Woodsense: Wood Pores

Tiny holes that deserve a closer look

Your project’s joinery may be absolutely airtight. Every surface may be sanded to perfection, and every angle may be spot-on accurate. But after applying a finish, you’re disappointed with the result. For example, you make a small wall cabinet of white ash and lay down a coat of gloss finish. It initially looks great, but then the sunlight playing across its surface seems to magnify hundreds of tiny craters and dozens of shallow furrows. What’s going on?

The answer lies in the tiny pores and vessels that are part of the architecture of the wood itself. I suggest a double-barreled approach to help you get the appearance you want. First, have a general knowledge of how the wood assembled itself in the tree. That will lead you to the second part: knowing how to overcome prominent pores with an appropriate finish.

Watertight white oak. Another type of structure, called tyloses, may completely pack cell cavities. In white oak, the bubble-like tyloses block porosity so effectively that the wood becomes watertight. This enables the construction of barrels to age whiskey.

Tree growth basics

The trunk of a tree is analogous to a bundle of soda straws—a series of tubes that conduct water and minerals up and down the tree. Each growing season, the sap moves throughout the sapwood, the living portion of the tree. In the vascular cambium—a very thin layer between the inner bark and sapwood—the tree creates new cells, increasing the diameter of the tree by one cell width at a time. The first cells of a growing period are called earlywood; subsequent cells form the latewood. Together, they create a growth ring. The outer group of rings are sapwood, the inner, heartwood.

As growth within a tree matures from sapwood into heartwood, profound changes occur at the cellular level. The contents of previously living cells die, and that space may be filled with water, resin, or other biochemical extractives that often give heartwood its darker hue. 

The basic tubular structure of sapwood and heartwood is the same. Depending on the species, these tubes, or “pores” may be more or less pronounced, which can give rise to the finishing problems I mentioned earlier.

Start at the end

The easiest way to see tree rings and wood pores is by looking at a stump or the end of a board, commonly called end grain. In softwoods, you’ll see rings but not pores. That’s because conifers rely on microscopic resin canals to transport nutrients, so these trees are considered nonporous.
By contrast, hardwoods have a plumbing system that utilizes much larger vessels. Some of these cell types are real jawbreakers to pronounce, but it isn’t essential to distinguish them in this overview, so for convenience we’ll call any open structure a pore.
Some hardwood pores are large enough to see with your naked eye, but a small hand lens or loupe with 8x to 10x magnification lets you see them—plus even smaller features—with greater clarity.

Three categories of pore patterns

In each hardwood species, the pores arrange themselves into one of three possible configurations. The first is called ring-porous. That simply means that the pores are concentrated along the newest growth region in the earlywood portion of the growth ring. This helps clearly define the starting line for each ring. This ring-porous pattern is typical for many—but not all—hardwood species grown in temperate climates with distinct seasonal patterns, such as North America. A few examples are red and white oak, black walnut, white ash, shagbark hickory, chestnut, and Osage orange. In general, it is these ring-porous species that benefit from using a filler as part of a glossy, film-building finish.
The second pattern is called diffuse-porous, meaning that pores of virtually identical diameter appear throughout the entire growth ring. This arrangement can make it very difficult to discern ring patterns. Diffuse porosity is very common in tropical hardwoods that can grow almost continuously throughout the year. Tropical examples include bubinga, Brazilian rosewood, cocobolo, koa, bloodwood, bocote, and Gaboon ebony. Some domestic examples are hard maple, boxwood, and basswood. These species generally do not require fillers.
The third category of pore distribution patterns is halfway between the bookends of the first two classes. It is called either semi-ring-porous or semi-diffuse-porous. (A distinction similar to a glass half empty or half full.) Whatever you call it, one distinguishing characteristic of this category is that the pore diameter decreases as they march into the latewood portion of the ring. Persimmon is one example, others include Spanish cedar, holly, cherry, and butternut. I recommend experimenting with these species to see whether or not you think a filler improves them.

Face grain furrows. When a cut reveals a side section of vessels, the voids can appear as coarse furrows, as seen in this illustration of ring-porous red oak.

Prior planning prevents poor results

High or low. High gloss finishes emphasize prominent pores. If you don’t want to invest time in filling pores, consider a low-luster oil finish.

Proper preview. Scraps from your project are the perfect testing ground for new stains, dyes, or finishing techniques. For a proper preview, sand the scrap to the same level as your completed project.

Pore over these wood facts

Sap-moving stacks. Cells moving sap within the trunk are stacked on top of each other, and fluid conducts from the end of one cell into the next. The cell stack is collectively known as a vessel.

Shades of mystery. Heartwood formation processes are extremely complex, particularly those related to lumber color, and are not fully understood.

Empty cell. The walls of tree cells are not living elements. Their contents may be living (protoplast), water, or a wide variety of biochemicals, or the cell may simply be empty.

Busy grain. “Grain” may be the most hardworking word related to wood. One author lists 50 different ways that the term is used to describe timber.

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