Greying wood: why it happens and how to regenerate it

Has your weathered wood turned grey and deteriorated within a few years? Are you wondering how this could have happened and now fearing that you will have to change the damaged material or even redo the whole job? Don't worry, we will explain in this article:

• what happens to wood when it greys;
• what can be done to regenerate wood;
• what can be done to slow down the ageing of wood.

For centuries, wood has been used in various sectors (furniture, construction, doors and windows, etc.) thanks to its physical and mechanical properties. No less important, however, are its aesthetic requirements: in some areas, colour plays a predominant role in the choice of wood. Unfortunately, however, colour changes over time, especially if the material is exposed to the external environment.

Once you have understood why wood ages and how this process takes place, you will be able to consider what to do with your grey, weathered surfaces. In most cases, all you need to do is clean them with a specific cleaner and protect them to give them a new lease of life!

Why does wood turn grey? To answer this question, we must first understand what this material is made of. So... are we ready to delve into the infinitely small? Don't worry, you won't have to read a scientific thesis. We will tackle just a few small but fundamental concepts together. Let's start our quick tour!

When we talk about wood, we must remember that it is not just a material, but a real organism and is composed of cells. Plant cells, unlike animal cells, are surrounded externally by a semi-rigid structure called the cell wall, which has the function of protecting the cell and providing mechanical support. 



What does the cell wall consist of? Here are the main substances:

•    cellulose;
•    hemicelluloses ;
•    lignin;
•    pectic substances;
•    protein;
•    water.

There are also other substances, such as tannins or terpenes, which are found outside the cell walls and characterise individual wood species. These substances are grouped under the name of extractives.

Wood has been used for centuries in construction, furniture, doors, windows and flooring because of its important physical and mechanical properties. Equally important, especially for the end consumer, are its aesthetic requirements, which can condition the choice of one wood species over another. A preponderant factor is certainly the colour, which, however, varies over time.

Let us consider wood in its fresh state. Its colour depends on two factors: 

• the colour of the cell walls, which is yellowish-white;
• the colour of the extractives, which can be colourless or various shades, depending on the physiological activity of the plant or other phenomena, including duramification. It is precisely the extractives that give the wood its variable colouring from species to species. These can range from the white of birch to the red of paduk, from the violet of rosewood to the black of ebony.
  
  

Now that we have taken a look, albeit a quick one, at the composition of wood, we can understand together what happens when this material is exposed to an environment.

Let us clarify a fundamental concept right away: the degradation processes of the wood surface begin immediately after its exposure to weathering.

Ageing manifests itself through colour changes, cracking and surface roughness, due to changes in both cell wall compounds and extractives.

One of the main degradation factors is sunlight and, in particular, ultraviolet (UV) rays, which trigger a photochemical oxidation process that leads to a rapid change in the colour of the wood. 

Lignin and extractives are the substances that undergo the most alteration: they absorb solar radiation, which causes the formation of oxidants and free radicals. These compounds in turn trigger other chemical reactions, which involve further degradation of the substances in the wood.

These decay products are generally yellow-brown in colour, which is why oxidised wood is more yellow than freshly cut wood. This colouring is mainly observed in light-coloured woods such as fir, pine or larch. In dark woods, on the other hand, a discolouration takes place that results in a turning towards pale brown.

The timing of the colour changes varies depending on the wood species and the climatic conditions. In general, for softwoods the first stage of yellowing can be very short, sometimes even a few days. It is clear that if we protect the wood with specific treatments or products, ageing will be delayed. But we will return to this concept later in more detail.





Having clarified this first mechanism, how does wood go from yellow to grey?

If the wood material is placed outside, it can be subjected to the action of rain, which washes away the degradation products of lignin and extractives, which have formed due to the action of ultraviolet rays. The result is a greying and impoverished surface. The phenomenon initially affects only the most superficial part of the wood, gradually going deeper as time passes.

Thus, the combined action of light and water accelerates the ageing and deterioration process of wood. However, there are other circumstances that can influence its deterioration. These certainly include the shrinkage and swelling phenomena that occur as a result of humidity variations induced by rainfall or night-time condensation.

We must not forget the high importance that biological attacks can have, but these will be addressed in a future article in our blog.




In general, we can say that the action of weathering causes not only a change in colour, but also cracking, increased roughness and a slow deterioration of the wood surface. 

Wood that has been exposed to the elements for centuries can even undergo real carbonisation: the colour thus turns from yellow to brown to black.

If you are a lover of the concept of recycling and reuse, you should know that aged wood is usually not to be thrown away (unless it has been affected by a severe fungal attack).

If the decay has only affected the most superficial part of the material, you only need to clean the greying wood with a specific cleaner, such as Lignum Vivo. 





If, on the other hand, the deterioration is deeper, then it is advisable to sand the surface by removing the first layer of material and then apply one or more varnishes/woodstains in order to increase the durability of the wood.

As we mentioned earlier, darker wood species tend to discolour. In this case, the application of an oil, such as Lignum Nature, can be sufficient to regenerate the wood.
The video below shows you how easy the restoration can be.

As we have said before, the processes of degradation of wood begin immediately after its exposure to the elements. This is precisely why it is more correct to speak of 'slowing down' rather than 'preventing' ageing. 
Is it possible, therefore, to slow down the ageing of wood? The answer is affirmative. There are different types of treatment that protect wood from the action of light and water and they are divided into product applications and heat or chemical treatments. Let us look at them together.

Application of products

LThe application of paint or wood stains can slow down the ageing of wood. Unlike heat and chemical treatments that change the composition and structure of wood, protectants act on the surface, acting as a shield against atmospheric agents. 

To do this, first of all the basic requirements of continuity and adhesion to the substrate must be met. Particularly in the case of varnishes, if the product does not have good elasticity and is not able to follow the shrinkage and swelling movements of the wood, the risk of cracking (even if only microscopic) is high, and the protective function of the product would be lost. 

With the use of protectors, light radiation can be shielded thanks to:
•    UV filters;
•    pigments and dyes;
•    resins.

Water, on the other hand, can be stopped by:
•    resins;
•    waxes;
•    additives with water-repellent function.

Heat treatment

Heat treatment is a method of slowing down the ageing of material; but in detail, what does it consist of? The wood undergoes a drying cycle at temperatures between 170 and 250 °C, in a controlled, oxygen-poor atmosphere. This implies a change in the chemical composition that consists mainly in the depolymerisation of the hemicelluloses, i.e. their fragmentation. This also leads to a change in the physical-mechanical properties of the wood. 

This process therefore increases the dimensional stability and durability of the material, as well as changing the colour of the wood, which usually turns a browner shade. However, colour stability is not guaranteed for all wood species: it is generally higher for conifers and lower for hardwoods.

The main disadvantage of this type of treatment is the decrease in the mechanical characteristics of the wood, due precisely to the depolymerisation of the hemicelluloses: their size, in fact, goes hand in hand with the strength and elasticity of the material. For this reason, heat-treated woods cannot be used for the construction of structures, which usually require high levels of mechanical resistance. The main applications are in the furniture and flooring sector.

Chemical treatment

One of the most frequent chemical treatments to slow down the ageing of wood is acetylation: the material is treated with acetic anhydride and at the end of the process the hydroxyl groups (-OH) of the polymers that make up the wood are replaced by acetyl groups (CH3 CO-). 
What does this entail?

Reducing the number of hydroxyl groups reduces the wood's sensitivity to water. Acetylated wood is therefore less sensitive to photodegradation and less hygroscopic, and therefore less prone to swelling and shrinkage and to attack by biodeteriogens.

This type of treatment also has some disadvantages including:

•    an increase in the acidity of the material, which can trigger oxidation processes of metal parts that may be present in the wood;
•    a slight darkening of the material and the possible appearance of greyish haloes on the surface;
•    the preservation of the porosity of the material, so that water absorption by capillarity can still occur in the heads of beams and joists exposed to the weather;
•    the high cost.

With this article we wanted to provide you with the most useful information on ageing wood, how to prevent it and how to renovate greyed wood. If you have further questions and curiosities, please do not hesitate to write to us! 

Bibliography:

• Tecnologia del legno, G. Giordano, UTET

 

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