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Preventing Wood Stains During Kiln Drying

Discolourations (stains) in wood have the potential to seriously downgrade and reduce the value of lumber. Even in cases where stain is admitted within grading rules, the presence of stain can have an impact on the “perceived quality” and acceptance by the customer.


June 29, 2012
By Peter Garrahan FP Innovations

Topics
Sticker stain is related to slow drying conditions sometime after piling the green lumber and is often associated with slow drying conditions in the kiln.

There are many types of stain and many points in the lumber manufacturing process where stain can develop. To cover all of the potential types, causes, and preventive measures for stain would require far more than a journal article. Most stains develop as a result of the exposure conditions of logs or green lumber prior to drying and, for that reason, there is a considerable amount of material already available on that subject. A recent special publication by FPInnovations deals extensively with the processes of stain development and identifies prevention/remediation treatments that address most of the pre-kiln drying stages. The publication is entitled “Wood Discolourations and Their Prevention” – Special Publication Number SP-50.  For further information on this publication and how to order a copy visit our website at www.fpinnovations.ca and follow the link for “Publications”. This article will deal strictly with the subject of potential for stain development due to conditions present within the dry kiln.  

Types of Stain
There are two basic types of stain; those we classify as biological and those that are a result of chemical reactions between naturally present compounds. These are often referred to as microbial versus non-microbial.  Microbial stains include sap-stain caused by fungi that attack the sapwood portion of a log or board, decay-related discolourations (also caused by fungi), and mould-related discolourations. Chemical or non-microbial stains include brown stain in pines or hemlock, sticker stain, and interior greying or darkening in light-coloured hardwoods. Regardless of which type of stain you are dealing with, one of the common factors in virtually all stain formations is the link with slow or poor drying conditions.  When slow drying conditions exist in a kiln, stain can develop quickly. This is because many of the processes associated with stain development are enhanced with the temperatures typically employed in the early parts of kiln drying.

Stain Development in a Kiln
Stain is often not detected until lumber reaches the planer mill. For that reason, the kiln-drying operation (including the kiln operator) is often the first area that is questioned. As mentioned previously, most, but not all, stain develops as a result of conditions prior to kiln drying. Therefore, it is good practice to closely monitor for the presence of incoming stain either in the logs or the green lumber.  Sap-stain and mould will be readily visible on green lumber or logs. Quality-control personnel and kiln-operating staff should inspect for and document the presence of stain in incoming logs and green lumber packages. If it is found, then obviously the problem needs to be addressed at the supply end. Unfortunately, chemical discolourations such as brown stain in white pine may not be visible until after drying and therefore it may be more difficult to determine if its presence is related to the kiln-drying operations or the pre-kiln-drying operations. However, if sap-stain has been detected prior to kiln drying, there is a good possibility that some chemical discolourations will also be apparent after drying (in species that are susceptible to such discolourations).

If it is determined that new or additional stain has developed during the kiln-drying process, it then becomes necessary to determine the factors that have contributed to it and what can be done about it. The following is a list of possible causes of stain development in a kiln and what can be done about each.

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As mentioned in the table below, the pattern of stain development can often help to identify the possible cause(s). For example, stain found only near the ends of boards is usually related to log handling and storage practices and NOT something related to the green lumber or kiln-drying operations. Stain found only in specific packages coming from specific areas of the kiln, however, is usually related to one or more of the issues identified below.

If everything is operating well, a dry kiln is usually the safest place to have your lumber. In fact, the stain prevention strategy for many mills is to minimize or entirely eliminate any storage of green lumber between the sawmill and kilns. This gets the wood into an environment where you have complete control. Following the points listed below will help make sure that your kilns are indeed a safe refuge for your lumber.


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