WHAT IS PITH?

By DANA SPESSERT, Chief Inspector

A few weeks ago, I was at a lumber yard grading Walnut and realized there might be a need for an article about pith and how it applies to Selects and Better.

Several boards contained full-length pith, with some boxed and others showing on one side or the other, which raises a question, to which side of the board does the pith apply?

To begin with, let’s look at the definition of pith in the NHLA Rules Book. On page 85 in the 2023 NHLA Rules Book, in the definitions section, pith is defined as “The small soft core occurring in the structural center of the log.”

In the Standard Grades, pith is limited in the following ways for some grades:

FAS

On page 14, under the heading of FAS, paragraph 56 states: “No piece shall be admitted which contains pith, boxed or showing, exceeding in the aggregate in inches in length the surface measure in feet.”

For the pith limitation for FAS, I believe it is straightforward - if the pith is boxed or showing and the grade is determined from the poor face with the reverse side sound, then the pith cannot exceed in length in inches the surface measure.

1 Common

On page 17, under the heading of No. 1 Common, paragraph 72, it states: “No piece shall be admitted which contains pith, boxed or showing, exceeding in the aggregate one-half its length.”

The pith limitation for 1 Common is straightforward; if the pith is boxed or showing and the grade is determined from the poor face with the reverse side sound, then the pith cannot exceed in length one-half the length of the piece.

F1F & Selects

Because the pith limitation is not spelled out specifically for the grades of F1F and Selects, this can cause an issue where graders wonder which side of the board the boxed pith applies due to the nature of grading from both faces for these two grades.

On the better face of F1F and Selects, because we are grading each face independently, the pith that is showing and any boxed pith would be measured for the pith limitation with the above Rule for FAS.

On the 1 Common side, the pith showing, as well as any boxed pith, would apply to that face, and the 1 Common pith limitation would apply. The bottom line is boxed pith applies to both faces.

The other question is, Does “knot pith” count for pith limitation?

The answer is no. Pith resulting from knots is not measured for the pith limitation as the definition of pith eliminates this as an option. The definition of pith states: “ . . . structural center of the log.” As you can see, the knot pith is not in the structural center of the log but in the structural center of the knot.

As always, please send your hardwood lumber grading questions to Dana Spessert at d.spessert@nhla.com or call 901-399-7551.