WHY IS WALNUT LUMBER STEAMED?

Walnut flooring is one of the most unique and beautiful floors available.  Along with its beauty comes some misunderstandings about the expectation of color and prevalence of knots and other natural characteristics.
To better understand Walnut, one needs to understand how the species grows from a sapling into a harvestable tree.  As a tree matures it adds diameter and height.  In this growth process the juvenile limbs fall off and fiber is added to the perimeter of the tree each year.  As the tree gains diameter, the higher grade, clear lumber is added to the perimeter of the tree as fiber grows over the knots left behind by juvenile limbs.  When wood fiber is added to the perimeter of the tree, it is added in what is called the sapwood layer, which is a layer of the tree immediately under the bark.  This portion of a Walnut tree is 1-2” thick from the bark. The sapwood is a network of thick-walled cells that form a pipeline, carrying water and minerals up the tree from the roots to the leaves and other parts of the tree.  As wood fiber is added, it starts off as sapwood, preforming its duty to move water and nutrients up the tree.  As more wood fiber is added to the outside in successive years, the sapwood becomes heartwood and is replaced by newer sapwood around the perimeter of the tree under the bark. The time frame from sapling to harvestable tree is 40 to 60 years depending on region and growing conditions.  Northern region timber grows slower, increasing its value by creating tighter growth rings and more even color tones.
Freshly sawn Walnut is a greenish-brown color with vanilla, nearly white sapwood. If Walnut is not steamed it will have a rich, chocolate-colored heartwood and vanilla-colored sapwood after kiln drying. This contrast is stark and for many who desire heavy to all heartwood, the removal of the sapwood can be costly as the perimeter of the log contains the clearer, and hence more valuable lumber.
Generally Walnut lumber sawn in a commercial sawmill is steamed before it is kin dried.  The steaming of Walnut has one purpose ­– to change the color of the sapwood to a color closer to the brown heartwood color.  Freshly sawn Walnut lumber is loaded into a specially constructed aluminum chamber in solid packs where the boards are dead piled against each other.  Then the Walnut is steamed by running steam through vats or troughs of water in the floor that emit a fully saturated steam as it heats the chamber.  Oxidation occurs when the sugars and starches in the cell walls are exposed to high levels of heat and humidity.  This is what changes the lumber color.  Some believe that the color change in Walnut occurs because the dark heartwood portion of the boards causes a leaching or dying of the color to the sapwood as the water ran off during steaming.  This is not the case.  The steaming process simply causes a chemical reaction in the sapwood that causes it to darken.  Any effect of the heartwood touching the sapwood in the steaming process has been shown to be minimal.  The process of steaming takes 3-4 days with most of the color change coming at the beginning of the cycle.
Manufacturers like LaCrosse that control their Walnut supply with 4 company-owned sawmills have an advantage as they can control the steaming consistency, and the resulting color consistency of their product.  Large mills like the Midwest-owned mills manufacture enough Walnut to make sure the process is repeatable from load to load. The process of ensuring Walnut steams properly starts in the woods when the tree is felled.  Immediately after harvesting Midwest waxes the ends of the logs and anyplace where the bark has been removed to keep the log fresh.  Logs are quickly transported to the mill where stocks are sawn regularly to make sure the resulting lumber is fresh.  Green lumber is quickly moved to steamers and protected from drying out at all.  The main culprit that keeps Walnut from steaming properly is letting the perimeter or skin of the boards dry out.  Occasional producers may have to wait to accumulate enough to steam.  At Midwest we run 6 dedicated Walnut steamers, each holding about 3 truckloads of kiln dried lumber, all designed to properly steam the product.   There are a lot of poorly designed steamers that do not do as good of a job as our steamers.
Once steamed, the Walnut is lowered to a safe handling temperature.  It is then put onto kiln stickers that allow airflow all around before being put into a dry kiln.  For lumber that goes into solid flooring (generally sawn just over 1”) it takes about 14-18 days to dry the wood to 7% (+/-1%) moisture content.
Just because Walnut is steamed it does not mean no sapwood in the flooring.  The deeper the wood is cut into when milling, the more the steamed sapwood will show up.  When selling flooring to an installer, retailer or homeowner, the salesperson should set clear expectation on sapwood.  LaCrosse Select & Better Walnut flooring allows a maximum of 10% of the surface area to be well-blended sapwood (well steamed).  The Natural grade does not have a sapwood limitation.  However, solid Walnut flooring is generally made from low-grade lumber coming from the center of the tree that is predominantly heartwood.  Where salespeople err is when buyers are looking for wide, select or clear Walnut floors.  The wider and clearer the floor, the more likely it came from the outer perimeter of the tree, as a result, there will likely be more sapwood in the floor.  The same holds true for narrow clear floors, especially if someone is looking for long lengths.  Boards cut from the perimeter of the log will have more sapwood.
Defects are another Walnut attribute salespeople should understand.  Walnut is a species with a high concentration of sound defects, caused by the juvenile limbs that fall off each growing season as new fiber is added to the tree.  In logging terminology this is referred to as “pinny”.  Salespeople should let buyers know that Walnut will exhibit a lot of sound tight burls and small knots.  This is what makes this species attractive to most buyers.  But at a time when distributors, installers, retailers and manufacturers are all working on tight margins with demanding customers, setting realistic expectations before the sale to avoid surprises is very important.

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