Fumed Oak

January 2003


Ammonia fuming is a traditional process for darkening and richening the colour of oak. Supposedly it was discovered by observing the way that oak beams in stables were darkened on exposure to the fumes from horse urine. Thanks to Haber and Bosch, we can now do this with convenient ammonia, and we don't need to keep a horse in the workshop.

Stickley cube chair In modern times, ammonia fuming is best known through the work of Gustav Stickley.

Is it hard ?

Ammonia fuming is one of my favourite finishing processes. It's not only easy, it's almost impossible to get wrong. If it doesn't work quite right, then just do it again - there's no old finish to strip off.

It's also very safe.

Is it long-lasting ?

Yes. You aren't applying a dye that could fade, you're changing the chemistry of the timber's own pigments. Oak will darken over centuries, but it isn't going to fade.

How does it work ?

All you need to do is to expose wood to the fummes of ammonia gas. A colour change is visible after half-an-hour or so, but 24 hours is usually best to give an even colour. Fuming only works where the wood is exposed, so any parts that are resting on others will need to be moved half-way and re-exposed.

The chemistry is an oxidation of naturally occurring tannins in the timber.

What do I need ?

Ammonia

I buy mine in a hardware store, as strong domestic cleaning ammonia. This is about 20% - 25% and perfectly adequate for overnight or 24 hour processes.

Supposedly you can also find ammonia in a drafting or technical draawing supplier. No doubt this was available once (it was part of a process for duplicating blueprints) but I've not found it there in recent years.

Don't use anhydrous ammonia. That's really nasty stuff; highly toxic and unneccessary.

You'll also need a couple of small flat dishes, to put the ammonia in. These should be plastic - I use take-away trays. Don't use metal, especially aluminium foil ! Ammonia is corrosive to aluminium, sufficiently so to eat a hole through the bottom of a tray.

A box or tent

Ammonia fuming
boxThis is my ammonia-fuming box. It's a piece of old lab equipment, and reasonably airtight. You don't need anything this complex though; just a simple tent made of polythene sheet and sticks.

Safety gear

Ammonia is noxious, but it's quite easy to control. It's not essential (with only 25% ammonia) to use a mask, but it makes it more pleasant. Ammonia dissolves easily in water, so it's important that your mask has eye protection, or you might as well not bother. The mask should use filter cartridges suitable for ammonia, but a basic charcoal vapour filter will reduce the nuisance. Dust filters and paper masks aren't any use though.
S10 mil-surplus respirator Personally I use an ex-military S10 mask.

Working with industrial scale quantities, or with anhydrous ammonia, is a real hazard. For our home-workshop quantities though, it's just a nuisance.

There should be no smell of ammonia outside a good fuming tent. I can happily work near mine, but if yours is leaky, then just use it outdoors. Ammonia is corrosive to metals, especially aluminium. Keep it away from your tools.

Dispose of ammonia by diluting it with plenty of water. Then pour it down the drain, or onto soil (it's a useful fertiliser and nitrogen source).

The only real hazard from ammonia on this scale is splashing it onto your eyes. If your mask doesn't already protect them, then goggles or a head shield are a wise precaution.

Splashed ammonia anywhere should be washed away with plenty of water.
Consult a doctor after eye splashes, or if skin splashes appear to have caused an injury.

How do I do it ?

  1. Assemble your fuming box or tent.
  2. Place the ammonia dishes centrally in the bottom of the tent. For a small piece of furniture, I use one take-away tray. For a whole boxful I'd use two, otherwise about one tray per piece.
  3. Load up the fuming box with the wood to be fumed. Stack it carefully, so as much as possible is exposed to the fumes. Use softwood stickers underneath to lift pieces off the floor. You'll probably need to re-arrange the trays a little.
  4. Check the timber is all in place, you can reach the ammonia tray and that you can close the tent afterwards.
  5. Mask up
  6. Pour in the ammonia. 1/4" depth is plenty. Close the box, or seal up the tent.
  7. Ignore it for at least a couple of hours.
  8. Check for progress (optional). Examine the stack of timber to see if any areas are shadowed by their supports. Move them around if needed.
  9. Leave it overnight. Don't mess with it, you won't help.
  10. Mask up
  11. Open the tent and remove the ammonia trays. Throw them into a bucket of water, then pour the lot onto the garden or down a drain.

Fuming works equally well on finished items, or on dimensioned stock. If it's inconvenient to fume a large assembled table, just do the pieces before assembly. The depth of the finish is usually adequate to survive light finish sanding, but planing or deeper machining will remove it. I often load up the fuming box with two projects; the one I've just finished, and the raw or dimensioned stock for the next.

What woods can I use ?

You need a wood with a high tannin content. Oak is good (and traditional).

Tannin content varies from tree to tree, but not much from board to board out of the same flitch. It's worth keeping your boards together (this is always good advice, as it allows you to match grain patterns).

Oak sapwood doesn't contain much tannin. It may look the same colour when fresh, but fuming just won't darken sapwood.

To better manage my stock, I fume some of my boards before placing them in storage. This way I can avoid the sapwood, and I can better match shades between boards.

History

15th century oak chest The pieces we think of as "Jacobean" oak are now almost black. They weren't originally this dark though, and were probably unfinished when first made. This darkening is the natural (and inevitable) oxidation of oak exposed to air. Our ammonia fuming is just a short-cut to this.


The Arts & Crafts movement of the 1850's, or the Craftsman movements of 1900, returned to oak furniture as part of their political approach to "honest" furniture. Despite their claimed simplistic approach, Stickley made considerable use of modern machinery for production and finishing of furniture. He was particularly emphatic over the correct finishes for the oak used. Modern Stickley reproductions rarely copy these exactly; some of Stickley's techniques were intended to tone-down the most apparent features of the timber, whereas the reproductions are trying to highlight it as a badge of their authentic materials.

What should I use as a finish afterwards ?

You can use anything. Fuming doesn't affect the wood in any way that affects finishes.

My usual finish, which I find appropriate for Craftsman work, is "shellac over oil". This gives a "quality" finish suitable for good furniture, but it's less fussy than true French polishing. It's also quick and simple to apply, and easily repaired after scracthes or damage.

  1. Apply two coats of a commercial finishing oil, such as Liberon. This is mainly tung oil, with a thinner and a chemical drying agent. The purpose of this is to seal the wood, rather than to produce a finish. A thinned oil is easier to apply on bare wood than pure oil.
  2. Apply a couple of coats of unthinned pure tung oil. This should "body up" the finish, filling the pores.
  3. Allow to dry thoroughly. At least overnight, but ideally give it a couple of days in a warm environment
  4. Apply a light cut of shellac, using a mop. Repeat as necessary. This should form a film of shellac over the oil
  5. Apply shellac with a rubber, a cloth pad. This produces a smooth semi-gloss surface.

What about "green casts"

If you fume red oak, rather than white oak, you may have problems with a greenish cast developing. This depends an awful lot on the timber you're using. It's more of a problem for Americans, as I've never seen it on European timber, only imported American. Most oak in England is imported from France, and this is a white oak that doesn't show it. I haven't tried fuming the Live or Holm oaks.

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