Q. We buy medium-sized loads of hardwood kiln dried lumber. Can you please define what “kiln dried” means?

A. Your question is great, because there is no firm definition of the term “kiln dried” in our industry. Different people will have different understandings when talking about this term. For this reason, it is important that you have a clear understanding between you and your supplier about the quality of lumber that you require, including what kiln dried means.

The following is a short summary of some thoughts that should go into the process of working with your supplier. First, moisture content of the lumber is critical for efficient production. For this reason, I suggest that the average moisture content of a load of lumber should be 6.8% moisture content. Further, I suggest that 95% of the pieces in the load should be within 0.6% of the average. Stated another way, 95% of the pieces should be between 6.2% and 7.4% MC.

In addition to specifying the moisture content, it should be clear on how moisture will be measured. Any good commercial moisture meter made in the USA should prove satisfactory if properly used.

It is surprising to me that many people when talking about final moisture content do not use a decimal point to specify correct or desired level. This practice should be eliminated, as the decimal number is critical.

In addition to moisture content, we also want to make sure that the outer fibers of the lumber have not been over dried. Overdried means that conditions in the kiln were such that the surface dried under 5.5% moisture content. Add this low surface moisture content the outer surface becomes very brittle, and machining defects, and even some gluing defects will be more common.
It is also very important that there be no residual drying stress in the lumber. Such drying stress is often called case hardening. This defect results in immediate warp when machining the wood.

Of course, with many species,we have color requirements or expectations. These need to be communicated with the supplier, as color is controlled often by drying conditions.

I personally prefer that the communication between you and your supplier not be considered as developing specifications. Specifications seem to be sometimes used as a club and certainly don’t facilitate improvement in drying quality.

It is important to appreciate that cool, drying quality is determined mainly by the kiln operator and the procedures used. Most equipment that is commercially available is adaptable or controllable to produce whatever quality lumber is required. So, the kiln operator, and drying supervisors, become essential in any discussions about the definition of kiln dried.

Q. We have a few questions regarding bowing particleboard, stability, linear expansion and the EMC of different areas of the country. Thanks for your consideration. 

A. Just to make sure that everyone has the same product, particleboard is a product using small chips similar in size to the chips we would see when sawing wood, but not so small that we would call them dust. The largest dimension could be 1/4 inch. These chips are then coated with adhesive. The adhesive is activated with heat and pressure in a mechanical press, often making a dozen or more sheets of particleboard at one pressing.

The properties of particleboard are determined by the density of the board and the amount of adhesive. Therefore, each manufacturer’s product will have different properties.

Regarding linear movement, when the moisture changes, particleboard has the chips oriented somewhat randomly, which means movement is in all directions, and overall much less than solid wood. Further, the denser boards and the more adhesive used restricts movement.

Some foreign-made boards have low density, poor adhesive and non-wood material, exhibiting poor quality control from piece to piece.

Due to the heat used when pressing, particleboards move less than solid wood  The standard EMC tables are for solid wood that is being dried for the first time. The EMC values for particleboard are likely under 1/3 of the solid wood values, which means we seldom have concerns about moisture changes in normal service. Of course, particleboards should always be stored in a dry environment.

I hope you appreciate why I cannot answer your questions more precisely.

If the chips are larger than particleboard chips, the product can be called flake board, waferboard, and similar. If the chips become smaller so the are only a few fibers, then the product is called medium density fiberboard (MDF).

Q. Our glued panels normally have a wonderful surface that is smooth, hard, and finishes to a premium product. However, once in a great while, we will find a small area that seems to have microscopic fuzz. With today’s thin finishes, we see these areas as not meeting our quality. What should we do, if anything, to correct this?

A. I know exactly what you were referring to, and there is an easy cure. We have a technical name in the wood business for this issue called tensionwood. Tensionwood is formed in all the living trees of hardwood species, especially cottonwood, yellow poplar, and even red oak.

Tensionwood is formed in the living tree due to various causes including stress from outside that often cause a curved stem. Tensionwood is also formed around the larger knots. Simplified, the miniature, skinny soda straws that we call cells forming the product we call wood are made of two main chemicals — cellulose (cotton essentially) and lignin (the strength giving component that also glues the cells together). When tensionwood is formed in the tree, the result is that there is much more cellulose and much less lignin. So, the tensionwood cell is rather weak. This weakness shows up when we machine or sand it at the surface. The week cells are easy to push over rather than cut especially when the cutting tool is dull.

When machining tensionwood, it is critical that we have sharp tools. This includes sharp sandpaper, as the minerals in the sandpaper do get dull. With that dullness often comes a little more pressure, which again pushes the tensionwood fibers into the surface rather than cutting them off.

So the simple solution is to use sharp sandpaper, which translates into using fresh sandpaper, rather than using sandpaper until it gets quite dull. 

I have also heard of using sanding sealer before final sanding, which stiffens the tensionwood cells so they can be cut easier.
Although sharp sandpaper sounds rather simple, there is no simple way other than experience to determine or know when sandpaper needs to be changed. 

However, when looking at sandpaper if you think it needs to be changed, it probably needed to be changed a while ago. What I’m trying to say is that most of us use sandpaper beyond its optimal life.

One other comment about tensionwood. With more cellulose in the wood cells, they become somewhat more absorptive of finishes. This extra absorptivity can create a slight difference in appearance in the finish. A finishing expert will have to work with you. If this difference in appearance is an issue.