Why Is My Wood Green?!

If you’ve ever bought something made from wood and suitable for outside use, you might have noticed that occasionally that wood is green. Is this a problem? A fault with the merchandise that you’ve just bought? Nope, there’s no need to worry. We’re here to explain what that green stuff is and why it’s perfectly normal.

The Big Question

Let’s get you some answers right off the bat. Why is your wood green? Because it’s been treated or tanalised. In some cases (most cases actually) this treatment causes light wood to turn a delicate shade of green, or to have green patches, and it’s a normal side effect of the treatment. Wait, what’s tanalisation? And pressure treatment? Let’s take a quick look.

Tanalisation and Pressure Treatment: What’s the Difference?

Pressure treatment is a process that well, treats your wood under pressure. We’ll get to why you’d want to do this in a minute. Tanalisation is essentially the same thing. Tanalith E is a chemical used during pressure treatment, but not all pressure treatments use Tanalith E, some use a different brand name of chemical. All wood for outside use can be pressure treated, but not all pressure treatments use Tanalith E. So how does this process work?

How Pressure Treatment Works

During pressure treatment, a whole bunch of wood is put into a big tank. That tank is then sealed up so that it forms a vacuum. Next, the pressure treatment fluid (which may or may not be Tanalith E) is put into the tank. Because the tank is a vacuum the immense pressure forces the fluid into the wood. The fluid will penetrate a few millimetres into the wood. This means that it’s not only the surface of the wood that’s being treated but also below the surface.

What exactly is this fluid? Different brands have different makeups. But the main ingredient in the fluid is copper, which is awesome for protection against wood rot and against insects in general. There are other chemicals too, but the composition and amount really depend on the company doing the treatment. Usually, there’ll be chemicals that protect against things that copper doesn’t cover, such as kinds of fungi. The chemicals are all safe once the wood has finished treatment, so you don’t need to worry about your kids or pets being close to the treated wood.

And Why Is Treatment Necessary?

Pressure treatment is a good thing when it comes to the wood you’ve just bought. Let’s face it, you’ve probably spent a fair amount of money on that furniture or shed or cut wood, and you’d like your investment to last a long time. Pressure treatment increases the lifespan of wood, protecting it against insects, fungi and rot. In fact, pressure treatment protects the outside of wood against rot for about thirty years and the inside for around sixty years!

Obviously, this means that pressure treatment is a good thing. Especially if you want to be able to leave that bin store or shed or garden bench out in all weathers.

The Colour Change

Initially, pressure treated wood usually has a green tinge to it. This might be all over, or just in patches or spots. If you remember your high school chemistry, you’ll know that it’s common for copper to turn things green. But this is only temporary.

Over time, as the wood weathers, that green colour should turn into a light honey colour, and after even longer will become a sort of silver-grey colour. This colour change does not mean that the effectiveness of pressure treatment is changing, it’s just a natural process that occurs when wood is exposed to the elements.

But It’s Splashed!

In some cases, treated wood might look like it’s been splashed with green paint. Rest assured, it hasn’t. If you look closely these green patches might even look like crystals or salts. This happens sometimes when there’s sap in the wood that undergoes pressure treatment. This will fade over time once the wood is outside. If you see bubbles of resin on the wood you can flick them off with a sharp knife, but don’t sand down the wood. If you sand off all that green all you’re really doing is destroying the treatment and your wood will rot far sooner.

It’s Mouldy!

It’s not uncommon to see mould on pressure treated wood. This is because the wood gets very humid during treatment, and is most common during hot weather. Mould doesn’t affect the strength of your timber at all. But if you see blue or green looking mould you should be able to easily remove it by hand (a quick wash down should be fine). Leave the mould alone and it will disappear by itself once the weather starts to cool down.

Is There Anything Wrong With My Green Wood?

No, there’s nothing wrong with your green wood. In fact, there’s everything right with your green wood. The fact that it’s green proves that it’s undergone pressure treatment and therefore should last you a good long time!