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Sawmill & Treating Insights: Tips to Preserve Color of ‘White’ Wood
Since color and brightness of white woods are attributes that make it attractive, it is important that such traits be maintained throughout the processes from cutting the tree through drying the lumber.
By Dr. Brian Bond
Date Posted: 12/1/2007
Over the last few years the demand for ‘white’ woods has increased significantly. Just what is a white wood?
(Editor’s Note: White woods as described here are grade hardwood designations, not differentiations between white wood pallets and painted pallets.)
Species that typically fall into the category of white woods are those that have large amounts of sapwood and are desired for their bright, lighter or whiter color. Hard maple is usually the first species that comes to mind when discussing white woods, but soft maple, ash and even yellow-poplar can be considered white woods.
For white woods, the desirable portion of these species is the sapwood, which is the lighter colored outer portion of the log or trunk. The sapwood contains living cells, some of which are involved in transporting water and sap and some that are responsible for storing starches and sugars. The sapwood typically has a higher moisture content and greater permeability (faster drying) than the heartwood. The heartwood is composed of dead cells that have polyphenolic compounds such as fats, oils, waxes, resins, gums, tannins, aromatic and coloring materials.
‘White’ Wood, Sapwood
Since color and brightness of white woods are attributes that make it attractive, it is important that such traits be maintained throughout the processes from cutting the tree through drying the lumber. It is important that the wood not develop blotches or stains nor lose its brightness.
One of the main problems with white woods is that they are predominantly made up of sapwood; they contain sugars and starches that fungus can feed on and living cells that can produce chemicals and stains. The brightness of wood also can be affect by temperatures used in drying.
To avoid discoloring the wood, it is important to understand the problems associated with discoloring and how they are caused.
Once a tree has been cut, measures should be taken to ensure the best possible color and to avoid staining. During storage, logs are susceptible to fungal and enzymatic (chemical) staining.
Fungal stains result when the hyphae of the fungi penetrate deeply into the logs, and portions of the wood are used as food by the fungus. Common fungal stains found in logs are referred to as sap stain or blue stain.
Enzymatic or chemical staining results from the sugars in the wood, interactions with exposure to oxygen, and chemical reactions that occur at the cellular level. The chemical reactions that occur have been compared to how an apple slice will turn brown with exposure to air. Warmer air temperatures and higher humidity levels tend to increase the rate that chemical reactions occur, and they can increase chemical staining in some of the white wood species. Unlike fungal stains, there is often no indication of enzymatic staining until lumber is sawn from a log. Gray staining in hard maple is one example.
The more time that logs sit in storage conditions, the greater the chances for both fungal and enzymatic staining. Because of the warm temperatures that occur in the Northeast from April to November, it is especially important to prevent lumber discoloration. One method to prevent moisture loss and reduce staining is to end coat logs with a wax-based sealant. End coating has been shown to reduce both end checking and enzymatic staining in logs.
Strategies for Log Yard, Green Lumber
Probably the most common method of preventing moisture loss in logs during storage is watering them with sprinklers during the summer. Watering the logs lowers the air temperature and reduces the amount of available oxygen around the logs. The reduced oxygen levels help prevent sapwood staining and decay fungi from infesting the logs.
However, using warm recycled water for sprinkling can inadvertently introduce bacteria into logs that also can lead to unwanted wood discoloration. In addition, water with high concentrations of minerals, such as iron, may leach into the logs and cause a darkening of the wood.
In my opinion it is best to avoid watering white wood logs. If you must, though, I suggest using fresh cold water with low mineral concentrations.
Ultimately, one of the best practices to control wood color in the log yard is to process the logs as soon as possible after they are felled – within two weeks.
Unlike in the log yard, where the strategy is prevent moisture loss, when storing green lumber a consistent and even rate of moisture loss is the goal to ensure bright color and avoid staining.
Green lumber is more susceptible to discoloration caused by fungal and enzymatic staining because the bark is not present to help control moisture loss and protect against fungal infection. Research consistently recommends prompt kiln drying after sawing to minimize discoloration in white woods.
It should be a priority to sticker the green lumber as soon as possible (ideally within 12 hours) to help minimize darkening and discoloration. Lumber should then be placed in the kiln and the drying schedule started. If lumber cannot be kiln-dried immediately, then it should be stored temporarily in an area with good air flow — ideally, a shed with a bank of fans.
For white wood species, air temperature and relative humidity are two critical factors in determining the lightness of the lumber after the kiln drying process.
Avoid using older schedules that use dry bulb temperatures of 130ºF and starting relative humidity values of 86% or higher. They can lead to darker colored lumber that can be characterized as pinkish-brown or reddish-brown in color. Ideally, it is best to keep dry-bulb temperatures below 110oF until the wood is below the fiber saturation point (28-30% MC). It is thought that at the higher air temperatures, the sugars in the wood caramelize and create the darker colored appearance. Using low dry bulb temperatures (<110ºF) and low relative humidity levels (<70 percent) also can help ensure that interior darkening of the lumber does not occur. Keep in mind that for the secondary manufacturer, interior color is likely just as important as surface color.
Make sure your kiln is able to reach and maintain set-points. Older kilns may have been designed to dry material that already is well air-dried, and they may not have the air flow and venting capacity required for white woods.
It is very important that the wet bulb depression is reached quickly and can be held. Drying white woods at lower temperatures is usually combined with higher air flow (400+ feet per minute) to achieve rapid, uniform moisture loss. You might reverse fans every two hours to ensure high moisture loss throughout the load. Try to achieve moisture loss of 5% per day — minimum — in most white woods.
If you follow these guidelines, you should be able to produce bright white wood lumber free of chemical and fungal stains.