Fine Points of Steaming Walnut

Details on equipment and procedures for steaming Walnut to change the color. November 12, 2008

Right now we steam our walnut at 205 degrees dry bulb and 202 wet bulb. I was wondering if we could go to 210 with a zero degree depression. Would that hurt the lumber? Would it break down any cells? Is it true that the colour change happens in the first 24 hours? We are just having a problem with the colour. We steam at 205 with a 3 degree depression for 66 hours and are wondering if maybe the hours are fine but the 3 degree depression is drying the lumber a little, which is affecting the colour.

Forum Responses
(Sawing and Drying Forum)
Professor Gene Wengert, forum technical advisor:
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.”

Non-pressure steaming of walnut is done in special vats or buildings with provisions for wet steam at temperatures from 190° to 212° F. (I prefer 195 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. (The lumber could be put out for air-drying immediately after steaming and stickering, but I have not seen this done often.)

From contributor S:
A local mill steams their walnut outside. They have a big, really heavy waterproof tarp that goes over the lumber and they shove a steam hose under it. I don't think they monitor the temperature at all. Kind of low-tech, but it works for them.

From contributor G:
We steam walnut like contributor S explained and a problem we find is that the color does not go through the board but will sometimes surface out (in 4/4). If 8/4 lumber is cut against it and all you have left is a bunch of sappy boards, will any amount of steaming make it black?

From contributor D:
I built a control system for a steamer a few years ago and I did as I do with vacuum kilns. I used a temperature sensor inserted into a piece of wood in a pack to control the steam. The operator would set the desired temperature and the system would spray as needed to keep the wood up to temperature.

If I was using a water filled trough, I would like a system a little different than Gene's. Instead of perforating the pipe, return the condensate to the boiler. You will still heat the water but won't lose expensive chemicals.

From Professor Gene Wengert, forum technical advisor:
The condensate in a steamer is full of chemicals from the wood and one might not like to have them in a boiler.

From contributor D:
No, Gene, it should be a closed loop. Only the boiler condensate is returned to the boiler. The steam pipe is immersed in the water like you mentioned but without the perforations. I thought you made this suggestion a couple years ago.

Professor Gene Wengert, forum technical advisor:
Okay. I missed what you were talking about. The closed pipe in hot water is too slow for good steaming, but it will work eventually. What is needed is several runs of pipe in the water trough plus a cover to prevent splashing. If the temperature is cool or takes too long to achieve (such as in the canvas-covered affair mentioned above), then the color change will not be thorough.

Getting back to the questioner's issue, we always steam at 100% RH, which means we worry only about the WB. I have not seen a steamer that also has DB controls or any heat, as that would not be necessary and would indeed be a negative as far as quality goes.

From the original questioner:
Our steamer has 3" pipes along the bottom that are half under the water. Our steamer can also be used as a kiln as it has fans and vents if needed. When we steam we do not use them. I was just wondering if we steam at 212 (our steamer will go that high), will it affect the boards and collapse any cells since it is so hot? And does the colour change take place in the first 24 hours especially if we can get it that hot? Because if so we are wasting lots of energy and time.

From contributor D:
The idea of monitoring the core temperature is focused on conserving time and energy. I have experimented with cherry, for example, and found that sapwood will turn reddish as low as 158°F so I might add only enough heat (steam) to raise the temperature of the core to 160. With this parameter (temperature) under control, you can then experiment with time.

Professor Gene Wengert, forum technical advisor:
I doubt that you can steam at 212 because the wall and vent covers will be cool and will not let you develop 100% RH. In fact, this is one reason that we do not use a kiln for steaming as there are cold spots (are your vent covers insulated?) that will not let the WB get to 190 F or so. A cold spot will condense the moisture and prevent the WB from getting higher than the temperature of the cold spot. The second reason is that steam will deteriorate most buildings quite rapidly.

Your pipes must be totally under water to assure 100% saturated steam. Regarding steaming time, a good deal of color change takes place in the first 24 hours, but as I stated, 72 hours for 4/4 is commonly used to provide the best color development throughout the wood.

Collapse will not occur if you are at 100% RH. Your statements have me concerned that you can not achieve 100% RH however. I have never seen anyone use 212 F at atmospheric pressure to steam walnut as it is impossible to get 100% RH at that temperature in a steamer building. Without 100% RH, you will damage the wood.

It is wise to remember that there is a patent on steaming that was recently awarded to a gentleman (Danny Elder) in Texas and he has been (in the past) rather strict about people who infringe on his work. I do believe that you are okay if you steam walnut, beech and cherry in the manner I stated above, as this is the way we have been doing it for decades and we have used these species in the past as well. But variations, perhaps as contributor D has mentioned, will likely fall under his patent.

From contributor G:
This might be a little off the subject, but I recently heard that you can "roast" red oak and make it look like walnut. Has anyone heard about that, and how it's done? Or is it patented?

From contributor D:
Gene, a patent isn't worth the paper it is printed on if the holder doesn't have the money to defend it. A potential violator doesn't have a thing to worry about if 1) he has documentation proving that he has used the process described in the patent and 2) he has the money for an attorney.

The roasted red oak is old stuff. Ask someone who runs a WoodMizer VK2000.

Professor Gene Wengert, forum technical advisor:
The money to defend oneself from a patent infringement case is no less than $300,000 and even then you do not know if you will win. This is why most cases are settled out of court for a lesser amount.

Ethically, I do believe that we should all honor a patent. Without that respect for the law and order of this country, we will fall apart.

Regarding the roasted red oak question, it is indeed true. I do not agree with contributor D's comment about the vacuum kiln he mentions providing such a product. This new product involves heating oak to a high temperature and actually charring the wood slightly, giving it a dark color. I find this interesting as we have used heated oak in wine barrels (called toasted oak) and have charred whiskey barrels for years, if not eons. The new product comes from Europe and goes by several different names. I have seen a heat treated product so I know it works and it is for real. Actually, in the late 40s there was a similar product tested at the US Forest Products Lab.

There is also a heat treated oak product that is dried in a heated press which results in collapse of the oak, giving it a corrugated, dark appearance which is also being sold. I am not sure how they avoid honeycomb.

From contributor D:
Gene, I ran a VK2000 for maybe a year. I used to have a red oak square that looked like a piece of wenge. Wish I had kept it.

Ethically, I don't think people should try to patent processes that have been used for years. Like the one about using spray to elevate humidity on a new load. Or the clown that patented what VacuTherm has done for years. Or the one about sitting on a swing perpendicularly! Rubbish.

From contributor C:
"The roasted red oak is old stuff. Ask someone who runs a WoodMizer VK2000."

Well, I've been running mine since '01, with everything from 4/4 red oak to 9X16X25' white pine, and have had two problems of discoloration. The first was in fact on red oak; 8 boards out of a full load, and my fault for using a blanket I shouldn't have. That was within the first 18 months of buying the kiln.

The second was on reclaimed cypress that gave problems to a much larger operation running 10 larger vacuum kilns, and had previously run several RF kilns that now sit idle. That was an interesting experience, and I was fortunate to be involved with it as I learned a lot about the vacuum kiln market.

Contributor D, you have made it clear you dislike the VK2000. However, it is not nearly as bad as you project it to be.

"I ran a VK2000 for maybe a year."

Perhaps this is part of the problem? It's just curious that there are so many people still running the machines and having success with them. Just because you couldn't make it work doesn't mean that it doesn't.

I respect the efforts and experience you have put into the field of vacuum drying, as well as your willingness to share knowledge you have accumulated. I just wish you didn't feel the need to continually denigrate a particular product. Undeniably, there are issues and concerns with the VK line, and they are not for everyone, but run properly, they can provide great results. Most of the problems coming out of the VK's are a result of operator error, not the machine itself.

From contributor D:
I actually got the VK2000 to dry 3" red oak squares at BWP so I know it can be used to dry wood. I recently helped an old customer for whom I modified the vac system on a VK2000 ten years ago so I know some people are willing to put up with the problems. But thermisters and electric blankets will drive most people crazy.

Professor Gene Wengert, forum technical advisor:
I used a VK-1000 for 18 months and we had problems with the gasket sealing well. We never burned any oak or made it toasty. We did have a learning curve with maple color and never did get the wood really white, such as we can do with conventional kilns. All in all, the system worked well providing good lumber quality. (Why do we expect a new system to work 100% perfect? Even the present systems - steam heated kilns or DH kilns - do have problems from time to time. Oftentimes the problems are related to prior handling or wood quality and not the drying system.)