[EricM81]

In the jewelry industry we mix a solution of denatured alcohol and boric acid.  This solution is applied to precious metals prior to the application of a torch to prevent oxidation.  When burned, the boric acid creates a glassy surface barrier that is easily removed by pickling (heated sodium bisulfate solution).

Assuming I'm using denatured alcohol containing the cheapest/nastiest additives, would 99% IPA burn safer from my lungs standpoint?

Would something work better/safer? It's stored in a semi-airtight container on our benches and alcohol is added once a week due to evaporation.

[Enthalpy]

Hi Eric! (Messier 81? An astronomer? Welcome anyway!)

I expect denatured alcohol as well as IPA to burn cleanly, producing very little harmful fumes, so the vapours of the alcohols are more of a concern than their fumes (you don't swallow the denatured alcohol of course, do you?). What in detail boric acid does in a flame is less clear to me.

Do you have something to suck the gases, fumes and vapours away from your nose? More useful than picking one alcohol versus an other.

[EricM81]

Thanks, Enthalpy.  A venting solution would be optimal and is the long term strategy, but it's difficult.  At every torch, there's two or three positions where a torch is applied.  A respirator is used during longer jobs, but no one puts them on for the quick and dirty.

Boric acid (mixed with borax) and alcohol is an ancient barrier flux.  It prevents the build up of firescale, otherwise known as cupric oxide.  The alcohol is a convenient way to flow the boric acid powder evenly over the piece and you hit it with a lighter before turning on your torch.  It burns green and when it goes out, the piece is coated with a glassy surface that inhibits the copper in the alloys from interacting with oxygen from the torch or atmosphere.  It's so ancient and habitual that we do it even for metals that don't even need it.

I'd heard from another jeweler that his doctor suspected the long term use of denatured alcohol had damaged his lungs and kidneys.  I was thinking that if IPA vapors were cleaner, I could substitute a 90% concentration for the denatured and live longer. 

[Dan]

I'd heard from another jeweler that his doctor suspected the long term use of denatured alcohol had damaged his lungs and kidneys.  I was thinking that if IPA vapors were cleaner, I could substitute a 90% concentration for the denatured and live longer.

Depends what your ethanol denatured with. I expect it is the denaturing additive (e.g. methanol) that would most likely be the concern here.

If IPA works for your application, I don't see a downside in switching to it.

[Intanjir]

Modern denatured alcohol formulas have long been moving away from methanol.
At some point many governments realized that the point was to make it irreversibly undrinkable and not to outright poison their citizens engaging in occupational or (ill-advised) recreational activities. So they now seem to want to add things that give you a wicked 'hang-over' or taste abominable or just result in a generally unpleasant experience by whatever means but without lasting serious toxicity.

As a result recent formulations seem to greatly utilize simple ketones.
The EU recently agreed on denaturing with 3% isopropyl alcohol and 3% MEK (by volume) and bitter flavoring. Isopropyl is metabolized into the simplest ketone: acetone.
So if you are in the EU, it would be somewhat odd to switch to pure isopropyl out of fear of 3% isopropyl but I suppose you might still do it out of excessive concern over the MEK.

Most formulations are probably not as benign as this one though. But even in the US none of the totally general formulations incorporate methanol, though some of the formulations only allowed for specific activities have methanol. That said the potential for 2.5% nitropropanes in some of the general formulations is less than appealing.
http://www.ecfr.gov/cgi-bin/text-idx?c=ecfr&sid=f7866313787bfcfb2b9cd25ba6af81b5&rgn=div5&view=text&node=27:1.0.1.1.17&idno=27#sp27.1.21.c

Ethanol fumes are of minimal concern so denatured alcohol has the rather considerable virtue of diluting the fumes of its denaturants. So a 3% isopropyl mixed in ethanol will have in the rough vicinity of 3% of the fumes of pure isopropyl. Given this fact I would not be concerned at all if exposed to fumes from the above EU formulation.
If you live in the states and are concerned about the formulation you might examine the above linked page with the full list of formulations allowed for various applications.
In particular the only denaturant in Formula No. 3-C. is isopropyl alcohol and it is very likely allowed for your application. I would personally rather be exposed to fumes from 95% ethanol with 5% isopropanol than to those from 100% isopropanol.