During the last century, global development models have allowed us to bring together culture and traditional techniques from different parts of the world. For the carpentry sector, an age-old craft, the combination of knowledge is synonymous with progress. In this way, the multitude of resources to study, experiment or put in practice their worth have made of the unknown, products of actuality.
This is the case with the charring process in wood. Its application produces an original and beautiful finish, as well as functional, becoming an attraction and a sign of sophistication based on the natural knowledge of antiquity. It is simply a technique for preserving wood that consists of burning the surface of the wood with fire, creating a visible layer of charcoal.
The method has been recovered from traditional Japanese culture, where it is known as Shou Sugi Ban (Sugi is the Japanese name for cypress). Its creators valued the material in its natural state, trying to avoid alteration or contact with other components or products. To this end, by means of burning, they managed to keep it protected, naturally favouring its durability and providing it with greater protection against xylophagous agents and fungi, as well as atmospheric agents and the sun.
Nowadays, this practice, carried out according to its original approach, is generally executed on cedar, larch, cypress, ash, etc., although it can be applied to other species, depending on the use to which it is to be put. The hardness of the species used, including natural variations within the same species, influences the final finish and its application. As a result, each intervention will have genuine and unique results that do not guarantee a homogeneous finish. However, the surface must be treated in several stages to achieve the desired result, so that the thin film of charcoal created after the surface is gently burnt takes care of its protection, extracting moisture and working as a protective chemical component.
Depending on the application for which it is required, the wood will come in many different formats, from boards, planks, sticks, beams, etc. To start the work, the exposure surfaces to be burned must be determined. Generally, for exterior use, it is recommended that the chosen wood has an acceptable natural durability, avoiding durability classes 3, 4 and 5, according to the UNE-EN 350-2 standard, which will help to improve its performance. Likewise, a previous design and constructive solution should be determined, showing the risk of suffering external aggressions and how it can be minimised by carbonizing. It is advisable that the pieces exposed to the elements be burnt on all four sides.
It can be carried out either using industrial butane/propane gas torches or industrial ovens. In both cases, specific safety measures and protocols, approved machinery and periodic technical inspections are required.
A constant speed with a regular exposure time and the same distance between the face and the burning source will result in a more uniform finish. The thickness of the resulting layer will be proportional to the protection given, but also to the fragility of the protection, so it is important to take this into account. 3-4mm thickness of charcoal may be sufficient (if ember flame is used, the finish will be much more unpredictable).
Just as precise as the burning must be the subsequent cleaning of the treated area. Using a brush with bristles of varying hardness and maintaining constant pressure in the direction of the grain, the charred residues are removed before the board is cooled with water. In this phase, the resulting surface will be defined, depending on the desired finish, more or less of the affected thickness will be removed. The more layer removed, the less protection. Brushing will also help to homogenise areas that are more exposed to the flame than others.
Once the surface has been treated, it is sprayed with cold water to interrupt combustion and cool the material. Allow to dry completely before proceeding with the last step.
Our piece is now finished, with a charred and shiny surface. Now we must apply a final treatment, a sealer to the charcoal layer. From this moment on, the handling has to be careful to take care of the finish layer. Apply the chosen product evenly to the surface with a cloth or brush, always in the direction of the grain. The excess of the product used should be removed and left to dry. It is advisable to repeat this operation once more. The delicate and irregular finish should not be exposed to possible shocks or impacts from objects.
This final treatment, in addition to intensifying the protection, will allow the wood tones to vary, from a natural colour to the most intense black. The final appearance will also depend on the species of wood, the degree and time of exposure to the flame and the strength of the brushing.
It is a complex, laborious process that requires careful security and safety measures.
Charring can boast a successful and appreciated natural ageing. Over time, the burnt relief will be maintained as long as it does not suffer physical aggressions. However, depending on the climate and/or exposure, the variation will be different in different areas. If in any case the charcoal has detached and could pose a risk to the whole installation, a new treatment is recommended, even in situ, to restore full protection.
If the intervention is located inside the building, maintenance will be minimal or non-existent.
The execution projects mainly demand the aesthetic nuance, but it is important to remember that this method made it possible to improve the useful life of the wood and to use local species in constructions that are still standing today several centuries later.
Although the estimated life cycle of this material will vary depending on the numerous factors to which it is exposed, construction solutions and the species of wood, durability will always be improved, avoiding fungal and insect attacks, as well as degradation from the sun. In addition, proper maintenance can lead our elements to last for generations.
In addition to being a renewable resource, the wood must be certified as coming from forests under controlled forest management. The use of fossil fuels is unavoidable in the carbonization process, but taking into account the life cycle of the material and the absence of chemical products in its processes, it is considered a sustainable material, with a “zero” carbon footprint.
The correct execution by means of appropriate constructive solutions together with a carbonized finish makes the wood a strong resource against water or inclement weather. It also has a good behaviour to photo degradation, causing the desired silver patina effect on its surface with the passage of time. Although the finish allows the wood to breathe, its surface repels water and prevents the appearance of mould. The behaviour against the spread of fire is also improved.
Insect and pest resistance
This treatment is not affected by termites or other xylophagous agents. Although in areas with termite infestations it is not recommended to use any type of wood joinery, due to the inevitable attacks, the option of charring can be an alternative.
Variable in colours and textures, depending on the duration and intensity of the processes applied, and can lead to very different solutions.
Although it is a process with admirable results even with natural imperfections, the material must be applied according to guidelines that minimize the possible deterioration of the pieces.
With the curiosity that charred wood arouses, Grupo GUBIA likes to make you participate in the developments and processes always from the knowledge and experience, but in this case also through the technique, essential to understand the constraints and limitations for our projects.
Those who know us know that we like challenges, but, above all, that wood must be the mean to overcome them and make them tangible.