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Oak Tanning & Discolouration

Oak hardwood is widely used in street furniture seating and benches as well as building cladding and other surfaces.

This is because of its durability and life span of 15-25 years, but mainly because we enjoy the look of the grain detail and colour.

So, what is the hang-up on using oak hardwood?

  • Tanning Runs Out of The Timber
  • Patchy / Discolouration to the Timber

This is simply the natural process that happens in an external environment.

IMG_4275 Oak3

 

 

 

 

 

 

 

 

Looking at the images above you can see why oak is used, its grain and charcheter stands out from the rest.

 

Tanning

What Is It?

Tanning mainly knows as tannins is within all fresh sawn oak and is the collective name given to the acidic chemicals held in solution within the liquid sap.

This is still present in dry oak when the water evaporates leaving the tannins in the solid sap.  They get their name from the leather tanning industry which used to use oak bark to tan leather.  Interestingly, it is also responsible for the oak flavour wines obtain from oaken barrels.

 

Why Does It Come Out?

When the timber has been introduced to wet conditions such as rain, the tannins will naturally wash out of the timber and runs to the surface directly below.

A good downpour of rain normally results in the loss of tannins, this will continue to happen for up to 6 months.

Tannins will only wash out of the outer few mm of the oak, so if you cut or plane it the fresh surface will release tannins all over again

 

Can You Prevent Tanning?

No is the honest answer, you can put in measures to slow down the process which would be introducing a regular oiling maintenance program. This will put a barrier on the outer timber to repel water going into the timber but this can’t guarantee the non-loss of tannins.

Oil doesn’t soak into fresh sawn oak very far as the water will stop it.  You must dilute the oil with white spirit and the oil will soak into the wood further, giving a stronger barrier to the elements.

 

Does the Tanning Damage the Ground Below?

Tannins will stain concrete & paving if left to dry. The brown staining can be removed by using a jet wash system to clean the damaged surface.

If the timber is over natural ground such as soil, it will simply drain off into the earth

Logic Bench Leaching_1

 

Discolouration

Why Has the Oak Changed Colour?

Again, this is just the sap / tannins draining away from the timber. The colour change does initially look like it has patchy stains or blemishes, but this will even its self out if given the time to naturally weather.

Weathering is a slow process so it can’t be rushed, the best results come from patience.

tannis Staining

Can You Remove the Streaks in the Oak?

The most common technique uses Oxalic acid solution to get rid of any stains or streaks.  This treatment will return the oak surface to an even tone like its original colour and the protective finish can be re-applied after the Oxalic acid solution has been used.  Oxalic acid is highly corrosive so ensure you follow the manufacturer’s instructions and wear the correct safety equipment.

 

 

Below you find a series of images showing  the tanning process and how it stains the concrete base and then how its removed.

 

  1. Oak Hardwood Bench – Tanning has started to drain from  the hardwood and stain the concrete base

Logic Bench Leaching_5

 

  • 2. The Timber Bench was removed showing the damage to the concrete base

Logic Bench Leaching_2

 

  • 3. Acid solution was applied to the surface of the concrete

courtyard 9-3 acid applied

 

4. The concrete is washed down and another acid course applied

Courtyard 9-6 after first scrub with acid

 

5. After washing the concrete surface again, the tanning has almost gone completely

Courtyard 9-7 after scrubbing and wash down with fresh water

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