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Wood Fuming (AKA Smoking)

Staining wood without stain

Wood fuming aka smoking
Wood samples colored by Fuming.

Wood fuming is an alternative to coloring wood with a stain. Treating wood with a stain to give it a darker, richer color is an age-old practice.

While products and methods for this have improved, staining still has some serious drawbacks. Most stains sit on the surface of the wood so scratches can expose the natural color of the wood beneath. Furniture parts such as chair legs are quite vulnerable to such damage.

Also, stain is not very uniform and will usually absorb more readily in some areas than others. Without careful surface preparation, stain will accentuate imperfections like sanding marks. 

Fuming (or smoking) is a clever alternative to staining. In this process, the vapors from liquid ammonia do all the work for you, provided that you are working with oak, beech, cherry or butternut. The gas penetrates deep into the wood pores and reacts with the naturally occurring tannic acid to darken it.

A nod to English ingenuity

Fuming with ammonia began unknowingly during the dark ages of medieval Europe. This was an era where horse barns were plentiful and 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. Someone (an astute stableman or perhaps a woodworker) finally realized the darker wood might be associated with the strong urine vapors. Eventually, the active ingredient (ammonia) was identified and was promptly put to use coloring wood.

American arts and crafts 

wood fuming (aka smoking)
Stickley-Morris Chair

By the beginning of the 20th century, the fuming process was highly evolved and refined. At this point Gustav Stickley in the US decided to utilize the process in the manufacture of Arts and Crafts style white oak furniture.  The resulting color was so popular that it is now synonymous with the style. So much so, that it’s difficult to imagine a Morris chair finished any other way.

The photos shown below demonstrate the typical degrees of darkening in a fumed-oak furniture piece. What cannot be seen in the photographs is how far the color has penetrated into the wood. Typically, the wood will darken as much as 1” deep on the end grain and between 1/16” and 1/8” deep on the face grain. This revolutionary because it permits a workpiece to be trimmed to final length and lightly planed or sanded after the coloring process, with no adverse affects.

Safety First

While the process of fuming is simple, working with ammonia requires some precautions. Ammonia is a caustic, invisible gas, slightly lighter than air. Ammonia fumes can cause serious respiratory damage if breathed, and will burn skin immediately on contact. When handling ammonia, wear rubber gloves and a cartridge respirator with filters rated for ammonia exposure.

WARNINGS

Be careful because standard carbon filters for organic vapors do not provide protection from ammonia fumes. You will find proper respiratory masks and filters are available here. Use a full-face respirator or a pair of swimmer’s goggles because ammonia fumes can also be absorbed through the eyes. Goggles will also protect your eyes against accidental splashes.

Wood fuming aka smoking
Ammonia respiratory mask

Keep ammonia containers tightly closed and out of the reach of children. Any spills should be cleaned up as indicated on the product label. 
Use commercial concentrations of ammonia that are primarily available dissolved in water (about 25% NH3 by weight.) Grocery stores carry ammonia for household cleaning, but the concentration is too low at only 5%. Don’t waste your time with low concentrations as it will take much longer and achieves a less dramatic result.

Wood fuming colors
Wood fuming process exposure gradients

Setting up a work area

To fume a piece of wood you need a suitable airtight enclosure. Small work pieces can be fumed under a plastic box or bowl. For larger pieces, a simple framework (slightly larger than the piece to be fumed) made of 2×2’s screwed together and covered with polyethylene will work fine. Use clear plastic so you can evaluate the staining progress. Note that Ammonia will attack some plastic materials, but not polyethylene.

Select a safe location like a detached garage or garden shed for the process. Avoid using any spaces beneath or adjacent to living or sleeping spaces. Pour some aqueous ammonia into a shallow glass container and place it under the piece. For our test, we used about 1-1/2 cups of aqueous ammonia held in three tuna tins. Put the enclosure cover over the work piece then check that there are no gaps where the gas can escape.

The fuming process

After 24 hours, check progress. Use 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 rinsing it down the toilet or by repurposing it to clean your oven rack. NOTE: DO NOT mix bleach and ammonia!

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 work 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 final application will bring out the wood’s characteristic dark-brown color. The fumed piece should be kept where it can safely off-gas for two to three days.

Note: Don’t place your newly fumed piece on an oak floor or beech dining table. Residual fumes can easily penetrate urethane finish and leave permanent dark streaks wherever your fumed work piece touches it. Once the off-gassing period is over, you will be safe.

Lightly sand the fumed work piece and apply a finish as you normally would for any other piece. While any clear finish will work, a penetrating oil, such as Min Wax tung oil, will yield the most aesthetically pleasing results.

Alternative Method

As an alternative to the fuming process you can experiment with brushing ammonia directly onto your wood. You will get similar but quicker results. You will need to play with various concentrations of ammonia in water to find the end color you want. Use your safety equipment along with foam or synthetic bristle brushes for applying to the work. Wash with water to clean up.

Summary

Some manufacturers are now offering wood flooring pre-colored by fuming. This is an exciting and seemingly perfect application since floors are susceptible to damage and scuffing that can easily scratch off traditional stains. Scratches on a fumed floor would only need a light sanding of the mark to remove it. This would mean a Fumed floor should stay very attractive for a very long time.

Be careful though because wood treated with this process seems to be sensitive ammonia cleaner and to UV as shown in the photo. These shadow-like stains are from furnishings placed for only a week or two.

wood fuming (aka smoking)
Fumed wood flooring-with flaws from furnishings

Resources

Flooring

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