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?

  1. Exposure to weather, including UV and rain
  2. Staining due to natural or introduced causes
  3. Chemical changes within the wood resins, or “extractives”

Introduction

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

UV weathered deck

UV weathered deck

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”

timber extractives in ironbark decking

timber extractives in ironbark decking

  • 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.
Iron staining after/before being exposed to grinding and water

Iron staining after/before being exposed to grinding and water

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?

  1. 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.
  2. 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
  3. 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.
  4. For water marks caused by extractive bleed, use Grey Deck Cleaner.
  5. 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.