Q: Is there any kind of a chemical that can be used to detect the presence of antifungal dip in the loads that we receive? Currently, we require that our vendors dip their loads of white oak and red oak and we're running into stain issues. We'd like to know whether or not they are, so we can process the undipped ones first to preserve them and also to file complaints.

 

A: The dipping process that you refer to is done to green lumber at the sawmill. The lumber is immersed briefly in a mixture of chemicals. Some of the chemicals remain on the lumber's surface and provide an insecticide and fungicide barrier on the lumber's surface to prevent new infestation. Most fungal damage is done to the sapwood by a blue-colored fungus. Hence, the stain is called blue stain or sap stain.

Yes, indeed, the companies that supply the chemicals for dipping wood do have tests for the presence their chemicals. Each chemical dip will have a different test procedure, so when buying from several mills, it will be tough to test for dipping unless they're all using the same chemical for dipping.

However, the dipping only prevents fungal stains. Most often in oak, the blue or grayish stain in the sapwood is caused by the enzymatic oxidation of sugars in the sap. (I believe that this is what you are dealing with.) Sometimes these are called gray stain, sap stain, oxidation stain, enzyme stain or chemical stain. Sticker stain or sticker shadow is in the same classification. So is pinking and browning in hard maple and a few other species.

These stains aren't controlled by dipping in a fungicide. They can be bleached away, and some dips do have a bleaching effect on the lumber's surface, so the stain may only show up after planing. These stains begin to develop in the log while it's stored in warm weather, so old logs are much more likely to develop this stain in the lumber during drying. In warm weather, the stain can begin to develop within 24 hours. This makes it extremely hard to control. Years ago, this stain in gum lumber was prevented by steaming the lumber prior to drying. I've also recently seen a patented heating process that apparently deactivates the enzyme in fresh oak lumber and thereby prevents the stain.

 

Q: We're concerned about the loss of strength for staples in wood where the wood is exposed to some wetting and drying. We're also concerned about splitting of the wood when we nail near an end of a piece. What ideas do you have? Thanks in advance.

 

A: In answering your question, I'm going to discuss the withdrawal strength of the nail, not the lateral or shear strength. Also, note that a staple is considered as two nails, so this discussion also applies to staples.

If you drive a nail into fairly wet wood, the withdrawal strength is as high as for a nail driven into dry wood. But, if the wet wood dries out, the withdrawal resistance may drop substantially as low as 25 percent of the initial values. On the other hand, if the wood fibers deteriorate or the nail corrodes and becomes somewhat roughened, withdrawal resistance may stay fairly constant or may even increase. It's hard to predict and so any potential increases shouldn't be counted on.

Before you give up entirely on nails (but you may have to use screws in some cases), nails or staples in dry wood that doesn't change moisture much, except for seasonal changes (more humid in the summertime and drier in the wintertime) may also lose withdrawal strength as the wood fibers relax over time. To minimize this effect, staples and nails are designed with non-smooth surfaces, using barbs, groves and rings. Adhesives can also be added to the nail or staples.

Splitting varies with species

Some species of wood are more prone to splitting than others. Usually the denser the wood, the more likely it is to split, although swirly grain offsets this tendency.

Regarding the splitting problem, dry wood (under 9 percent MC for softwoods and under 6 percent MC for hardwoods) is much more brittle and prone to splitting. So always check the MC when you have splitting to make sure that the MC is not the main problem.

A second critical item that affects splitting is the tip of the nail or staples. Nails with long, sharp points accentuate splitting in certain species, which may reduce withdrawal resistance. A blunt or flat point without any taper at the end greatly reduces splitting, but its tearing and destruction of the wood fibers when driven reduces withdrawal resistance. A nail tapered at the end but then terminating in a blunt point (for just a few nails, you can blunt the point yourself) will cause less splitting. The withdrawal strength in dense woods remains high, but in lower density woods, such blunting does lower the strength.

A good discussion of more details about nailing can be found at www.woodweb.com/Resources/wood_eng_handbook/Ch07.pdf.

 

Q: Is there a rule of thumb for how much of a gap we should have between the boards as we make packs for the kiln? We've been operating under the assumption that a total of 6.5 inches of gap on a 72 inch wide layer was desirable.

 

A: For narrow piles (under 8-feet in width), there's a very small, often immeasurable, benefit in leaving a space between the individual pieces of lumber. So, it's not done. In wider loads, there can be an improved drying rate in the interior of the pile during initial air drying.

Of course, if you air dry for a long time, then this benefit isn't seen at the end of the air drying. In the kiln with fans that blow the air through the load, the spaces only reduce the total kiln capacity. (In your case, this reduction is about 10 percent). The spacings don't help drying at all and in some cases may even result in poor air flow distribution in modern kilns. So, again, it's not done today.

You may wonder why this spacing between individual pieces was ever done. The answer is that 50 years ago before kilns had internal fans of the size we have today and had heating coils underneath the lumber, a lot of the air flow in the kiln was the result of the hot air rising through the packs. The spacing provided little chimneys for this to happen. Also, with large packs of lumber that were more than 8-feet wide, these chimneys encouraged internal circulation in air drying.

 

Q: What is marine plywood compared to regular plywood? Is it worth the extra cost for an outdoor project?

 

A: There are two plywood adhesives . . . interior (which doesn't stand up to frequent wetting) and exterior (which isn't affected by long-term wetting and would be called waterproof). Marine grade uses exterior adhesive. Incidentally, this is the same adhesive as used in the inexpensive construction CD-X plywood or any other exterior grade or "interior grade with exterior glue" plywood.

With marine grade, the lowest grade veneer that can be used is "B," which means that there will be no serious voids on the surface and in the interior plies. It's the voids and the severe crossgrain around the knot holes that cause poor adhesion in some areas of CD-X. So, marine grade will have good adhesion throughout the lamination. Also, marine grade can be sawn without getting a void in one laminate on a freshly cut edge. The edges will be totally solid.

The two species used for marine grade are Douglas-fir and western larch, although I've seen keruing and other species used in a plywood product called marine grade. You can get the same performance from panels such as exterior AC in most cases.

Marine grade has no natural decay resistance. It has no chemicals added to enhance decay resistance, unless it has been subsequently pressure treated (= $$$). Marine grade has no special waterproofing in or on the wood, unless it has been added as a special feature.

If you don't get the wood wet very often and if it's sealed, as you state, there's no difference in performance indeed by using a different, less expensive grade and species with an exterior rating.

A final note: Most interior grade panels do not use exterior adhesives (although CD-X does). But if the plywood seldom gets wet, the interior adhesive will be just fine.

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