From Gene Wengert, forum technical advisor:
Steaming Black Walnut Lumber: Black Walnut (Juglens nigra) is a premier species for the lumber manufacturer, demanding top prices. The heartwood color and the moderately light grain appearance are unsurpassed by any other species. On the other hand, the sapwood of walnut is white. The contrast between heartwood and sapwood is extreme and is difficult to modify or soften in the finishing process. As walnut’s appearance is its key selling point, many producers of walnut will edge the sapwood off and sell lumber that is 100% heartwood. This is potentially expensive due to the yield loss. Other producers may sell the wood and receive reduced prices for lumber with sapwood; that is, the white sapwood is considered a gradable defect unless the lumber is steamed to darken it.
The issue of how to improve the appearance of sapwood is becoming more critical in recent years as tree diameters are smaller, meaning more sapwood, percentagewise. Fortunately, a steaming process, pioneered by Hartzell Company of Piqua, OH, and by Conway Corporation of Grand Rapids, MI was developed over 50 years ago. This steaming of green walnut lumber has proven very effective for darkening the sapwood; it has also been used for beech (mainly done in Europe) and cherry.
Steaming Walnut for Color: The steaming process for walnut varies somewhat from mill to mill, as there has been very little published research looking at the process. One European study on steaming times and temperatures concluded that 212 degrees F at 100% RH for 16 hours gave the best uniform color between sap and heart. They did note that cooler temperatures were not as effective, even when steaming was longer. They also noted that most of the color change occurred during the first four hours. In addition to darkening the sapwood, the steaming process also makes the heartwood more uniform in color.
Perhaps the best modern article on steaming, which includes three pictures of do-it-yourself steamers, is in Drying Eastern Hardwood Lumber which John McMillen and I wrote in the mid-1970s. (U.S. Dept of Agr 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). The article was researched and prepared by Ken Compton.
The best steaming results are achieved by treating green lumber with wet steam in as tight a structure as possible at temperatures that give the most color in the least time. The lumber is not stickered, but is steamed tight-piled. There are two approaches to steaming. Steaming at elevated pressures and steaming at atmospheric pressure (sometimes called non-pressure steaming, or just called steaming).
A. Pressure steaming. Art Brauner and Ed Conway (Conway Corporation in Grand Rapids) developed the optimum conditions experimentally. Then they settled on steaming at 6 pounds per square inch pressure and 230° F for 5 hours. A longer time is needed in the winter. This procedure not only darkens the sapwood; the heartwood loses its purplish cast and become chocolate brown. Although the coloration is rapid and time saving, the lumber must be cooled in the pressure vessel or end checking and honeycombing occur. An alternative is to take the load out of the retort and cover the wood with a tarp until cool. The lumber is then stickered and kiln dried.
B. Non-Pressure Steaming. Non-pressure steaming of walnut is done in special vats or buildings with provisions for wet steam at temperatures from 190° to 212°.
F. Any structure is suitable so long as it is made of materials that will stand up under wet heat up to 212° F. There are no fans in steaming chambers. (Note: walnut should not be steamed in a dry kiln because of the time required and the corrosive effects of steam and volatile extractives.)
In order to achieve wet steam, which means that the humidity is 100% RH and therefore the lumber will not dry during the steaming process, low- to moderate-pressure steam (15 psi is probably the highest pressure and even lower is better) is introduced into the 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 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 for drying. (I suppose the lumber could be put out for air-drying immediately after steaming and stickering, but I have not seen this done.)
(Commercial Kiln Drying Forum)
From contributor O: