Extractives are chemicals that are present in each species of wood. Their purpose is to help make the wood less susceptible to rotting or getting damaged by the elements. Some extractives, like Maple Syrup and Turpentine, have been harvested and put to good use. Other extractives are purposefully left in the wood in order to render the wood more durable. When extractives are left in the wood, they can have a variety of different effects, both positive and negative. This series of articles will focus on some of the ways extractives can affect different species of wood.
Impact of Weeping Sap
Softwood species are prone to producing large quantities of sap. This sap is especially inclined to weep before the wood is dried. As the drying process takes place, the sap has a tendency to become extremely hard and may become crystallized over time.
Hardened Sap Makes Wood Brittle
Cutting wood with a high sap volume can be tricky. If the wood has dried out to the point where the sap is hard, that hardened sap can wreak havoc on blades during the milling process. For example, reclaimed old growth Pine that has accumulated years worth of dirt and hardened sap can be especially challenging to drill and cut. It has been known to split easily.
Hardened Sap Renders Wood Finish-Resistant
Not only is wood with hardened sap hard to cut, but the sap also serves as a repellent to a number of different types of finishes. To combat this problem, a primer coat can be added. This will help prevent the sap from weeping through to the surface so additional coats of finish will adhere to the wood more successfully.
Drying Weepy Wood Can Help Reduce Staining
When fresh wood with a high sap content is first cut, it’s a good idea to give it plenty of time to dry out. When the boards begin to dry out, they tend to do so unevenly, with the interior continuing to retain a higher level of moisture than the exterior. When the boards are out in the hot sun or when they come into contact with water, sap can start rising out of the boards’ interior and end up causing stains to appear on the wood. Certain species, such as Western Red Cedar and Spanish Cedar, may require slow, careful kiln drying at extra high temperatures to try to alleviate this problem.
In addition to staining problems, a board with weeping sap can be resistant to glue as well. This is all the more reason to be careful about allowing wood with a high sap content enough drying time before applying adhesive to it. Kiln drying may be necessary for certain woods with high sap content. After proper drying, it’s important to also pre-finish the wood. Mineral spirits or Naphtha wiped over the boards after planing can remove much of the initial sap weeping. After wiping the board, wait until the weeping stops before continuing to work with the board. Other than reclaimed timbers that are repurposed and weep after years or decades of initial use, most boards won’t weep much at all after this initial pre-finishing treatment.
Taking measures to prevent weeping sap from causing stains and other problems is an important step in preparing boards for use. In our next article in this series, we’ll take a look at how rot-resistant extractives and tannins can also weep, and what to do in these situations.
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J. Gibson McIlvain Company
The McIlvain family has been immersed in the premium import & domestic lumber industry since 1798. Headquartered just outside of Baltimore, the J. Gibson McIlvain Company is one of the largest U.S. importers of exotic woods. As an active supporter of sustainable lumber practices, the J. Gibson McIlvain Company has provided fine lumber for high profile construction projects worldwide. Call (800) 638-9100 to speak with a J. Gibson McIlvain representative.