is 30-40% Calcium Hydroxide and so 60-70% Sodium Carbonate, ie about 1 part CaOH to 2 parts Na₂CO₃ (by weight).

[Rocso]

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
both objects."

This also explains why it's important to keep the paste wet after application.