Wood Fuming (AKA Smoking)
Staining wood without stain
Treating wood with a colored stain to give it a darker, richer color is an age-old practice. While means and methods have improved, staining has serious drawbacks.
Most stains sit on the surface of the wood so a scratch or ding can easily scrape it away and expose the natural color of the wood beneath. Furniture parts such as chair legs are quite vulnerable to such damage. Also, stain is unpredictable and will usually be absorbed more readily in some areas than others, and without careful surface preparation, stain will accentuate small imperfections such as sanding scratches and areas of uneven porosity. Ammonia fuming (or smoking) is a clever and interesting alternative to staining. In this process, the vapors from liquid ammonia do all the work for you – provided you are working with oak, beech, cherry or butternut. The gas reacts with the naturally occurring tannic acid in the wood to darken it.
Fuming with ammonia began in medieval Europe where horse barns were typically built with oak timbers. Over time, the wood beams nearest the livestock always darkened. As anyone who has ever spent time in a barn knows, livestock urine can be a strong source of the pungent gas. Eventually, some astute stableman/woodworker realized the darker wood might be associated with the strong urine vapors. Before long the active ingredient (ammonia) was identified and put to good use to attractively color the wood.
By the beginning of the 20th century, the process had evolved and refined to a point that Gustav Stickley, who manufactured Arts and Crafts-style furniture (primarily using white oak) in the United States, decided to utilize it. The characteristic color was so popular and has become so closely associated with the style that it’s difficult to imagine a Morris chair finished any other way.
The photos shown here demonstrate a typical degree of darkening in a fumed-oak furniture piece. What cannot be seen in the photographs is how far the color has penetrated the wood. Typically, the wood will be darkened as much as 1” deep on the end grain and between 1/16” and 1/8” deep on the face grain, permitting a workpiece to be trimmed to final length and lightly planed or sanded after fuming, without affecting the color.
While the process of fuming a piece of furniture is simple, working with ammonia requires some precautions. Ammonia (NH3) is a caustic, invisible gas, slightly lighter than air. Gaseous ammonia can cause serious respiratory damage. In aqueous form, it will cause burns immediately on contact with skin. When handling ammonia, wear rubber gloves and a cartridge respirator with filters rated for ammonia exposure.
If you decide to experiment, be careful as standard carbon filters for organic vapors will not provide any protection from ammonia fumes. Proper filters are available from any well-stocked safety supply retailer. Ammonia fumes may also be absorbed through the eyes, so, if you do not have a full-face respirator, wear a pair of swimmer’s goggles when handling ammonia. This will also protect your eyes against accidental splashes. Keep the container tightly closed and out of the reach of children. Any spills should be cleaned up as indicated on the product label. Get and use commercial concentrations of ammonia. It is primarily available dissolved in water (about 25% NH3 by weight) for commercial use. (Look under “Chemicals” in your local phone directory.) Grocery stores carry ammonia for household cleaning, but the concentration is low – typically about 5% NH3. Don’t waste your time with low concentrations. While this will still work to a certain degree, it takes much longer and achieves a less dramatic result.
The Fuming Process
When a piece of wood to be fumed is completed, construct or locate any suitable airtight enclosure. Small pieces can be fumed under a plastic bin. For larger pieces, a simple framework (slightly larger than the piece to be fumed) of 2×2s nailed or screwed together and covered with polyethylene will work quite well. Use clear plastic so you can evaluate progress. Ammonia will attack some plastic materials, but not polyethylene.
Select a safe location such as a detached garage, workshop or garden shed for the fuming process. An attached garage may be used if you can adequately ventilate it with a cross breeze. Avoid using a basement workshop beneath living/sleeping space. Pour some aqueous ammonia into a shallow container (preferably glass or metal) and place it under the piece. (For our footstool, we used about 1-1/2 cups of aqueous ammonia held in three tuna tins.) Put the enclosure over the piece. Check that you have no gaps through which the gas can escape.
After 24 hours, check progress. Don’t forget your safety eye protection and breathing equipment. The wood should be taking on a gray/brown appearance. Remove the tray and discard the ammonia by flushing it down the toilet. DO NOT mix bleach and ammonia! Of course, it could still be used to clean the racks in your oven. Refill the container with a fresh batch of aqueous ammonia and return it to the enclosure. Again, check that you have no gaps through which the gas can escape.
Allow your piece to fume for another 24 hours, then remove the enclosure and discard the aqueous ammonia. (Note: continued exposure will not deepen the color any further.) The application of a finish will bring out the characteristic dark-brown color. The fumed piece should be kept where it can safely off-gas for two to three days. Resist the temptation to place your newly fumed (and not yet fully off-gassed) piece on your oak floor or beech dining table, since gaseous ammonia can easily penetrate urethane finish to leave permanent dark streaks wherever your fumed piece touches it (this can happen in only a couple of hours).
Lightly sand the fumed piece (if desired) and apply a finish as you normally would for any other piece. While any clear finish will work, a penetrating oil, such as tung oil, will yield the most aesthetically pleasing results.
As an alternative to the fuming process you can also experiment with brushing ammonia directly onto your wood. You will get similar but quicker results. You will need to play with the concentrations of ammonia to water in order to find the appropriate shade and to get uniform results over larger pieces. Use polyfoam or synthetic bristle brushes for the work and wash with water to clean up.
Some manufacturers are now offering wood flooring colored by fuming. This is an exciting application since floors are susceptible to scratches and scuffs that can easily scratch off traditional stains. This means your Fumed floor should stay very attractive for a very long time. Be careful though because wood treated with this process seems to be sensitive to UV as shown in the photo. These shadow-like stains are from furnishings placed for only a week or two.