Iron Solutions for Ebonizing Wood

Advanced tips on making and applying dark black stain with solutions of iron in water and various solvents. August 26, 2008

I am refinishing an oak hardwood floor and would like to stain it to an ebony color. I have been making samples using the “liquid nightmare” approach, distilled vinegar and steel wool, and have been happy with the end results. I have worked with the iron concentration in the solution, the time and exposure to oxygen and have found a formula that I like but every formula that I try leaves a rust colored film that must be wiped off with a dry or wet rag before sealing.

The problem with wiping with a wet rag is that this raises the grain in the wood and finish depth is relatively thin so even a light sanding will expose the original color at the edges. I have filtered the solution through coffee and paint filters multiple times but this does not seem to help. Does anyone have any suggestions for eliminating the film or cleaning the floor without raising the grain? Thanks in advance for any advice.

Forum Responses
(Finishing Forum)
From contributor R:
I think the filters you’re using are not fine enough to filter out the residue. Try putting a few folds of a cotton T-shirt over the top of a plastic bucket and then put a rubber band around the T-shirt to keep it in place. Tap the center of the filter to create a little bit of a crater. Pour the "nightmare' into the plastic bucket being careful so as not to splash it everywhere. Also, after you remove the steel wool, or the nails, let the solution sit for a day or so before you strain it into the bucket.

I’ve stained a few floors and I found I got good coverage and evenness by applying the stain from one of those plastic pressurized units the gardeners use for applying fertilizer.
Some of the better sprayers have additional tips you can buy separately. The one I have is a two gallon unit and one of the tips I bought separately gives me a fan that’s nearly two feet in width.

I’ve also used the same unit for on the job spraying of frame and panel wall panels like you would find in an old library. It turned out to be a real time saver and money well spent.

From contributor C:
Contributor R’s post is accurate, but I’m wondering if you’re raising the grain with warm water before your final sand and application of the brew? This is normal if you want to keep grain raising to a minimum.

On hard woods like oak, you may have to add a small amount of diacetone alcohol to your brew for better penetration. DAA has the ability to open up the cellular porosity to help the deeper penetration of the substance applied over it or with it. 5-10% at most - above that it does no further good. Also in my experience I’ve found with almost all chemical or natural stains that more than one coat gives better results. Keep in mind almost all of the mordant/stains that were commonly used at the turn of the century and later were intended for vat dying, but came to be used as stain later by the turner's especially as a stain.

In vat dying the wood would be soaked for days in the chemical and dye vats to acquire a through color on the veneers - so to get adequate penetration it is usually (though not always) better to re-dye the surface with warm not boiling brew's. You can also use glacial acetic acid in alcohol to make your iron acetate concoction or as RBC says "nightmare" - to cut down the grain raising (normal store vinegar is only 5% acetic to 95% water). This can be doubled with a 10% glacial acetic in 90% alcohol to put the iron containing material. I also prefer the fast rusting cast iron powder to the steel wool or nails, this way it can be weighed or measured or both to be able to get repeat results. I also - though not necessary - apply a saturated tannic acid solution to the wood before any dying to help overall uniformity when the stain is applied, though with a floor it may not be as necessary.

The film you’re getting, (which I have so far not encountered), may be due to a over abundance of the iron leaving a powdery iron residue on the surface. I'm also wondering if you sanded the wood too smooth since you seem to have so little penetration.

From contributor P:
To the original questioner: in your post you say the "rust colored film" can be wiped off with a dry or wet rag. If you need to wipe it off, why not just use a dry rag and avoid the whole raised grain issue?

I'd try an experiment first to see if you really do need to wipe off what appears to be a discoloration. I use dyes quite a bit in my stained finishes and occasionally get a surface discoloration called "bronzing." It's caused by an excess of dried dye on the surface of the wood and is always a different color than the dye itself. I've learned that it doesn't actually pose any problem at all and I can apply the clear coats right over it. The bronzing color disappears the instant the clear coat wets it out.

When you dissolve iron in vinegar, you're making iron acetate. An alternative to making your own iron compound is to buy some iron sulfate (aka, ferrous sulfate). It's also called copperas, green copperas, or green vitriol. You just mix it in warm water. This makes it easy to get the same grey to black color intensity no matter how many batches you mix, as well as saving a lot of time waiting for the "brew" to come up to strength. Ferrous sulfate is inexpensive and available from a number of on-line suppliers. You can make the color darker by mixing more concentrated solutions as well as applying multiple coats.

To get a true ebony or black color on wood, I like to start with a dye and then use a black pigmented stain over it. The combination of dye and stain produces a deep black color (controlled by the intensity of the dye) that also does a very nice job of allowing the grain and shimmer of the wood fibers to show through. There's an example of this finish at this link

Staining and Blending Difficult Woods

On the subject of color penetration that Contributor C introduced, it's normal for dye to only go as deep as 4 thousands of an inch on hardwoods and 12 thousands of an inch on porous soft woods. I have never seen any information that indicates a solvent like diacetone alcohol will increase the depth of color penetration into wood and I've read a lot of reference material related to wood finishing. Diacteone alcohol is a solvent for lacquer and other finishes.

After reading his post, I did do a search on the internet and couldn't find anything that supported using this, or any other, solvent to improve dye penetration into wood. I did find one reference that stated dye dissolved in water has the deepest penetration, but I didn't make any effort to verify or discount that sole reference.

I did find a couple web pages that are related to dye penetration into wood. The first is an experiment conducted by a woodturner who wanted to color the wood throughout so that it remained colored even after turning. He immersed the wood into the dye and let it soak for days without any success (using a couple different solutions/solvents). Then he immersed the wood in dye and placed it under a vacuum. He had some success using the vacuum, but not complete.

Although you can't expect the stain you're using to go very deeply in the wood, the nice thing is that you can do seamless touch-ups to the color if you do get any cut-throughs. Just re-apply the stain to the affected spot(s) using the same number of coats as you did on the rest of the floor, let it dry, and apply the clear coats. It will blend in without the change in color that can happen using pigmented stains.

From the original questioner:
All of this information is excellent. I am wrapping up another project so I have some time to experiment with the process before I finalize my decision.

There have been several questions so let me see what I can answer. Are you sanding before staining? I am and I have been playing with 80-120 grit on a random orbiting sander one strip at a time. It’s not exactly the process but close enough. It does not seem to have a significant effect on the penetration. I may try the DAA but if I can get rid of the residue by using a settling process and a t-shirt filter then penetration will not be a significant issue.

Why are you wiping the residue off? Unlike the residue that you have seen in some of your dying processes, at the concentration that I am seeing it causes a little splotching if not wiped off between applications of the iron acetate. When I try to clear right over it I get some red color trapped in the deep grains. I am considering using a Tampico Brush on a buffer as a final finish but if I can avoid one step of a multi step process by eliminating the residue all together even better.

The finish on the on the article on staining and blending difficult woods look beautiful. The client thought that they wanted black. I used iron acetate then India ink diluted in alcohol as a topcoat before sealing and finishing the samples. They are beginning to prefer the deep chocolate brown and black tones that two-three coats of the iron acetate creates. The iron acetate is great for touch ups if you do get any cut-through. Then there is the benefit of it being “natural”. I hear that a strong black tea mixture is high in tannic acids so I will probably look at doing a warm wash with this before my final sand to minimize grain raising through the finishing process. Thanks for all the great information.

From contributor P:
Denatured alcohol has the same effect on bare wood. It "opens up the wood pores" which has the effect of trapping more stain at the surface of the wood and making the color darker. If you use a non-grain raising (NGR) dye that's reduced in alcohol it will also open the pores of the wood as well as adding the color of the dye to the overall color of the finish.

But you can't dump denatured alcohol into an oil-base stain because the alcohol and mineral spirits don't mix well. I believe that's the reason that SW is using the diacetone alcohol; it's compatible with a wide range of organic solvents. So instead of wetting the wood with denatured alcohol (or another alcohol) to open the pores, letting it dry, and then applying the stain they just add the alcohol to the stain.

It doesn't actually make the stain (or dye) go deeper into the wood, it just swells the surface fibers of the wood so that more stain lodges there. There are better ways to produce dark colors on wood that offer a lot more control and repeatability. Dying the wood before staining, toning the wood after sealing the stain, and glazing all can be used separately or in combination. To get that "muddy" lifeless look (like laminate), first stain the wood and then add some of the stain to the finish for use as a toner.