Sapwood is the outer layers of growth between the bark and the heartwood of a log (Living Cells). This area of growth may be stained either by exposure to certain kinds of fungi or chemical reactions. Blue and Brown Stains are two types of discoloration covered within this course.
SPF Characteristics 103: Discoloration Defects
This course will help define terms used within the National Grading Rule in regards to various discolorations within wood.
SPF 103: Stained Sapwood
SPF 103: Blue Stain
Blue Stain, also known as Sap Stain, is a discoloration of the wood caused by sap-staining fungi and can range in color from light gray to blue, or dark blue to purple in severe cases. The fungi need oxygen, warm temperatures, and wood moisture content above 20% to grow and survive. Sap Stain fungi do not destroy wood fiber therefore do not reduce the strength properties of wood.
SPF 103: Brown Stain
Brown Stain, also known as Coffee Stain, is a non-fungal staining of the wood caused by chemicals already present in the wood. High moisture content and high relative humidity are precursors to brown stain. As extractives in the wood undergo enzymatic changes in high relative humidity and high temperature environments, a browning of the wood can occur, it is very similar to the browning of an apple after the inner flesh is exposed to oxygen. Precursors to brown stain often develop in the log and will become more noticeable during air drying of the lumber; specifically, during hot, humid summer months and it may be exacerbated by unfavorable kiln drying schedules. Brown stain can range from a very light brown to a chocolate color, and in some instances may even appear pink or reddish-brown. It is prominent around knots and other areas that are difficult to dry, such as around stickers placed between the wood for drying, or any area that was deprived of adequate air flow and evaporation during drying. Brown Stain is not a strength reducing characteristic.
SPF 103: Stained Heartwood
Heartwood (Heart) is the inner core of the log comprising the annual rings containing non-living cells. In some species of wood, the heartwood has a prominent color difference than the sapwood, usually darker.
Stained Heartwood and Firm Red Heart are a marked variation from the natural heartwood color. It ranges from pink to brown and is not to be confused with natural Red Heart. Natural color is uniformly distributed through the annual rings, whereas stains are usually in irregular patches. Firm Red Heart is considered a stain and is a stage of incipient decay (will be defined in SPF 104 course), characterized by a reddish color in the heartwood.