Wood color change can be caused by a variety of factors; extractives represent a major one. What are extractives? They’re basically chemicals naturally found in a tree, typically making the wood rot and weather resistant. (Sometimes extractives can even be tasty, such as maple sap used to make maple syrup!) Some extractives are useful after being extracted from the tree, while others remain part of the wood once the tree is sawn into lumber, offering the kind of characteristics that help determine its end use. A knowledgeable contractor or builder will know how the extractives will affect the finished product, whether it will cause resistance to glue or finish or will lead to color change.
Sap is one kind of extractive that will affect your lumber’s coloring.
How Sap Affects Lumber
One type of extractive that some lumber species produce is referred to as sap or resin. Softwoods in particular are prone to weep sap while the wood is still green. Once it no longer flows, the sap dries and hardens; sometimes it even crystallizes, causing the wood to become brittle.
Sap can cause the wood to be particularly hard on tool blades, especially if the lumber is reclaimed old growth Pine whose working properties have been changed over years of exposure to the elements. The combination of hard sap and decades of silica (a.k.a. dirt) will often cause damage to cutting tools as the lumber is cut to size and re-milled. When it’s time for drilling, the brittle wood can more easily split as well. The hardened sap can also be resistant to some finishes, making the affected boards require a primer coat in order to increase adhesion by sealing in the sap.
How To Avoid Sap Stains
Even newer lumber can be properly dried only to later display stains from weeping sap that don’t surface for days, weeks, or even months. The reason for this potential delay is that wood does not dry evenly; the interior parts of a board will also have a higher moisture content than the exterior, so resin can remain unset for some time, leading to a later release. Sometimes sap flow is caused by heat or water and can even occur after installation, causing previously perfect-looking lumber to appear blotchy and include signs of weeping sap.
One species that commonly falls prey to this kind of issue is Spanish Cedar; because of this issue, it is typically dried to a higher temperature than any other species in order to promote setting of the sap. Heating Spanish Cedar to a higher temperature requires a longer amount of time to ramp up the temperature in order to avoid causing damage to the wood; this scenario translates into longer lead times before boards are ready to be sold.
To learn more about sap and other extractives that can affect lumber color, take a look at Part 2.
