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Mark Sawyer
8-Sep-2017, 14:04
I've been reading about using Potassium Dichromate as a "stain" on mahogany on the woodworking forums in preparation for a camera project. It's not really a stain, but an oxidizer that darkens the wood. It was used quite a bit in the old days, as it is less likely to be blotchy and gives a nicer result, (it doesn't muddy the grain like a conventional stain).

Unfortunately, it's also toxic, carcinogenic, and can be absorbed through the skin or respiratory system. That kinda puts me off on using it. But it also makes be wonder, as it was used on many of our older cameras and other wooden ephemera, is there a danger of absorbing it through handling equipment that has an oil finish instead of being sealed by lacquer? Is it an issue when refinishing cameras? From what I've read, it doesn't break down much over time...

Jim Noel
8-Sep-2017, 15:16
If there was a danger in handling old wooden cameras treated with dichromate, i wouldn't be alive. I have been handling such cameras for over 75 years.

Sal Santamaura
8-Sep-2017, 15:24
..., is there a danger of absorbing it through handling equipment that has an oil finish instead of being sealed by lacquer? Is it an issue when refinishing cameras? From what I've read, it doesn't break down much over time...I have no expertise that would enable a valid answer, but...


If there was a danger in handling old wooden cameras treated with dichromate, i wouldn't be alive. I have been handling such cameras for over 75 years.I am certain that Jim's answer is no more valid than the claim of someone who says "I've been smoking for 75 years and don't have lung cancer. There's no danger in smoking."

There are too many factors related to individual genetics for such extrapolation to be wise. Hopefully, someone who really knows the answer will post.

Greg
8-Sep-2017, 16:11
http://forums.finewoodworking.com/fine-woodworking-knots/finishing/potassium-dichromate-oxidizingcoloring-wood
https://www.shellac.net/POTASSIUM_DICHROMATE.html
http://www.woodweb.com/knowledge_base/Is_potassium_dichromate_worth_the_risk.html
http://www.emgw.org/Resources/Documents/Papers%20and%20Articles/ChemicalStains.pdf

think I'll stick to Minwax....

LabRat
8-Sep-2017, 16:19
The main reason for staining wood is that if one just finishes hardwood and applys a finish, that even from the same species of wood that there is a very great difference of the color/depth/and look of the wood, so from a mfgs point of view (that are selling a product that is uniform looking), a way that will even out major differences is desired...

Dichromate is very orange, but darkens from light/age and penetrates well and evenly, and will even out many differences in wood... If surfaces are sealed, even tung oil should provide a barrier from contact... It would transfer the color to one's hands quickly (and there is the well known industry term "dichromate poisoning" ) so if it couldn't be sealed, it would not be used at all (even in the "bad old days")... Probably a bigger issue if used for something that gets a lot of handling, like musical instruments...

You probably have a bottle in you darkroom chem locker you can try, but I expect it would be more toxic to apply, but benign while it sits there (and used) over many years... But it can be handled safely, even in liquid form (I have a brief foray into holography where it is used to form the image)...

Steve K

Mark Sawyer
8-Sep-2017, 16:28
http://forums.finewoodworking.com/fine-woodworking-knots/finishing/potassium-dichromate-oxidizingcoloring-wood
https://www.shellac.net/POTASSIUM_DICHROMATE.html
http://www.woodweb.com/knowledge_base/Is_potassium_dichromate_worth_the_risk.html
http://www.emgw.org/Resources/Documents/Papers%20and%20Articles/ChemicalStains.pdf

think I'll stick to Minwax....

Yup, read those, and more. All kinds of answers, from "wear a respirator, facemask and gloves, and only open the container outdoors while standing upwind" to "don't worry about it". Potassium Dichromate is also used in photography for gum printing, carbon printing and photogravure, with varying sorts of precautions taken.

I'd like to use it, but yeah, Minwax sounds safer. I also have a couple of old studio cameras that someone before me lightly refinished with just an oil finish (no lacquer or polyurethane type sealers). Should I seal them or handle them minimally?

Yeah, probably worrying to much...

Mark Sawyer
8-Sep-2017, 16:33
If there was a danger in handling old wooden cameras treated with dichromate, i wouldn't be alive. I have been handling such cameras for over 75 years.

I've only been handling them around 40 years. But a coat of lacquer or such might mitigate the problem. What about an oil finish? Or does it reduce to something else after oxidizing the wood? If I do use it, is there something to treat the wood with after darkening to eliminate the worries?

I have former students now it their early 20's who may handle the cameras too...

Mark Sampson
8-Sep-2017, 16:54
I'd forget about it and go do some gum printing. It was in the late '70s when I took a gum printing class- IIRC you use potassium dichromate as a sensitizer in that process. We did use rubber gloves, but I never produced anything I liked, and have never gone back to it.
On-topic, perhaps, there's a hell of a lot more old mahogany furniture out there than there are wooden view cameras. If there was indeed a problem, might we not know about this already?

Mark Sawyer
8-Sep-2017, 17:42
On-topic, perhaps, there's a hell of a lot more old mahogany furniture out there than there are wooden view cameras. If there was indeed a problem, might we not know about this already?

Quite right. But this is the twenty-first century, and everything is going to kill us...

Harold_4074
8-Sep-2017, 18:26
Potassium dichromate is a strong oxidizer, and all of the "drying oils" used in traditional varnishes are strong reducing agents (cellulose is no slouch, either) so it is very unlikely that you are going to be poisoned by potassium dichromate leaching through an old finish. A modern polymer? Maybe, but unlikely, since the salt is water-soluble and these finishes are usually formulated to be waterproof.

A stronger consideration might be the resale value to a person who believes that anything hazardous on an industrial scale will also be lethal just due to proximity. Trying to explain reality to some of these people may shorten your life expectancy more than exposure to the hazard under discussion. (Stress and frustration are not good for your cardiac health :) )

chris_4622
9-Sep-2017, 15:51
Using it for one project will not cause over exposure. Just wear gloves and use it in a ventilated area. I wouldn't recommend using an oil finish unless it has driers in it.
I have a lot of experience using it on Mahogany but I've always used shellac as a finish. The real problem is if you don't have experience applying it you could end up with uneven areas.

Mark Sawyer
9-Sep-2017, 16:30
I'm not so much worried about the exposure during application, as precautions can be taken. It's more the handling after the camera is built. As in, after drying and darkening the wood, should I wipe it down with a damp rag before finishing? And would a water-based polyurethane finish provide sufficient protection for long-term handling?

From what I've read, it's much easier to get an even, non-blotchy finish with potassium dichromate than with conventional solvent stains. I'm still caught between the two...

el french
10-Sep-2017, 00:18
Won't other safer chemicals give you the same look? Do a search on ebonizing wood.

chris_4622
10-Sep-2017, 06:28
I'm not so much worried about the exposure during application, as precautions can be taken. It's more the handling after the camera is built. As in, after drying and darkening the wood, should I wipe it down with a damp rag before finishing? And would a water-based polyurethane finish provide sufficient protection for long-term handling?

From what I've read, it's much easier to get an even, non-blotchy finish with potassium dichromate than with conventional solvent stains. I'm still caught between the two...

No do not wipe it with anything, it is ready to be top-coated. I don't have much experience with water based finishes. I would consider a solvent based finish. After the first thinned coat you will have to sand lightly.
When applying the PD use a brush. It's difficult to explain but you don't want to wet a portion and let it soak in without doing the whole area because repeatedly wetting it will cause it to become darker. So avoid that by working one area quickly and moving on. Cameras are not big so you shouldn't have a problem.

jim10219
12-Sep-2017, 11:28
It'll be fine after it's applied. I do a lot of gum printing. While the potassium dicrhomate is pretty dangerous in crystal and liquid form, the gum print itself isn't dangerous once it's dried. If your oxidizing wood with it, just be sure to wipe off any excess after your done and give it time to stabilize. Sealing the wood be a good idea as well. Once the wood stabilizes, it should no longer pose a threat. It breaks down during the oxidation process. They use this stuff in commercial concrete as well. The main danger is when you're applying this stuff.

Tim Meisburger
12-Sep-2017, 12:03
For what you want it for, I don't think it will cause you any problem. You can also stain mahogany with lye, or lime, in solution. Lye is caustic, so you need to use care, but I use it without much worry to make soap.

Mark Sawyer
12-Sep-2017, 12:21
Won't other safer chemicals give you the same look? Do a search on ebonizing wood.

Ebonizing is sort of the same, an iron-based oxidizing, as I understand it. But I don't care for the cold greyish-brown to black tones.


No do not wipe it with anything, it is ready to be top-coated...


...just be sure to wipe off any excess after your done and give it time to stabilize..

Oh, you guys are a big help! :rolleyes:

I'm guessing it's okay to use carefully, I just wish there were some reliable info on how it breaks down over time. That chromium must still be there somewhere...

goamules
13-Sep-2017, 06:03
I guess I started this. I told Mark that I like the way it works, having used it some on mahogany. I don't know where I read it, but did read that most mahogany furniture used it for generations. Since in America mahogany was the most popular wood for furniture in the 18th and 19th centuries, I have to believe generations were eating off of it, playing cards on it, laying across pianos made of it.

I didn't think about it when I used it after brushing it on and letting it dry. I think rubbed oil on it. Yes, rubbed. Maybe a mistake, but I have a feeling if furniture makers were doing this for entire careers, a couple times won't hurt. You do hear about miners, assayers, and occasionally daguerreotype photographers getting sick from mercury poisoning. That was probably a worse substance.

Steven Tribe
13-Sep-2017, 06:58
I remember seeing that mortality rates for people working with hard woods (specifically mahogany and walnut) in the days before the mechanical removal of dust were pretty awful and specifically linked to cancer of the air passages. So any problem with dichromate would have been masked. I think the problem is of the same order as Thorium in post WW2 lenses - just show due caution in reworking the wood surface - the Chromium is still there.

Tim Meisburger
18-Sep-2017, 01:58
After researching this, I ordered some to use on the camera I'm making:p

Pere Casals
18-Sep-2017, 03:55
Leather shoes not treated with Dichromate may be difficult to find... so final product may not be hazardous if precautions used to change oxidation state of remaining Chromium.

But working with Dichromate can be very hazardous, and it's better to avoid it.

Mark Sawyer
18-Sep-2017, 09:45
I'm going to try it (carefully) on a few mahogany scraps, and perhaps a lens box or two, and compare it to more conventional stains (which win no safety prizes either).

Pere Casals
18-Sep-2017, 13:20
I'm going to try it (carefully) on a few mahogany scraps, and perhaps a lens box or two, and compare it to more conventional stains (which win no safety prizes either).

One tip. By mixing a Dichromate solution (orange colored) with used BW developer it hapens that chromium changes the oxidation state to a way safer stuff, it changes color from orange to dark green. I've been using this tip to lower Dichromate toxicity after finishing job, when using it as bleach for BW reversal. So I suggest to use that, or equivalent process.

"By mixing it with used developer" ... "The orange chomium(VI) is reduced to the green chromium(III) which is much less toxic and harmful to the environment and is no longer carcinogenic."

https://www.photrio.com/forum/index.php?threads/ammonium-dichromate-vs-potasssium-dichromate-in-reversal-processing.101014/

You also can "clean" tools, gloves, etc with a chemical that reduces the VI to the safer state.

domaz
18-Sep-2017, 13:34
One tip. By mixing a Dichromate solution (orange colored) with used BW developer it hapens that chromium changes the oxidation state to a way safer stuff, it changes color from orange to dark green. I've been using this tip to lower Dichromate toxicity after finishing job, when using it as bleach for BW reversal. So I suggest to use that, or equivalent process.

"By mixing it with used developer" ... "The orange chomium(VI) is reduced to the green chromium(III) which is much less toxic and harmful to the environment and is no longer carcinogenic."

https://www.photrio.com/forum/index.php?threads/ammonium-dichromate-vs-potasssium-dichromate-in-reversal-processing.101014/

You also can "clean" tools, gloves, etc with a chemical that reduces the VI to the safer state.

This advice seems a little risky to me. Mostly because you aren't specifying which developer you are mixing with. It's unlikely that mixing used Pyro developer would have the same results as mixing it with used D-76 for instance (it could even be disastrous). You can reduce Hexavalent chromium to the safer CrIII using Asboric Acid. That is probably the safer approach and is documented in a scientific research paper (https://www.researchgate.net/publication/8227753_Reduction_of_ hexavalent_chromium_by_ascorbic_acid_in_aqueous_solutions).

Pere Casals
18-Sep-2017, 16:22
This advice seems a little risky to me. Mostly because you aren't specifying which developer you are mixing with. It's unlikely that mixing used Pyro developer would have the same results as mixing it with used D-76 for instance (it could even be disastrous). You can reduce Hexavalent chromium to the safer CrIII using Asboric Acid. That is probably the safer approach and is documented in a scientific research paper (https://www.researchgate.net/publication/8227753_Reduction_of_ hexavalent_chromium_by_ascorbic_acid_in_aqueous_solutions).


You are right, a list of safe developers should be added...

Xtol contains ascorbic acid as main developer. I've used that tip with Xtol and D-76 with good results.

I would not mix dichromate with Pyro without knowing safety...

It is true that mixing Dichromate with used developer has been a common practice in reversal process with Xtol, D-76, Dektol (also Vit C there) and Ilford PQ universal. This has been done extensively in cinematoghraphy, reversing Tri-x, Double-X , etc it has been common in the past, often with Dichromate bleach. But as you suggest, mixing chemicals can be hazardous, so before mixing a developer with Dichromate one must know that it can be done with involved developer, you are right. Goog to know that Ascorbic Ac is a good way...

In fact what I wanted to point is that Dichromate should be reduced to CrIII as soon as possible after usage, including drops remaining in glassware, gloves, etc