Cause of Loose Knots in Pine
Looseness or tightness of a knot in Pine is a characteristic that is formed during tree growth, not a product of the kiln process. June 13, 2014
Question
I dried a load of fresh 4/4 red pine like I have in the past but this load seemed to have had an outstanding number of loose knots. What are some causes of this problem? Dried to fast or too slow? Or could it have been the nature of the wood?
Forum Responses
(Commercial Kiln Drying Forum)
From contributor W:
I am no expert but I suspect that there were some dead limbs that got covered with subsequent diameter growth. These dead knots then shrunk differently than the surrounding green wood as the wood was dried, losing the knots.
From Gene Wengert, forum technical advisor:
I agree with WDH. I suspect that you have a wood problem and not a drying problem. Loose knots are knots that are held in the lumber with resin and the geometry of the situation. They are not an integral part of the main piece of wood. So, when the resin evaporates in drying along with the extra shrinkage the knot becomes loose. If the geometry is favorable, then the loose knot can fall out.
From the original questioner:
I suspect it is nearly impossible to detect this problem before sawing the log? One of those take the bad with the good situations.
From contributor W:
I think that you can detect it somewhat. If the stub of a dead limb is visible on the surface of the tree bole then there will be some dead knots, at least in the first boards that come off that side. As you go deeper into the log and reach the point where that up-to-now dead limb was alive (green), then those knots should be tight. So look at the bark for indicators of dead limbs as a first sign. Also, bark on pines and many other trees will continue to show what is called a knot scar (a roundish dimpled scar on the bark surface) that tells you that somewhere down in that log at that spot there was once a limb. It might be grown over for a number of years, but the bark has memory and will still exhibit the scar. In fact, in North Carolina in a long term pruning study conducted by the company that I work for knot scars were still visible on the exterior of the bark ten years after a green limb had been pruned. That meant that there was a least several inches of clear wood under the bark at that point, but below that there was an old limb lurking. The Forest Service established a set of log grade rules that focused on these knot scars and other quality indicators. You may have heard terms like"3-face clear" which means there are no knot indicators (scars as I referred to them above) on three of the four faces of the log. That tells the buyer that there is a considerable amount of clear wood under those clear faces. When I saw logs on my little WM LT 15, I use these knot scars to determine how I should cut the log and which face to open. Hopefully I did not over explain this or expound on the obvious.
From Gene Wengert, forum technical advisor:
In pine, sometimes with branches that are part of the surrounding wood we will call the resulting knots in lumber red knots. Branches that were dead and had the wood grown around them so the resulting knots are not attached except by resin and geometry are called black knots (due to the black color of the bark and color of the dead wood). Red knots stay tight in drying; black knots loosen in drying.
