Ring Count

[Bob Smalser]

“Hi Bob, Could you possibly create a post explaining why and how tight ring counts improve lumber qualities? I know that tight ring counts are preferred, but why is it better?”My pleasure…it’s really quite simple, looking at a close-up of the growth rings:

http://pic3.picturetrail.com/VOL12/1104763/3075040/53114962.jpg

In early spring the wood grows quickly with large, thin-walled cells…”earlywood”…seen as the lighter colored portion of the growth ring. As the season progresses and growth factors such as light, temperature, nutrients and water become more limiting, the cells get smaller, thicker-walled, denser and more resinous…”latewood”…seen as the darker portion of the growth ring. You can actually see the cells get smaller in the photo as they darken. Smaller, denser cells with more natural resins are stronger…and a higher proportion of them in the wood makes for stronger and more durable timber…significantly stronger and more durable timber.

Even in the tropics there is usually some limiting factor to create latewood…like an annual dry season…and it is the rare tree that doesn’t show some sort of growth ring.

http://pic3.picturetrail.com/VOL12/1104763/3075040/53116512.jpg

But it also helps to understand something of how the tree grows…many folks think of growth rings in three-dimensional form as cylinders, but they are really cones as shown above. That’s useful to understand when ripping bending stock from wood originally milled from a cant as most sawmills do…there is almost always some grain runout in straight, sawn boards that should be identified before selecting stock for bending. That’s why riven or split boards like the Vikings’ early lapstrake planking are almost always stronger than sawn boards, and for most store-bought wood, boards milled from nearer the pith will have less grain runout.

http://pic3.picturetrail.com/VOL12/1104763/3223936/39972549.jpg

There are some sawmills that can compensate for log taper without making a cant, taking their waste from the pith rather than the slab…the Lucas and Peterson swing-blade mills come immediately to mind. If you can find a local sawyer using one and explain your requirements, you can obtain higher quality bending stock.

http://pic3.picturetrail.com/VOL12/1104763/3075040/53116506.jpg

Wood is composed of hollow, spindle-shaped cells of cellulose cemented together by lignin and arranged parallel to each other along the trunk of the tree. Trees grow by adding cells in the (again, cone-shaped) cambium layer immediately beneath the bark…the trees’ existing cells do not grow larger. Thus new wood is laid down atop of old and the diameter of the trunk increases…stretching, cracking and sloughing off the bark as the tree grows.

The wood immediately beneath the cambium layer …the sapwood…stores and transports the tree’s water and nutrients. As sapwood contains much water and minimal resins, it is never rot resistant…even in rot resistant species…and is undesirable for use in boats. Generally, the faster the tree species grows, the higher percentage of sapwood…yellow pines from modern plantations take decades to develop heartwood and today’s SYP lumber generally contains no heartwood. Black Locust is an exception…a fast-growing pioneer species that generally only contains a half-inch of sapwood and high rot resistance and strength in its heartwood…hence an excellent boatbuilding timber to propagate.

Heartwood consists of dead, inactive cells that no longer transport or store food and water. The transformation of sapwood to heartwood is accompanied by a general loss of water and a dramatic increase in the “extractive” content…the resins, tannins, gums, oils and minerals that give heartwood its distinctive darker color and in some species resist rot.

Most woods like also have “rays”…which are horizontal cells that transport water radially across the grain. In White Oak these cells are larger and more conspicuous, especially in quartersawn wood. “Tyloses” also occur in some hardwoods like W. Oak, ash and hickory. These are ingrowths that clog the sapwood’s pores as it transforms to heartwood and prevent water transmission. This makes White Oak excellent for boats and cooperage, but also prevents the penetration of preservatives in the wood, so there is a trade off. Softwoods transmit water differently than hardwoods…they transmit water from longitudinal cell (tracheid) to longitudinal cell and don’t have pores…. that’s how you tell a “softwood” from a “hardwood” botanically, not from the leaves. The Ginkgo or Maidenhair Tree in your local park is certainly a broad-leafed tree…but it also has tracheids and is really a “softwood” botanically.

http://pic3.picturetrail.com/VOL12/1104763/3075040/52135894.jpg

As a practical example above, here’s a Doug Fir log in my stack yard recently harvested from a neighbor’s farmstead. The tree above is what we call locally a “mini-old growth”. It was a seedling in 1900…well before the area was first selectively logged in 1936…yet it is only 21” in diameter. You can see that as a seedling, it had a tough time competing with its larger forest neighbors, and there are 20 rings to the inch until its larger neighbors were thinned in 1936. Then with more, but not full sun, the tree grew at a rate of 8 rings to the inch…and you can see those rings tighten up some as its neighbors like the one below also shot up…until the area was completely cleared for a farmstead in the 1970’s, where it got full sun and grew at an even faster rate of 4 rings to the inch.

http://pic3.picturetrail.com/VOL12/1104763/3075040/52135880.jpg

In contrast, its close neighbor above was also the same size…around 21”…yet this tree didn’t sprout until around 1975 when the farmstead was cleared, and has spent its entire life in full sun…3 rings to the inch.

There’s little doubt which log will become boat framing and which log will become studs.

[ 05-08-2004, 11:15 AM: Message edited by: Bob Smalser ]

[L.W. Baxter]

A timely and informative post for me, Bob.

This afternoon, I ripped some boards out of another DF beam. (I got about 40 bd ft, most of it better than 20 rings per inch, some clear lengths to 14').

I was planning to post a question regarding coloration, I think it ties in here. You can see from the picture below, one of the boards is stained purple. I was wondering, is this a form of rot? I have several very nice boards that have this coloration. I hope that they are useable.

http://www.imagestation.com/picture/sraid115/pb5b59d1c4424305fc4b4ba890110b39b/f8c006da.jpg

Also, I've noticed variable bands of color running through the boards I've been getting out. Some of the heartwood will be quite reddish, and other parts very pale. This doesn't seem to correlate directly with the position in the log, or with the width of the rings. A curious phenomena.

[Bob Smalser]

Lovely stock.

Color often comes from the soil minerals and is a bit unpredictable...potassium and manganese salts make purple I believe, I doubt seriously it is decay, which is generally dark, reddish brown.

[L.W. Baxter]

Well, the color is so striking, it made me wonder. I've seen the coloration before in stud lumber, of course, but never really thought about it. I'm glad to hear your opinion on it, as you say, it's very nice stock.

I've recently accumulated enough fine, clear stock that I'm thinking of planking something with it, instead of cutting it up for frames....carvel planking of 1" fir? hmmm...

[Ron Williamson]

But, but, but... IIRC in ring-porous hardwoods like oak,ash and hickory,low ring counts and fast growth make stronger material because,the porous part of the wood(latewood) is weaker.Fewer rings per inch mean more solid wood per inch.
R

[Bob Smalser]

Some excellent comments here and elsewhere:


Originally posted by Ron:

But, but, but... IIRC in ring-porous hardwoods like oak,ash and hickory,low ring counts and fast growth make stronger material because,the porous part of the wood(latewood) is weaker.Fewer rings per inch mean more solid wood per inch.

Originally posted by M:


One thing you didn't mention was the difference between ring-porous and diffuse-porous wood. The way you described earlywood and latewood applies only to ring-porous, or open-grain, species like oak, ash, chestnut, elm, and hickory. But in diffuse-porous, or closed-grain, species like beech, sycamore, maple, cherry, birch, and sweetgum, the pores are distributed "randomly" thoughout each annual ring. That's why many woodworkers disparage open-grain woods, especially if they're fast-grown, as "coarse." You've got that density gradient going back and forth (which is exacerbated in fast-grown wood), causing tearout and other manifest inconveniences.

With the oaks--which I'm most familiar with--fast-grown wood is stronger than slow-grown, because most of the wood in wide rings is the denser latewood. But slow-grown oak is a much nicer wood to work with because its texture is more even overall.

Another point regarding ring-width and strength is that most "softwoods" are topsy-turvy from ring-porous hardwoods, in that most of the wood in each ring is the less-dense earlywood. This causes slow-grown softwoods to be harder and stronger than fast-grown.

I think most folks on this board who have an opinion would agree that the definitive text for all this sort of info is R. Bruce Hoadley's _Understanding Wood_, Taunton Press, 2000.Written in the context of boatbuilding softwoods…your comments are excellent…I probably shoulda left more-complicated hardwoods out entirely. I agree with you, and have certainly read that before...but not having much oak out here to play with, I wonder what the strength differences really are?

I doubt shipwrights (and ships) of the 16th and 17th Centuries when those original virgin oak forests were used thought it to be much of a disadvantage…after all, those uneven-age forests had younger, sun-grown trees to select from, too.

Doug Fir has been downgraded in strength a couple times since the 1930's as an engineering material...minor downgrades...the last downgrade quite recent. My lumber grader the other day thought the reason for the downgrade was because,"…it wasn't as strong as we thought it was." My answer was yes, but there's little doubt in my mind as to why when you look at the two example logs from my stacks.

Perhaps the difference between new and old oak is akin to the DF dynamic only in reverse?

[ 05-08-2004, 11:12 AM: Message edited by: Bob Smalser ]

[NormMessinger]

And people have been know to leave the forum because of a few rotten boards in the bilge. This is the sort of thing them miss. Good on yer, Bob.

[Andrew Craig-Bennett]

May I second that?

[J. Wellington Wimpy]

..vote for Bob.

[Krueg]

Bob, I'm eating this stuff up.

The people here willing to share their knowledge with less experienced folks like myself are what makes this place great (still). Thanks.

Eric

[Bob Smalser]

More from a sawyer in Australia:


Bob I have never heard of fast grown wood being stronger before. Is this only for Oak, Ash & Hickory or for all hardwoods? I can grade wood here (Australia) Pine can fail visual grading if any of the growth rings in the inside 100mm diameter are over 13mm whereas ring width does not get a mention in hardwood grading.
I read somewhere that the sap wood in hickory is the best part and that the heartwood was not as strong do you know anything about this? That's what the discussion above is about. The argument is that in "ring-porous" like oak, ash and hickory as opposed to "diffuse-porous" woods like cherry and maple...woods that have pronounced, open pores in rings....that those ring-porous woods are stronger with fewer rings per inch.

The logic is there, well enough...the question is, how much stronger or weaker and is it significant?

We don't have many of these woods out west in any quantity and I don't mill them much....no oak expert here, by a long shot...but the family milled oak when I was a kid back east and having used a bit of it...including 200+ year-old W. Oak...I suspect the issue is overstated:

1) What you lose in more pores you gain in a higher proportion of smaller, latewood cells just like in softwoods.

2) The gains in terms of durability from higher extractive content in the latewood are well worth any minor strength issues.

3) I haven't seen any results of lab testing on this to determine any differences on strength and would want to before drawing firm conclusions. Doug Fir, in contrast, is tested frequently because it's our Number One structural wood over here.

I just ordered am 8-dollar copy of Hoadley's out-of-print book, "Understanding Wood", because that's where I've been told the issue is raised....I'll let you know...."Because I said it is" or modern lore passed down through magazine writers doesn't cut it - I want to see the numbers.

Wood for bending is a different issue and wood from young second-growth trees is preferred because of pliability. Faster growth and correspondingly larger cells are desirable here.
__________________
Bob

[ 05-10-2004, 12:01 PM: Message edited by: Bob Smalser ]

[Ron Williamson]

Hey Bob,
When you get your copy,turn to page 131.If I find time(and if I was more clever) I might be able to scan and post it.
The Wood Handbook may have better numbers.
I don't have any idea of the strength differences.I would guess that, when given the choice of such spectacular new world material,the old boys would choose to upsize the scantlings on a clear stick of timber, when in doubt.Thereby also saving hewing time.
R

[pjwalsh]

Very nice discussion,

I am intrigued by the idea that fast grown oak may be stronger than slower grown - but I am also skeptical - I am thinking that strength relates to the proportion of cellulose fiber per unit volume. Ok, a proportion of each ring is latewood with more cellulose and the remainder has less, but is there any reason that this proportion would change for a wide ring versus a narrow ring? If not then the proportion of cellulose per unit volume for the tree as a whole will not be different.

(Vlate/L=f*pi*Ro^2 where f is the fraction of each ring radius element that is latewood and Ro is the max radius of the section and L is a unit of length along the trunk).

I should think there would be little difference in strength - more in appearance and possibly water transport through the material.

Has anyone any practical observation of the variation in latewood as a proportion of each ring for faster grown wood versus slower?

edit:

On thinking a bit more maybe the wider rings would contribute to a reduction in parallel to the grain shear strength for a softwood in which there is little tangling of the cellulose fiber from ring to ring, but this effect would be less evident in a hardwood in which there is more cellulose going across the grain.

[ 05-11-2004, 10:26 PM: Message edited by: pjwalsh ]

[raycon]

PJwalsh the 2nd post on the ring count discussion at the woodweb
http://www.woodweb.com/cgi-bin/forums/sawdry.pl?read=34 0092 (http://www.woodweb.com/cgi-bin/forums/sawdry.pl?read=340092)
may give you the answer.

I coarsely flipped through the one "Timber Engineering" (thats the title as well) text book I have and "Understanding Wood" to see if spring wood/summer wood was taken into consideration when selecting/grading stock for hardwood timbers.
The topic received more attention on the cosmetic/finishing side of things than structural. No % differences(physical property tables) were given nor end use suggestions regarding when to use one over the other.

Interesting topic.
Upon edit:
http://www.semi.vt.edu/Presentations/SEMI-Nov02_Zink-Sharp.pdf
(The woodweb post led me to look up the physical differences between the fast growth versus early growth ring porous hardwoods. PGwalsh reads like he's an engineer trying to wrap his head around why one may have better mechanical properties over another. Just from the images and density data a relationship between [fast growth vs slow growth ] and physical properties can be hypothesized ...)
No #'d data though...what is data in terms of this discussion published mechanical property tables or the raw #'s before Safety factors are applied?

[ 05-12-2004, 01:01 PM: Message edited by: raycon ]

[Bob Smalser]

...may give you the answer. Only if you like more of the "...it is because I said it is..." type of answers.

I'm still looking for engineering strength data...for DF as an example, older editions of the USDA data show that it used to be stronger than in recent tests.

I'm not dissing anybody's answer, but I'm betting somebody out there in forestry land did a thesis or study on this and it's just a matter of finding it.

[pjwalsh]

Indeed Raycon - you have me pegged,

I am a graduate student studying polymer science and engineering at the University of Massachusetts. Wood is one of the original polymer composites and I was trying to understand the strength issue from that standpoint. If I have a minute tomorrow I will pop over to the library and see if they have Hoadley's book. Actually, I think he works a few buildings away on the Umass campus so I may just go over and ask him directly.

I could not find the ring count thread on the woodweb forum - which subforum is it in?

On the density idea, wood is a lignin matrix with long cellulose fibrils as the reinforcing phase and shorter hemi-cellulose fibrils too. Cellulose is more dense than lignin so wood with a higher density probably has more cellulose per unit volume and will be stronger. Are faster grwon trees more dense or less dense?

Bob, I'll bet the reason D. Fir strength numbers are being revised down is the shear strength issue I mentioned before. Most structural applications involve multiaxial loading and so shear strength often becomes the limiting factor.