NC State Extension Publications

Introduction

Brown stain in white pine can result in considerable degrade. Losses in the rough mill can be quite costly especially when cuttings are generated for use in light-colored suites. To the kiln operator, variations in the severity of brown stain among hacks of lumber within the same kiln charge as well as between charges in different kilns can be quite frustrating. Numerous drying schedules have been tried by kiln operators with variable success. Is there something more than just the kiln schedule that can aggravate brown stain?

The Case Study

In late summer, 4700 board feet of 4/4 mill run white pine lumber was cut from fresh logs at a mill in northwestern North Carolina. The boards were 10-12 feet long and most ranged from 4-8 inches wide. After sawing, the boards were distributed at random into three separate bundles. The bundles were then shipped within one-half day to a nearby furniture plant where they were dipped into a tank containing an anti-blue stain solution only. Study bundles were then handled and processed as follows:

Treatment No. 1 – stacked on sticks and kiln dried immediately. Dead pile time was less than one day.

Treatment No. 2 – stacked on sticks and air dried for 39 days followed by kiln drying.

Treatment No. 3 – dead piled for one week after sawing, then stacked on sticks and air dried for 32 days followed by kiln drying.

Kiln Drying

The kiln used in the study was a steam-heated, unit package-type with a 24-foot wide load. Lumber in treatment No. 1 was dried in one charge while lumber in treatment Nos. 2 and 3 were dried together in another charge. The schedule was essentially the same for both charges and based mostly upon time. During the latter stages kiln samples were used to check moisture content. The schedule was as follows:

SET POINTS TIME (hours)
Dry Bulb, °F Wet Bulb, °F Treatment 1 Treatment 2 & 3
130 100 19 20
140 100 7 7
150 100 18 16
160 110 8 25
170 120 23 18
180 130 215 71
180 170 11 12
TOTAL: 1212 days 7 days

Results

All kiln dried lumber was surfaced two-sides, full length of the board to 1516-inch. Each surfaced board was appraised from the worst (stained) face and tallied according to the severity of brown stain. The severity of stain was classed as follows:

Severity of Stain Percent of Surface Area Stained
None 0
Moderate up to 50
Heavy 51-90
Complete > 90

The percent of board feet volume by severity of brown stain within each treatment is shown below.

Treatment No. Percent by Volume by Severity of Brown Stain
None Moderate Heavy Complete
1 100 0 0 0
2 10 55 34 1
3 3 27 51 19

Discussion

Results of this study were most striking! There was no brown stain in lumber placed into the kiln within a day of sawing. Moderate to heavy brown stain developed in most of the lumber that was partially air dried prior to kiln drying. Heavy to complete brown stain developed in most of the lumber dead piled for one week followed by partial air drying prior to kiln drying.

Suggestions

Purchasers of green, white pine lumber should work in close harmony with sawmills. Careful scheduling should be used so that fresh cut lumber is brought to the user’s lumber yard as soon as possible. Schedule lumber receipts to coincide with cutting at the mill and kiln space availability if at all possible. Upon arrival at the user’s yard, priority should be given to the stacking of the white pine. Kiln dry as soon as possible. While colder temperatures during the winter help reduce the brown stain potential, there are periods of warm temperatures. Cutting at the mills, scheduling receipts, and kiln drying should be on a year-around plan.

It is also suggested that white pine users perform their own in-house studies. Keep track of the history (mill cutting dates, shipping, receiving, stacking, air drying time, and kiln records) of your lumber and compare with the severity of brown stain – what you do with your lumber prior to kiln drying will affect the severity of brown stain.

Authors:

Associate Professor and Extension Specialist
Wood & Paper Science
Project Leader (retired)
USDA Forest Service

Publication date: July 1, 1988

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