I mill, dry, and use walnut on a regular basis, especially now that walnut is back in style. It wasn’t that long ago that everyone was tearing out dark cabinets and putting in the maple to lighten things up. Now, walnut is used regularly and often stained to an “espresso” color, which means really dark. Some folks would swear off staining walnut because it is naturally so nice, but I am not one of them. I, personally, almost always stain walnut, even if I just want it to look like walnut (more about this in a later posting). This got me thinking about the color of walnut, and no discussion about walnut color can start without talking about steaming.
Steaming has one purpose and one purpose only – to change the color of walnut. Here’s the deal. Walnut logs, like the one shown in the first photo, have a rich, chocolate-colored heartwood and light vanilla-colored sapwood. The contrast is stark and to those that want all dark heartwood, the light sapwood is seen as a defect, something to be removed or hidden. But removing it can be costly to mills in that they will be discarding the outside of the log, which usually has the clearest lumber with the fewest knots. Additionally, the sapwood can be inches thick, and not using it results in a loss of board feet production. So, to make the sapwood more acceptable, mills use a separate process to steam the lumber after it is cut to darken the sapwood.
When I say “mills”, I mean larger production mills, those that are focused on efficiency and willing to spend the money to steam the lumber. At smaller mills like mine there is no steamer and much of the sapwood is removed on the mill. If a piece of lumber has a significant amount of sapwood, I usually grade it lower because I know it is less desirable.
The steaming process, as shown in the second photo, makes the walnut lumber more uniform in color – but at a cost. The sapwood does darken, but at the same time the heartwood lightens and the entire board turns to a washed-out gray color. Compare this with the walnut heartwood that is not steamed and is usually a medium-dark brown which will lean in color towards red, green, or even purple.
The idea of steaming walnut (at least my guess) must have come from sawyers that milled walnut on a regular basis. They surely noticed that when walnut logs sat for awhile before being milled that the sapwood would begin to darken. In these types of logs, much like the third photo, the colors can become quite homogeneous. In some cases it is difficult to distinguish between the heartwood and sapwood if the logs are milled at the right time. Somewhere along the line, this natural process was forced along with steaming.
Steaming, as mentioned before, is a separate process. Lumber is milled, then put in “dead” stacks (without sticks to separate the lumber), and then put in a steam chamber. After steaming, the lumber is stacked with sticks and put in a kiln to dry.
I have met many customers that were under the impression that the kiln changes the color and this is not the case. The steaming changes the color. Kiln-dried walnut and air-dried walnut look the same if they haven’t been through the steamer. Just remember that steaming is an entirely separate process from the kiln.
The last thing you should know is that there is a lot of more detailed information available about steaming walnut (optimum temperatures for best color, optimum timing, etc.), but most of it is geared to those that are actually doing the steaming and can get boring in a hurry. Since most of us won’t be steaming our walnut, I think it is best to stop here.