Editor’s note: This previously unpublished column was produced before Gene Wengert died in 2025. It is printed here in memoriam.

Q. Please explain the difference between sapwood and heartwood. Does this difference affect the gluing, machining, etc., properties when making cabinets?

A. Indeed ,sometimes we “experts” use words we think everyone is familiar with. So, your question helps keep us on the straight and narrow. 

There are one or several layers of cells in the growing tree that are alive, called the cambium layer(s). The cambium is where new cells are created, bark cells to the outside, and wood cells to the inside. The cells that the cambium creates are full size and do not grow later in the wood or bark. 

This fact that the cells are full size is why you can put a nail in a tree 6 feet above the ground and 30 years later that nail will still be 6 feet above the ground. 

A tree increases in height by adding new cells to the tips of the branches and central stem. With cambium growth, the tree does increase its diameter.

More than 9% of the wood cells are oriented vertically within the tree. The wood sells often are compared to miniature soda straws about 100 times longer than they are in diameter. 

The length of these cells ranges, depending on species, 2 to 3 millimeters, or a bit longer for hardwoods and closer to 5 mm for softwoods.

When the wood cells are initially created, they are called sapwood because they are conveying liquids from the route system to the leaves. In other words, they are conveying water up to the leaves, and the leaves, then make sugar in a water liquid, and other chemicals, that are brought down through the bark.

As the years go on after the sapwood cell is created, in a number of years, depending on wood species and weather conditions, the sapwood cell becomes inactive in conducting fluids, upward and often times acts as a storage area for the tree. The chemicals vary depending on species. 

For example, in black cherry, the chemicals have a reddish color, which is the desired red that we like in cherry. In walnut, the chemicals have a green color or a very dark brown color, which again are the colors we like in the wood. Each species has its own color of aged sapwood cells, which we call heartwood. 

Some of the chemicals have a peculiar odor; for example, sassafras smells like root beer, and eastern red cedar smells like the bedding used for gerbil. In brief, we can say that heartwood is age altered sapwood.

In some species, the chemicals put in the old sapwood block liquid movement. A good example is white oak; most white oak species are impermeable to alcohol and water, making them ideal for whiskey barrels.

In most species, sapwood and heartwood do not require special techniques for processing into wood products. Often the heartwood is lower in moisture content than the sapwood, meaning that different drying techniques for the lumber need to be appreciated in order to avoid overdrawing the heartwood. 

In my experience, the only one trouble that we have with heartwood is that fine dust can create allergy reactions in some woodworkers. Also, some chemicals in the heartwood are water soluble, and that can create some finishing discoloration issues. 

Because the sapwood is more porous, often finishes, and even gluing adhesives will penetrate the sapwood more than the heartwood, creating differences in finish appearance and glue joint strength. because heartwood includes the region within the first 20 years growth (in a mature tree), we will see more knots in that heartwood region, and also tend to get more. risk of warping with moisture content changes. 

As a summary, unless there is a bothersome color change between sapwood and heartwood, manufactures will not be concerned about the minor difference between sapwood and heartwood.

Q. Does the powder post beetle get into wet wood? I have read your articles about them, and they are in dry wood, but I think I have some that came in wet wood before we got into the kiln. Thanks.

A. There are several different powder post beetles that affect wood, but normally the one that we find in North American infects dry wood. This one is the lyctus powder post beetle that prefers 20% MC down to 7% MC and is found in hardwoods (leaf trees) only. This MC range is where eggs will hatch, pupa and larva will grow and eventually (10 to 12 months) leave the wood as a mature insect.

It is common, after they leave, to find the fine powder dust, technically called frass, in their holes. The dust can come out of the holes when the wood is hit with a hammer or other device, to loosen the frass.

There is another common wet powder post beetle that we find in logs, air drying stickers and lumber. It prefers above 25% MC and both hardwoods and softwoods. It is usually called an ambrosia beetle. However, it is a true powder post beetle with frass in the exit holes. Its life cycle is several weeks, so they quickly multiply in warm weather air drying. In fact, in a log missing some of its bark, ambrosia powder post beetles can be found in warm weather.

SPECIAL ALERT. In any kiln that exceeds 140 F, we can be certain that all eggs, pupa, larva and insects in the wood are killed. A few kilns for lumber may reach only 115 F or so, however, and so do not sterilize the wood. The temperature in the kiln does not affect preexisting damage. Further, the frass from air drying will be present after kiln drying lumber, potentially creating small piles that could be misinterpreted as coming from lyctid powder post beetles — false conclusion.

Q. We had some 4/4 white oak lumber that had been partially air dried. When we put the Lumber into the kiln, the next day we noticed in the moisture samples or kiln samples that there was a lot of surface checking. The moisture of the samples was around 30 to 35% MC. The dry kiln temperature and humidity were as required in the standard kiln schedule, which we follow religiously. Our kiln operator indicates that these are pre-existing checks that are re-opening. Can you confirm this?

A. The easiest moisture level to create new surface checks (the keyword here is NEW) in white oak is above 45% MC. So, the lumber you are putting into the kiln is not high enough in moisture to create new checks. At the 35% MC and lower, the surface fibers are stronger than at higher MCs and the drying stress is also lower. Below 45% MC , we can only re-open and potentially worsen preexisting checks. 

Note: If you had known that these pre-existing checks were present, the “Drying Hardwood Lumber” book, suggests that the schedule in the kiln be backed up one step in order to minimize damage.

I suggest that you add to the kiln operator’s duty that he check for surface checks on Lumber coming into the kiln. A simple test for cutting a sample for surface checks is given in “Drying Hardwood Lumber.”

The real issue here is that in the previous drying, before you received the lumber, the lumber was dried too quickly and developed surface checks. These checks closed when the lumber was exposed to high humidity, such as in a tight bundle or as a sticker bundle in rain or fog. 

Note also: Visual inspection that finds these checks near or at the completion of drying, tells us that the lumber was drywing too quickly above 45% MC, and small checks developed. Then the surface was subject to rewetting. Finally, using standard kiln schedules resulted in visible checks at the end of drying.

Q. What is the pith?

A. The word pith refers to the first year’s growth of the stem or branch. This is often considered to be non-wood in the first year. In practice, the pith refers to that dark area at the center of the tree, which may occupy two or three years of the initial growth. 
This growth is not very strong. This weakness protects the plant from a heavy bird landing on the new growth or from snow weighting down the young stem from breaking. Generally, the pith is very hard to finish and make it look good so the pith is avoided in furniture, cabinets, and in flooring.

The pith can be included in furniture and cabinets, if it is hidden from view or blends in with other natural tree defects. 

As the tree grows gradually for the next 10 to 15 years of its life, the wood around the pith gains strength. So, minimize the pith and surrounding (15 rings) in pieces requiring high strength.

The pith and rings around the pith also are prone to twisting tape shrinkage.

Q. Do you have a favorite book about harvesting of the last big pine forest in the south?

A. Although this book deals with harvesting of Southern Pine and the impact it had on the daily lives of many people in Arkansas and Oklahoma, I suggest “Sawmill: The Story of Cutting the Last Great Virgin Forest East of the Rockies.”

The complexity, the size the number of layers and many other characteristics were very surprising to me and at the same time very interesting.

The author also discusses transportation of the logs to the sawmill, the importance of various saw milling jobs, marketing to Northern locations, land deals, and fortunes made almost overnight.

Interesting part of our wood industry and also easy reading.