Essential Timber Knowledge

Fresh Sawn or Weathered Timber Appearance?

The Transition from Fresh Sawn to Silvery Grey 

An underreported aspect of using timber is the significant aesthetic change it undergoes after being placed outdoors.  This is a natural process and will happen to all timbers regardless of the species.   

Fresh sawn, different species of timbers display a large variance in colour, ranging from Ash which has a very white colour to Cumaru, which is a rich brown.  This variance makes it easy to distinguish between timber species.  However, over the course of approximately 6 months, the appearance of all timbers changes to the same driftwood grey which means the only way to distinguish the species is through the grain pattern.  We call this driftwood grey ‘weathered timber’. 

The Science behind the Transition 

The colour change is due to UV penetration from the sun and moisture, such as rain. 

The colour of the wood is held in the cellulose structure of the wood.  In turn, a major component of the cellulose is lignin.  Lignin holds the colour, whereas cellulose has a whitish/grey colour.  When the timber is exposed to sunlight, radiation from the sun’s ultraviolet rays breaks down the lignin.  This broken-down lignin is then washed out of the wood by moisture such as rain.  The cellulose that remains on the wood surface has the weathered silvery grey colour. 

The cellulose will eventually degrade as well but is much more resistant to UV penetration and leaching. 

How long does the weathering process take? 

As a rule of thumb, the weathering process takes 6 months.  However, this number will wildly vary depending on several factors. 

The most obvious of these is the location of the timber.  If it is sheltered from rain and in the shade in large parts of the day, the colour transition will take a lot longer than 6 months.  On the other extreme, if the timber is placed on an exposed hilltop with no shade and facing all elements, the transition will be less than 6 months.  Most street furniture has some elements of protection from nearby buildings or trees so 6 months is a good guide. 

Another factor that affects the length of the weathering process is the oil content of the timber.  Timbers with higher oil content will take longer to transition.  Tropical hardwoods, such as Iroko and Opepe are very oily and therefore will retain their original colour for longer. 

Are there any methods to protect against weathering? 

Some people prefer the fresh sawn look; others prefer the appearance of weathered timber. 

There is no right or wrong answer, it is a purely subjective opinion.  However, for those who do prefer the fresh sawn look it would be interesting to know that there are methods to maintain the fresh sawn look or at least slow the transition. 

The most common ways of maintaining the fresh sawn appearance is to use a coating, of which there are two types: 

Varnish

1, Coatings that form a layer on the timber surface, such as varnish.  This is not recommended.  If a bench was coated with varnish and the varnish is scratched, the water proofing is compromised.  Water will spread between the varnish layer and the surface of the wood.  This water will then go mouldy and go a green colour, which destroys the attractiveness of a wooden bench.   

Penetrative Coatings

Coatings that soak into the timber, called penetrative coats, such as linseed oil or stains.  This is a better option because it allows the timber to breath properly as it does not cover the timber surface.  Also, a product such linseed oil is a natural product and more environmentally friendly than varnish.  The disadvantage with penetrative coats is that it requires a yearly maintenance program to keep the level of protection up. 

Aside from coatings, another way to slow weathering is to place the furniture so that it is sheltered from direct sunlight or/and rainfall.  Of course, this will not be possible on all projects, but for example a tree placed behind a bench will offer a certain level of protection. 

Of course, the best option may be to work with weathering rather than against it.  After all, the fresh sawn appearance only lasts for 6 months which is a fraction of a bench’s 20 year plus lifespan.  By this, we mean that silvery grey tends to suit being surrounded by vibrant colours.  If a weathered timber bench is sat on top of a grey powder coated planter edge, it will look rather bland. 

How does Mildew and Fungi Growth affect weathering? 

Mildew and fungi growth can be a challenge in the UK given our damp climate.  Mildew is the most frequent, and well-known type of fungi growth.  The thing to understand with mildew is that it only affects timber when the moisture content is still high.  This is because mildew requires water to grow.  When fresh sawn timber is used in benches, there might be some mildew growth as the moisture content is likely to be relatively high.  However, the problem should disappear of its own accord over the course of several months as the timber dries out to meet the surrounding humidity level, usually 15% – 25%.  At these moisture levels, mildew cannot form. 

However, this is not really an issue for seating because mildew is only ever a very light, thin layer – as soon as the bench is used, the mildew will disappear. 

Conclusion 

Leaving timber to weather naturally is the most environmentally friendly, easiest and cost-effective method.  We would advise that a project is designed with the weathered timber appearance in mind rather than fresh sawn. 

If you would prefer to keep the fresh sawn timber appearance, we recommend a natural penetrative coating such as linseed oil.

Please note that by weathered timber, we are referring to timber that has spend a long time outdoors. Typically, dried timber would have a fresh sawn appearance as it would have been dried indoors. For more information on timber drying, read our blog here.

Sources 

https://www.silvatimber.co.uk/what-is-weathering/

https://www.abodo.co.nz/resources/articles/understanding-how-timber-weathers

https://www.russwood.co.uk/cladding/weathering/

https://ecochoice.co.uk/news/blog/item/how-wood-weathers

https://www.wood-finishes-direct.com/blog/its-summer-can-you-stop-wood-going-grey/#:~:text=When%20sunburn%20and%20windburn%20peels,the%20job%20on%20its%20behalf.

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