Fun with Vinegar-and-Iron Dye Stains

Iron soaked in vinegar makes an interesting dye that can be black, gray, or brownish-red, depending on the application and use. Here's a discussion of the chemistry and methods involved. July 29, 2007

I was unsuccessful in creating an ebony finish using vinegar and nails for a project I am working on. However, what I did get was a beautiful reddish brown finish on pine and maple.

What am I laying down and leaving behind on the wood? And would this finish be something that would be good to use on an outdoor stained project as a replacement to a pigmented stain due to the fact that there is a chemical reaction taking place? Much of my work is exterior and I am continually looking for optimum finishes to handle the difficult weather conditions. I finish the works with D-Dur for UV protection, but I am curious as to the properties of this staining process and its potential value on exterior work.

Forum Responses
(Finishing Forum)
From contributor G:
You will be able to get a darker effect with your solution if you use high test pickling vinegar (7% acetic acid) and put 0000 steel wool in it rather than nails. This will give a stronger iron acetate solution. You can also spray the wood with tannin (strong tea) to make the color darker. Iron acetate is a metallic salt and thus should be fade resistant, but that is theory on my part. I have never used it outdoors.

Another approach would be to get a stain made of lamp black or carbon black. Carbon won’t fade. You can also have a shading lacquer made with LB in your D-Dur, which will help a lot with the UV resistance. You are getting UV inhibitor added to the D-Dur, aren’t you?

From contributor L:
The iron reacts with tannins to create the greys and blacks. Your maple stain is created by rusting of the iron solution and is also an old traditional finish (often used by colonial gunsmiths). Woods such as oak will have more tannins and will blacken when treated with the iron solution. Otherwise tannins must be added to the wood to get a black reaction.

The old time gunsmiths would heat treat their maple stocks after treating them with the iron solutions. This method creates a sudden color change that is dramatic and interesting. It is especially effective for figured maple.

From contributor J:
I'm interested in getting that gray weathered look on some red fir I'm using for a fireplace mantel. I read about the vinegar and steel wool with rusty nails. But when you say to "cook," is that as in heat? And for how long would you leave this brew standing?

From the original questioner:
I am delighted to hear that this would be an exterior finishing alternative. But in discussions with Chemcraft, I am sure the inhibitors were added to the D-Dur.

I was so excited by the colours this mixture of steel wool and nails was making that I immediately started using it on a wood sample for a potential customer. I added a pinch of black pigment and applied 7 coats of the mixture to a large ornate maple appliqué. Three coats of flat lacquer and the result was a deep rich brown that is just not yet black, with hues of reddish brown peeking through. Just gorgeous. I made a second batch without the steel wool and the colour was a faded grey. So I am going back to the original formula but will add the 7% acetic acid and see what happens. I was trying to achieve an ebony finish, but what I got was better and magnificent. I prefer it for the application.

Your replies have given me some real things to experiment with. I enjoy getting right out and trying these tips. Finding something new out about finishing is exciting. I am going to try adding the heat element to see what you described. That sounds like it could look amazing. Any more details on that process would be great!

I guess the down side could be that you will get vast colour variation, on large projects, from different reactions to each mixture and various cuts of wood. Controlling the colour could be a real challenge I imagine. The upside is that this is cheap and very rich looking. If you could get it down, it could be very applicable to a rustic exterior work.

My formula was specifically 3/4 of a mason charge of white no name grocery store vinegar, filled with Bull Dog 0000 grit steel wool. It sat 36 hours. I added 10 forged square head nails I purchased at Lee Valley. That was the reddish brown base and then I added a flat teaspoon of black pigment. 24 hours more and I off poured the juice and applied with a brush on maple.

From contributor L:
I have seen the maple finish done on a gunstock. The gunsmith I watched used a blowtorch to activate the color after wiping the iron solution onto the maple gunstock. He got a rich orange hue which appeared as the solution dried under the heat of the blowtorch. Since this technique dates from the colonial flintlock smiths (who favored maple), I am guessing that they would use whale oil lamps or candles/torches because I don't think that they had blowtorches at that time.

From contributor R:
You can get the same effect without all that work by using ferrous sulfate which is what you are making with the vinegar and nails. To get more of a grey or black you will need to pre-treat woods that have little tannic acid with a mordant of tannic acid. Tage Frid's book on finishing (3 book series) has some great chemical stain and mordant formulas for different wood species. (George Frank's book is very good also.)

From contributor C:
Caution: Know your chemicals before you use or recommend them for others to use!

Ferrous sulfate is not the same substance as acetic acid and iron/steel solved derivatives. Iron/steel acetic acid derivative is or forms a salt known as Iron ll acetate, AKA ferrous acetate. M. formula (CH3COO2) FE. It is a light brown or off white solid ionic compound of iron. It is highly soluble in water and is used as both a mordant and a dye.

Iron ll sulfate/sulphate, M. Formula (FeSO4) is a blue green crystal commonly called green vitriol, copperas, or sulfate of iron. Its uses are many, most commonly iron gall ink, grey wood and wool dye, dark greys when used with tannin, lawn conditioner, iron supplement, and other.

Ferrous sulfate produces greys on woods and will not produce reds or browns on wood unless it's reacting with something in the wood, whereas ferrous acetate will. You are better off buying small amounts of either and making weighed formulas of each for use as dyes or mordents, as the affect of each is quite different depending on the formula strength. If you have a small project that has to be an ebony stain, nothing will achieve it better than chrome mordant - Potassium Dichromate and hematoxylin/logwood - extract. Apply one coat PD, let dry, then a coat of Hematoxylin. Repeat steps again, sand 400 and then repeat again. Warning: chrome mordant is toxic. Read and use all safety precautions outlined. I have been using it safely for 38 years with no problems - be safe and do the same. Again - please do not recommend or use chemicals you are not fully familiar with.

From contributor J:
I was able to achieve that gray weathered look on my mantel piece, but now I need to seal it somehow without it darkening. I tried a polyurethane satin. It gave it a wet darkened look. Trying to retain the dried out grey look but still protect it from water or other spillage that could leave stains. This is a fireplace mantel in the house.

What is D-DUR and LB?

From the original questioner:
D-Dur is a 2 part urethane which is formulated for outdoor applications. It boasts UV protection, and the ability to allow wood to breathe, letting vapour escape while at the same time acting as a good barrier to moisture. Not really what you're looking for. It's a really tank and costly. As far as your flat finish, wax comes to mind as a potential solution.

From contributor J:
How about paraffin? Would that seal the mantel that I have grayed without changing the appearance?

From contributor C:
Any coating you put over chemical stain will change the color to some degree. This a problem with trying to obtain a certain color with chemicals. You get just what you're looking for, then it changes when coated. In my experience it is best to get a carnauba wax such as Mohawk's blue label paste wax natural and melt it over a double boiler and then add pigments, in your case mostly white with a touch of black. Let it cool while stirring and then let it set till next day. Then it can be applied by rag or brush and the excess wiped off. Steel wool will keep the shine down to a low satin appearance, giving it a natural look.

From contributor P:
Look up the Lee Valley Tools website. They sell good quality products. You'll find wax and some brush-on fast drying lacquers by Deft that create a real interesting flat natural look. Different grades of wax give different sheens. You should find some interesting info to view on different types.