Smoked oak
Smoked oak is oak wood that has been modified in color. During "smoking" the wood is fumigated with ammonia or ammonia (ammonia water ). It changes color and takes on a brown to dark brown or black hue . Natural color variants are retained, as the color depends on the amount of tannic acid present in the wood .
The reaction of the ammonia with the wood constituents (acids, resins, etc.) leads to the formation of aging and light-stable organic complex salts. These not only change the color of the wood, but through the " plasticization " the wood becomes more supple and less brittle and also more resistant to insect and fungal attack. Therefore, smoked oak is suitable for the production of high-quality, robust solid parquet, which can also be laid as industrial flooring.
Our native oak species ( English oak and sessile oak ) are particularly suitable for the type of wood treatment described here , as they have a high proportion of tannins. The American red oak, which is also grown in Central Europe, as well as many other oak species of American origin, cannot be used because of the insufficient tannin content. The sapwood is not suitable for smoking and is therefore usually removed before treatment. A previous surface treatment with paint or oils prevents ammonia from penetrating.
Procedure
The methods of smoking have evolved from observations made in old farmhouses where people and cattle lived under one roof. The ammonia gases from the animal excretions gradually turned oak furniture. The method could be used in a targeted manner by placing furniture to be smoked in horse stables, because the urine of horses contains a particularly high amount of ammonia.
An old artisanal process is building site smoking. For this, the oak wood is completely installed. Then the parts to be smoked are covered airtight and several salmiak spirit bowls are set up. The ammonia from the bowl escapes and reacts with the oak wood. This creates a discoloration on the surface, which is only several millimeters deep.
More modern processes have in common that they can be operated on an industrial scale. The aim is a smoking process in which the oak wood has the same color through and through (core smoking). A distinction can be made between processes that work under normal pressure or with the aid of a vacuum . The latter usually shortens the smoking time and leads to darker shades. The procedures of the individual manufacturers vary in the amount of vacuum, the amount of ammonia and the duration of the process. In addition, batch or continuous processes can be used.
After smoking, the finished timber is ventilated. The sawn timber should then be ventilated to such an extent that the non-bound ammonia components are removed from the wood. The remaining part is bound in the wood. This remaining portion causes the permanent discoloration, as it has reacted with certain tannic acids in the wood.
When gluing smoked oak, make sure that the wood is properly ventilated, since conventional white glues do not harden properly if there are still shares of unbound ammonia. However, there are no problems after properly ventilated. Here, too, there are different methods: once in a conventional dryer with the help of increased temperature or simply outdoors.
The dark color of smoked oak is more stable to UV light than natural wood or thermowood.
As an alternative to creating a smoked color, there are smoked stains or paints. However, these are only superficial treatments, they do not create any deep color changes.
literature
- Christoph Marquardt: The process of smoking oak and the most important influencing factors. Rosenheim 2005 (University of Applied Sciences Rosenheim, diploma thesis, 2005).
- C. Groetsch: Optimization of the smoking process of oak wood with ammonia. Rosenheim 2006 (University of Applied Sciences Rosenheim, diploma thesis, 2006).
- Karsten Aehlig, Sebastian Weidlich, Enrico Zönnchen: Investigations into the emission behavior of smoked oak. In: wood technology. Vol. 52, No. 4, 2011, ISSN 0018-3881 , pp. 24-28.