Fumed oak is wood that has gone through a process of ammonia fuming. The result is that wood becomes dark, and the process brings out the wood grain pattern. The process involves subjecting fumes of a strong ammonia solution to wood for a particular period. We also call it smoked oak.
Fumed oak is typically white oak that we expose to a strong ammonium hydroxide solution. The usual period is overnight, but you can expose the wood for more time if you want the wood to be darker. The advantage of fuming over staining is that it does not block the wood grain yet changes its color.
Fumed Oak: History
Wood fuming was discovered by accident in England towards the end of the 19th Century. Oak boards stored in a stable in England became dark rapidly.
The person who discovered it deduced that the ammonia fumes generated from horse urine caused the wood to darken. In 1901, Gustav Stickley, an American furniture designer and manufacturer, introduced wood fuming in the United States.
The process became popular it soon became standard practice throughout the world. Stickley also outlined a fuming method applying ammonia fuming to the entire architecture of a room by placing bowls of liquid ammonium hydroxide and sealing it.
Although at the time it was considered a hazardous procedure, today’s advanced personal protection equipment makes it a relatively safe process.
The Oak Fuming Process Explained
When we talk about fumed or smoked oak, we allude to wood in either flooring or furniture that has undergone color-changing treatment. We put the oak into an enclosure into which ammonia fumes are released.
The enclosure could be a tank or a tent and the amount of ammonia is relatively small. The ammonia fumes in the air change the color of the wood. There is a common misconception that we fume or smoke oak by applying ammonia directly to the wood.
However, this is not so. There is a chemical reaction between the wood and atmospheric ammonia. The process here is that ammonia reacts with the tannins present in the wood.
The wood becomes darker when the tannins are nearer to the surface. The darkening is also directly proportional to the time of exposure to ammonia fumes. Depending on the exposure time, the wood becomes either a rich brown or almost black.
As mentioned above, the extent of darkening depends on how long we expose the wood to ammonia fumes. Twelve hours is sufficient for moderate darkening, but 72 hours yields a much darker color.
It is worth mentioning the temperature in the chamber. The temperature does not affect the darkening of the wood. But a change in the temperature can change the tone of the wood.
Higher temperatures create red tones whereas, with a low temperature, you can get greenish tones. The effect of temperature on the ammonia fuming process of oak paves the way for considerable creativity while smoking the wood.
Today wood fuming is a fairly common process particularly with arts and crafts furniture manufacturers. Ammonia fuming of oak for flooring tends to be done by professionals, but amateur woodworkers can do small pieces.
Factors That Affect Fuming
To understand fuming you need to understand the factors that affect the process. Here are a couple of factors that have a bearing on the results of ammonia fuming of oak:
Effect of the Atmosphere
A major reason for the color variation while fuming is the way the chamber is constructed and the orientation of the planks placed in the fuming chamber.
It is very important to control the environment or the atmosphere within the chamber, and it is also important to consider the environment outside the chamber. External humidity and temperature can also bear what happens inside the chamber.
These are the parameters that we need to monitor during fuming procedures. It can vary the results, and it is how atmospheric conditions can vary the results.
Influence of Tannins
Another factor that can affect the darkening of wood during fuming is the presence of tannins. If you are fuming wood, whether it is oak or any other type of wood, the tannin content is the primary consideration.
The production of natural tannin is dependent on the growing conditions of the tree. The tannin level depends on the time of year that we harvest the tree. Trees are believed to produce more tannin during autumn and summer. So, wood that comes from trees harvested during those seasons could contain more tannin.
Ultimately, the tannin content of wood has a major factor in the fuming color process. It is the tannins that react with ammonia to create a color change.
The Importance of Tannin in Wood
Tannins come from complex organic substances called phenolic acids and tannic acid. PlaPhenolic compounds are found in plants and trees from all over the world. Tannin is very important in the process of ammonia fuming because it reacts with ammonia to effect a color change in the wood.
The rule of thumb is that the higher level of tannin, the more drastic color change will occur. If you are fuming wood for flooring, the color will vary from plank to plank, depending on the tannin content.
The color variation in fumed oak for flooring provides a striking contrast if you grade the planks according to color and lay them out.
It is an interesting point to note that tannin is one of the primary reagents for curing leather. So, the tanning industry also relies heavily on the use of tannin.
Determination of Tannin Content in Wood
Determining the tannin content in wood is a highly complex and lengthy process. It involves several stages like extracting the bark of the wood, crushing it, powdering it, determining the moisture content against other substances, determining soluble solids, and ultimately measuring the tannin content.
Therefore, anyone engaged in the ammonia fuming of oak would probably involve themselves in measuring the tannin content of oak or any other wood, for that matter.
However, it is sufficient to say that tannin is a substance in wood that causes it to darken, especially when exposed to ammonia. It also causes natural darkening without exposure to ammonia or other substances, but the process is usually very slow. It can take months or even years for any noticeable darker shade of the wood during natural darkening.
As a woodworker, you need to realize that each board is unique in mineral content, chemistry, knots, and shading. Due to this, the results that you get from the fuming process may also vary.
The desired result is to add depth and a pleasing patina to the wood, which you cannot match through staining or any other wood coloring process. If you control the process well, you can get some good results.
Ammonia fuming of oakwood may not be practical if you have a small setup. However, you can procure readymade fumed oak lumber and create interesting variations in your woodworking projects.