Wood subjected to certain environmental conditions degrades and can be attacked by all types of biological agents, such as fungi or xylophagous insects, which in turn accelerate further degradation.

To contain this process and delay it to the maximum, certain measures can be taken, being the autoclave treatment for wood, one of the most common and effective resources known for this.

It is also common to refer to wood treated by this method as couperized, tanned or impregnated.

What is autoclaved treatment for wood?

The autoclave is an airtight metallic container capable of withstanding high pressures and which allows to apply treatments that improve the wood’s resistance to rot and degrading action of biological agents. The process basically consists of the injection of chemical substances, mainly copper salts, using vacuum techniques.

The most common process (Bethell or Full Cell System) consists of 3 phases:

  • Phase 1: Empty. The wood is introduced into metal chambers in which a vacuum is generated for a certain time. This removes air and moisture from the wood cells.
  • Phase 2: Pressure and / or Impregnation. Inside the chamber and even under vacuum conditions, the product starts to be applied. And little by little we leave the pressure equal to the ambient pressure and we increase it so that the wood impregnates and absorbs the compost.
  • Phase 3: Empty. The product is extracted into the chamber and a vacuum is applied again to dry or clean the wood from the remains of the applied product.

There are other techniques and variants, such as empty cell (Rueping, Lowry) or Cyclic (OPM and APM). Each with its advantages and disadvantages.

What compound or products are applied?

In this sense, there are important differences. Each company, depending on the product it wants to achieve (type of wood use, finish or color) can use different compounds.

Protectors, varnishes, paints and even stains can be used. To this is added the growing concern with the environment and the use of certain chemicals.

The water-soluble salts of copper (tanninized) stand out, with creosotes, organic preservatives or alkyd resins being the most common.

Color of autoclaved wood?

We are used to seeing wood treated with greenish tone. It is mainly due to the use of water-soluble salts, the most used with the Bethell method, which, as already mentioned, is the most used.

However, this need not always be a result. Products can be used to color the wood or even techniques that respect its natural color more.

Advantages and disadvantages of autoclave treatment for wood

This treatment has interesting advantages and advantages for wood. They stand out among them:

  • Increased durability. Greater resistance to the action of insects and fungi.
  • I know improves performance against adverse environmental conditions. In other words, species that are not naturally suitable for external use may become suitable after treatment. For example, pine or fir.
  • Greater stability. Related directly to the above resources. In other words, wood moves less in the face of environmental changes.
  • Fire retardant agents can also be applied during the process.
  • The costs associated with maintaining the wood are reduced.

Types of autoclaved  treatment for wood

We can classify autoclaved wood according to two criteria. On the one hand, according to the risk class that the wood reaches after treatment, and on the other, according to the type of wood used.

Classes of use or risk

The autoclave treatment, at least that carried out by companies and qualified professionals, obeys certain criteria and we will be strict in order to be able to catalog the uses for which the resulting wood is suitable.

The EN-335 on Wood Durability standard defines these classes of use:

  • Class of use 1: the least demanding. Covered and protected from the weather and not exposed to moisture.
  • Usage class 2: Covered, protected from the weather, but there may be occasional humidity.
  • Usage class 3: Indoor, but outdoor. It is not in direct contact with the ground.
  • Usage class 4: Outdoors and in direct contact with the soil, even with fresh water.
  • Usage class 5: In contact with salt water.

Whenever we buy wood we must request the technical sheet or labeling. In this documentation, reference should be made to the type of use for which it is suitable.

autoclave types

What are the most used woods?

The Pine it is by far the most used. This is due to its excellent quality / price ratio and good treatment performance. Normally, after an autoclave with water-soluble salts, the result is suitable for use in class 3 and up to 4.

Other common woods are oak, elm or akoga.


The uses are essentially any, although from a practical point of view people use autoclaved wood for outdoor use:

  • Outdoor deck: gardens, pier …
  • Tracks.
  • Wooden walkways.
  • Structure construction: pergolas, porches …
  • Outdoor furniture.
  • Fences, agricultural fences, posts …