Why is Walnut Purple Sometimes?

The purple color of some Walnut boards is natural. Here's more background with comments and observations from woodworkers. February 16, 2014

No, not a new species, but rather the standard color of American walnut for quite a few years now. My understanding is that the purple hue of walnut is the result of steaming during the kiln drying process. The reasoning is that this will turn the light colored sap a medium brown, hence balancing out the board when sapwood is involved, and increasing the value. The unintended side effect is that the heartwood develops a purple hue. Many people I speak with bemoan this ruining of the beauty of natural walnut. Assuming I am correct about the cause, does anyone else out there have any comments or complaints on the subject purple walnut? Personally I wish the industry would stop doing this.

Forum Responses
(Architectural Woodworking Forum)
From contributor G:
It has been my experience that steaming ruins the purple color and turns the walnut to a dull washed out brown while darkening the sap. The purple I see in walnut here in the Midwest is very attractive in non-steamed walnut.

From Professor Gene Wengert, forum technical advisor:
The purple color is the result of improper steaming. It will fade quickly with exposure to light.

From the original questioner:
Thank you, Wood Doc. I'll put a piece in the sun today and see what happens.

From contributor P:
I have also found that unintended consequences do change color when exposed to the light of day.

From contributor M:
I have seen heavy shades of purple even in material we have sawn on our own mill and dried in our small solar kiln. We just used a bit of walnut that has never been steamed that had a very pronounced purple hue. It isn't a bold purple but it's there and jumps out under certain light. I have often wondered about mineral or something similar to what happens when you open a poplar log. You can watch the colors flash off right before your eyes while it's sitting on the mill.

From contributor Z:
The purple hue is a direct correlation to the proximity the tree has grown to a water source. Being close to a water source, i.e. a stream, does not guarantee the tree will be have a purple hue, but 9 out 10 times, a tree that does not have a constant water source will not have the purple hue. Having used walnut way before steaming was used to try and balance the color of the heart and sapwood, I used to get really good walnut, and always considered the purple hue to be some special walnut because it would only occur in about 10% of the wood, depending on where it was logged.

From Professor Gene Wengert, forum technical advisor:
I do agree that the purple is a natural color in walnut at times. My earlier answer was for walnut that had been steamed and still had the purple hue or color. In any case, sunlight will make the purple color go away on the dry, exposed wood. This reaction makes me think that we have some sort of oxidation going on.

From contributor S:
What happens to it over time? Does it yellow?

From contributor B:
The purple is mineral deposits in the ground around the walnut tree. It can also be caused by metal in the tree, nails, wire fence, etc. If a tree grows where there is plenty of water, the sapwood will be thin since it doesn't have to fight for enough water. If there is little water around the tree, there will be a thick sapwood area so it can suck up all the water it can.

I set aside cool looking wood/grain for special projects. Walnut and poplar with purple stain are two that I have used a lot. One project I did was a walnut lap desk with purple stained walnut. To bring out the interesting effects of the purple staining I paste filled the walnut with purple paste filler (colored). It was very cool.

I first saw the purple staining in 1970 when I started woodworking. I was lucky to have a local sawyer named Simi Agin who at the time had been cutting wood for 30 years. He was an encyclopedia of knowledge. He also supplied George Nakashima with many of his walnut slabs.

From contributor K:
I have about 5000 b.f. of home sawn air dried walnut from northern Illinois and it always has that purple hue when I begin to work it. I find that over time, as it ages, the brown tone becomes more intense and brilliant as the purple changes. I have an armoire and four-posted bed I built for my wife as a wedding present which is about 10 years old. The aged walnut brown color is the most beautiful walnut color I have ever seen on any aged walnut product.

From contributor B:
I've had the pleasure of working with walnut for 42 years. I'd say about 45% of it had a light overall purplish tint to it once freshly surfaced. I didn't always like this since I love the warmer gold through red brown tones which come with age and exposure to light, typically called patina. I think the questioner was mainly talking about the streaking of more pronounced purple color caused by mineral deposits in the ground (really pronounced in lighter woods like poplar), also caused by metal in the tree, from nails, spikes and fence wire. If I could only have one wood to work with, it would be walnut, not steamed.

From contributor P:
I personally like the purple color and so do a lot of my customers. I don't steam any of my walnut because I want only the natural aspects of the log to come out. That's the way mother nature grew it, so why not show it off as something different instead of trying to make it look like every other hohum walnut piece we've all seen a million times?

From contributor Q:
There are natural purple tones in our Oregon grown walnut. None of the walnut here on the West coast (Oregon, Washington and Northern California) is steamed prior to drying and the wood retains much more of the varied tones than the muddy brown (in my opinion) tones of steamed walnut.

The main reason the walnut in other parts of the country is steamed is because the trees are relatively small, so have a higher proportion of sapwood which is white in color. The walnut in the West grows quickly and the trees attain very large size. Six foot dbh trees are not uncommon.