FNR-117
forestry & natural resources
MARKETING AND UTILIZATION 
PEG Prevents Drying Defects in High Value Wood
Daniel Cassens
Forest Products Marketing and Utilization
Freshly cut wood may contain at least one-half of its weight in water or sap. As this water is removed, either by air or kiln drying, the lumber shrinks, and warping, checking, and splitting may result.
Wood warps and splits because it shrinks different amounts along different grain directions (Figure 1). In the longitudinal direction wood shrinks very little. Flat sawn lumber (tangential grain direction), on the other hand, shrinks about 6 to 9 percent, whereas quartered sawn lumber (radial grain direction) shrinks about 3 to 5 percent when dried from green to about 6 percent moisture content. These differences in shrinkage present serious problems when drying specialty wood items such as tree or limb cross-sections, bowls, carvings, and sawn items which have especially diverse grain patterns. If these items are allowed to dry naturally, objectionable cracks and splits are almost certain to develop in them.
Polyethylene glycol, or PEG, can be used to treat green wood to prevent splits and warping that result during drying. PEG is a white, waxlike chemical that resembles paraffin. It is a solid at room temperature, is nontoxic, noncorrosive, odorless, colorless, and has a very high fire point (580 °F). It dissolves readily in warm water. When green wood is soaked in a solution of PEG, the chemical moves into the wood structure, prevents shrinking, and thus reduces splits and other drying degrade.
Treating Schedules
Wood to be treated with PEG should be tightly wrapped in plastic, and submerged in
Figure 1. Characteristic shrinkage patterns and associated distortions as affected by grain direction. (Courtesy U.S. Forest Products Laboratory, Madison, WI)
water or frozen as soon as it is cut and until it is placed in the PEG solution. This precaution will keep the wood green for easier treating and prevent checking. The addition of borax (1-2 percent solution) will help to reduce the development of mold, bacteria, and stain fungi. The piece to be treated should be cut as near as possible to its finished size, thus making sure that the wood which will ultimately be used is well treated. Wood cut from dead trees and logs will still contain substantial quantities of water; however, it may be best to soak such wood for two to three days in water and then treat with PEG.
Table 1 shows treating schedules for walnut for different combinations of temperature,
COOPERATIVE EXTENSION SERVICE • PURDUE UNIVERSITY • WEST LAFAYETTE, INDIANA