Q We are making wooden blind slats. One big issue, besides the supply of basswood logs, is avoiding warp in manufacturing and the finished product. Can you go over some of the key points?

A There are a few manufactured wood products that have special, minimal warping requirements. These products include pool cues, croquet handles, premium drumsticks, roof shingles, and blind slats. 

The procedures to obtain flat, with minimal warp risks, costs money and requires more careful manufacturing. Overall, this is a complex subject. 

Here is a brief summary of the manufacturing requirements.

Choose the correct species. We require a straight grain species, which includes straight logs. When the grain is not naturally straight, the log is split (a split follows the grain) and the sawing is parallel to split face. With blinds, basswood is preferred as the grain away from the core is straight, and machining and drying the wood is easy.

If we do not split the log to find the grain, then we saw parallel to the bark and not to the core mod the log. At this point, a thick piece of lumber, often called a billet, is sawn with the idea that after drying, a rip saw will saw the individual slats. We avoid making slats from wood within 20 rings from the center of the log.

Once the billets are sawn, they are dried carefully. Stacking is perfect with 18 inches or closer sticker spacing. Then the standard kiln schedule is used for a white color. At the end of drying, the moisture must be uniform within the billet (shell to core, edge to edge, and end to end). The average target is 6.8% MC.

Drying stress (casehardening) must be removed at the end of drying using the conditioning process following standard procedures. (see Drying Hardwood Lumber from the US Forest Products Lab.). This is where many people fail.

When ripping the slats from the billet, any immediate warp indicates stress (growth stress is rare, but drying stress is sometimes common). Any warp over time indicates an MC change, which means the 6.8% MC target was not met, or the relative humidity in storage or manufacturing area is not 34% RH, plus or minus 2% MC.

Although thin rip blades mean higher yield, thin blades are prone to wandering, which means uneven thickness. Avoid untested thin blades. Keep teeth as sharp as possible. Gang rip saws are common. With a gang, be on the alert for one blade wandering before the others, causing a jam and excessive down time.

Completed or finished blinds sent to be dried in wetter than average humidity — 34%RH will likely have small humidity changes and may warp if sawing and drying procedures were not followed. A finish will slow and minimize MC changes but does not stop MC changes.

Q We have some hardwood lumber that was submerged in a flood of a nearby creek. Rumors abound about the value of such lumber, including our insurance company’s opinion. What is your experience and conclusion?

A The flood water is full of grit particles that embed themselves in all the minute nooks and crannies on the surface of the wood. This grit will dull planer knives and other tools almost instantly. So, the lumber is not useful as regular lumber would be. 

I have heard of people power washing the lumber to remove the grit, but this is not totally effective in grit removal.

In my opinion, the lumber after flooding may have usefulness for pallets, but not for the higher value uses that regular lumber has. Burning the wood is not a good idea, as the grit will tend to coat the grates or other hardware. 

By the time the special handling of such lumber is considered, my opinion is the lumber’s net value after being submerged in a flood is zero.

Q We use mainly eastern white pine (EWP) lumber. We buy lumber already dried. From time to time, we notice that machining defects increase as though the lumber is brittle. Any comments or guidance?

A It is true for almost all softwood species that they develop machining defects, like chipped grain with 1-inch or longer slivers, if the lumber is:

• a)     dried to under 10.0% MC, especially the outside shell regions of the lumber where the machining tools do their work.
• b)     dried at temperatures of 160 F and hotter
• c)     dried in the kiln using under 8.0% EMC (which will dry the outside layers of the lumber under 8.0% where the machining tools do their work). Once the shell is over-dried, it will show brittle machining even if the shell moisture is raised at the end of drying.

Note that the core MC is not as critical as the outside shell MC. Unfortunately, some EWP drying operations use techniques more suitable to construction lumber, rather than the more demanding techniques for furniture and cabinet uses. Because EWP is fairly stable, we can use the higher final MC without much concern.