This article aims to learn more about the three most common types of stains or chemical reactions that naturally occur in wood that may have the appearance of mold but aren’t and how to best prevent mold from happening.
Given the right conditions, mold can grow on any type of newly-sawn wood. Later, we will learn how to avoid the conditions that promote mold growth.
Many pallet users are rightly concerned about this in the Midwest in the late spring and early summer months when humidity and temperatures are higher. However, before you call or e-mail your pallet supplier to come and pick up that load of pallets, verify what the discoloration really is. Because not all wood discoloration is mold, it is likely a type of stain that just naturally occurs in wood products.
1) Iron Stain / Tannin Acid Reaction
Iron Stain
Iron Stain is one of the most common discolorations found on wood. Many hardwoods, such as Oak, Birch, and Maple, as well as redwood, cypress, and cedar, are susceptible.
These types of wood are particularly vulnerable to iron stains because they contain large amounts of tannin acid. The black-grayish discoloration is caused by the reaction between natural extractions in wood and iron/steel materials, such as nails, chainsaw blades, and saw blades used in the milling/wood-cutting process. The staining can be throughout the lumber caused by water movement when naturally drying or even when being heat treated. However, the affected area is less for woods that have an initial lower moisture content. However, it is possible for staining to occur if water is reintroduced to the surface of the wood, like rain or some other source. The iron staining is cosmetic on the surface of the wood and does not affect its strength.
Iron Stain
Tanner Acid Reaction
2) Brown Stain / Zebra Stain
Zebra Stain
Brown stain can be found in Western Hemlock, some types of pines, and other species. It only appears after the wood is drying out or has been dried in a kiln. The unaffected areas appear in their natural light yellow or white color, while the affected areas appear dark brown, making for a noticeable difference in the surface colors on primarily the ends of the boards or stringers.
Below the surface of the affected pieces, sometimes the brown stain will look black after the wood has been dried out. This is known as zebra stain. Zebra stain happens when iron or manganese darkens the browning and makes it turn black. This type of staining does not affect the properties of the lumber/wood and is safe for any environment.
Brown Stain
3) Blue Stain
Blue Stain can be the closest to resembling mold on wood. But again, blue stain is not a mold that should be a cause for concern. It is a common cause of the discoloration of wood and lumber. Certain darker-colored microscopic fungi cause a blue or gray discoloration in the sapwood of the wood as it is drying out. The name blue stain is a misnomer because not all blue stain is blue. Blue stains can be shades of gray, black, brown, and a variety of other colors as well. Here is also more good news: blue stain has no adverse effect on the performance or strength of stained wood. It can be used for exactly the same purpose intended, like non-stained lumber. Blue stain poses no health risk or concern and is safe to handle.
Blue Stain
How to prevent conditions that promote mold growth on pallets
For mold to grow, it needs several ideal conditions:
- Humidity or Moisture
- Stale Air
- Warmer Temperatures
It is impossible to eliminate mold spores because they are present everywhere, but you can help reduce the mold spore density by looking and inspecting areas that may be creating favorable conditions for mold or if mold is already present.
Look for areas outside the building that could be a mold-breeding environment. Do you have old wood scraps, crates, plastic films, grass clippings, or leaves next to the building?
Make sure that rain gutters direct water away from the building and that the parking lots or warehouse dock areas have proper drainage so that water can’t accumulate or puddle.
If pallets are being stored outside, be sure they are covered to keep out of the rain and in an open area with lots of air movement and sunlight. If pallet surfaces become wet, mold spores can germinate and grow. Keep gaps and spaces between the pallets.
Extra precautions are needed for new green wood pallets that are being stored indoors when humidity and temperatures are high, mainly above 50 degrees.
For example, for outdoor pallet storage, be sure there is good ventilation or climate control in the warehouse. Consider installing fans to keep airflow moving, keeping stacks of pallets in open areas, and having gaps between the rows of pallets away from any raw goods that could be a source of mold.
Airflow Trailer