Potassium Dichromate or Lye for Darkening/Oxidizing Mahogany

F

Ford Fitzkee

Member
I was hoping to get some advise on people's experience using either potassium dichromate or lye to oxidize mahogany and other types of wood.  I am well aware of the toxicity of the chemicals and required safety precautions. 

The piece in question is a large federal desk/library table seen in the photo that I recently finished construction on. It is made out of some really nice African Mahogany that I got from a local lumber yard (no Genuine Mahogany within a 2 hour drive, unfortunately). I am very pleased with how the African Mahogany worked out.  I was able to match the boards fairly well at the lumber yard after digging through the stack a bit.  The top has especially nice ribbon striping.  Tearout wasn't a problem despite the reversing grain of the ribbon striping with a sharp jointer plane and high angle smoothing plane, with the chip breaker set extremely close to the edge. 

The issue is how pale the mahogany is after planing.  I want to finish the piece with shellac, and did some test pieces with amber/orange shellac as seen in the photo, but the result not nearly as dark as I was hoping for.  I am also going to try a test peice using Garnet shellac, but I don't think it will make a huge difference.  I want that deep rich aged mahogany look seen in period mahogany furniture.  I would like to avoid dyes and stains if possible due to issues with fading and the wood oxidizing naturally, thus changing the color years down the road.

I am leaning toward using Potassium Dichromate or Lye to oxidize/darken the wood before applying the appropriate shellac (blonde, amber, or garnet).  Despite the use of chemicals, this seems like a more natural outcome to me than stain or dye since you are simply accelerating the oxidation process that would occur naturally over the decades.  I will obviously do as many test pieces as required before putting anything on the desk.  I would love to hear about people's experience with these products, preferences between the two, successes, failures, and techniques.

Thanks,
Ford
 
Personally I stay away from any chemical stain because you have no control over it.  The look you are after can be achieved with dyes/stains/glazes.  With using different shellacs-garnet/blonde etc...you will need to do many mulitple coats to see the color difference and effect they have.

 
Ford,

I have used Potassium Dichromate quite a bit in the past few decades.  In can be helpful when a person is trying to get a match with old wood.  These have been my concerns:  1) In spite of being an oxidizer, it does not work in precisely the same way as 'natural' oxidation.  It's color is somewhat different, going more toward reddish-purple on Honduras mahogany.  Also, it will color the wood uniformly, which is not the way wood ages.  2) In my experience, all wood changes color, usually by fading, upon exposure to light.  Artificially colored wood seems to change more dramatically.  Wood colored with Potassium Dichromate seems to fade toward a greenish hue, which can look pretty bad in extreme cases.  Because you are obviously willing to experiment with different materials to get the effect you want, I would suggest using a combination of Potassium Dichromate with aniline dyes to get a ground color, followed by some glazing for an 'old' look.  Some shellac and wax and "Bob's your uncle."  PSP
 
Some good points were made, pretty much right on. Myself, I wouldn't use potassium dichromate for what you are trying to do. With P.D. you get what you get, with lighter or darker shades by thinning. No control for correcting color or changing it. Use dyes or stains, you have better control. Unless you are putting this in direct sun light, don't worry what will happen in the future. As the stain fades the natural ageing will take over. Seen the neighbors kid bite a table a few days ago, can't see using things more dangerous than they have to be. Gave a class on dyes this morning, potassium dichromate didn't come close to the color we got from dyes. For mahogany I start with red or orange base color and build on that. If you haven't seen it done, it takes some nerve. Working through my 35th year finishing wood, where did the time go?
Randy
 
Olde Mill sells button lac with a really dark rich color. A few years ago they had some black shellac. I've found the button lac had the most effect on the color, and was about twice as dark as any of the garnets I've used.

I've never used the black but have heard it makes a great base and darkens the wood quite nicely.
 
I appreciate all the suggestions.  Has anyone used lye as another alternative, or is it similar to the potassium dichromate?  I think I am going to do some experimenting with aniline dyes and different concentrations of Potassium Dichromate (with necessary safety precautions) on scraps to see what I get.  Luckily all these options are fairly inexpensive so it can't hurt to experiment a little.  I will definitely post the the results I get.

-Ford
 
Ford, I first used Potassium Dichromate in the late 70's on violins, since then I have used Ammonia, Potassium Dichromate, Potassium Permanganate, lye, drain cleaner, oven cleaner, tannic acid, different bleaches, muriatic acid and nitric acid and others that I can't remember. They all give a different effect, in my experience the difference between pigment stain, dye stain and chemical stain is that with pigments and dyes you are adding a colorant to the wood, with chemical stains you are causing a chemical reaction that changes the color of the actual wood, by combining all three you can create effects that that can't be done with just one or two. The only book I know of that deals with chemical staining in any depth is " Classic Finishing Techniques " by Sam Allen. Yes chemical use can be dangerous, you must be cautious, you can create beautiful white oak finishes with dyes and stains but if you want a real fumed oak finish you have to fume the oak with Ammonia.
 
Not to disuade you from experimenting with anything but often times I am asked to repeat a color I have already done on a piece.  You will not be able to do that well with a chemical stain. 
 
Now your on the right track, experimenting is cool. As long as you don't have to answer to someone for it. Lye is especially interesting on cherry. Chemicals, stains and dyes are all tools to a finisher, especially when you know what they will do before you do it. I studied with the  master of chems, George Frank, when I attended and taught at the wood finishing course at DCTC. His books are full of chemicals and color photo's of samples.  His first book Adventures in Wood Finishing has his history, and the second, Wood Finishing With George Frank, both have recipes.  His stories will answer a lot of your questions and give you a chuckle besides.CBWW has a good point, as a restorer I don't use chems very often. The consistency isn't there and for the most part are not reversible. Yes, sometimes I make mistakes and need to back up, chems don't do that. Be careful.
Regards
Randy
 
I wanted to give an update on how the darkening of the mahogany went.  I decided to try the potassium dichromate after doing a number of sample boards with different concentrations.  I am thrilled with the results.  It really brought out amazing color and character in the wood.   See the attached photo.  I opted for a 1 teaspoon potassium dichromate to 2 cups water concentration based on my samples.  I applied 2 coats with a sponge to ensure full coverage and no missed areas.  This ended up a bit darker than I was shooting for, but was still close to my goal and looks awesome.  I think it turned out darker than the sample because on the sample I blotted up the excess, reducing the effect.  On the table, it took more time to get the surface covered, and by the time I went back to blot up excess it had fully soaked into the wood.  Lesson learned.  It certainly is tricky to control, but it is doable.  The 3 factors affecting the range of darkness are the concentration, the  amount applied to the surface, and the length of time it is left on the wood.  Concentration has the largest impact.  The  other 2 factors can be kept constant by applying the same number of coats to all surfaces evenly but thoroughly (not allowing to puddle) and let it fully absorb into the wood.  Therefore the effect can be consistently controlled only by the concentration used.  It seems like the range  of appropriate concentrations based on my samples was about 1teaspoon PD/ 6 cups water, which yielded a  small but noticeable darkening effect, to 1 teaspoon/1cup of water, which is really dark and a little too dark for my taste.  These concentrations may have different effects on different species and pieces of wood.  The African Mahogany I chose for this table came from 3 different boards that were all close in color, but were certainly not from the same tree.  The color match after applying the PD was just as good, and maybe a little better.  I raised the grain before applying the PD so there wasn't much raised grain after application.  The darkening started right away, with the full effect taking about 1 hour.  It didn't cause any splotching and yielded a very consistent darkening effect, while still showing the natural variation in color and shades in the grain of the wood.  Essentially it darkened everything the same amount so lighter areas of the mahogany were still lighter than the darker areas after the PD was applied. The only change I would make next time would be to use a 1 teaspoon/3 cups water concentration for a slightly lighter color.  Anyways, that pretty much sums up my experience.  My apologies for the long winded follow-up, I just wanted to  give everyone an accurate description of my personal experience with the potassium dichromate.  Thanks to everyone for the help and suggestions.

-Ford
 
You must log in or register to reply here.
Back
Top