Q What is the best RH to run the plant in the wintertime and why?

A Here is a short answer. Previous columns here have gone into more detail.

First, if solid wood dries 1% MC more, it will shrink (which includes warping) about 1/4% more. However, if dry wood gains 1% MC, not much swelling or size change will occur. If the wood gains 2% MC, a little swelling will occur, but not much. The reason is the hysteresis effect if you want the technical term. The conclusion is that we basically want the wood to dry and shrink very little in manufacturing or in the customer’s home or office. A little moisture gain is usually not fatal. This conclusion assumes or requires that the kiln operation provides lumber at the correct MC with little variation.

Second, most homes and offices in North America have air that averages around 30% RH on cold winter days, which will dry wood to 6.0% MC; we call this condition 6.0% EMC (Equilibrium Moisture Content). In the summertime, the average interior humidity is around 50% RH or 9.0% EMC.

So, in a furniture or cabinet or flooring plant or shop, we do not want the wood changing MC very much between the plant and the customer’s home or office. We especially do not want drying. Therefore, in the wintertime, we do not want the plant to be much above 35% RH.

Many hardwood drying operations should target a final 6.8% MC with nothing drier than 5.8% MC at any time during drying, and nothing wetter than 7.8% at the end of drying. This is equivalent to a 0.3% MC standard deviation. This humidity level will keep any further drying to a minimum. In fact, a finish on the product will buffer any short term lower humidities pretty well.

Note that heat lowers the RH, so we do need to be very careful when adding heat. In fact, when adding heat, we likely will need to add moisture to the air. In practical terms, the RH can vary plus or minus 3% without concern. Now, in the summertime, it would be good to maintain the same wintertime EMC values, but this is usually impractical. So, we try to keep the RH under 50% RH, with 40% being a good target. Remember that 9% MC wood in the summertime will potentially dry to 6% MC in the wintertime. A 3% MC change can result in 3/4% shrinkage.

The kiln operator can achieve any reasonable MC value with little variation, with the proper equipment and proper operation. The technical instruction for the kiln operator is, “Never let the kiln go under 5.0% EMC (maybe even under 5.5% EMC when the products being made are extremely moisture sensitive) at any time within the kiln, start to finish.”

Oftentimes, in my technical visits, I find that instructions and targets for the kiln operator are lacking, so the operator goes by the general information and instructions that are “in the book.” BIG MISTAKE. Customized requirements are needed, with a moisture content measuring system that gives feedback to the kiln operating personnel so they can essentially “be perfect and get better.”

Q What can you tell me about black locust? The logs are available here in West Virginia and the lumber seems to have good prices.

A Every month here at FDMC, we feature one wood species in our Wood Explorer column. Archives are available online at WoodworkingNetwork.com. Check it out.

As a furniture manufacturer, the biggest issue is that the lumber is very hard to dry without surface checking. It checks more easily than oak, so it requires especially slow drying conditions. The checks are often very hard to detect in dry lumber without cutting a surface-checking sample. Of course, once it is planed, checks may show up, but they were created early in the drying process.

Q How can we estimate the weight of lumber? Thanks for your help.

A Obviously, the weight depends on the species, moisture content, growth rate in some species, size, and other factors including natural variability. The following report can be used for lumber at any moisture content. Let me know if you have any questions. https://forestandwildlifeecology.wisc.edu/wp-content/uploads/sites/111/…

Q We seem to have a depressed area at the glue joint. We glue several pieces into an angle and then wait 24 hours before we sand them smooth. Can you suggest why we see this sunken area? Thanks.

A The most common reason for this defect, called a sunken glue joint, is that the adhesive, which contains water, causes the wood at the joint to initially swell in height. Then, if the piece is sanded or planed smooth before the moisture leaves (or the swelling goes down), then the swelling goes down after the panel is made smooth. This delayed shrinkage at the joint then creates a sunken joint.

The cure is to wait maybe three to five days before smoothing the surface. The wood needs to be warm during this time as the warmth encourages water to move. I have seen some people put sawdust between the panels to develop air between each angle in a stack, which encourages quicker drying of the joint.

If this answer does not seem correct in the similar complaint from one of our readers, there is a rare case where the heat in finishing softens the glue, so the joint moves and create a depression.

This sunken glue joint defect is more obvious when using glossier finishes.

As the critical factor is moisture in the adhesive, you might want to explore using a non-moisture adhesive such as PUR (polyurethane) made by a reputable manufacturer.

Q The current market is demanding a “white,” or sapwood, sort in 4/4 thickness F1F/Btr and 1Com grades. How can we and our splicers reduce, or eliminate, the light gray stain in the sapwood of our yellow poplar, specifically our 4/4?

A The gray stain in this species and other white-colored species is often called enzymatic ideation stain, which involves the oxidation of the natural sugars and starches in the sapwood.

This oxidation requires warmth, moisture and time.  The reaction can begin when the log is first cut in the woods. Lumber from old logs is very difficult to dry and achieve a white color.

What is required is that the lumber, once sawn from fresh logs, must be stickered and put into a fast drying situation ASAP — within hours, not days. This low humidity (12 to 15F depression in kiln language) and high airflow (more than 400 fpm) situation quickly eliminates the moisture also, so the oxidation is stopped. In addition, it helps to start drying no hotter than 105F — cooler can be better. Note that often a kiln cannot achieve the suggested low humidity because the water from the lumber leaves faster than the vents can exhaust it. In this case, loading less lumber into the kiln is suggested.

Because we need fast drying, the stickers used need to be quite dry so that the wood they are in contact with will dry quickly and white in color. If stickers are even slightly wet, then sticker stain results — a darker section in the area in contact with the sticker.

The fast drying must continue for most of the kiln schedule to avoid slow drying of the core; this slow drying would result in the darker color in the core.

These instructions for yellow poplar are the same as those used to achieve white hard maple, ash and other white wood lumber.

Q We store our incoming green oak lumber in a building that is adjacent to where we store our kiln-dried oak. Is there a risk of having the green lumber infecting the dry lumber with the powder post beetle?

A The short answer is “No.” The reason for this answer is that the dry wood powder post beetle, called the lyctid PPB, does not inhabit green or partly dry lumber (wood above about 25% MC). It especially likes wood under 15% MC.

There is another PPB, called the ambrosia beetle, that does get into wetter wood, and could be seen in the drier wood debris on the floor where you store the green lumber. It is important to keep the floor free of wood debris. Hence, dirt is not the best. But, this ambrosia beetle to so common that the air yard is likely the source. So, the rule is: “Keep wood debris out of the air yard, and green storage and kiln-dry handling areas.”

Foundations in the air yard should be treated. Stickers are sterilized when in the kiln when temperatures exceed 133F. So, avoid using old sticks left outside or kept in storage for a long time.

The ambrosia beetle does not live in dry wood, but, the tunnels and small 1/16-inch diameter holes can be seen, even though the insect is not active any longer.

In case your question arises from having found active, lyctid PPB in kiln-dried wood, this is impossible if you dried the wood yourself. Why? Once the eggs are laid, it is virtually impossible that the beetles hatch and leave the wood sooner than 10 months later. Even if you did have eggs laid on recently dried lumber, the chances are really good that if you plane the lumber within a few weeks after drying, the eggs will be removed with the planer shaving.