Drying Thick Southern Yellow Pine Export Lumber to Minimize Brown Stain
Joseph Denig, Assistant Professor and Wood Products Extension Specialist
Stephen J. Hanover, Associate Professor and Wood Products Extension Specialist
North Carolina State University
Southern yellow pine flitches and joinery exported to Europe are remanufactured and sometimes finished with clear coatings. Brown stain may detract from the desired appearance. Thus, presence of brown stain in export stock may decrease its value and be reason for a claim against the exporting company.
What is Brown Stain?
Brown stain is a chemical discoloration within the wood that ranges from light brown to deep chocolate color. This stain is not caused by fungus. It should not be confused with the so-called “kiln burn” that often shows as a charred discoloration on the board surface after high-temperature kiln drying. The stain may develop on the surface, slightly under the surface or throughout the piece. More typically the stain concentrates directly under the surface. Areas under the sticker are generally not stained. Sometimes it is very difficult to detect the stain as the normal latewood (summerwood) color blends with the brown stain colors. Heartwood is generally not affected.
Important facts are: (1) the stain is caused by the precipitation of water soluble extractives; (2) these extractives are carried along with the free water within the wood as it dries; (3) the extractives precipitate at the location in the wood where liquid water turns to vapor. This is usually at or just beneath the surface. Water in the wood directly under the sticker moves laterally and/or into the sticker. Thus the stain is not generally found directly under the sticker; and (4) the solubility of the extractives increases with higher temperatures. Any drying practice that increases solubility of the extractives (e.g. higher temperatures) usually increases the depth and color of the stain.
Literature from the 1930’s to the present has discussed brown stain in the white pines (both east and west) but not in Southern yellow pine. No foolproof solutions have been developed but several processing variables that can be controlled at the mill and some dip solutions have shown promise in reducing the intensity of brown stain in the white pines. Processing variables discussed in the literature include:
Processing For Improved Results
Length of log storage Quick turnover
Time between sawing and stacking Stacking immediately
Time between sawing and kiln drying Kiln drying within 1-2 days
Kiln temperatures Lower dry bulb temperature and lower humidity
Pre-dipping Mixed results
Brown Stain Control Study
North Carolina State University Wood Products Extension personnel conducted a study to develop processing recommendations for the Southern pine industry that would minimize and/or eliminate the development of brown stain. The study included two phases. The first phase consisted of studying processing variables and their effect on brown stain at several mills. The second phase was testing kiln pretreatments and schedules at the Wood Products Laboratory, North Carolina State University and at cooperating mills.
Results from Phase I – Visitations
Six Southern pine mills producing export material were visited by North Carolina State University Wood Products Extension personnel during the spring of 1985. The following observations were noted during the visits.
Length of Log Storage
The length of log yard storage varied from zero to two years. Mill personnel interviewed believed that logs stored under water sprinklers for several months exhibited more brown stain than fresh logs.
Time between Sawing and Kiln Drying
The time between sawing and kiln drying varied greatly. Practices ranged from dead piling at the green chain for several days and then stickering the material for either air or kiln drying to immediate stickering after sawing followed by kiln drying within one to three days. The effect of the length of time between sawing and drying was not clear.
Kiln Drying Schedules
Kiln drying schedules ranged from low temperature to high temperature. Generally good results were observed with low temperature schedules; bad results were observed from high temperature (above 2120F) schedules. Very good results were obtained from a dehumidification kiln with a maximum dry bulb temperature of 1400 – 1500 F.
Mill Productivity and Kiln Size
Most mills have difficulty in cutting a sufficient volume of export stock to fill a dry kiln within a relatively short period of time. As a consequence, loads of mixed thicknesses, and/or moisture contents and partial loads were often dried. Some mills dried both dimension (on one track for example) and thick stock (on the other track) within the same kiln, keeping the thick stock in the kiln for 1* to 2 runs. The consequences of these variables with regard to brown stain are not known. An alternative observed was a small kiln. This small kiln dried a full charge of similar thicknesses soon after sawing.
Results from Phase II – Kiln Tests
Two kiln tests were conducted to evaluate the effect kiln schedules and pretreatments have on the degree and depth of brown stain. The results of the kiln tests were as follows:
1. 12/4 Effect of Dry Bulb Temperature
The effect of dry bulb temperature on the degree of brown stain was evaluated using a high temperature, conventional, and low temperature schedule.
12/4 – High Temperature versus Low Temperature Schedule
Number of Final
Kiln Schedule Days M.C.% Results
High Temp (2120+) 2 6 medium-heavy brown stain
Conv. Temp (l700-1800) 5 11 medium brown stain
Low Temp (1400 -1500) 7 15 slight brown stain
2. 10/4 – Pretreatment by Dipping Versus Non-Dipping Followed by Low-Temperature Versus High-Temperature Drying
Sixteen foot fresh cut flitches were designated to be either low or high temperature dried. Each flitch prior to kiln drying was cut in half. Half of each flitch was pretreated by dipping in sodium azide (pH=10). The other half was not dipped. The end-matched sample set from each flitch was then either low- or high-temperature dried according to the following schedules:
10/4 – Kiln Schedule, Time, and Final M.C. %
Temperature Time Final M.C. %
Schedule D.B. 0F W.B. 0F (Hours) Ave. Range
High Temp 230 170 29 15 10-23
Low Temp 140 120 126 15 12-19
After drying, the flitches were surfaced to various depths and evaluated for brown stain. A four point brown stain visual grading system was used. Results of the study were:
10/4 – Degree of Stain by Depth and Treatment
Treatment Degree of Stain by Depth (inches)
1/16 2/16 3/16
High-Temp-Dipped 2.4 1.4 0.1
High-Temp-Not Dipped 2.7 1.5 0.1
Low-Temp-Dipped 1.7 0.2 0.0
Low-Temp-Not-Dipped 1.9 0.1 0.0
The kiln temperature had a significant effect on the degree of brown stain in both test charges. Temperatures above 2120F resulted in considerable brown stain. Lower temperatures gave much better results. A very mild schedule, keeping temperatures at or below 1500F produced the best results, but the drying time was increased by 40% over a conventional schedule. For all test schedules most stain was removed by surfacing at a depth of 3/16-inch. High temperature dried flitches still had slight to medium stain at a depth of 2/16-inch, while nearly all stain was eliminated at this depth in flitches dried in lower temperatures. Dipping in sodium azide proved not effective in reducing brown stain.
To minimize brown stain in yellow pine flitches and joinery, firms should (1) use low temperature or conventional kiln schedules, (2) avoid dipping in azide as it offers no advantage, and (3) keep the length of log storage to a minimum.
For quality control practices, firms should (1) document processing procedures including length of log storage, time between sawing, stacking, air/kiln drying, and kiln conditions, (2) determine from each kiln load if indeed brown stain exists. This is not easy because most of the time the stain is just under the surface. A hand block planer, or a portable hand power planer can be used to remove the surface within a small area of selected pieces to determine both degree and depth of the stain, and (3) correlate procedures with absence or presence and degree of brown stain. Adjust procedures if possible to reduce the staining problem.
Evaluate your long term commitment to the export market. Study the feasibility of installing a smaller, low temperature kiln if you intend to pursue this market.