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Weekly UpdatesBark has the possibility of being used as an outdoor wall covering. American chestnut (Castenea dentata) and yellow-poplar (Liriodendron tulipifera) have been used for this purpose. (1) Although bark siding has a long history of use, it remains a small, niche application and little is known about its processing characteristics.
To prepare poplar bark for use as siding, the green bark is stripped in sheets from freshly harvested trees in the spring and early summer when the vascular cambium is active and the bark can be easily removed. The strips are then cut to size and stacked flat to dry. Kiln-drying may be employed. As with lumber, drying is required to ensure the dimensional stability of the bark and to reduce the risk of biodegradation.
Moisture meters are commonly used for the rapid determination of the moisture content (MC) of the wood. These meters are based on the variation in electrical properties (conductance or power loss) of wood associated with variation in MC. (2) Variation in temperature, density, and chemistry can also affect electrical properties, and correction factors are applied (manually or automatically with some meters) when using moisture meters on lumber of different species or different temperatures. No information is available on the use of lumber moisture meters with bark.
This project was conducted at the request of a local manufacturer to determine moisture meter correction factors for yellow-poplar bark.
Materials and methods
Seventeen pieces of fresh yellow popular (Liriodendron tulipifera) bark were collected from the manufacturer in east Tennessee, stored in plastic bags, and brought to the lab in early June. To determine the MC difference between the outer bark and the inner bark, three pieces of bark were cut at the inner/outer bark boundary and the MC was measured using the ovendry method.
For the moisture meter calibration, a 8.5- by 5.5-inch (22 by 14 cm) sample was cut from each of the 17 pieces using a band saw. Each edge was sealed using a wax emulsion (Anchor-Seal, Danvers, Massachusetts). Immediately after cutting, the samples were weighed (nearest 0.01 g) and monitored for MC using three different moisture meters:
1. Delmhorst RDM-2S (Delmhorst Instrument Co., Towaco New Jersey). A digital-display conductance meter with an upper reading limit of 56 percent. The meter was set on Douglas-fir, as this requires no correction factor. A 15-E electrode was used, which has eight uninsulated pins that penetrate 1/8 inch (3 mm). According to the manufacturer's website (www.delmhorst.com/products_elect_list.html), this electrode is intended for use with veneer, cardboard, leather, and paper. The two rows of four pins were oriented along the bark fiber direction. The pins were inserted into new bark at each reading; the old pin holes were avoided.
2. Delmhorst RC-1B. An analog-display conductance meter with a display limit of 65 percent. A 26-ES 'hammer' type electrode with 1/2-inch (12 mm) un-insulated pins was used. The pins were inserted parallel to the grain direction in new holes each time a reading was taken.
3. Wagner L609 (Wagner Electronics, Rogue River, Oregon, www.wagnermeters.com/). A digital display, power loss (pin-less) meter. The output range of this system was limited from 4 to 22 percent MC (in whole-number divisions). The meter was held against the inner bark surface, with the bark lying outer bark side down, on a lab table. The lab table itself produced a reading on the meter of 11 percent.
The samples were measured daily for 18 days. At the time of each measurement, the samples were also reweighed. After repeated measures while drying in ambient conditions, the samples were moved into a climate chamber with a temperature of 72[degrees]F (22[degrees]C) and 65 percent relative humidity (12% equilibrium moisture content [EMC] for wood). The samples were stored in the chamber until reaching a constant weight at which point they were measured again. After the last measurement, the samples were ovendried at 103[degrees]C and reweighed. Ovendry (true) MC was calculated at each measurement time.
Results and discussion
As expected (3,4) the initial MC of the inner bark was high (average 124%) and much greater than the MC of the outer bark (average 33%).
Figures 1 through 3 show the moisture meter values for the whole-bark samples plotted against the ovendry MC values for each of the three meters used. There was a positive linear relationship between the meter values and the ovendry values. In Figures 1 and 2, however, this relationship appeared to be different above and below the fiber saturation point (FSP), which is approximately 30 percent MC. The presence of free water (MC > FSP) significantly affected the electrical properties of wood, and moisture meters are not recommended for use with lumber with a MC above FSP. For this reason, the resistance meter data for bark were divided into high- and low-MC groups (above and below 30%). Linear regression equations were then calculated for each data grouping. Using the regression equations for the lower MC groups, correction factors for each meter were calculated for the range from 10 to 30 percent meter readings (Table 1).
There was considerable unexplained error in the relationships among the meter readings and the ovendry values ([R.sup.2] from 0.41 to 0.83). This may have been due in part to the large difference in MC of the inner and outer bark initially and variations in the ratio of inner and outer bark thickness among the samples. Regardless of the explanation, the application of correction factors results only in a one-to-one relationship (slope of the regression [approximately equal to] 1) between the meter values and the ovendry data; the error in the relationship is not reduced ([R.sup.2] values are not improved). The (corrected) meter readings of bark MC may have a lower precision than is expected when using meters with wood. It is likely, however, that highly precise determinations of MC will not be necessary for bark products such as siding. The bark samples were not measured at low MC levels (< 12% ovendry) so these data do not provide correction factors for meter readings below 10 percent. But, bark products are generally intended for outdoor applications where EMCs (for wood) are most often above 10 percent. (5)
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(1) Anonymous. 2008. Natural Poplar Bark Siding History. www.naturalbarksiding. com/poplar_bark_history.html. Furniss Enterprises, Inc., Lake Toxaway. NC. Accessed 2/10/08.
(2) James, W.L. 1988. Electric moisture meters for wood. Gen. Tech. Rept. GTR-FPL-6. USDA Forest Serv., Forest Products Lab., Madison, WI.
(3) Koch, P. 1972. Utilization of southern pines. USDA Forest Serv. Agriculture Handbook No. 420.
(4) Koch, P. 1985. Utilization of hardwoods growing on southern pine sites. USDA Forest Serv. Agriculture Handbook No. 605.
(5) Simpson, W.T. 1998. Equilibrium moisture content of wood in outdoor locations in the United States and worldwide. Res. Note RN-FPL-0268. USDA Forest Serv., Forest Products Lab., Madison, WI.
Josef Muehlbacher
Adam M. Taylor *
The authors are, respectively, Undergraduate Student, Univ. of Applied Sci. Salzburg, Austria (jmuehlbacher.htw2005@th-salzburg. ac.at); and Assistant Professor, Univ. of Tennessee, Knoxville, Tennessee (AdamTaylor@utk.edu). This paper was received for publication in September 2008. Article No. 10535.
* Forest Products Society Member.
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