CHAPTER 8, LESSON 2 of 3
GOAL: To learn how a log is cut into boards and how the way it is cut affects how the wood behaves later.
Sawing a log into boards is called conversion. Most logs are converted to yield the maximum number of board feet by a method called plainsawn. This lesson will explain the three types of stock that result from this conversion method and discuss how their attributes affect the appearance and seasonal movement of the wood.
| From Log to Board
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| Conversion means sawing logs into boards. It can be done in many ways, from a minimal operation by one person with a band saw mill in the forest to the massive operation shown here in this high-volume mill.
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| Three types of boards: flatsawn, riftsawn, quartersawn
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| When all the rings form a curve horizontal to the surface — less than 45° — the board is flatsawn. A riftsawn board has rings between 70° and 45° to the surface. A quartersawn board has rings between 90° and 70° to the surface and might display many ray flecks.
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| Wood shrinks and expands with changes in humidity almost twice as much in the tangential direction as in the radial direction. That means, for example, that seasonal changes in a flatsawn board will be greater than those in a riftsawn board or a quartersawn board.
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In the plainsawn method of converting logs to lumber, the logs are sawn into slices like a loaf of bread, but down the length. A cylindrical log cut this way yields boards that look much alike on the surface. For instance, if same-thickness boards are cut from two logs of the same species and diameter, the 10th boards from each log will look generally alike, as will the 20th boards.
Wood harvested this way falls into one of three groups: flatsawn, riftsawn and quartersawn. Each group is defined by the angle at which the annual rings on the end grain meet the surface of the board. A quartersawn board has rings between 90° and 70° to the surface, and a riftsawn board has rings between 70° and 45° to the surface. When all the rings form a curve horizontal to the surface — less than 45° — the board is flatsawn.
The first and last few boards from the log will be flatsawn, while the middle boards will be primarily quartersawn. Nowhere will you find a wholly riftsawn board; it will be part of one of the other two groups. Indeed, a board from the middle of a large diameter log will be made up of all three groups. Clearly, nature doesn’t precisely honor our neat description, but the terms work well enough.
A flatsawn board exhibits strong cathedral grain. Quartersawn boards have straight grain with the rays breaking the surface as very noticeable large smudges. Riftsawn is straight grain, absent the flashes of ray tissue. The rays are there but are more noticeable on the end grain than on the flat surface.
After the log has been cut into boards, the boards must be dried. A truck loaded with newly felled logs destined for a sawmill could just as aptly be called a water tanker as a logging truck. That’s because practically half the weight of each log could be due to water. Depending on whether it is softwood or hardwood, there can be a considerable range of moisture content (MC) in a newly felled tree. As well, the sapwood will contain more moisture than the heartwood. The average MC is about 75%, but it can exceed 200% in some species. The desirable MC for furniture-making is 8% to 10%, so a lot of moisture must be removed.
Two ways of removing moisture are air drying and kiln drying. Done correctly, both methods will deliver wood that is sound, flat and without stain mark from stickers. Done incorrectly, both will deliver distorted, split and/or case-hardened material best suited for firewood.
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