Q. Can you explain or give insight into wood used outdoors, especially the UV weathering gray appearance?

A. First, I want to be clear that a stain to me is a penetrating stain that goes into the wood, staining is several hundredths of an inch thick, but the stain does not form a film. Film form products are like a paint and not a penetrating stain
We probably expect or suspect that the ultraviolet (UV) part of light weakens and destroys the inhibitors in the wood or finish. Older studies show, the depth of penetration of surface fibers of woods; fibers are made partly of cellulose (cotton) so are partly broken down by UV, which penetrates about 0.01 inch.

Now, exposure to outdoors is more than UV. There is rain water that washes off loose fibers, exposing fresh wood.
Further, the light coating of broken wood fibers, dirt, and microorganisms encourages growth (is food for) gray to black colored mold and mildew fungi which require water, as well as these just mentioned sources for food.

Further, when the sun hits the dark surface, the wood surface temperature goes up, the relative humidity drops to very low levels right at the surface, which results in shrinkage and the opening of small surface cracks. This means that the best UV absorbing will occur on a sunny day in the afternoon.

Depending on the wood and its orientation, some pieces become an uneven surface and develop puddles after the rain. These puddles extend mold and mildew daily growing time.

I know that the US Forest Products Lab developed more 50 years ago a clear, waterproof penetrating stain that did indeed retard weathering. It was just a waterproof finish. This suggests to me that water is an important part of the weathering process. This FPL Natural Finish had to be applied again in one year, then five. The finish had pentachlorophenol in it as a fungicide. This toxic chemical is likely why we do not see this on the market today.

Of course, we also must consider the fasteners used. Stainless steel or aluminum fasteners will not create the ugly streaks that iron creates.

In summary, blaming UV alone  does not consider the other factors and conditions.

Q. Some bleach was applied to steamed walnut after drying. The wood turned extremely red. What is this?

A. When a low pH (caustic, the opposite of an acid) is applied to many species of wood, a color change often occurs, sometimes getting white, but sometimes (in cherry and walnut), turning red. I do not have a long list of species and color change. I have seen where drywall mud (caustic) when allowed to contact cherry and wiped off with a wet rag leaves ugly red marks. Finishing chemists can often be your best friends.

Q. I had a special order to make some case goods and millwork using acacia. The customer is complaining about some light-colored spots that have wood that is very soft. What did the customer do to get these punky spots?

A. I am afraid you are “barking up the wrong tree” as the defect is created in the living tree, perhaps after an injury, or created when the harvested tree or logs are stored in a wet, warm location for a few months.

As a simplified expansion, wood is made of two components: cellulose (cotton) and lignin (glue and stiffener). There is a group of fungi, called white rots, that eat the brown colored lignin and leave the white to yellowish-white cellulose behind. Being mostly cellulose or cotton, these pockets can be very soft and often result in eroding so there is a depression.

It is common that we do not sell wood with rot or decay, so the lumber should have been more carefully screened. The one case where decay is sold is in wood called spalted.

Not all white rot is bad. The delectable shiitake mushroom, which is grown on wet wood, is in the white rot group.

Q. We are drying some hard maple billets that we then resaw into thin slats. The main issue is that the color when we are done is brown. We need the wood as white as possible. I am using the kiln schedule in the US Forest Products Lab Kiln Manual, beginning at 130 F. Our competitor has a much whiter product. Help please.

A. This is a common question that we have addressed before. First, you are using the wrong schedule (way too hot and way too humid) and also the wrong manual. You should be using DRYNG HARDWOOD LUMBER, by the US FPL; copies you can find on-line. In DHL, schedules will suggest that for the whitest color, start at 105 F. The humidity is also critical — 10-12 F depression.

As a quick summary, the color is determined by what happens above 45% MC. At lower MCs, we might make the discoloration darker, but it is at the higher MCs that initiate and determine the final discoloration.

Discoloration is an oxidation reaction that takes time, warmth, and moisture. So, at 105 F we have little discoloration, so long as the humidity is not too high. At 130 F initial conditions above 45% MC, the 45% MC discoloration can occur within six hours. Actually, the discoloration shows up at the end of drying, but it is at the high MCs that the oxidation begins and becomes hard or impossible to reverse. So, at 40% MC we typically do not see the ultimate discoloration in the wood.

If you need the whitest possible color, around 90 F initial conditions will be essential.

Incidentally, some people will start at 105 F, dry the outer layers very white, but then go to 130 F. Before the core is under 45% MC. As a result, the core is darker than the shell. Staying at 105 F until the core is under bi-colored wood is certainly a defect.

Sometimes the oxidation can begin while the logs are stored in the woods or log storage, so minimal storage and prompt sawing in warm weather is essential, followed by proper drying within 12 hours or less after sawing.