Pallet manufacturers and sawmills that make cut stock or other pallet material may need to dry their green lumber and pallet parts for two reasons. One is to prevent mold, and the other is to prevent chemical or enzymatic staining, which occurs naturally and particularly with species with large amounts of sapwood. Of course, mold causes stains, too – as fungus consumes sugars and starches in sapwood.
“Any time you’re shipping and using packaging material, there’s always the opportunity for high moisture content in green wood to move into the packaging material or lead to mold,” noted Brian Bond, a Virginia Tech professor and extension specialist in the area of wood products manufacturing and an expert in drying wood. The presence of mold depends on the density of mold spores in a given area and moisture, which it needs to grow.
Most pallet manufacturers do not kiln-dry their lumber or pallets because it is not required. Those that do likely have customers that require very low lumber moisture content – less than 15%. For those kinds of pallets, buying kiln-dried softwood (or hardwood) lumber is an option.
For others, drying lumber to below the fiber saturation point is sufficient for appearance and to prevent mold. The goal is to get the surface dry as quickly as possible below the fiber saturation point, which is generally about 26-30% moisture content. A hand-held moisture meter can measure the moisture content of the lumber.
Chemical stains are a result of the oxidative darkening of chemicals produced by a particular living cell that is located in the sapwood. While these stains often don’t show up until later in the drying process, they often get started with the poor handling of logs. Logs should be removed from the woods and processed rapidly, particularly in warmer weather. Do not let them sit for more than two weeks in the warmer months. This type of stain is not a threat to products but can be considered visually undesirable to the customer.
Once logs are sawn into boards and cants, the opportunity for both types of stains will remain. Certain species are prone to chemical staining, such as white pine, hard maple, soft maple, ash, hickory and yellow-poplar. Lumber of these species should be placed on sticks in an area with good airflow within 12 hours of sawing. For typical 4/4-inch lumber, sticks should be 7/8—3/4-inch thick. Automated stackers can mechanize the process, although workers may still be required to position boards.
The goal is to provide enough air flow to enable moisture to be drawn away from the surface. The longer the lumber remains in a dead-pack, the greater the chances for chemical staining. This stain is often not detected until the lumber is further processed after drying.
Two common non-kiln options for drying are air-drying or fan sheds. However, humidity and poor drying conditions will impact the effectiveness of both methods – high humidity, poor air movement, and too much lumber in a small area can all affect the rate of drying. If the lumber is outside, unprotected from rain, that can negatively impact air-drying.
“Fan sheds are a great way to get wood down to the fiber saturation point quickly,” said Bond. Fan sheds have the benefit of protecting the lumber from rain and direct sunlight. A bank of fans is used to force air through lumber stacked with stickers. Air flow is critical for drying wood above the fiber saturation point, or 30% moisture content. Above 30% moisture content, the higher the air flow, the greater the drying rate. By adjusting the fan speed, the drying rate can be adjusted.
The air flow through the material should be checked. A hand-held anemometer will measure air flow across the length of the lumber. (The rate of flow typically varies more from the bottom of the pack to the top.) Make sure the drying loss or moisture content loss per day is the rate desired.
Use lots of open space between the material to achieve faster drying. Also, don’t store too much material in a closed area. One thousand board feet of lumber holds about 2,000 pounds of water – which can be released into the surrounding environment. The more wood, the more water, and in a small space, which means high relative humidity and the opportunity to grow mold or stain.
Fan sheds are commonly used on ‘white’ woods or species prone to stain. Be careful when using a fan shed with species that are prone to check. Don’t put too much material in the fan shed. Make sure the air flow rates are high enough to achieve the desired moisture loss. Fans should not be run when the humidity exceeds 90%; this will result in re-wetting of the surface.
For many pallet manufacturers, air-drying may provide the necessary moisture loss to prevent mold and fungal staining. It’s a great way to reduce the moisture content of wood with no energy cost. However, poor air-drying conditions can lead to large amounts of degradation (checks, splits, stains, warping) and drying times vary throughout the seasons. Slow drying conditions in the winter can lead to increased inventory levels.
Surface checks and end checks are a common form of degradation in air drying, particularly for oak or thick stock in any species. Both types of degradation occur during the first 1/3 of moisture loss from drying. If the drying rate is too high during this stage, rapid shrinkage of the fibers on the outer layer of the wood or the ends can occur, leading to wood failure.
Cover lumber that is air-drying to reduce splits and checks from direct sunlight and keep rain off the material. Low-grade lumber or metal roofing can be placed over the material to prevent rain and direct sunlight. On the ends of piles or stacks, a covering, which should be an open mesh material, can be used to slow the drying process. It is particularly important to keep the ends of species like oak covered to reduce exposure to the sun and prevent checking and splitting. The covering material is similar to what is used for tennis courts and batting cages. Make sure there is enough space between stacks of material. Keep the material 12-16 inches off the ground. For high-grade material and thick stock, use end coating. Use a shade cloth on the ends, too.
Do not let the lumber air-dry below 25 percent moisture content. The wood will degrade as it cycles from wet from rain to drying.
An excellent source for estimating how long to air-dry lumber is a report from the U.S. Forest Service; it is available at: www.fpl.fs.usda.gov/documnts/fplgtr/fplgtr121.pdf.
Perhaps the best method of preventing mold and fungal staining is treating the lumber with chemicals that protect the wood. Lumber dip tanks and the appropriate chemicals serve this purpose effectively and efficiently.
Suppliers have upgraded dip tanks over the years to improve efficiency. Packs of lumber are usually held above the tank after dipping to allow excess solution to run off the lumber. Dip tank systems also have features to tilt the pack to hasten run-off. They may have other automated features for infeed and outfeed loading, unloading and staging. If you are considering a dip tank and chemical prevention for mold, make sure your customers accept the chemical treatment that will be used. Dipping is often done immediately after the lumber is cut and still in the dead-packed condition. The quicker the material is dipped after sawing, the better it acts as a mold preventative.
However, if you are handling pallet material and want to prevent fungal stains without dipping the lumber in chemicals, rapid drying – as outlined above – should achieve the same results. Care must be taken, though, as some species – like red and white oak – will check and split if they dry too quickly.
If you follow these guidelines, you should be able to achieve the desired success in lumber drying. Of course, there are significant differences related to wood species and thicknesses of material. You must use the appropriate best practices to achieve the proper drying rate for your specific material.
There is another plus to drying lumber. As the moisture content decreases, the strength of the wood increases. The lumber is stronger, and the pallet is stronger. Design values for pallets – bending strength, tension strength, how much weight the pallet will support, and other factors – made of green lumber are significantly lower than those for dry lumber.
