Understanding Extractive Bleed

By Joint Coatings/Forest Products Committee; Extractives Task Force

Wood Extractives: Distribution,Properties, and Classes:
Color, odor, and natural durability of heartwood are characteristics imparted by a class of chemicals in wood known collectively as extractives.1 Wood is converted by the tree from sapwood to heartwood by the deposition of extractives, typically many years after the growth ring undergoing this change was formed by the tree. Extractives are thus not a part of the wood substance, nor do they play a major role in the mechanical strength of wood, but rather are additional materials added as a part of the natural aging process of the tree.

Most commercial species that are valued for their appearance owe their merchantability, at least in part, to characteristics imparted by their extractives. In the wood of most species, extractives are most highly concentrated in knots, which are remnants of branches. When knots appear in boards, they can be problematic for extractive bleed in three main ways.

First, knots often have the highest concentration of extractives per unit volume.

Second, knots are effectively all end-grain in plain-sawn boards and thus are perfectly positioned to bleed extractives efficiently.

Third, knots commonly check and crack during drying or over time in service, thus opening routes through the knot for rapid movement of water (water is used in this article to indicate liquid water), air, or other substances (such as turpentine in pine resins), which often carry extractives with them.

For these (and other) reasons, clear boards (those without knots) have always been preferred for most applications in which wood is finished. As we harvest ever smaller and younger trees, the availability of clear boards will decrease and their price will increase, so it behooves us to learn to mitigate the negative effects of extractive bleed from knots. The changing wood resource also makes the harvest and use of sapwood increasingly more likely, but within that sapwood, knots will each likely bear a small core of heartwood, so the knot–heartwood–extractive dilemma is likely inescapable.