(Which) Stain & Heat ?

[PCello]

Dear Maestros,
Can anyone tell me which substance is being applied here? I have seen this type of stain multiple times, but I have never figured out what is being used.
Thank you very much!

https://www.instagram.com/reel/C7qmZpcoopX/?igsh=MWpzcHRid2k4cGsxcg==

[Advocatus Diaboli]

Nitric acid. 

[Wood Butcher]

Can someone explain what the chemistry is behind this? Why does it need the heat to oxidise, if that is what is happening, or this simply speeds up the process?

[PCello]

2 hours ago, Wood Butcher said:

Can someone explain what the chemistry is behind this? Why does it need the heat to oxidise, if that is what is happening, or this simply speeds up the process?

I asked ChatGPT the same: 

 

Lignin and Nitric Acid

Lignin is one of the main components of wood and reacts sensitively to chemical influences. Nitric acid (HNO₃) can oxidize lignin, leading to a brownish to dark brown discoloration. This technique has historically been used in violin making and furniture restoration to artificially age wood.

How does the reaction work?
    1.    Oxidation of lignin:
    •    Nitric acid acts as a strong oxidizing agent and alters the structure of lignin.
    •    This process produces quinones and other colored degradation products, which cause the darkening effect.
    2.    Acceleration through heat:
    •    Heat can enhance the reaction by accelerating the breakdown of organic components in the wood.
    •    The heat lamp in the video may be intensifying this effect—causing the wood, already treated with nitric acid, to darken further.

Why is this method interesting in violin making?
    •    It creates a natural aging effect, similar to the patina of old instruments.
    •    Unlike pigmented stains, it does not obscure the wood grain but enhances its natural texture.
    •    The coloration penetrates deep into the wood, making it long-lasting.

Caution:
Nitric acid is highly corrosive and can damage both the wood and health if not used properly. For this reason, some violin makers opt for milder alternatives or diluted solutions.

 

[Wood Butcher]

This seems like something which would be fraught with problems and danger. I will not pretend to understand a great deal about chemistry, but I would be concerned about any strong chemicals continuing to react in the decades after manufacture.
How could anyone be sure the reaction is fully neutralised, or that it would not affect the varnish in some way?
I even wonder what the effects of long exposure to intense UV in a cabinet for 8 weeks are. Surely this must be degrading the wood in some way, as part of forced oxidation process.

I know a lot of people are wanting to darken the wood as much as humanly possible, and recently there was a thread where someone was wanting to expose their violin to the gasses of trench warfare from 1916. I sometimes wonder if chasing the look has taken precedent over everything else for some, and potentially at the expense of the longevity of an instrument.

[David Burgess]

I don't know what it is.
My guess is that it's some sort of water emulsion applied to the surface, leaving a particulate (hence the whitish appearance), and that whatever the remaining particulate is, this melts into the wood with the heat from the heat gun.

[violinsRus]

I agree, it is nitric acid.  I would avoid using it. 

[Don Noon]

I haven't used nitric acid (and never intend to), but I'm curious how it could leave an opaque white surface that goes clear with heat.  David's idea struck me as the most likely, and it seems different from anything else.  I'd be interested in messing around with it... as long as it doesn't involve nitric acid or chlorine gas or phosgene.

[Davide Sora]

I don't know what substance it is, but just holding a heat gun of that type (not a hair dryer) so close to the wood and the gluings is enough to make me discard that method.

However, seeing the reaction, I would guess that some oxidizing chemical is involved.

[violinsRus]

1 hour ago, Don Noon said:

I'm curious how it could leave an opaque white surface that goes clear with heat

IMO there's no opaque white surface, just white wood with raised grain. That is what I've seen nitric do, not good to mess with the stuff.

[David Burgess]

10 minutes ago, violinsRus said:

IMO there's no opaque white surface, just white wood with raised grain. That is what I've seen nitric do, not good to mess with the stuff.

I've use nitric acid a hell-of-a-lot in large-scale boxwood peg production, and have never experienced anything anywhere close to what is seen in the video.

[Spelman]

6 hours ago, Wood Butcher said:

...recently there was a thread where someone was wanting to expose their violin to the gasses of trench warfare from 1916...

Tf?

[Christian Pedersen]

A 10% nitric acid solution allowed to dry followed by the heat gun is most likely what is shown. I have tried this. The 10% solution also works extremely well to stain boxwood. Likely much safer that at full strength as I was taught.

--Christian 

[David Burgess]

12 minutes ago, Christian Pedersen said:

A 10% nitric acid solution allowed to dry followed by the heat gun is most likely what is shown. I have tried this. The 10% solution also works extremely well to stain boxwood. Likely much safer that at full strength as I was taught.

--Christian 

I have never seen nitric acid alone, or diluted with water, produce such a sharply delineated color change upon heating. I think there's more to this story.

[Christian Pedersen]

[David Burgess]

Christian, watch the video again.
There were sudden differences in color between areas to which the same potion had presumably been applied, when heated. There was no gradient. Just a "wham-bam" color change.

You have a sudden color change between the area to which nitric acid was applied, and the untreated wood, but that's not what this video is showing.

[Christian Pedersen]

5 minutes ago, David Burgess said:

Christian, watch the video again.
There were sudden differences in color between areas to which the same potion had presumably been applied, when heated. There was no gradient. Just a "wham-bam" color change.

You have a sudden color change between the area to which nitric acid was applied, and the untreated wood, but that's not what this video is showing.

I didn't video the step I used the heat gun, but the reaction is exactly the same as in the video. I just didn't let the nitric acid solution dry fully first.

[David Burgess]

6 minutes ago, Christian Pedersen said:

I didn't video the step I used the heat gun, but the reaction is exactly the same as in the video.

Would you be willing to video that?

[Christian Pedersen]

 

 

[M Alpert]

1 hour ago, Christian Pedersen said:

A 10% nitric acid solution allowed to dry followed by the heat gun is most likely what is shown. I have tried this. The 10% solution also works extremely well to stain boxwood. Likely much safer that at full strength as I was taught.

--Christian 

Can this be trusted? I mean, that it wouldn't keep degrading the wood, over time?

I've never tried nitric acid, just heard horror stories, how it has turned tonewood to cardboard...

But oxidation with nitrites sure works, and lots of people use that. It seems like the reaction neutralizes itself as it plays out, but does anyone know 100% for sure?

Why would a mild solution of nitric acid be different?

A skeptic, who really wants to know...

[Christian Pedersen]

8 minutes ago, M Alpert said:

Can this be trusted? I mean, that it wouldn't keep degrading the wood, over time?

I've never tried nitric acid, just heard horror stories, how it has turned tonewood to cardboard...

But oxidation with nitrites sure works, and lots of people use that. It seems like the reaction neutralizes itself as it plays out, but does anyone know 100% for sure?

Why would a mild solution of nitric acid be different?

A skeptic, who really wants to know...

I'm not advocating it's use on anything other than boxwood fittings. 

[Don Noon]

1 hour ago, Christian Pedersen said:

 

 

Yeah, I guess that's a match, thanks.  I was thrown off by how white the wood was, and thought it had something opaque on it.  Looks very nice, but wouldn't feel comfortable with acid on the wood.

[dan gall]

I am an amateur, but I'd want to try this on a strip of rib stock and later a cheap in the white violin and use destructive testing to verify it has not penetrated the wood too far and left unreacted acid behind.  I imagine I'd apply it, then use a scraper to peel off a patch for a baseline, then bake the rest, then peel off another patch incrementally, saving the shavings from each depth in small jars.  then I'd add the same amount of water to each and test the ph.  If I was really on my game I'd figure out how much acid per unit area first and use that and the amount of water to calculate an estimate of the true ph at each depth.

Then I'd try wiping it down with a gentle basic solution (maybe sodium bicarb) with known and not undesirable reaction products (a salt and water).  Then I'd repeat the ph testing on another patch.

If and only if the nitric acid behaves well or can be made to behave well with a base would I use it.

[LCF]

It looks to me like there is an opaque coating on the wood?

 

Also it resembles what happens when you apply a sublimation dye to plastic and apply hot air.  I use that method on pvc. Sublimation dyes are very common eg security pass cards and driver's licenses, photos on coffee mugs.  

[Christian Pedersen]

1 hour ago, dan gall said:

I am an amateur, but I'd want to try this on a strip of rib stock and later a cheap in the white violin and use destructive testing to verify it has not penetrated the wood too far and left unreacted acid behind.  I imagine I'd apply it, then use a scraper to peel off a patch for a baseline, then bake the rest, then peel off another patch incrementally, saving the shavings from each depth in small jars.  then I'd add the same amount of water to each and test the ph.  If I was really on my game I'd figure out how much acid per unit area first and use that and the amount of water to calculate an estimate of the true ph at each depth.

Then I'd try wiping it down with a gentle basic solution (maybe sodium bicarb) with known and not undesirable reaction products (a salt and water).  Then I'd repeat the ph testing on another patch.

If and only if the nitric acid behaves well or can be made to behave well with a base would I use it.

I think this is a rather risky technique for the instrument and more importantly your health. The results as attractive as it might look are still short of ideal to me and long term may be similar to ozone treatment.