Q. We are thinking about using southern yellow pine for moulding, shelving, and paneling. Any guidelines?
A. Indeed, southern yellow pine (SYP) is a bit different than most other softwood species. It shrinks a bit more when the moisture content (MC) changes, but more importantly, it warps (especially twisting, when the MC changes. This warp is due to the grain in the tree, especially the grain angles for the first 15 to 20 years.
Therefore, in addition to perfect stacking when drying, it is also very important to get as close to the final MC as possible (7% MC is a common moisture content in interior environments) if we do not want warping in use.
At the same time, drying under about 10% to 12% MC greatly increases machining defects, as the wood is getting so much stronger and the grain angle factors, too. Planers, moulders, routers, etc., need to be powerful as the MC drops and strength increases. The bottom line is that it is rare to find SYP used for many wood products except structural pieces. Also, the price of SYP this past year has tripled, making it quite expensive compared to other species suitable for millwork.
Another issue is sap. The pitch or sap in SYP is plentiful. It consists of many different chemicals. Some of these chemicals are fairly liquid and runny at room temperature. Some are liquid and runny at cooler temperatures, too. And some become liquid and runny only at warmer temperatures. This pitch can drip or ooze out of the wood and also migrate through paint and varnish, and even plastic overlays. The pitch interferes with gluing and clogs up sandpaper. It can coat saws and knives. In the kiln, it can coat the walls. When SYP is used for firewood, the pitch can coat chimneys and create creosote, as well, which in both cases is creating a fire hazard.
So, during lumber drying, what we do is heat the wood to at least 160F (180F is even better and faster), which will evaporate the chemicals in pitch that are liquid and runny at 100F and maybe a bit hotter. After 24 hours at 180F and around 48 hours at 160F for lumber under 2-inches actual thickness, the pitch or sap that remains in the wood will be hard at room temperature. This process of evaporation is called "setting the pitch."
Should the wood be subsequently heated in processing (such as heat from machining or sanding, or in a finishing oven), the heat can soften the remaining pitch and create issues when softened. Of course, when cooled, the pitch hardens. Due to this softening, which can happen in a home when the wood is, for example, near a fireplace or in direct sunlight or has a hot pan on it, etc., we generally prefer 180 F treatment temperature "just in case." Certainly 160F is adequate most of the time.
Q. We are trying to make a butcher block, end grain kitchen cutting board, but the core of the lumber has a whiter color than the shell. About 3/4 of the pieces are affected. What is going on? Am I too picky?
A. One key principle when drying wood is that when the specific spot within a piece of wood is still "wet" (above 30% MC), differences in drying conditions — temperature and humidity — will create differences in color. In your case, the white core results because the lumber was air dried for a while at cool temperatures and modest humidity, and so dried slowly. This gives us the dark color in the shell as the wood had time to oxidize. However, when the wood went into the kiln with the core still being wet, the kiln was set hotter and maybe drier than the outside, so the core dried quickly and lighter in color.
We encourage drying people to never "partly air dry” before kiln drying, but rather fully air dry when uniform color is needed. You are not too picky. Hopefully your supplier will work to adjust things with this. In the future, and to help get an appropriate adjustment, add "uniform color, shell to core" to your order request.
Q. Can I sterilize wet wood before drying, and then not worry about insects, like powder post beetle?
A. Sterilization of wood only sterilizes the wood for that moment. Once wood leaves the sterilization equipment, it is subject to infection just like unsterilized wood. Infestations can come from wet wood insects like the ambrosia powder post beetles and dry wood insects like the lyctid powder post beetle that are especially damaging in completed wood products.
Q. I have some ash that is getting sticker stain? Why and what can I do?
A. Ash is indeed one of the worst species, or perhaps we should say "unforgiving" when drying is not close to perfect. ticker stain, also called sticker shadow, is likely.
Sticker stain results because of the difference in the speed of drying under the sticker versus the speed of drying in the wood between the stickers. Slower drying is darker. To get faster drying under the sticks, we use very dry stickers — 7% MC maximum. Use stickers with fluted surfaces to encourage drying under the sticker. Note: All this drying variation and therefore color variation occurs above 45% MC, although the color difference usually only shows up after drying is complete. Drying under 45% MC might darken the stain, but it is caused by what happened at the higher MCs.
From a technical viewpoint, this stain is caused by the enzymatic oxidation of sugars and starches in the wood. It seems that logs stored in warmer weather before sawing and maybe lumber stored too long before stacking, are more prone to developing stain.
I recently gave a webinar for the NHLA on drying maple that discussed color control. This discussion applies to ash as well. You can watch a free recording of the webinar at nhla.com/education/webinars/dryinghardmaple-register/.
Q. Our local sawmill has some willow that they just sawed. It has an unpleasant odor, but the sawmill yard said it would go away after drying. Will it?
A. The odor is caused from the activity of an anaerobic bacteria that was in the tree. The bacteria created fatty acids, among other things it did, and this fatty acid then turned rancid. Indeed, the odor seems to go away when dried, but if the dry wood is exposed to higher humidity or liquid water, the odor can return. So, it seems avoiding this smelly wood, which we see in other species also, is probably the best action.
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