Chemical stains include a variety of discolorations that occur in wood during drying but are typically not noticed until the drying process is over. A previous blog focused on sticker stain, so this post addresses other chemical stains.

Interior Graying

Interior graying is most common in southern oaks, hackberry, maple, and ash, but it can also occur in many other wood species.

Brown Stain (Coffee Stain)

This stain is most common in white pine and in thicker boards of any pine species.

Chemical Stains

Chemical stains develop when naturally occurring chemicals in wood react with air (an enzymatic oxidation reaction) to form a new, typically dark-colored compound. A key point is that the chemical precursors, which eventually lead to discoloration, develop in the wood above 40 percent moisture content. Once these precursors develop, staining is inevitable. Stain control must begin when the moisture content is high.

It is often difficult, and sometimes impossible, to detect when these precursors have formed. Air-dried lumber may show no outward signs of discoloration, yet stain may still appear after kiln drying. In some cases, the surface of the lumber may already be below 40 percent moisture content when the precursors form. In this situation, stain will develop only in the interior of the board and will not be visible until the lumber is machined, planed, or ripped.

A stain can be tested with a drop or two of concentrated oxalic acid. Oxalic acid will bleach chemical and iron tannate stains but will not affect fungal stains.

Requirements for Chemical Stain Development

Chemical stains will form when wood is dried slowly under warm or hot conditions. These environmental conditions trigger enzymatic oxidation reactions. The longer poor drying conditions persist, the deeper the stain penetrates and the darker the discoloration becomes.

To prevent chemical staining, drying issues must be addressed when the lumber is above 40 percent moisture content. Slow drying can be caused by several factors.

Causes of Slow Drying

• High relative humidity during the initial drying stages

• Slow air velocities or long airflow paths

• Snow-melting (thawing) schedules that use high relative humidity

• Overloaded predryers or kilns that cannot achieve the desired humidity quickly

• Warm and humid ambient conditions

High Relative Humidity

The most frequent cause of chemical staining is high humidity in predryers or kilns. Accurate kiln controls are essential to ensure that controller readings accurately represent the internal drying environment.

For most light-colored woods, the recommended initial settings are 130°F with a 10°F wet-bulb depression (65 percent relative humidity). Predryers, which use lower dry-bulb temperatures, require even lower humidity levels. Failure to maintain these conditions when the wood is warm will likely lead to staining.

Airflow Considerations

Even when incoming air has an acceptable humidity level, slow air velocity or long airflow pathways can cause humidity to increase as the air travels through the lumber stack. Humidity in the interior of the pile may reach levels high enough to cause stain.

Never dead stack green lumber.

Snow Melting (Thawing) Schedules

Snow or ice melting schedules typically operate at 80°F and 100 percent relative humidity for 24 hours. These conditions are ideal for stain development.

Overloaded Predryers or Kilns

A predryer or kiln overloaded with wet lumber, or forced to operate under a schedule requiring extremely low wet-bulb temperatures during hot and humid weather, may experience excessively high internal humidity.

Humid Weather Conditions

Although spring weather often provides excellent air-drying, periodic warm temperatures combined with high humidity can create ideal conditions for stain formation.