Why Timber Changes Colour
Exposure to Weather | Sapstain Mould | Bluestain Mould | Iron Staining
Earlier this month (May 2016), Preschem was asked to give a presentation on why timber changes colour at the Victorian Woodworkers Association. This got us thinking that this topic would make a great general knowledge post. After all, this will assist with some of those decisions on what to do when building something out of wood.
Why does timber change colour?
- Exposure to weather, including UV and rain
- Staining due to natural or introduced causes
- Chemical changes within the wood resins, or “extractives”
Before we get into the nitty gritty, we need to understand a few properties of wood that will effect the colour over time.
The colour of timber is caused by natural pigments. This is defined by a group of chemicals known as quinones and polyphenolic compounds. Quinones are the true pigments, as the polyphenolic compounds are clear. However they can become important over time, as will be discussed later.
All timber contains what are generally referred to as “extractives”. Extractives have been discussed earlier in a few previous articles, but are the pigments, tannins, and other resins. They can be ‘extracted’ with a solvent such as water or mineral turpentine, hence the name.
Another key component of timber that can effect it’s change of colour over time is lignin. Lignin is the glue that holds the timber fibre together. It is the most important structural component of timber.
1. Exposure to weather, including UV and rain
Weathering is the biggest single, and most important cause of discolouration. There are two main mechanisms behind this:
- Exposure to rain bleeds out the extractives, including the natural pigments (quinones). This process lightens the colour of wood.
- Exposure to UV breaks down the lignin into simple sugars. These sugars act as a food source for moulds, which can be dark and blotchy in appearance. But in situations where they are controlled, such as in dry or coastal climates, they don’t develop fully so turn the timber a silver-grey.
2. Staining due to natural or introduced causes
- Biological staining
- Sapstain or Bluestain moulds are usually an issue when the timber is freshly cut. They thrive on the extractives and high moisture content of green timber. They tend to be a dark blue-grey or black in appearance, hence the term “Bluestain”. The damage that they do over time is superficial as it usually effects the outer layer only.
- Wood rot will also change the colour of timber, especially brown rot. As the name suggests, brown rot is brown in colour
- The main cause of staining through introduced causes, is Iron Staining. This happens when wood is sprayed with welding spatter or grinding metal. The iron then rusts when exposed to water leaving small black blotches over time. In addition, the Iron also reacts with the tannins, which are quite dark. This could also be classed as a chemical change as well, but it’s been classified here.
3. Chemical changes within the wood resins, or “extractives”
- Remember that we discussed the polyphenolic compounds with the quinones in the introduction? When they are exposed to air and a little UV, they oxidise to form quinones. So they go from being clear to a defined colour. This darkens the wood over time.
- Exposure to light, even ambient light, causes tannins to slowly oxidise. This can create a yellowed appearance over time. It is very common in light coloured timber like Blackbutt or Radiata Pine.
- Introduction of alkali or acidic material like glue or finishes, can cause unexpected reactions with various components of extractives.
- Leeching of extractives can cause water marks on timber, especially when freshly cut. The water draws out and concentrates the extractives on the surface.
How do you prevent these colour changes?
- UV Weathering is the most difficult to control, while keeping the wood looking natural. Using a properly formulated and lightly pigmented product like Aussie Clear or Aussie Coat. They will slow the weathering process down over time, but won’t completely stop it. So you need to regularly maintain them over time. Think of it like sunscreen if you’re out in the sun. You have reapply throughout the day to prevent you getting badly burnt, but it won’t completely stop it.
- Iron Staining prevention is very straight forward. Don’t conduct any metalwork on or near unsealed timber. Also, don’t use a metal based scouring pad or steel wool to prepare the timber surface. Always use sandpaper made with glass grit.
- Prevent fungal decay by using a registered timber preservative like Timber Preserver or No Rot Gel, but note that these products can effect the colour, or are unsuitable for weather exposed situations.
- Prevent sapstain/bluestain moulds by using a properly formulated finish like Aussie Clear or Aussie Coat, that contains an ingredient to prevent it’s growth.
How do you remove unwanted colour changes?
- For UV weathering, use an oxalic acid timber cleaner like Grey Deck Cleaner. A pressure washer is also a useful tool. But if the weathering is quite heavy, a light sand prior to the application of Grey Deck Cleaner is recommended.
- For iron staining, you must
- first sand back the timber,
- Re wet the timber and leave for 24 hours to ensure that you have removed all the iron
- Apply Grey Deck Cleaner
- For sapstain or bluestain moulds, use Grey Deck Cleaner or D-Mould to kill it. Then use a formulated finish like Aussie Clear or Aussie Coat to prevent it’s recurrence.
- For water marks caused by extractive bleed, use Grey Deck Cleaner.
- To remove yellowing or darkening of interior timber, lightly sand the finish and/or wood itself.
One thing to note with all the issues discussed with the exception of timber decay, is that the factors that cause timber to change colour usually effect the exposed surface layer only. They do not effect the timber internally, and most do not do any real or long term damage.
Great article Rohan, many thanks!
Thanks for the detailed information regarding timber colour change. Its useful to prevent timber and also to repair the damage.
You’re welcome. Feel free to also ask other questions on timber and its properties. It may be worth another post that’s more specialised on a specific topic.
Hi. Our blackbutt dining table and chairs are getting quite dark yellowish colour which i don’t like it at all.
The afternoon sun has directly on it. Is any chance we can fix it? If so, what do we need? I really want the original colour back.
Thank you for your enquiry. With your permission, I would like for you to answer some questions that I’ll send to you via email and also ask for you to respond and include some pictures. Once my solution works, if you could send me some pictures back. I would then like to add this as an update or maybe even a follow on post about restoring weathered timber to it’s original state. Preschem does value your privacy, so will not disclose your identity or location, but I’m sure that your experience will help other people understand that this is not as big an issue as they may think.
We want to stop pine rough sawn weather boards from going grey and don’t want to use a coloured stain. Will a clear sealer stop this ? Maybe even a concrete sealer?
Thank you for your enquiry. Which species of pine are you using? This is important because if it’s Radiata Pine, it requires to be treated to H3 as it is not durable (Class 4 above ground as defined by AS 5604 has a probable life expectancy of 0-7 years without treatments). Other species of pine may or may not be durable and/or treated.
If it’s treated (radiata) pine, then the treatments will assist in keeping the timber from grey weathering. As I stated in this post, grey weathering is a type mould that grows after the UV from the sun breaks down the lignin in the timber into simple sugars. The mould feeds off those simple sugars. But the primary treatments are also toxic for the mould, as well as fungal decay. In this case then you can use either Radial Timber Sealer or Exterior Pine Clear. Neither products have a pigment, but as a result, neither have any UV protection either. So this solution is only valid if your weatherboards are treated softwoods as per AS 1604.
Otherwise, you will have to use Aussie Clear or Aussie Clear light to prevent UV weathering. But these are pigmented with a “transparent” pigment rather than the “opaque” pigments used in stains. These “transparent” pigments are the only effective UV protection which will drastically slow down the UV weathering process. Aussie Clear Light as the name suggests, is suitable for light coloured timbers. It will approximate wet straw. We can supply you with a sample can that you can brush out onto an offcut to see if you’re happy with it.
I have a budgeroo cabinet inside why does the timber turn dark my has been oiled my friends has been sealed and hers has gone dark too
I just finish sanding my wooden floor. I like the natural look of pine wood floor. Could you tell me what I should do to preserve the ‘raw’ look of pine wood after sanding? Hope I can do something to keep the colour light and no yellow or shine as well as the pine floor hardens to stop scratches and marks.
That is the million dollar question. Unfortunately Preschem doesn’t have a product to suite your needs. However…
In terms of hardening the floor, you are limited by the natural hardness of the timber. Most pine species will tend to be of low to moderate hardness. There are no floor finishes that will improve that situation that I’m aware of. I would recommend that you make sure that your furniture have soft pads on its feet and if possible, have a bigger foot. This will spread the weight over a larger area so decrease the likelihood of marking. Fabric furniture pads are available at most hardware stores.
You could use a traditional linseed oil (or tung oil) with thinners and terebin drier, which will naturally deepen the colour and highlight the grain features, but this will have to be re-applied on a regular basis like a decking oil.
In terms of the hardest wearing floor finish, then consult a specialist paint store. I know when I did my floor a decade or so back, the 2-pack epoxy finishes were still the hardest wearing and most resistant to scratching etc, but it has yellowed with sunlight. But be prepared to move out for 1-2 days as the off-gasses as it cures are very strong.
The water based finishes don’t tend to yellow and may have come a long way in the last 10 years in terms of their scratch resistance. So speak to that specialist paint store. They have excellent product knowledge.
I hope that this information has helped.
I have just finished laying a red gum decking that has been stored for about 3 years. I sanded the deck and purchased a tin of natural finish ultra deck and coated a few 4 meter lengths with one coat. I was hoping for the reds and pinks would appear as they do when the deck gets wet. But no, instead I have a dull yellow tint that looks a lot like merbau.
I have now sanded all off it off and am looking for a better option that gives me reds that I am after. Do you have a product that will do the job?
Thank you for your enquiry. You caught me at a good time as I’m off on leave tonight.
In answer to your question, we do. Aussie Clear. Unlike the Ultradeck, which is a “low build” film forming modified acrylic finish, Aussie Clear is a true penetrative oil. It’s not better than, or worse than the Ultradeck, but it is different in how it performs over time. I have more details on Aussie clear with these links, but remember that Aussie Clear will darken the timber and it’s designed to bring out the natural features of the timber. This includes the natural colour. It is also lightly pigmented as is discussed further in the first and second link.
A quick rule of thumb test if you have an offcut handy. Apply some cooking oil to the redgum. That is more or less what will happen when you apply Aussie Clear but it will be darker.
If you are still unsure, then we can arrange a sample can for you to try, but this won’t arrive to you before Easter, unless you live locally in the Cheltenham area. but we close at 4pm tomorrow.
What timber is best to use as external timber battens – we want them to grey off to a natural look.
As external timber battens they need to remain true with minimal movement – but still grey off?
With external batterns, the most important consideration is the choice of timber. As you want it to be weather exposed, it will need to be either of class 1 (preffered) or class 2 natural durability timber as per AS 5604, or H3 treated pine as per AS 1604. Note that class 2 timbers and H3 treated pine have approximately the same life expectency of 25+ years fully exposed. This is it’s resistance to fungal decay above ground. And if this is a load bearing structure, you will need to know its structural rating as well and then apply that the the span tables in AS 1684.
For the timber to grey off, it needs to be exposed to UV from the sun. Therefore if you wan’t to apply a finish or preservative to the timber, to get one that will not stop that process. I therfore recommend Radial Timber Sealer. This will act as a short term water repellent/sealer, which will assist with the dimensional stability while new. It will allow the timber to season without the rapid drying issues that can occur and therefore minimise warping, splitting, cupping issues that are typical even of kiln dried timber when it’s exposed to the weather. The good thing is that it does not contain any UV agent to prevent grey weathering.
If you wish to follow on an application on UV weathered timber, then I recommend the TWA Woodcare Clear Wood Preservative. It’s the only product that won’t significnatly darken UV weathered timber once it dries.
Thank you for your informative and comprehensive article Rohan. Just over 2 years ago we had installed in our new house an engineered Blackbutt timber floor. I was thrilled with the result as every board had its own distinctive grain pattern, and the colour was just as I had hoped. However the other day I noticed that under a floor mat the colour was much lighter than the rest of the floor. I read your article and noted its mention of Blackbutt. I love the present colour of the floor but I was wondering if you could give me some idea of how darker I can expect ti get before the colour stabilises. I don’t fancy the idea of the grain possibly getting obscured.
Thank you for your kind words. Without knowing what type of finish you are using then the yellowing is either the finish or you are experienceing the oxidation of the polyphenolic compounds into quinones. As the discolouration is not happening under the mat, then this would indicte that it’s more likely to be the finish.
I personally have Silvertop Ash flooring at home and this is very similar in appearance to Blackbutt. And I have yellowing on most of the floor except under furniture etc, so in my case it’s the finish. I found that after about the 3 year mark, it hasn’t gotten any worse. I still love my floor as I have the “Wormy Chestnut” version, which has the odd light imperfections such as ambrosia, gum veins etc. And there is a colour variation in there as well. I have blond, light pink to mid brown tones. It adds character rather than being perfectly uniform.
I wish to protect my Baltic pine table and bench seats from water condensation, but try not to change the wood colour. These pieces are under a metal roof with clear sheeting at regular intervals and situated under a large entertaining area. They are protected from the rain,
Sorry for the delay in response, I’ve just returned from leave.
In this application you could use Radial Timber Sealer. This is an unpigmented exterior finish that is designed to be water repellant and therefore stabilise timber exposed to the sun and rain without preventing the UV weathering where the timber turns silvery grey). In your case as it’s not exposed directly to the sun and rain, it won’t change colour quickly. Over time it will change in colour and likely yellow off. In this arrticle I’ve discussed above the oxidation reaction of polyphenolic compounds to quinones, which act as pigments. Radial Timber Sealer will not prevent this reaction from occuring.
I hope this is clear.
We have a new silvertop ash decking. We want the weathered grey look, but also want to protect and preserve the timber. How can we do both ?
Thank you for your question. Preschem has a product called Radial Timber Sealer. This is a product that’s designed to help control the moisture content of the timber, while not inhibiting the UV weathering effect at all. It will last about 6-9 months in most situations.
However, should you decide to re-apply Radial Timber Sealer once it’s UV weathered, it will go a dark grey like a wet power pole. This could last up to 2-3 months. Especially if this deck is fully exposed.
Hi there we have a white mahogany ( white stringy bark ) deck and wanted the weathered / light silver look , we applied two coats of a oil without any UV blocker to protect it which has worked well in the exposed areas but I was just wondering if there is a way to mimic the UV weathering to areas that are covered and dont get sun exposure like under the eaves?
So you used Radial Timber Sealer? If so, you could add a little of the “Grey Mist” pigment bottle from Cutek. I have been playing in the lap to also achieve this so there is another little option you can try but be careful and try a little section first or an offcut.
-Get some OO or OOO steel wool and place a small amount in a cup of white vineagar.
-Leave it for a few days
-remove/strain the steel wool and lighlty dab the timber.
BUT be careful! This is where the offcuts come in handy to test first. You may have to dilute your iron stain mixture it to get the right amount of grey. This isn’t a permanent chemical staining process as effects the “extractives” in the timber. If they bleed out with the weather they will also bleed the staining.
My new decking has been down about 3 weeks It has not been stained or painted yet. Today I used water to spray cob webs from the doors of garage. I then walked across the deck which was slightly wet from rain and spray. In a few minutes I noticed that I had left black footprints over the decking. I used no chemicals when spraying garage doors, only water. Can someone help? I will gladly send photos.
I’ll be happy to assist and yes send me some photos. I might make a supplimental post to walk through the troubleshooting process to help others if that’s all right. I will send you an email to respond to as well with some things to do to see if the stains go away and questions. Your answers and description of what happened to my suggestions will help with the photos.
Some good information here. UV will cause havoc without proper treatment.
Not really. It’s such a slow process that it takes decades for anyting more than superficial cosmetic damage from the mould to occur. A highly durable timber species (above ground) like Iron Bark or Merbau, will most likely need replacement for timber decay first. The mould that grows on the simple sugars, is not attacking the timber structurally, unlike brown rot for instance.
Great conversations happening here, very informative.
We use Iron bark logs for garden edges and have noticed that they start to show black stripes between 3 – 6 months. We water blast them first after they have been debarked to get them clean as sanding them was very labour intensive. Would the water blasting contribute to them and what would we need to do to keep them looking as clean as possible. Happy to send some pics.
Sorry for the slight delay, it’s been a busy start to the year for use here, and going back into lockdown (I wanted to say more but restrained myself…).
If it’s allright with you, I will contact you and discuss offline, then summarise this on a blog post of it’s own.
Hi, In January I put down a rough sawn hardwood deck milled from Populus Yumenis after being allowed to air cure with fillets for two years. After advice I protected with a mixture of 41. 5% linseed oil, 41.5% parboiled oil and 7% turpetine. After about a month the beautiful timber started to blacken. I’ve tried various timber cleaners which improved about 50% but then proceeded to blacken after being reoiled I have even tried oxalic acid and bleach. Your advice would be greatly appreciated, thankyou.
Hi Tim, you’ve unfortunately rediscovered why linseed oil (raw or boiled) is not recommended as an exterior finish, unless very carefully formulated. It is a sapstain mould nutrient. Even then I would only recommend use on treated pine as the treatments will also assist in mould prevention. Untreated timber offers no additional treatment.
To fix, I recommend that you sand the deck first with an 80 or 100 grit media. Sand it just enough to remove the black staining. Then hit it again with a bleach containing sodium hypochlorite. This can be found in many forms such as the original White King, pool chlorine or Domestos toilet cleaner. Do not use neat, but dilute 1 part bleech and 5 parts water. Scrub the timber with the solution using a deck preparation brush then wash off thoroughly with water.
If you use a pressure cleaner at the washing stage, use the car washing nozzle as you can erode the timber with the high pressure jet nozzle.
I would then wait a month or so and see if the mould reappears. If it doesn’t, then use a properly formulated decking oil such as Aussie Clear or Aussie Clear Light. Aussie Clear is formulated with a powerful mouldicide to prevent sapstain mould. This should fix your problem. Unfortunately, it will require a bit of elbow grease to remedy that.
I would love to know how you get on.