We are regularly asked about tannin leaching, which can be a common problem for anyone working with timber furniture installed on a concrete surface as it can leave an unsightly stain on the surface below. This is a guide to explain what tannins are and some of the questions that we get asked.

What are Timber Tannins?

Tannins in hardwood timber are the natural properties of the timber itself and that stored within the internal grains of the timber. When moisture or water permeates through the timber it will bring these tannins to the surface, causing black or brownish stains.

Tannin issues in landscape furniture?

When the tannins are brought to the surface of the timber it is called “leaching”. When this occurs is can run off the timber and when washed onto surfaces such as concrete or pavers it can discolour the surface. It’s important to note that when staining like this occurs, the stains are due to tannin, not from an applied oil coating ‘washing off’. This isn’t a particularly nice sight to see on new concrete under brand new furniture!

Above. Examples of tannins leached onto concrete.

Can I prevent or control timber leaching?

You have to remember that timber is a completely natural product and this process will occur at some stage.  We get asked a lot to remove the tannins from timber before we deliver our furniture. But things aren’t quite so simple! Below are a list of questions we get asked.

Can you leave the timber out to leach before oiling?

If we do this then the timber doesn’t have any protection and could split or warp during the leaching process making it unusable. This would waste a lot of timber.

Can you clean the timber with Oxalic Acid?

This is possible but it is a time consuming and expensive process that only cleans the surface of the hardwood. The used acid then has be properly disposed of and this process is not generally an environmentally friendly one. Finally,  if you read the instructions on the timber cleaners it will tell you to weather the timber first to get the full effect, thereby letting nature take its course anyway.

Can you use Recycled Timber?

Don’t get us wrong, we love old timber! It has character and we love nothing more than recycling materials. The hard thing is to find recycled timber that is high enough quality for the public to use and also be cost effective. But that’s not all! A lot of recycled timber is sourced from old wharf frames, sleepers and power poles. So these large timbers are then cut down to smaller battens for your furniture. This then exposes all of the tannins in the center of the material and the whole process starts again!

Can you make the furniture and leave it outside?

This is the only real way to effectively remove tannins naturally. The small problem at CSA is that we cant keep up with demand and to get your furniture to you as soon as possible there simply isn’t enough time!

So what do we do?

The best advice that we can give it for projects where the furniture is installed on concrete is to allow enough time so that after delivery you can sit the furniture aside. Wet it down or leave the rain to do its work and when you are ready, give it a bit of a clean, re-oil and install it.

 Can the type of timber help?

Yes – it can. For long lasting outdoor furniture, only Class 1 rated hardwoods are used. Commercial Systems Australia sources only the best FSC Australian hardwood timbers. These are seasoned and then kiln dried to try and condition the timber when it is first cut to size.

 Can we seal the concrete?

Yes you can. This can be more difficult on larger concrete areas, but sealing small concrete slabs or concrete walls is another option to help prevent stains on the concrete.

What do we do if the tannins do leach?

Tannins leaching isn’t the end of the world, but it does require a little cleaning. We know that no one likes to do it but it should always be allowed for in your job. There are plenty of natural concrete cleaners on the market. If the environment allows for it, use a diluted bleach and water mix, pour it on the tannins and give it a quick scrub with a brush. Wait a few minutes and rinse it clean. This should restore the concrete to its original condition.

Are there any alternatives?

There is one and it is becoming quite popular. You may want to look at incorporating recycled composites into your project. We do agree that it may not have the same character as good old Australian hardwood, but it doesn’t require any oiling and also, no tannins!

We hope the above guide provides some useful information. If you have any further questions please feel free to contact us on (03) 9723 4111.

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