Is shellac that has been dissolved with methyl hydrate (methanol) toxic, once dry?

Question

I've dissolved de-waxed shellac flakes in methyl hydrate (which contains methyl alcohol), and I would like to know if the alcohol retains some of its toxicity after it has dried. Intuitively it's so volatile that I don't think anything remains, but I'm not sure.

Note on methyl hydrate vs denatured alcohol: Before I purchased this product, I thought methyl hydrate was the same thing as "denatured alcohol" (aka "methylated spirits"). Although the two types of solvent are interchangeable in function when it comes to dissolving shellac, it seems they are very different chemically -- with methyl hydrate being apparently methanol, and denatured alcohol (aka methylated spirits) being ethanol mixed with another type.

In the particular case of the product I used, "Methyl Hydrate", there are no ingredients listed on the back label. The front mentions "99.9%" pure. Pure of what? Just methanol, or pure mix of 99.9% ethanol with methanol? The fluid from this bottle is transparent. I've seen purple ones too in videos, but maybe they were denatured alcohol.

I've sent an email to their support. But received no reply yet, after a week.

Possible Substitutes?

• 95% grain (drinking) alcohol from the liquor cabinet. Nothing more concentrated than 95% is sold in my area. It might not be concentrated enough, and might contain sugars. Bad idea?

• 99% isopropyl (rubbing) alcohol from the medicine cabinet? Bad idea?

Edit: Feedback from the local poison center

Before posting this question, I'd initially wiped a surface with a methyl-hydrated rag in preparation of applying shellac. And I stopped in my tracks, realising it was perhaps the wrong solvent to use.

I placed a (very quick) call to my local poison center, (Vancouver BC, Canada), to determine what to do after spilling methyl hydrate on food contact surfaces (like a kitchen counter, dining table, or wooden cutting board). They made several (somewhat controversial) recommendations:

1. methanol is highly toxic. should wear adequate protection. sure.

2. methanol "does not just evaporate". This claim is dubious, but I figured they assumed it had been mixed with water, or booze. Methanol is water soluble, after all. When I mentioned that it was precisely the point of pure methanol as a solvent, i.e. to evaporate quickly, they seemed perplexed. And it made me very suspicious about the veracity of the other claims.

3. in the case of the wooden cutting board. they mentioned it would be preferable to just discard it, or clean it thoroughly with dish soap. For a plastic cutting board, they suggested a run through the dishwasher. For a melamine counter top, they suggested cleaning it thoroughly with water and soap. I would have expected them to suggest letting it dry, followed by cleaning, but that would have been a surprising answer, given (2). At least there was some consistency.

Answer 1

Methyl hydrate = methanol. Also called wood alcohol.

There is already methanol in most versions of denatured alcohol and in the general equivalent sold in the UK and elsewhere, and these are of course commonly used to dissolve shellac.

And while methanol is quite toxic it is completely volatile, i.e. all of it evaporates, so none remains in a dried shellac film*.

However that's not the only concern since the safety of the user must also be considered. The difference in toxicity to the person applying the shellac of pure methanol versus ethanol with just a small amount of methanol added is significant. And some sources warn against casual use of pure methanol because it is much more dangerous than in a dilute form.

So take appropriate safety precautions if using methanol for this purpose — a well-ventilated room might not provide sufficient protection from the fumes, so wearing an appropriate respirator could be considered the minimal safety requirement.

The front mentions "99.9%" pure. Pure of what? Just methanol, or pure mix of 99.9% ethanol with methanol?

I think this will be a small amount of dissolved water.

95% grain (drinking) alcohol from the liquor cabinet. Nothing more concentrated than 95% is sold in my area. It might not be concentrated enough, and might contain sugars. Bad idea?

That should be perfectly suitable if you can get it where you live (it's most definitely not available everywhere!) and you don't mind the price. In fact some shellac users choose 190-proof drinking spirits as their preferred solvent for it and won't use denatured alcohol, despite how commonly it is used for the purpose.

The main reason some don't use this is cost, since drinkable ethanol is heavily taxed or has a levy placed on it in most countries while denatured alcohols, and other spirits that aren't drinkable, are not.

99% isopropyl (rubbing) alcohol from the medicine cabinet? Bad idea?

Should be OK but you'd want to do some testing to see how it works for you.

Some shellac users (and other sources) specifically warn not to use it and cite, among other reservations, that it has a slower evaporation rate than denatured alcohol. But this slower evaporation might be considered a benefit, not a negative at all, since the ultra-fast drying of shellac regularly trips up learners who aren't used to it. Bottom line, despite certain individuals stating that it simply doesn't work there are others who use it for their shellac so obviously it can.

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*It's not generally appreciated that this is also true of finishes that contain mineral spirits/white spirit, no matter how much of the finish is solvent (and "Danish oils" may contain 70% or more) eventually every trace of it will be gone.

Answer 2

Two additions to Grpahus's comprehensive answer:

Skin absorption of methanol is also an issue (chemistry.se), so gloves should be worn. Latex isn't much good; nitrile and PVC are not much better but for short-term use are recommended by some sources. 100% methanol is worse than 10% in ethanol, though how much worse is another question (and one for chemistry, not woodworking). I say all this is someone who's fairly casual with isoproponal and acetone (I use them in the lab almost daily).

All (relevant) alcohols are hygroscopic and will absorb water from the air if left open; of course the other 5% in food-grade 95% ethanol is also water. This is just something to bear in mind if you might not want to add water (not really an issue for shellac by the sounds of things, but can cause problems with some other products).