Q. I have just run into an expression that is new to me. What is meant by ‘red knots’ and ‘black knots’?
A. The two terms, red knots and black knots, actually are used with softwood lumber. To understand the difference, we need to go back to the living tree.
Assume that we have an actively growing branch in the stem of a tree. Each year as the tree’s diameter increases, new wood from the stem is formed around the branch and the branch itself increases in size itself. The connection between the stem wood and the branch wood is very intimate; that is, the wood cells touch each other and liquids flow between them and so on. It is a living connection. When the wood in this area is eventually sawn into lumber, the knot will be tight, there will be no bark and the color is often a bit red. Hence, this type of knot is called a red knot.
Now assume that we have a branch that has died but is still connected to the stem. There may even be bark on this dead branch and because of the sap or resin in the tree, the branch does not rot or fall off quickly. So, as the stem increases its diameter each year it grows around this dead branch and its bark. There is not an intimate, good connection, as the branch is dead. When this region is sawn into lumber, the dead branch wood will be dark colored (perhaps fungal staining), plus there might be included bark. Overall the knot will be dark and so is called a black knot. The knot is held in the lumber by its physical shape and perhaps some resin. When the black knot is dried, it shrinks a bit more than the stem wood and the resin evaporates, so a black knot will common be loose and can even fall out, leaving a knot hole.
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