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wood anatomy --- growth rings

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Some information that is helpful before you read (or in addition to reading) this article is contained here in the overview article http://www.woodbarter.com/threads/wood-anatomy-an-introduction.18349/


GROWTH RINGS:

growth rings header.jpg

If you have a "mystery wood" that you want to ID, there are a number of things you can do including sending a sample to the USDA and waiting months for a reply. A better method is to take good pics and post them here on Wood Barter asking if anyone can ID it for you. That often works, but not always. This article is one of a pair of articles that show how, with a pretty modest amount of effort, you might well be able do it yourself.

The two articles are about how to use a simple 10X loupe and some fine-grit sandpaper to expose wood anatomy to an extent that allows for some pretty nifty analysis of the wood. This article is about growth rings and the other is about parenchyma. Parenchyma is less familiar to woodworkers than at least the rudiments of growth rings, but most folks are pretty startled when they realize the details that are available with so little effort to make both growth rings and parenchyma visible and useful in wood ID.

The parenchyma article is at: http://www.woodbarter.com/threads/wood-anatomy-parenchyma.18346/

Most trees have a year's growth cycle that generates identifiable characteristics that taken together are called "growth rings". These growth rings have many characteristics, including the very basic question of whether or not you can even SEE any growth rings since some tropical woods don't show growth rings at all and a few woods that have them don't show them very well. Growth rings can be strongly visible or pretty much invisible (even though present). They can be smooth or coarse, have a high rings-per-inch count or a low count, and so forth. For hardwoods, there are long cylindrical vessels that run up and down the trees like soda straws and the end grain cross section of these vessels are known as pores and the size and arrangement of pores is a major characteristic for hardwoods.

Most of the characteristics of growth rings are readily visible in end grain cross section with a 10X loupe (some with just the naked eye) and this brief article just mentions and illustrates some of the major characteristics to give you an idea of both the variety and the usefulness in using growth ring characteristics in identifying woods.

visibility: How easy is it to make out where the rings begin/end? There are several characterizations that have to do with visibility. Here are three of them:

growthrings1.jpg


count: Are there a lot of rings per inch or not very many?

growthrings2.jpg


texture: Are the rings coarse or smooth?

growthrings3.jpg


basic growth ring structure in hardwoods: Hardwoods have pores, which are the end grain cross section of the little "straws" running up and down the tree to carry nutrients. The size and organization of the pores is a major characteristic of hardwoods and it is categorized in a couple of different ways. The most basic categorization is ring porous, which has large pores in the earlywood, dropping of sharply to much smaller pores in the latewood and then diffuse porous, which has same-sized pores throughout the earlywood and latewood. In between the two are two more vague categories of semi-ring porous and semi-diffuse porous. Semi-ring porous has a more gradual transition from large earlywood pores to small latewood pores and semi-diffuse porous has a somewhat less than totally uniform distribution of pores, often being that there are fewer/smaller pores at the end of the latewood. Here are examples of all four categories:

growthrings6.jpg

pore density: The pores in hardwoods come in sizes and arrangements such as widely spaced ("sparse") and so tiny and numerous that they are formally called "uncountable" for the obvious reason. Here are some examples:

growthrings7.jpg

pore groupings: Pores often grow in easily identifiable groups, such as radial strands and wavy bands.

growthrings4.jpg

dendritic groups: Another type of pore groups is "dendritic" groups. Dendritic means "branching", and dendritic groups often look a bit like tree branches growing up ("up" when you orient the wood with the pith down and the bark up)

growthrings5.jpg

All of the characteristics shown here, and several more, are readily visible with a 10X loupe on a decently cleaned up end grain.

So I hope this has whetted your appetite to try some of this for yourself. Most of the samples in this brief article are exotics. My site has extensive pics of the growth rings for most domestic woods. The parenchyma characteristics are also very informative for domestics, and that is the subject of the other of these two brief articles.
 
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