In this new Q&A series, I will be answering some questions that the American Hardwood Export Council has commonly been asked at events and trade shows in which they have participated. They all have a common theme relating to the export of American hardwoods to destinations all over the world.

We often hear from importers who are offered quotes for hardwood shipments on a gross/green tally or a net tally basis. In this series I will answer questions about the differences between the two, the advantages of kiln drying and also all about what is probably the most versatile timber species that grows in America’s hardwood forests.

What is the difference between gross/green tally and net tally and what are the advantages and disadvantages between the two? 

Sawmills often price lumber shipments on either a net or gross tally and there is a price differential between the two. The difference between them is that the net tally is how the lumber is described after kiln drying and the gross or green tally refers to lumber before it has been kiln dried. The volume in cubic meters or the board footage will differ between the two due to shrinkage that occurs when the water is removed from the lumber during the kiln drying process.

So, no matter what the final moisture content of the lumber is, the net tally is the actual volume of the wood after it has been in the kiln. Lumber sold on a gross or green tally basis will be about 7-10 percent more in volume than the same lumber sold on a net tally.

Buying on a net tally basis will give a manufacturer a better idea of yield, as the shrinkage factor has already been considered. The upfront cost may be less if buying on a green or gross tally, but it is recommended that American hardwood lumber is purchased on a net tally basis.

It should be noted that there are provisions in the National Hardwood Lumber Association (NHLA) rules for shrinkage in kiln dried lumber. There are allowances made for shrinkage in width, thickness and volume. All kiln dried lumber is permitted to be ¼” (6.35mm) less in width than the required minimum width. Rough sawn lumber that is less than 2” (50.8mm) can be 1/16” (1.6mm) less and for 2” and thicker lumber the shrinkage allowance can be 1/8” (3.2mm) less than the minimum required for thickness.

Can you explain the kiln drying process for American hardwoods and is it really needed?

AHEC has often been asked about failures associated with using American hardwoods in furniture manufacturing. It is estimated that around 80 percent of problems associated with American hardwoods used in interior situations are moisture-related. In almost all cases, this is the result of using American hardwood lumber that has been sawn locally from imported American hardwood logs, rather than imported as kiln-dried lumber directly from the United States.

In this case, the lumber is never properly dried. Moisture-related problems can be largely eradicated by buying and using imported kiln-dried lumber. American sawmills have been shipping kiln-dried American hardwoods all over the world to countries with extreme climates for many decades and have experienced very few problems.

There are many steps to successfully kiln drying lumber. All hardwood species will dry differently because of their different cell structures. White oak, for instance, will take several months to dry properly, whereas tulipwood will only take a matter of days. Depending on the species, the lumber may need to be first dried by the old-fashioned method of air drying before being placed in a kiln.

Air drying will reduce the moisture content to around 20-22 percent, which is not dry enough to be used for interior joinery or furniture. For harder-to-dry species, such as the oaks, controlled air drying can reduce the amount of time that the lumber needs to be in the kiln. For example, a 1” (25.4mm) oak board that has been air dried for 60 days under good conditions will probably take another 30 days in a kiln to reach the desired moisture content of around 8 percent.

Different width and length boards of the same species may be included in the same kiln load, but not different thicknesses as the drying time will vary according to thickness. Kiln drying is complex and lengthy, but the American hardwood industry has mastered the process over more than a century. It is well worth the cost to ensure that lumber bought arrives in the factory ready for manufacturing.

Can you explain the difference between European poplar and American poplar?

One fact to establish is that American poplar or American yellow poplar, as it is more frequently known in the United States, is not actually a poplar at all. Liriodendron tulipifera to give it its botanical name, is part of the magnolia family. AHEC has sought to differentiate this species from European poplar by ensuring that it is known in export markets as tulipwood, referring to its botanical name and highlighting its tulip-shaped leaves.

Tulipwood trees are much bigger than European poplar trees, producing far better quality lumber. The trees grow straight and tall, allowing for the production of sawn boards in long lengths and wide widths. The lumber dries exceptionally well and quickly, while maintaining good dimensional stability, which helps to make it one of the most economical American hardwoods. Tulipwood machines particularly well and will take paint and stains beautifully, allowing it to look like more valuable hardwoods, such as walnut or cherry.

Tulipwood also possesses an exceptional strength to weight ratio, making it an ideal timber for niche structural manufacturing applications such as Cross Laminated Timber (CLT) panels and Glue Laminated beams. Tulipwood grows extensively throughout the eastern United States temperate hardwood forest, but most abundantly in the Carolinas. European poplar, by contrast, is much softer and is mainly used for paper pulp, plywood cores, industrial packaging, boxes and pallets.