What is steaming?

[Jamie Buxton]

Over the years, I've seen steamed beech and steamed cherry offered in hardwood dealers. I've read about steamed walnut. Two questions....

* What is steaming exactly? I believe that it involves some change to the kiln schedule. However, kiln schedules often involve adjusting the temperature and humidity -- even to the extent of spraying water. Steam sounds a lot like that, so what is really going on?

* What is the effect on the lumber? That is, how does steamed lumber differ from non-steamed lumber?

[Jim Becker]

Steaming during part of the kiln drying process is primarily used to increase yields on certain species, such as walnut, where the sapwood is a radically different color than the heart wood. It darkens the sap and lightens the heart to where they are about the same dull color...(yes that was an editorial comment...) That's why most kiln-dried walnut is a different color than air-dried walnut...the latter has a lot more browns, purples, greens and golds in it--a great richness. That said, there is still good cause to use kiln-dried (and steamed) walnut where the particular piece and style calls for extreme consistancy, such as a period piece, that will also receive a multi-step finishing process.

[Jamie Buxton]

Hmm.. If steaming is used to even the colors in walnut, what would it be doing in European beech? Beech doesn't have the big difference between sap and heart.

[Steve Clardy]

Do a goggle search on steaming. Theres lots of info out there. Such as

http://www.woodweb.com/knowledge_base/Steaming_beech_for_veneer_production.html

http://www.fpl.fs.fed.us/documnts/fplrp/fplrp378.pdf#search='steaming%20lumber'

[Dev Emch]

Steaming offers two advantages if you wish to call them that. Features if you will.

First of all, wood tends to dry from the inside out and not from the ouside in. So as a kiln begins to dry a load of wood, moisture from the outside of the wood is removed but the wood quickly replaces it with moisture from within its core. So some woods and kiln schedules that are to rapid will result in wood that has a non constant moisture content all the way through. This can result in honeycombing which is very bad.

To counter this, steam is injected into the kiln at precise levels and the kiln is allowed to establish a constant moisture level thoughout the drying process in the wood. The kiln then backs off this moisture level in controlled amounts and this results in a more constant moisture content throught the wood. You dont get the variations and subsquently, the honeycombing and other defects from these uncontrolled moisture fluctuations.

Now, if you air dry and work with lumber that is less than say 8 or 12 quarter, you will not need to worry about steam kilns. If your in a hurry or if your working lumber that is truely fat, a steam kiln may be an advantage.

Another thing to consider. Woods like walnut have dark heart wood and cream colored sap wood. Steaming will turn the sap wood to almost the same color as the heart wood. So now, you need to be extra careful in buying stuff like walnut as you may be paying for sap wood that has been steamed to a darker color. Buyer Beware!

Beech is a super cool wood. Many older wooden hand planes in the country were made of beech. Beech was the wood of choice. Now, handplanes have to remain as stable as possible. And side escapement planes such as beaders, hollows, rounds and profiles are no exception. The reason beech was used is that beech has some unqiue moisture properties that allow moisture to move evenly throughout the wood. There are some fancy names for this used by the hand tool makers who reproduce beech planes. See Clarke & Williams for example.

But this moisture property also wreks havoc on wet beech. Once dry, its great wood to work with and its pretty darn stable. But getting it there is no picnic. It is very unstable during the drying process and you will loose a huge amount of your green yield to drying defects.

By using a steam kiln, the lumber guys are able to better control the drying process with beech and actually allow it to produce reasonable finished yields of dry, stable, defect free lumber. In europe, beech grows like maple grows here. Its everywhere. So the european market has an appitite for good beech like we have for maple. So the steaming process allows them to use what they have given the nature of the wood.

You will also notice that european steamed beech has more of a darker reder hue to it then say natural beech. Again, a side effect of the steam kiln process.

Hope this helps...

[John Stevens]

So now, you need to be extra careful in buying stuff like walnut as you may be paying for sap wood that has been steamed to a darker color. Buyer Beware!

Beech is a super cool wood. [snip] Once dry, its great wood to work with and its pretty darn stable.

Dev, or anyone, please educate me on these two points--

1. Why is sapwood less valuable than heartwood, as long as the two look the same?

2. Everything I've read about beech, including the lit handed out by a company that sells steamed Euro beech, describes the wood as "moderately stable." Is that incorrect?

Thanks in advance.

[Jim Becker]

1. Why is sapwood less valuable than heartwood, as long as the two look the same?

They don't look the same on walnut and cherry...radically different. The sapwood on both is very "white" while the heart is very "dark". The sapwood also doesn't change color over time in the same way that the heart does...for cherry, that means the sapwood looks lighter over time. And for walnut, which actually gets lighter, there is still a big disconnect between the sap and heart. (prized by turners, but eschewed by most flat woodworkers who are not fond of Nakashima, etc.)

(Interestingly enough, for maple, we "prize" the sapwood and avoid the heart in many cases...)

[John Stevens]

...for cherry, that means the sapwood looks lighter over time. And for walnut, which actually gets lighter [snip]

The color change caused by steaming is temporary?

[Jim Becker]

The color change caused by steaming is temporary?

Not to my knowledge, at least with walnut. But I don't know about cherry...most of the cherry I buy is air-dried and any KD I get isn't steamed to the best of my knowledge. (The supplier the the KD air-dries first and then finishes in the kiln) Hopefully, someone with more knowledge can contribute to this conversation!

[Tim Sproul]

Steaming...when you are talking about 'steamed' euro beech and 'steamed' black walnut is NOT referring to the steaming done during kiln drying. I had this false impression of steamed woods too.

To make black walnut sapwood and heartwood 'closer' in color, the wood is steamed while green....and then is dried in any manner you might like - most often kilned since commercial operations are generally the only ones who steam wood and then kiln-drying tends to be economically cheaper than air-drying.

Steamed euro beech imparts a pinkish color to it. Without the steaming, the already bland steamed euro beech is very much bland.

I did not know that cherry was steamed in this manner.