What is Mineral Stain?

Mineral Stain

 The term mineral stain has been loosely used by the industry to denote stains of various kinds in hardwood lumber and veneer regardless of their origin. The colors associated with mineral stain can be described as light gray through tan, brown, olive green, purple, blue and black (16). Researchers have tried to restrict the use of the terms mineral “stain” and “streak” to refer only to the olive and greenish black areas common in hard maple and other hardwoods (10, 12). The National Hardwood Lumber Association defines Mineral Streak as “an olive to greenish-black or brown discoloration of undetermined cause in hardwoods”(7). The first written indication that mineral stain was a recognized as a defect was in the lumber grading rules published in 1898. The specific term mineral streaks and spots were officially included in the “Rules for Measuring Hardwood Lumber around 1937 (1). Mineral stain is found in many hardwood species such as hard and soft maples (e.g. sugar and red), cottonwood, magnolia, and willow, but it is also common in red and white oak, ash, and hickory (10, 16). Mineral stains appear on the cross sectional cut of logs in the form of either isolated spots, dark areas contouring particular annual growth rings, or both. It has also been reported that areas in wood where mineral stain is present have a tendency to split or crack open where the discoloration is deepest (12, 3).

 It has been suggested that the stains and streaks of discoloration in oak may be comprised of both organic and inorganic matter (16). The discolorations observed in mineral stained areas of hardwoods have been noted to be traceable to crystalline inclusions composed of various minerals in the cell lumens of the sapwood (9). These minerals are associated with carbonates dispersed among noncrystalline phenolic substances. While the inclusions cause no damage to the cell wall, they do have a significantly higher ash content making them harder, and giving mineral stained areas a higher specific gravity, than the normal surrounding wood (9,10). Mineral stained wood is more acidic (higher PH) than both normal heartwood and sapwood (10,14).

Causes of Mineral Stain

Mineral stain resulting from injuries

Researchers commonly attribute the onset of mineral stain as resulting from abnormal cell physiology occurring from wounds caused by insects, animals, weather, or human activity (logging) (5, 9, 11, 12, 13, 16). One researcher strongly stated that for hard maples, mineral stain is a direct result of wounds and the interactions of trees and microorganisms (13). While poor site quality can be associated with mineral stain formation in hard maple, factors such as a poorly performed logging operation, specifically logging wounds, will contribute more to the problem (6). However, such evidence for oaks has not been forthcoming. While it has been suspected that bacterial infection or fungal activity associated with wounds could also be a factor contributing to mineral stain in oak or other hardwoods, no conclusive results have been achieved regarding these pathogens (5, 9, 14).


Mineral stain resulting from growth site characteristics

Differences in the occurrence of mineral stain have been identified among trees of the same species at different geographic locations (2, 10). For example, Cassens stated that mineral-free, light colored red oak is more commonly found in certain areas of the country such as lower New York, Pennsylvania, southern Indiana, and southern Michigan (3). This suggests that geographic variation in site characteristics may influence mineral staining. Poor site quality and/or soil properties are thought to be probable causes of mineral staining in red oak. Oaks, sweetgum, ash, and cottonwood contain “much more” mineral stain than similar species growing on better drained sites (10). Procurement experts link increased incidence of mineral staining in red oak may be related to trees growing on soils with a high content of coal, or trees growing on slopes (15). It has been suggested that forest stand conditions which affect branching characteristics and tree vigor have the greatest influence on occurrence and severity of mineral stain (8, 13). It was determined that for hard maple, stand-growing conditions that promote rapid healing of small branch stubs results in trees with no mineral stain and that stand conditions that promote large low branches with large branch stubs to heal more slowly leads to a significant presence of mineral stain (13). Despite the general observations given in the literature, the causes of mineral stain associated with site quality remain vague. Specific site variables associated with high mineral stain severity have not been identified.


Mineral Stain Specific to Red and White Oaks

Sachs et. al (1966) grouped types of dark discolorations in red and white oaks into three categories based on their origins in the standing tree and differing morphological and chemical characteristics: 1 living in the sapwood, 2. at the sapwood-heartwood boundry and 3. in normal fully developed heartwood (10). Sapwood discolorations were related to an injury of living sapwood parenchyma cells by wounding or natural pruning, from infection by parasite organisms. With subsequent healing and diameter growth, sapwood stains become incorporated into regular heartwood but will usually be darker than normal heartwood colors and retain a characteristic appearance that distinguishes them from the other two stain types. An example included the sapwood stain caused by a borrowing carpenter worm and possible infection by bacteria. These dark discolorations have been referred to as pathological heartwood, wound heartwood, protective heartwood, and discolored sapwood. “Caused by an excessive production of pigmented compounds which appear to originate in the ray parenchyma cells and may be similar in chemical structure to complex polyphenols of normal heartwood” (10). While research at the Forest Products Lab did provide some insight as to the makeup of mineral stain, they were not able to determine its root cause. “It appears that mineral stain is caused by an interaction of a number of factors”. (10). Causes of stains that form in the sapwood-heartwood boundary are still uncertain, neither bacteria nor fungi have been detected in the discolored wood. Oaks containing stains of the heartwood-sapwood boundary are frequently found growing on moist soils, which may even become saturated for extended periods.

 While there are many theories that indicate potential origins of mineral stain, no definite conclusions have been made to explain its presence in oaks. Mineral stain has been found in young trees and in old trees, in trees on both poor and on good sites. It has been found in trees with apparent injuries that occurred during their development, and in trees with no apparent injuries (1). As a result, it is generally agreed upon by the academic community that understanding how and where mineral stain occurs in oak and other hardwoods is a complex problem.