Restoration Workshop: Darkening wood

Fuming with ammonia is a technique used primarily by furniture builders. The process of changing the colour of wood by exposing it to ammonia fumes can give wood a rich hearty tone that is almost impossible to duplicate with conventional finishing techniques -- which makes it perfect for building antique style projects. But fuming is also a technique that can come in handy for a furniture restoration project as well.

For instance, a job in which all the pieces of the project (a dinette suite, perhaps) were originally coloured by a stain that was mixed in with the finish. In such a case all the colour would be removed in the stripping process. Once all the original colour is stripped away these pieces would be perfect candidates for fuming.

The setup: Industrial furniture makers use industrial strength ammonia, but household ammonia works too -- it just takes a little longer. And a 'fuming chamber' may sound exotic, but all you need is a duct tape-sealed Rubbermaid container for small projects

Or a job requiring new parts (a missing rung on a chair), and the wood colour has accumulated from natural aging (no stain was used).

Why bother fuming? Why not just use a stain? Because there are some definite advantages to fuming. The biggest advantage is the ammonia fumes work all by themselves. You can be doing other things while the wood is being coloured. Fumed wood is also incredibly uniform in colour. There are no problems with blotchy stain application and no drips to mar the finish.

One other characteristic of fumed wood is obvious in open porous woods like oak. Stain tends to collect in the pores making them very dark in comparison to the surrounding wood; the grain really stands out. Fuming tends to colour everything uniformly, leaving the wood much more natural looking. Depending on the desired result, fuming can give you exactly the tone you're looking for.

Ammonia fumes work in combination with the tannin already in the wood. Oak normally has a lot of tannin in it, however not all species have tannin and the ones that do can have different levels of the stuff. If you happen to be refinishing a project that has low levels of tannin and you'd like to darken it, you can add tannin to the wood. Water-soluble tannin, or tannic acid, is available in powdered form from scientific chemical suppliers.

Not sure? Do a test. Make a saturated solution of tannic acid and apply it, with a foam brush, to one of two scrap pieces of the desired species. Let it dry overnight. Fume both (see procedure below) and see which one comes closets to the look you're trying to achieve.

Safety: In order to fume wood safely, there are four requirements. All are equally important and absolutely necessary. The first is a good fitting respirator with ammonia filter cartridges. Regular cartridges are not good enough. Ammonia filter cartridges are available at most safety equipment stores. The second is a tight fitting pair of goggles (not safety glasses) to protect your eyes from the fumes (try swimming goggles). The third is a pair of rubber safety gloves. And the fourth is an airtight fuming chamber. This can be as simple as a plastic garbage container sealed with duct tape for small pieces, or the back of a rented cube van for the larger pieces. The fumes will disperse after a couple of hours and won't hurt the van.

The Procedure

Assemble the following: respirator with ammonia filter cartridges, goggles, safety gloves, airtight fuming chamber, glass dish, ammonia (available where cleaning products are sold), bucket of water, wood to be fumed. A glass dish is preferable to a bowl simply because a dish exposes more ammonia to the air and glass won't react with the ammonia. Arrange the wood in the chamber so that all the wood needing to be darkened is exposed and not touching any other piece. Place the glass dish in the middle of the chamber floor. (If there is a lot of wood to be fumed and the chamber is bigger than a garbage pail, use more than one dish). Because you'll be working closely with a significant volume of ammonia you should wear a respirator, goggles and gloves. Fill the dish with ammonia and seal the chamber with duct tape.

The intensity of the colour is controlled by how long the wood is exposed. The numbers on the blocks represent hours

To help determine what is happening in the chamber, put some scrap pieces of the same species of wood in the chamber at the start of the operation. These are test pieces that can be removed and inspected periodically. You can also put a coat of clear finish (the finish you intend to use on the project) on the exposed area of the test piece. This will immediately give you a precise reading of the colour of the wood. If it is not dark enough, continue with the fuming process. This is a fairly long process, which means that so long as you do all the pieces that need fuming at the same time in the same chamber, there is no anxiety concerning timing.

The ammonia will stop working after a while, so about every eight hours put on all the safety gear, open the chamber, empty the dish into the bucket of water, refill the dish with fresh ammonia and reseal the chamber.

When the process is finished, put on the safety gear, remove the furniture from the chamber, empty the ammonia into the bucket, put the bucket outside, and remove the safety gear. Allow the furniture to off-gas for a day before applying a finish. Something to keep in mind: Fuming with ammonia does not add a chemical to the surface of the wood. This means that any stain or dye can be used over the fumed colour. Also, any finish can be put on the fumed wood. This allows for an infinite range of colour possibilities.