Drying checks on the surface or end develop above 45 percent MC because drying is too fast. Interior checks (honeycomb) are a surface or end check first. Control initial checking (often in air drying) and you control interior checking.

 

Stain

Lumber can become discolored through 

* chemical reactions (iron tannate stain)

* fungal infection (blue stain, mildew and mold)

* slow drying (esp. sticker stain) causing oxidation of wood chemicals

* exposure to air and light

* exposure to heat

 

Affecting yield

 

 

A. Affect grain and moisture on part warp after manufacturing

B. Lumber length and width

C. Drying stress (casehardening) and tree stress

D. Bacterial odors

E. Warped lumber

F. Discoloration

 

Did you know that the way a sawmill is operated has a tremendous influence on the quality, drying time and cost of the drying operations? Did you know that a sawmill influences warp, checking, splitting and color achieved in drying? This influence is so large, that even a top-notch kiln operator is unable to offset these influences and achieve high quality dried lumber. So, what sawing procedures affect drying and what can a sawyer do to make better quality (from a drying perspective) lumber?

 

 

 

Fresh logs

When a living tree is harvested and sawn into logs, not all the cells in the tree immediately die. Some of the cells in the tree, specifically, those called parenchyma cells, that contain the starches and sugars in the tree, do not die but can remain alive for many months. It is these cells that have chemical reactions occurring between harvest and lumber drying that cause or contribute significantly to many stains in lumber. These stains including sticker stain, grey stain, pinking, and brown stain.

 

These stains are grouped together and called chemical stains or enzymatic oxidation stains. [Note that these are not related to fungal stains.] These chemicals stains are controlled, not by using a special drying procedures (although we certainly can lessen the stain in a few cases), but by using freshly harvested logs, especially in warmer weather when the oxidation is most rapid.

 

End checks in lumber are often the same end checks (A check is a small crack.) that developed in the log or extension of these checks. Logs that are stored for more than a week or two before sawing, especially in warm weather, will develop serious end checking unless the log ends are coated ASAP after harvesting the tree and bucking the logs to length. End coatings are available commercially and are usually waxed based in water. The coating must be applied thick enough so that drying of the end of the log is prevented. In addition to end checking, if the ends of the log begin to dry, there will be enough oxygen for a fungus to begin growing. This fungus, which is dark blue in color and therefore gives the lumber a blue color after drying, feeds off the sugars in the sap of the tree at very high moisture contents.

 

Common names for this fungus' activity are blue stain, sap stain and log end stain. Fungal staining can be controlled by end coating the log ends to prevent drying. Fungicides are seldom used. Finally, there is apparently some strength degradation that occurs in stored logs. Although serious damage takes month to occur, it is well documented that lumber from logs stored for three months in the summer is more than four times more likely to surface check and develop honeycomb in drying. Avoiding storage of logs in warm weather is certainly prudent.

 

 

 

 

 

 

It is commonly stated that bandsawn lumber will check much less in drying than circle sawn lumber. Indeed, this is a true statement most of the time. Circle sawn lumber is often sawn with dull saws. It is rare to see circle saw teeth sharpened on the sides (called side dressing); yet it is the sides of the teeth that actually cut the surfaces of the lumber. A dull band saw will not run well and may even come off the wheels; hence, band saws are typically much sharper than circle saws.

 

In either case, a dull saw will tear the wood fibers, creating micro-checks and weakness in the wood surface. These checks can easily develop into drying checks and interior checks (honeycomb). Obviously, the key to preventing such sawing-caused checks in drying is to use a sharp saw.

 

 

 

 

 

 

It probably makes sense to everyone that 2 inch (or 8/4) thick lumber will take longer to dry than 1 inch (4/4) stock. Actually, the 8/4 takes 2.5 times longer. Lets translate that into lumber that is sawn at the sawmill with some thickness variation. Consider an example. Assume that a piece of lumber that is exactly 1 inch will take 10 days to dry in a kiln. However, piece that is 1-3/16 inch (and still counted as 4/4) will take about 15 percent longer to dry, or 11.5 days.

 

This increase becomes more important when we realize that the drying time in a kiln is almost always determined by the wettest piece, not the driest or the average pieces. Further, the cost of drying, including profit, is often assumed to be $25 per day per MBF. So, by including a few "over-sized" pieces of lumber in a 10 MBF kiln load, the cost is over $350 extra for the load.

 

In addition to longer drying, the process uses more energy; if the species is likely to surface check (like oak), then checking risks also increase dramatically with thicker lumber. Thickness control at the sawmill is the key. When this cannot be achieved, then using a planer to reduce the thickness of the thicker lumber is often done. The maximum green thickness for 4/4 is typically 1-1/32 inch or 1-1/16 inch. The process of green planing is called blanking if one side is planed or presurfacing if both sides are planed.

 

 

 

There are several different grain issues at the sawmill. Quartersawn vs Flatsawn. In almost all cases, flatsawn lumber is preferred. [Flatsawn lumber has the annual rings running from edge to edge; quartersawn has annual rings running from face to face. Quartersawn is also called vertical grain.] It dries faster (about 15 percent faster in many cases), has a pretty grain pattern, is stronger, shrinks less in thickness and has less sied-bend warp less than quartersawn.

 

On the other hand, it shrinks more across the width and cups more than quartersawn. On the other hand, sometimes the beauty of quartersawn grain is desired, along with its less shrinkage in width. Side-Bend Warp. Crook or side bend warp is more common when the annual rings are not centered from edge to edge. That is, when viewed from the end of a piece of lumber, the annual growth rings on the left edge should be a mirror image of the rings on the right.

 

Side bend is also worse when one edge of the lumber has either reaction wood (compression wood is softwoods and tension wood in hardwoods) or juvenile wood, while the other edge does not. Admittedly, it is hard to see tension wood, but compression wood is easily seen and avoided when sawing lumber. The risk of warp during drying in lumber from crooked logs must also be appreciated.

 

 

 

 

Bowing, that is lengthwise curvature like a ski, is most likely with crooked logs, or sawing a log so that the saw is not parallel to the bark. Cup warp. Cupping which is warping from edge to edge, is most likely with flatsawn lumber. The tendency for cup increases as the piece of lumber is sawn from a region closer to the center of the log. [Heart crack risks also increase.]

 

Basically, avoid trying to make excellent pieces of lumber from the heart center of the log...it is unlikely that such pieces will be flat after they are dried. Twist warp. Normally, lumber has the wood cells running parallel to the faces and edges...in other words, the grain runs along the length of the piece. Sometimes, however, the grain is at an angle to the edges or faces...that is, there is slope of grain (SOG).

 

If one splits a piece of lumber from the end, the split will follow the grain. When the split deviates and moves to the edge or face, then there is SOG. With significant SOG (even as little as 10 percent), the lumber is quite likely to warp, especially twisting warp. SOG lumber is also much weaker. Avoid SOG by sawing parallel to the bark; however, SOG is unavoidable when there is spiral grain in the log or when the log is crooked log.

 

  In summary, the technique of sawing a log greatly influences checking, color, warping, strength and shrinkage. Proper techniques will produce lumber at the sawmill that will dry faster and with less quality loss.