Tag Archive | sapwood
October 25, 2011  

Kiln-Dried Walnut Is Not Steamed Walnut

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.

This walnut sample was not steamed before drying. Notice the distinct line between sapwood and heartwood.

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.

In this steamed walnut panel, the sapwood can be seen as a lighter stripe down the middle.

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.

This sample was not steamed, but the log sat for months before milling, causing the sapwood to darken considerably.

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.

Walnut steamer at Mueller Brothers Timber with walnut just unloaded. Notice that all of the wood is brown with no light sapwood.

September 13, 2011  

White Wood, Sap Wood and Spalted Wood

Lately I have gotten a lot of questions about spalted wood and white woods. The main question I get about white woods is usually, “What white woods do you have?” They really are asking, “What species do you have that has a wide enough sap ring to produce white boards?” The customer usually ends up buying maple, but it starts a discussion about where white woods come from and what to look for in your purchases.
Every tree has heartwood, which is the center of the tree and sapwood, which is the outer layer of the tree, just inside the bark. The sapwood is white. Sometimes it is tinted a little, (poplar, for example, is slightly green) but it is always very close to pure white. The heartwood is always darker. Sometimes, it is only a shade darker (basswood) and sometimes it is chocolate brown (walnut) or even black (persimmon). This sapwood layer is thin in some trees and very thick in others.

This hickory was cut quickly, before it could spalt. Hickory often has sapwood that is thick enough to produce all white lumber.

Lumber that we consider white woods will have a much thicker ring of sapwood when compared to lumber that you would normally think of as being darker. Here is the trick, the sapwood needs to be thick enough to produce a reasonable amount white lumber. This is very often the case in species such as maple, ash, and hickory. In these trees, the sapwood is thick enough that we can use use it. In darker woods like walnut, cherry and oak the sapwood is usually only about and inch thick and is trimmed off like fat from a choice piece of meat. Within, the white woods it is possible to have a log that is almost entirely sapwood or a log that is almost all heartwood. It is most common that the white wood log is about half and half.

Recognizing the sapwood layer is the key to understanding the defects that can happen to white woods. Typical defects in white woods are end stain, sticker stain and spalting (although this is typically considered a positive among furniture makers). Sapwood is a live layer of the tree and will degrade or decay. I compare this layer to fresh produce. If exposed to hot and wet conditions the white wood will start to darken, then spalt (early rot) and then rot. In the same conditions, heartwood will not spalt and it will only very slowly rot. When purchasing white woods, pay attention to the color of the boards, especially if cut during the summer. If the logs are stored for a long period before cutting the ends will typically be darker. If the lumber is not dried quickly enough, it will have an overall darker color. And if it is dried on sticks that don’t allow for proper drying there could be sticker stains, which are dark stripes across the boards that very often do not plane out.

This is a good example of spalted silver (soft) maple. It shows the darker heartwood with minimal color change and the lighter sapwood with the dramatic spalting characteristics that woodworkers look for in light woods.

When shopping for spalted wood or looking to make your own, simply make sure that the log has a thick layer of sapwood, since this is the only area that will spalt. The maples spalt the best because of the sugar in them, but I have also seen good spalting in poplar, hickory and sycamore. I have even had some very nice spalted oak before, but again, this was only in the sap wood. All of the boards looked perfectly normal on the heartwood sides, except for some worm holes. Just remember white woods are sapwood and only sapwood spalts.

This dresser was built with WunderWoods lumber by customer Steve Palmer. It is a great example of consistently spalted lumber. This log had a lot of sapwood, which made some nice wide spalted boards.