Based on their causal factors and nature, wood discolourations can be put into two major groups, microbial and non-microbial discolorations. In our experience, non-microbial discolourations are not as well understood as microbial ones despite their common occurrence.
Microbial Colour Changes
Microbial discolourations are caused by micro-organisms. Many wood discolouring fungi/bacteria such as blue stain fungi, mould fungi, and bacteria, may produce dark or coloured stains when exposed to the right conditions of nutrient, moisture and light.
Non-microbial Colour Changes
Non-microbial discolourations can be mechanical (e.g., burn marks, dirt), chemical, biochemical or photochemical but may also have both chemical and microbial involvement. Analysis of these stains can be complex as many factors may be involved such as exposure to light and oxygen, substrate/coating pH, species, external contaminants, presence of metal ions and moisture content.
Discolouration and Extractives
All wood contains “extractives” which are chemicals that are solvent and/or water-soluble compounds that can be highly reactive. Extractives are non-structural chemical components that are mostly produced during heartwood formation. Extractives are extremely varied in their chemical nature and embrace many different classes of organic compounds, including tannins and other phenolic compounds, resins, essential oils, fats, terpenes, flavanoids, quinones, carbohydrates, glycosides and alkaloids. These extractives can be a major contributor to wood discolourations. For example, some common polyphenol extractives exposed to oxygen and light, oxidize to form quinones and other complex polymers. Generally speaking, dark woods tend to lighten and light-coloured wood tends to go yellow. Specific changes are very dependent on specific environmental conditions and species of wood.
Water Stain Discolouration
Exposure of raw wood to water can mobilize and cause leaching of water-soluble extractives presenting as unsightly water stain rings when the wood is dry. Water stain can be quickly and easily removed with CUTEK® Wood Reviver.
Natural Silvering of Wood
Uncoated exterior wood that is exposed to UV rays breaks down the lignin in the cellulose causing photochemical degradation. This is what causes wood that is bare or coated in clear CUTEK® Extreme to lighten/silver over time. The specific shade of grey and rate of change is dependent on species, exposure aspect and coating format used. If you wish to slow the rate of “silvering off” of your exterior CUTEK® Extreme coated wood then it is vital to add a CUTEK® Colourtone to your CUTEK® Extreme prior to application in all coats. Exterior wood that has “greyed off” may be quickly and easily restored to fresh looking wood by using CUTEK® Wood Reviver.
Discolouration caused by Alkaline Materials
Alkaline substances and chemicals such as bleach and alkaline adhesives etc., can react with woods high in polyphenolic tannins to produce stains. These resulting stains can often be distinguished from iron stain as they are usually brown in colour whereas iron stain typically has a black/blue appearance.
Discolouration Caused by Iron Stain
Iron stain is an unsightly blue–black or grey discolouration that is often incorrectly described as “mould” because of its frequently spotty appearance. Iron stain can occur on nearly all woods; however, some woods are particularly prone to iron stain because they contain large amounts of tannin-like extractives. The discolouration is usually caused by a chemical reaction between extractives and iron content in steel products, such as nails, screws, and other fasteners. Steel used in contact with wood must be protected from corrosion by using stainless steel or processes such as hot dip galvanizing. Problems with iron contamination can come from traces of iron left on wood from cutting, grinding or slicing; cleaning the surface with steel wool, wire brushes, or iron tools; using finishes stored in rusty containers; and using previous iron-containing or iron-contaminated finishes. Iron dust from metalworking and even plant fertilizers can be sources of iron along with removal of old rusted eavestrough, handrail construction and contact by steel capped boots. Merely striking wood with a hammer can cause iron stain on some wood. Urine on wood floors will also hasten the reaction with iron and wood extractives, producing the typical iron stain discolouration. Unprotected wood that get wet on or off site prior to fixing are particularly vulnerable as the water-soluble extractives are more readily mobilized to react with any iron contamination. Iron stain can be quickly and easily removed with CUTEK® Wood Reviver.
Discolouration in Wood with High Leucoanthocyanin Levels
Some wood types can contain high levels of naturally occurring tannins called leucoanthocyanins which normally appear colourless. In rare circumstances, the application of a higher acid value glue or coating (such as CUTEK® Extreme) can cause the leucoanthocyanin tannin to chemically change into anthocyanins and other derivative compounds. These compounds can then appear in the visible region of the light spectrum, showing more commonly as an attractive red note enhancement of the underlying substrate, however more rarely purple, blue, bluish green through black hues may be visible in some areas. The actual colour of the stain is a function of overall pH and environmental factors such as heat, light levels, enzymes, and oxygen. Not all woods are susceptible.
Natural features of wood types like this show why it is important to choose suitable wood variety for the intended environment, coating format and end use application. If you suspect that this may be a problem for your project, please use a Pre-Mixed Sample to help determine the suitability of our product for your project.
If you are unsure if CUTEK® Extreme is the right option for your project, please contact our friendly staff for further advice.
CUTEK® staff are experts in remedial treatment products and processes for many types of wood discolourations and stains. If you have a specific issue that you would like advice on, please contact us for further information.