In some western hemlock lumber pieces “brownstain” develops as the wood dries. Brownstain is deleterious to high-value appearance-grade stock because it reduces shop and better grades of hemlock to paint grade. The stain varies in color and can be superficial or subsurface. The incidence of subsurface brownstain, usually as a striped discoloration termed “zebra” stain has been seen more frequently with the increased kiln drying of western hemlock. When brownstain is subsurface, planing allowances have to be larger than desired, resulting in wasted fibre. A 1995 Forintek research project indicated that water storage of logs resulted in more surface brownstain during air drying than dryland storage but the numbers of logs were insufficient to determine if there was a significant difference. The project reported here was set up to investigate further whether storage influenced the development of brownstain and whether it occurred during kiln drying.
Three hundred cubic metres of hemlock logs were freshly cut, 60 m3 sent to a sawmill for conversion and the rest split between dryland and water storage. After two, four six and eight the stored logs were sawn in batches of 60 m3 into 100mm squares. These were examined individually before and after kiln drying and potential and observed brownstain marked of the faces. The squares were then planed and the extent of subsurface brownstain assessed.
Wet areas were apparent in both sapwood and heartwood of freshly sawn hemlock squares but were not useful in predicting the site of potential brownstain. Kiln drying gave the wood a toasted appearance in some of the drying runs which was removed by even the lightest planing. The most predominant form of brownstain was subsurface zebra (striped) stain which developed only during drying. Before planing, the presence of zebra stain was usually detectable as a shadow at the surface of dried lumber. This stain was up to 3 mm beneath the surface and therefore up to 3 mm had to be removed by planing to reach non-discolored wood. Zebra stain occurred only in the sapwood, usually the inner sapwood alongside the heartwood. The stain follows this zone along the boards. The zebra stain was more likely to be visible on edge-grain than on flat-grain, possibly reflecting moisture movement out of the boards. Some logs showed more zebra stain than others and appear to be predisposed to the stain. Both water and land stored logs, for short and longer terms, developed brown stain. Grey stain was also present but less common, and appears to be part of the natural character of hemlock wood. In general the longer the log storage time, up to 8 months, the less brownstain resulted, but based on the current information, changes in log storage type specifically to reduce development of brownstain are not justifiable.
In this work the lumber with the drier surfaces, which developed the least brownstain, was from the material that had been stored longest and included both water and dryland stored logs. There is an indication of interaction between log drying, storage time, air and kiln drying of lumber and the development of surface or sub-surface brownstain. Further investigation of these interactions may help to devise methods of reducing the incidence of hemlock brownstain.