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Potassium dichromate: chemically coloring wood
Potassium dichromate: chemically coloring wood
Rob Arnold
Published by linear
02-07-2007
Potassium dichromate: chemically coloring wood

The question comes up occasionally about using chemicals to change the color of wood. Usually the desired result is the color of aged wood--cherry and mahogany come immediately to mind as woods that develop rich, lustrous color when they are aged. Aging is free, but it's slow, and we're impatient woodworkers here in the 21st century.

Dye stains can darken cherry or mahogany quickly, but the wood beneath the dye does continue to darken with age, and you can wind up with a darker piece than you really intended.

So, is it possible to "hurry up" aging wood with chemicals? For some woods, yes. According to Understanding Wood Finishing, by Bob Flexner, Potassium dichromate reacts with tannic acid in wood. So woods with tannic content--oak, cherry, walnut, and mahogany--are good candidates for using dichromate coloring.

Dichromate is available from a variety of sources, and the price can vary widely. I obtained a pound on ebay for under twenty dollars. As you'll see shortly, this is a lifetime supply. Dichromate is an orange powder. It dissolves in warm water to make a vivid orange solution.

Dichromate is a hazardous chemical. Handle it with caution as you would with any wood finishing chemical. The powder is especially damaging to mucous membranes, so wear googles and a mask when you mix, and add the powder to the water instead of the other way around.

David Marks has an article up suggesting 1 teaspoon of dichromate mixed with 1 cup of water as a starting point. For you metric users, that works out to about 9 grams in 250 mL of water.

I mixed up a batch by these instructions, and used a foam brush to paint it onto samples of walnut, cherry, mahogany and oak. I'll let the pictures do the talking.



The effect on mahogany is astonishing. Within a half hour the wood takes on a deep, rich coloration. All the luster of the grain and figure remains--the light plays off the grain the same as before the coloration. The look is not muddy or blotchy.


Cherry can be a difficult wood to stain. but it takes the dichromate treatment well. I might mix a more dilute solution than the Marks formula above--cherry responds profoundly to the chemical.


Walnut is suprising. the dichromate depens the color without obscuring the grain at all. It's more of what you buy walnut for.


And on oak the effect is more subtle. It didn't knock me out at first but I really got to like it after a while..


Since this is a water solution, you'll probably want to take the time to wet your piece with water before coloring, then dewhisker your work after it dries. The water will raise some grain, and you'll want to avoid having that telegraph through your finish. By doing that first, you avoid having to sand the colored wood.

I'll put some more pics up in this tutorial once I get some time to take the test pieces through a final finish. I have read a lot of conflicting information about this technique (even Flexner discourages you from trying it!), so I had to try it myself. I highly recommend this method of aging for cherry, walnut, and most especially mahogany.

I put a few coats of wipe-on poly on the samples, so show a more realistic contrast:






Give this a try. It's well worth the effort to find a source.
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  #1  
By MilDoc on 02-08-2007, 12:59 PM
I've been wanting to try that, and thanks to your examples I will. The mahogany looks cool!
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  #2  
By LowerUnit on 02-08-2007, 11:07 PM
thank you for the insight - good information & nicely presented.
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  #3  
By TheRic on 02-08-2007, 11:34 PM
Great Info will have to remember this.......Curious on why Flexner discourges the use? Any proof there is no darkening over time?
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  #4  
By linear on 02-08-2007, 11:52 PM
Flexner's position is that aniline dyes give you a wide color pallette, and dichromate gives you one color effect.

My position is "look at that mahogany!" But I do see his point.

Proof, no, but the process of oxidization is what we are accelerating with the chemical here.
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  #5  
By scmhogg on 02-09-2007, 01:44 PM
Linear,

Thanks for the report. I particularly like the change in the mahogany. What type of mahogany is used in your sample.

You can obtain potassium dichromate and other chemicals used for aging or patina from the Science Company.

http://secure.sciencecompany.com/Gen...cals-C670.aspx

Steve
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  #6  
By linear on 02-09-2007, 02:43 PM
My yard sells it as "genuine mahogany." They also offer "Honduras mahogany," and "African mahogany." It is neither of those, nor is it Lauan. It looks and acts like the writeups I read for Swietenia macrophylla.
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  #7  
By linear on 02-11-2007, 09:50 AM
I just want to add a picture of a piece that was taken under sunlight instead of the shop lights. The effect on mahogany is very satisfying.

This business card holder was finished with dichromate and wipe-on poly.

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  #8  
By Jeffrey Schronce on 02-11-2007, 10:36 PM
Excellent write up! Based on your experience I have just ordered a pound of the stuff. Will heed all precautions.

Question, could be theoretical at this point, would reducing the concentration in the mixture lessen the effect or would it be more beneficial to use a lighter coat?

Edit : According to your write up it appears that would be the case.
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  #9  
By linear on 02-11-2007, 10:48 PM
Flexner and Marks both suggest you can do a more dilute solution, and apply repeatedly until you get the color you like.

I have done a little bit of experimenting, and that seems to be the case. But here's how I look at it. I wouldn't use any finish without running test pieces. So I started with the "reference" formula from Marks and added water until I saw some change. I think 1 tsp to a pint is a better base strength to dilute from. At the "reference" strength, I ran into some small crystals reforming on the surface where the water evaporated. I'd think you don't want to handle those. And the effect was only slightly diminished by dilution. And you can always add more coats to darken up your color.

I only just dampen my workpieces with a foam brush, so I didn't see much grain raising, and the dry time was around a half hour.
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