Is Vacuum Dried Wood Stronger?

With so many variables to control, it's hard to be sure whether any particular drying system gives better results in terms of wood properties. October 29, 2012

I heard a report that vacuum dried wood is stronger than wood dried at conventional temperatures (130 to 180 F). We do know that drying temperature affects the strength of hardwoods. So, if vacuum dried wood is dried at 100 F or so, it will indeed be stronger than conventional. However, if wood is dried conventionally at 80 to 110 F, such as in a dehumidifier, it will be just as strong as vacuum drying. In fact some vacuum kilns dry at 150 F and such vacuum drying will not make the stronger material. In other words, it is not the vacuum process but it is the temperature of drying that matters when considering wood strength. Has anyone heard of this vacuum strength study?

Forum Responses
(Sawing and Drying Forum)
From contributor D:
Gene, I don't know that I would say "stronger" but what I'm sure of is that it's less brittle, and as you say, DH kilns can do the same. It just takes a month of Sundays with DH.

One of our customers builds chairs and chairs do a lot of major bending. They say there is absolutely no doubt that vacuum dried wood has a higher yield. Baseball bat manufacturers claim that their vacuum dried bats have more "pop". A new bat company recently used the empirical method. They gave billets from one of our kilns, billets from their conventional kilns, and billets from another vacuum kiln manufacturer to a batter. The "other" vacuum kiln manufacturer is known for their high temperatures.

It turned out that there was little or no difference in color between our billets and their conventionally dried billets. Their batter didn't find a difference in performance. The problem with the conventional kiln is the time and care (and potential disaster) that is involved with each kiln charge.

People from PSU have been here and have offered to conduct tests like the comparison of strength. That sounds easy but when you sit down and consider all of the variables that must be controlled, you see that it's not done just by handing them a board or two. It could be done but we haven't done it yet. So I am sure that vacuum dried wood has different characteristics but I'm not sure that "stronger" can be claimed.

From Gene Wengert, forum technical advisor:
Because the MC is so critical in strength and brittleness (once over-dried, wood does not reduce its brittleness much if the MC is increased), it would seem that a comparison of various drying methods could be difficult unless over-drying is strictly controlled. In other words: Final MC is also important, but history (lowest MC value and temperatures used especially early in drying) is very important too.

From contributor P:
I know baseball bats were mentioned and Iíve noticed that when I watch ball games, the maple bats seem to break/shatter more than the ash. Is there any light on this apparent phenomenon?

From contributor D:
I don't think there is any doubt that maple is more apt to break than ash. A lot has to do with grain but there are other reasons. Companies who make maple bats for the pros are quick to point out that not all maple bats are created equal. Also, and I never like this part, some ash companies want their wood dried to 9%. Most maple companies want 6% to reduce weight.

From contributor P:
I am fearfully awaiting the time that a sharp bat shard pierces some poor spectator or player. Do you know if Adirondack bat (which used to be in Dolgeville NY) and Louisville Slugger, both bats from my youth, are still in business? I see unfamiliar logos on most bats today.

From Gene Wengert, forum technical advisor:
As usual I agree with Contributor D that the grain in maple is seldom perfectly straight, but rather has small deviations. We know that even small slope of grain will greatly affect strength properties. So, maple for a bat has some serious issues right away.

Also, we know that in conventional drying, when shooting for 7% MC, that some pieces will be over-dried a bit and these over-dried pieces will become brittle, even if the MC is brought back up a bit. Of course, with proper equalization in a conventional kiln and correct temperatures, all this over-drying can be avoided. One might ask why these proper drying procedures are not followed in order to avoid as many broken bats.

Again, if dried properly to avoid over-drying and also dried at low temperatures, we would have a better product. Then, choosing pieces with the best SOG, and we have a great product.

From contributor X:
I don't know much about vacuum drying, other than when I send Contributor D highly figured valuable wood to dry, it comes back perfect and is so much quicker than drying it here myself. From the time I package it here in Texas and ship it to Pennsylvania he dries it, repacks, and ships it back. It is always at least three to five times faster than me trying to dry the thick burls/highly figured slabs right here myself, and risking destroying them due to my lack of the proper type kiln and expertise for wood like that.

From contributor D:
Yes, Rawlings Adirondack is still making bats. I did a little consulting for them back in the 80's. They were fighting an old RF/vacuum kiln back then. I don't know how they are drying their wood today. Louisville Slugger is still around. I believe all of their billets are dried at Lewis and Hockenberry. L and H has six of our kilns. Another company doing a brisk business these days is Marucci Sports. They have three of our kilns. I worry about people getting hurt, also.

From contributor E:
My own opinion isnít worth much, but here around Charlotte, NC an old fellow with a huge Fuji vacuum kiln swears the wood is stronger vacuum dried. He was a major producer of persimmon golf club heads back in the day. He still personally plays with persimmon clubs in spite of the sport's titanium dominance. A few years ago, maybe six or so, he was still a small supplier to the Louisville bat company. He's a quiet type, retired mostly, sitting there with a half million dollar cylindrical kiln to play with. My personal experience with his wood, which I bought in large, thick dimensions, has always been free of cracks or tension. Just plain dead stable.