Q&A - American Hardwood Lumber
by Bob Sabistina- Technical Consultant to the American Hardwood Export Council (AHEC)
This month’s article will continue to answer questions I receive from buyers throughout the world. If you have a question regarding the American hardwood lumber grading rules or any other topic you would like to see in future articles pertaining to hardwoods from the USA, please contact me at:firstname.lastname@example.org
Question: We are located outside of Delhi, India and have a few questions regarding the American hardwoods that we have begun to use in our furniture factory. We are currently buying the wood locally and plan to expand our production to buy direct from AHEC members in America. Our main concern is the movement we are experiencing after the wood (we are using ash) is machined into parts and glued up. We have traditionally used teak and this was never a problem. We are concerned that our climate here is not suitable for North American species and need some help from you. Are there some extra steps we need to take in processing the ash? We have used European beech bought directly from a supplier in Italy and have had good success with it. Any help you can provide will be greatly appreciated.
Answer: My initial reaction to your question is that the ash you are buying locally has been improperly dried and what you are experiencing is a moisture content problem. We have shipped kiln dried American hardwoods all over the world, to every conceivable climate for many years and have had very few problems of this nature. Firstly, let me say that I have been to India many times and this is a recurring problem with American hardwoods coming to India in log form and being processed locally. Secondly, the beech you have bought directly from your Italian supplier has been kiln dried before shipping. Thirdly, teak is one of the few woods in the world that has its own natural oils and, when going through the drying process, shows very little shrinkage.
To get the most out of hardwoods from the vast American forest resource, the wood should to be properly kiln dried. It is estimated that nearly 80% of the problems associated with hardwoods destined for interior use are moisture related. Let me try to explain some of the important points with regard to hardwoods and their moisture content.
Moisture content (MC) refers to the amount of water contained in a piece of wood. When wood is allowed to dry naturally it is referred to as “air-dried”. Typically this process will remove the “free” water in the wood, which is not bound to the cell walls, and bring the MC down to around 25%. This is called its fiber saturation point (FSP). To remove the trapped water from the cell walls, artificial heat needs to be introduced. When this drying process starts, water evaporation begins, taking the wood below the FSP. Shrinking occurs as the trapped water is removed. This shrinkage occurs along the width of the piece, not lengthwise.
This drying process continues until a suitable MC is reached, normally around 9%. When this dried lumber is then introduced to its final environment the wood adjusts to its surroundings. When a stable condition of both relative humidity and temperature occurs, the wood will reach a constant MC, which is referred to as the equilibrium moisture content (EMC). As humidity and temperature indoors may fluctuate with the seasons, so will the EMC. This fluctuation in EMC is minor, but can be noticeable in applications such as flooring, where temporary separations appear in hardwood floors in heated winter homes. Proper drying to the suitable MC and consistent controls on the environment where the wood is finally used will reduce the movement that will occur.
Proper kiln drying is a very technical process that takes from just a few days up to several months, depending on the species, thickness, and the quality of the equipment being used. Hardwood lumber dries more quickly from the ends of the boards, which is why you see most KD lumber with painted ends. This “paint” is usually wax-based and seals the ends from drying out more rapidly than the rest of the board. Typically lumber is dried lower than the targeted final MC and a conditioning treatment is applied where hot steam is actually introduced back into the kiln to stabilize the lumber and even out the MC throughout the entire board.
It is very important to monitor the MC from the time you receive the wood to its final end use. To easily check the MC, you need to invest in a moisture meter. While not the most exacting of methods, they are usually hand held, which allows the user to check freely the wood throughout the plant. They usually have two pins that you insert in to the thickness of the wood and a digital reading will give you the MC. Multiple readings can be done on one piece or throughout an entire lift to give you an average of MC. It is vital when using a moisture meter that you follow the manufacturer’s directions, as factors such as outside temperature and species will affect the readings.
Another factor that proper kiln drying will influence is the stability of the lumber after it is machined. High temperatures will also kill any insects and eliminate the occurrence of fungus or decay. Proper kiln dried hardwood lumber machines more consistently and you will have successful and stronger glue joints.
For more information, please visit our website: www.americanhardwood.org
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American Hardwood Export Council
Tel: (44) 20 7626 4111
Fax: (44) 20 7626 4222