The purpose of this article is to learn more about the 3 most common types of stains or chemical reactions that naturally occur in wood that may have the appearance of mold but isn’t and how to best prevent mold from happening.
Mold can possibly grow on any type of newly sawn wood given the right conditions. Later will learn how to avoid the conditions that promote mold growth.
Many pallet users are rightly concerned about this in the late spring and early summer months in the Midwest area 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 discoloration of wood is mold. It is very likely a type of stain that just naturally occurs in wood products.
1) Iron Stain / Tannin Acid Reaction
Iron Stain is one of the most common discolorations found on wood. Many hardwoods such as Oak, Birch, Maple are susceptible as well as redwood, cypress, and cedar.
These types of wood are particularly vulnerable to the iron stain because they contain large amounts of tannin acid. The black grayish discoloration is caused by the reaction between extractions naturally occurring 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. Although 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 and on the surface of the wood and does not affect the strength properties of the wood.
2) Brown Stain / Zebra Stain
Brown Stain can be in Western Hemlock, in some types of Pines and other species. It only appears after the wood is in the process of drying out or has been dried in a kiln. The unaffected areas appear in their natural light yellow color or white, 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 has no effect on the properties of the lumber/wood and is safe for any environment.
3) Blue Stain
Blue Stain can be the closest to resemble mold on wood. But again, blue stain is not a mold that should be a cause for concern. it is a common cause for the discoloration of wood and lumber. There are certain darker-colored microscopic fungi that 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 stain 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 negative effect on the performance or the strength of wood that is stained. It can be used for exactly the same purpose as it is intended like non-stained lumber. Blue stain poses no health risk or concern and is safe to handle.
How to prevent conditions that promote mold growth on pallets
For mold to grow it needs several ideal conditions:
Humidity or Moisture
It is impossible to eliminate mold spores because they are present every where but you can help to reduce the mold spore density by looking and inspecting areas that can be creating the favorable conditions for mold or if it is already present.
Look for areas around the outside of the building that could be a mold breeding environment. Do you have old wood scraps, crates, plastic films, and grass clippings or leaves next to the building?
Make sure that rain gutters are getting the water a good distance away from the building and that the parking lots, or warehouse dock areas have proper drainage, so the water can’t accumulate or puddle.
If pallets are being stored outside be sure the pallets are covered to keep out of the rain but also in an open enough area with lot’s of air movement and sunlight. If pallet surfaces become wet, it is an opportunity for mold spores to germinate and grow. Keep gaps and spaces between the pallets.
For new green wood pallets that are being stored indoors when humidity and temperatures are higher mainly above 50 degrees extra precautions are needed.
Like the outdoor storage of pallets be sure there is good ventilation or climate control in the warehouse. Consider putting fans in to keep air flow moving, keeping stacks of pallets in open areas and having gaps between the rows of pallets and away from any raw goods that could be a source of mold.
An air flow trailer has fans and vents in them. The trailer keeps pallets out of the rain, it reduces humidity and condensation, by keeping air from getting stale. Pallets also continue to dry during the storage time.