When kiln-dried lumber is offered for sale, certain species, mainly walnut, beech and cherry, are often listed as being steamed. What exactly does this mean? Is it worth the extra money?
Sapwood and heartwood
To understand steaming, we have to first consider a basic concept in wood's growth. When the tree first forms a wood cell, the cell is white in color and is wide open, so that liquids can easily be conducted from the roots to the leaves. This wood is called sapwood.
As the sapwood cell ages, perhaps in five years, 75 years or whenever, depending on species, this sapwood cell will begin to accumulate chemicals. These chemicals are often dark in color (such as the black of walnut or the red of cherry) and may have a unique odor as well (such as the sweet smell of eastern red cedar or the root beer smell of sassafras). Soon, this aged sapwood cell no longer can conduct liquids, at which point its only role is to support the tree. These aged cells that are usually darker in color and have a special odor are called heartwood cells.
For most species, the white sapwood is not as pretty (to some people) as the dark heartwood. The sapwood will be stained dark in finishing or may be trimmed and eliminated totally. Note that a few species, such as maple, are virtually all sapwood and it is the white color that is desired, while the brown heartwood is considered a defect.
When lumber is steamed right after it is sawn and before it has a chance to dry, the white sapwood becomes much darker, looking at times almost as dark as the heartwood. The heartwood, when steamed, also changes color, becoming a little bit darker. Further, steamed heartwood often has more uniform color than unsteamed. These color effects develop throughout the lumber and not just at the surface.
Black walnut sapwood will become much darker if steaming is done properly, and the heartwood will take on a rich chocolate appearance, rather than its often greenish hue.
Cherry sapwood does not darken a great deal, but it certainly becomes redder. The heartwood becomes a very rich reddish color, looking like it has been aged naturally in the sun for a few years.
Beech lumber takes on a distinctive pink color. In fact, because almost all European beech is steamed before drying, this pink color is an essential characteristic of European beech. In truth, steamed American beech looks the same and the two cannot be separated from each other.
In a patented steaming process for red oak, the heartwood developed a definite pinkish hue.
In hard maple, steaming will pink the wood through and through.
Steaming for color
Steaming as a process to improve the appearance of sapwood is becoming more critical today as tree diameters are smaller, meaning more lumber with a higher percentage of sapwood. Fortunately, a steaming process, pioneered by Hartzell Company of Piqua, Ohio, and by Conway Corporation of Grand Rapids, Mich., was developed more than 50 years ago. The lumber is not stickered, but is steamed "tight-piled."
The steaming process varies somewhat from mill to mill, as there has been very little published research looking at the process. Perhaps the best modern article on steaming, which includes three pictures of do-it-yourself steamers, is in Drying Eastern Hardwood Lumber (U.S. Dept of Agriculture Handbook No. 528, pages 62-64. Although out of print, any public library can obtain a copy. On the Internet, a PDF file is at www.fpl.fs.fed.us/documnts/usda/ah528.pdf.)
Lumber steaming is done in special vats or buildings with provisions for wet steam at temperatures from 190 to 212 degrees F. Any structure is suitable so long as it is made of materials that will stand up under wet heat up to 212 degrees F. There are no fans in steaming chambers.
To achieve wet steam, which means that the humidity is 100 percent RH and therefore the lumber will not dry during the steaming process, low pressure steam (10 psi is probably the highest pressure and even lower is better) is introduced into the steaming chamber at floor level by perforated steam pipes in water-filled troughs. By bubbling the steam through the water, saturated steam is assured. Steaming times for 1-inch lumber are typically 24 to 96 hours, with 72 hours being quite common.
After steaming, the hot lumber is removed from the steaming chamber, stickered and put into a dry kiln.
Some people claim to steam their lumber, but they steam for a very short time, steam at cooler temperatures, steam at under 100 percent RH, and so on. By not following the requirements, the lumber is indeed steamed but the color enhancement is lacking. As a purchaser of steamed lumber, it would be prudent to indicate on a purchase order that the steamed lumber will have sapwood that is nearly as dark as the heartwood and that the heartwood color will be fairly uniform, compared to unsteamed lumber. If the lumber's color subsequently does not meet your specifications, you will have grounds for returning it and receiving full credit.
Proper steaming does not alter any of the wood's properties, other than color. However, excessive steaming can lead to weakening of the wood and shelling when machining. Steaming at less than 100 percent RH can result in cracking large cracks or fine hairline cracks, depending on species.
Have something to say? Share your thoughts with us in the comments below.