Advanced Timber Knowledge

Why has my external timber changed colour?

Understanding Timber Transformation: Fresh Sawn vs. Weathered Timber

The concept of fresh-sawn timber is quite straightforward – it’s a timber that hasn’t yet been exposed to the natural elements. This applies to both seasoned and non-seasoned timber, encompassing both Hardwoods and Softwoods. On the other hand, we are more accustomed to seeing weathered timber in our surroundings, from garden fences and gate posts to farm buildings and old waymarking signposts. This weathered timber boasts a classic silvery-grey finish that seamlessly blends with the natural environment and is widely admired.

But why does timber change colour?

Several processes contribute to this transformation, with the primary factor being exposure to the weather.

1. Sunlight Exposure: Sunlight, particularly the ultraviolet (UV) light in the sun’s rays, causes photochemical degradation of the wood surface. Lignin, the ‘glue’ that gives cell structure to wood, breaks down under UV light into simple sugars, which serve as a food source for certain moulds. Initially, this can result in temporary dark and blotchy patches. However, as the food source is consumed, the moulds become inactive, and the timber gradually turns silver-grey. Due to the degradation of the cell structure, the timber’s surface may appear ‘fluffy’ initially, but this will wear away over time, leaving a slightly rippled surface.

2. Rain Exposure: Exposure to rain leaches out the natural tannins and pigments present in the wood, resulting in the timber becoming a lighter shade than its original colour. The concentration of tannins varies depending on the timber species, and some may experience temporary surface staining. However, these surface stains are only temporary and do not affect the long-term appearance of the wood.

Can anything be done to preserve the fresh-sawn appearance of timber?

In theory, yes, but in practice, it’s not always straightforward. For any treatment to be effective, timber generally needs to be seasoned, though exceptions exist. To protect the wood from weathering, we must prevent UV and moisture from contacting the timber’s surface. There are numerous brands and treatments available, but they typically fall into one of two broad categories:

Surface Coatings: These coatings sit ‘on top’ of the timber and include options like paints, varnishes, and lacquers.

Pros:

Can alter the appearance of timber.
Can extend the timber’s lifespan.

Cons:

Cannot be applied to fresh-sawn timber.
Often requires a detailed application process.
Demand ongoing maintenance to remain effective.
May change the timber’s appearance.

Penetrative Coatings: These coatings ‘soak into’ the timber and include pigmented stains, opaque stains, and natural oils.

Pros:

Can alter the appearance of timber.
Easy to apply.
Suitable for fresh sawn, air-dried, and kiln-dried timber.
Can extend the timber’s lifespan.

Cons:

Require ongoing maintenance to be effective.
Generally, darkens the timber’s appearance.

What are Tannins, and how do they impact timber?

Tannins are acidic chemicals held within the liquid sap of timber. They are water-soluble, so when moisture permeates the timber, tannins are brought to the surface, causing black or brownish stains. Most tannins bleed from the ends of a timber board due to the cell structure. Imagine timber as thousands of tiny straws, with moisture struggling to escape from the side walls but flowing freely from the ends. Rain is the primary cause of tannin leaching, but it typically stops after 5-6 months. If the timber is cut or planned again, fresh exposure will release tannins once more.

Can leaching be prevented or slowed down?

While there is no foolproof way to stop leaching, certain measures can help slow it down. A regular oiling maintenance program can create a barrier on the outer timber, repelling water. However, this doesn’t guarantee the elimination of tannin loss, merely extending the process over a longer period. Diluting the oil with white spirit is advisable for better penetration into fresh-sawn timber.

The best way to prevent leaching is through careful timber species selection. Woods with low tannin content, such as Douglas Fir, are less prone to leaching compared to tannin-rich woods like Oak.

How can tannin stains be cleaned?

If tannin stains are noticed early, they can be cleaned with soapy water due to their water-soluble nature. However, if the stains are not addressed promptly, an oxalic acid-based timber cleaning product, often called timber tannin remover, may be necessary. Dissolve 1 kg of oxalic acid in 6 litres of warm water and scrub the solution onto the affected area using a stiff brush. After treatment, thoroughly rinse the surface with water, as oxalic acid is toxic.

By understanding the processes involved and taking appropriate measures, you can effectively manage the appearance and maintenance of timber as it transitions from fresh-sawn to weathered.