Drying Oak Lumber: Step 3B, Sample Boards

How to select sample boards for accurate measurement of moisture content and drying rate. August 16, 2012

From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
Of course, we do not oven-dry an entire piece of lumber. Rather, we cut a smaller piece, usually about 30” long, from the large piece of lumber. This piece should be free of knots and bark, as we will be interested in how wood dries and not bark or knots.

From this 30” piece, we cut a small 1” along the grain, full width and full thickness piece from each end. The 28” piece remaining is called a kiln sample or sample board; the two short pieces are called moisture sections. The moisture sections are weighed and oven-dried. The sample board is weighed. It is assumed that the sample board’s initial MC will be the average of the MC of each of the moisture sections (MS).

Initial MC of sample board = [MS (1) + MS (2)] / 2

If we know the initial MC of the sample board (to 0.1%) and also its weight (to four digits), we can calculate what the oven-dry weight of the sample board has to be.

OD Weight of Sample Board = ([Initial weight] / [Initial MC + 100]) x 100

Once we have the calculated OD Weight, we can weigh the sample board at any time during or after drying and calculate its MC.

MC = ([weight at any time / calculated OD weight) - 1] x 100

In order to avoid having the sample board lose moisture too rapidly from the end grain, the ends are coating with a moisture barrier. Commercial coatings for sample boards (different than end coating for lumber) are available or asphalt roofing cement can be used. Once coated, the sample board will behave as though it was a long piece of lumber.

We want the sample board to represent the lumber in the load. We may have 10,000 pieces of lumber in a load, so how do we choose the correct pieces (usually 10 or 12 per load) of lumber to be our samples? First, we do not want to know the average MC of the lumber in the load. Instead, decisions in drying are based on the wettest and the driest MCs. So, what pieces will be the wettest and driest?

The wettest pieces will be quartersawn, the most recently sawn, the thickest and all heartwood. We also know a wet piece will be bacterially infected; however, we may decide that since such pieces seldom dry without honeycomb and checking, we would rather have a more normal wet piece. If a load has mixed white and red oak, then each group should be sampled separately; although white is initially drier than red, white does dry more slowly.

The driest piece of lumber will be the opposite of the wettest.

Because we will not be perfect when choosing the wettest and driest, it is suggested that perhaps eight pieces be selected to be the wettest and four to be the driest. Once the MC is obtained from the moisture sections, then the two wettest and two driest pieces can be chosen for especially close monitoring during drying.

More detail can be found in Drying Hardwood Lumber or in Drying Oak Lumber.

Forum Responses
(Sawing and Drying Forum)
It seems that oak is a standard for wood that is typically hard to dry correctly. Would I be correct in assuming that these measures would be a good idea for drying other difficult woods? Also, one thing I guess I didn't pick up on was whether or not it would be advisable to run just the fans in a kiln at first, as a form of hurrying along an air dry period?

From Professor Gene Wengert, forum technical advisor:
The process for oak is similar or identical for most hardwoods. I have not yet gotten to the kiln or air yard, but usually air drying for oak is fast enough, so acceleration is not needed. There are fan shed dryers that would be similar, but they are seldom used for oak.

Step 1, Assessing the Raw Material
Step 2, Stacking
Step 3A, Measuring Moisture Content
Step 3B, Sample Boards