Related Article: Urban Walnut Trees: Their Value as Timber or Veneer
Defining Hardwood Veneer Log Quality Attributes
Everything that you ever wanted to know about veneers
Condensed report from United States Department of Agriculture Forest Service
Click Here to read original report or click on the "Download File" above.
Our objective at the outset of this project was to better define the attributes of veneer logs that distinguish them from sawlogs so our research into the impacts of forest management on wood quality will have maximum impact. Increasing the production of high-quality timber is our ultimate goal. Rather than obtaining specific attribute criteria on a handful of important defects, our work to date has addressed two allied objectives:
1) to understand which log characteristics are most frequently the determinant characteristics that separate high-grade sawlogs from veneer logs, and
2) to better understand how these determinant characteristics are appraised for different markets by different log buyers.
By meeting these objectives, we can focus future efforts on defining the criteria for the most important log attributes and identifying forest management practices that influence this.
The pinnacle of log quality for hardwood products manufacturers is the appearance grade veneer log — logs that can produce veneer that is highly visually appealing. But high-quality trees that contain these top-quality logs are relatively rare, representing less than 1 percent of the hardwood sawlog inventory in the northeastern United States.
Because of the high value of the veneer product that comes from appearance-grade veneer-quality trees and their relative scarcity in the forest, these trees command a significantly higher price than do trees that contain only sawlogs. The veneer logs cut from these highest quality trees typically cost 1.5 to 6 times the price of grade 1 sawlogs. Because of the exceptionally high prices paid for veneer-quality trees, a large portion of a quality stand’s timber value may be derived from only a small fraction of the trees in the stand. These price differentials can provide significant economic incentive for both the landowner and the logger to manage their resources to optimize the production and recovery of veneer logs.
When managing a timber stand, there are many things that can be done wrong that can damage trees and greatly reduce the price that the landowner receives when he decides to sell their timber. Timber value is lost if the timber harvest is mistimed, such as, if veneer-quality trees are removed before they are of sufficient size. Also, if potential veneer-quality growing stock is cut, injured, or crowded (poor between-tree spacing) during thinning operations, veneer log yields will be reduced.
Other timber management strategies that might be less obvious also affect the value returned to the landowner. By enhancing our knowledge of veneer-quality requirements, we can better understand how these other management practices influence the yield of veneer-quality timber. With lower value products (e.g., rubberwood, medium density fiberboard, plywood) coming into greater use in the construction of furniture, worldwide demand for U.S. hardwood veneer is rapidly increasing. This will lead to continuing price inflation for veneer-quality timber and stronger incentives to manage prime timber stands to promote the yield of the highest grade sawlogs and veneer logs.
Even though hardwood veneer logs typically are regarded as the upper end of the log-quality spectrum, there can be significant variation in characteristics between logs. Variation in veneer log price is based not only on quality, but also on species, markets (e.g., sliced veneer for fine furniture vs. rotary veneer for panels/plywood), customers, and the veneer manufacturer’s veneer procurement strategy.
For most appearance-grade veneer, the primary quality criterion is its “attractiveness.” Of course, “attractiveness” is judged differently from one person to the next. The three key aspects of “attractiveness” are wood color, grain pattern, and blemish or defect content. The effect of a defect on appearance depends on the type, size, and location of the defect, as well as the method of veneer production (e.g., how the veneer log is sliced).
Unfortunately, defects are sometimes revealed only after a log has been sliced into veneer. Defects that affect many veneer slices may render the log worthless for veneer resulting in a substantial monetary loss to the veneer manufacturer. The loss includes not only the purchase price but also log shipping and handling costs, the expense of log storage, and costs associated with log processing up to the point that the defect is discovered (processing costs for sliced, dried, and clipped veneer are high; approximately 10 times the cost for converting logs into lumber).
Since the 1950s, attempts have been made to identify defects and develop grading systems for hardwood veneer logs. In spite of these attempts, the purchase of logs for high-quality veneer is still largely based on company-specific grading criteria with acceptable log characteristics and prices changing in response to market forces.
The top five importers of hardwood veneer manufactured in the United States in 2002 were: Canada, Germany, Spain, China, and Italy (based on export value). Just 5 years earlier, the top five importers of domestic veneer were: Germany, Canada, Spain, the United Kingdom, and Belgium-Luxembourg (Italy was seventh and China was not in the top 10).
Veneer export figures for 2000 substantiate the escalating importance of the greater China region (China, Hong Kong, and Taiwan) in furniture production — 20 percent of the sliced hardwood veneer exported from North America went to the region (based on square footage) . European countries consumed about 60 percent of North American sliced veneer production in 2000 while the rest of Southeast Asia consumed only 12 percent. The total value of U.S. veneer exports increased by approximately 41 percent between 1994 and 2000 and by 84 percent from 1992 to 2000.
Beginning in the 1980s, an important difference between domestic and export veneer markets has been veneer sheet thickness requirements. While 1/32-inch thick veneer remained the standard thickness demanded by domestic buyers, export veneer buyers embraced 1/42-, 1/50-, 1/64-, 1/75- and even 1/100-inch veneer thicknesses. New veneer application and sanding technologies in Europe and Japan enabled super-thin veneer to be used. When slicing thinner veneer, much higher yields can be obtained (measured on a square-foot basis).
A second important production benefit for manufacturers of super-thin veneer is an increase in drying efficiency. Lower temperatures and faster dryer throughput rates reduce energy requirements and bottlenecks at the veneer dryer. It has only been since the late 1990s that thinner veneer (usually 1/42 inch) has become more accepted by U.S. veneer buyers. Veneer quality requirements vary among regional veneer markets.
European, especially German, veneer markets are particularly focused on growth ring and texture consistency. Tighter growth rings are more important in European veneer markets than they are in domestic markets. Japan, which uses super-thin veneer (e.g., 1/100 inch), also demands tight growth rings. Domestic veneer markets usually accept more marks and more figure than do European markets. The same is true for China and other Southeast Asian markets.
Veneer producers and brokers are important sources of information on the quality aspects of veneer. Veneer log buyers and brokers are knowledgeable about the effects that external log attributes will have on veneer quality. Many log buyers spend time in the forest looking at timber sales so they also are sources of anecdotal information on how differences between sites affect veneer log quality attributes. Recognizing the link between the external appearance of logs and the appearance of finished veneer requires an understanding of how trees grow and respond to stresses as well as considerable field experience. Changing market conditions and consumer preferences (e.g., demand for dark versus light-colored wood) also are key factors in the determination of acceptability as a veneer-grade log.
We asked each of the veneer industry experts the following questions: What are the most troublesome defects in each of your major veneer species?Which log defects are hardest to spot/recognize when evaluating the exterior of the log prior to sawing and slicing?Which log defects seem to have the greatest regional variation?
We also observed log grading and, when visiting veneer mills, the rotary cut, half-round, and vertically sliced veneer production processes. On several of these trips we were able to see the veneer being graded and talked with veneer sales personnel.
Veneer Product Requirements Dictate Veneer Log Requirements
Veneer log requirements are extremely variable, both between veneer operations and within a single operation. Variability is seen in the log species distribution, log sizes, and log quality/appearance. The variability is attributable to: product distinctions between veneer market segments (e.g., doors versus furniture), differences in individual customer requirements, regional differences in log supply and veneer demand, changes in market preferences over time, and the inherent variability of the log resource.
Veneer logs exported to international manufacturers must be of a quality that will produce veneer that meets their standards. Log brokers/resellers have log classification systems that address international veneer manufacturing requirements.
For example, in Asia, many companies slice veneer from the outside-in and rotate the log to create eight slicing faces. This type of slicing requires logs that are free of bark-to-bark cracks (cracks that run the full diameter of the log end). Also in Asia there are some companies willing to purchase “shipper logs” — less expensive logs that are color-streaked and contain pin knots. Along with “shipper logs”, “boule logs” also are low-end veneer logs that frequently are shipped to international customers. At times, Italy has been a particularly good market for cherry boule logs from the United States.
Veneer markets that are new, small, unfamiliar, or appear to be temporary may be risky for domestic manufacturers to try to serve. Log sales (rather than veneer sales) to these markets may offer a low-risk export alternative. Veneer manufacturers indicate a strong preference for the highest quality logs since they can be sliced for the highest-end markets. Unfortunately, there are few of these highest quality or “perfect” veneer logs. However, lower quality veneer logs, if bought for the right price, also can produce a good profit margin, provided the veneer mill has a diverse customer base.
Veneer log prices vary based not only on quality but also on anticipated veneer market value, species, and the veneer manufacturer’s veneer procurement methods. At the upper end of the veneer quality and value spectrum is veneer for architectural millwork (e.g., ornate moulding and wainscoting). After architectural millwork, the next most valuable veneers are those produced for the panel (4 by 8 feet room paneling) and door markets.
Other important veneer markets, in descending order by value, are: furniture or dimension products, flooring, stock panels (e.g. hardwood plywood), and backing or substrate panels. Prices paid for architectural veneer may be 2 to 3 times the prices paid for panel veneer and door veneer (which are within 5 percent of each other). Prices for furniture, dimension, and flooring veneer are only 50 to 70 percent the price of panel and door veneer.
Higher quality veneer is used to cover surfaces that will be highly visible and have a large surface area (thus the length of the clear area between blemishes and consistency of appearance are of paramount importance). Highly figured logs and flitches that will produce distinctive veneer usually are sliced for the architectural veneer market.
Terms used to describe some of these figures include: “fiddleback,” “quilted,”“curly,” “ropey,” “tiger stripe,” etc. Key quality requirements for architectural, door, and panel veneers are: large, blemish-free areas; consistent and desirable coloration; and tight growth ring patterns (e.g., more than 8 rings per inch).
The door market tends to have more stringent color requirements than does the panel market. Conversely, the panel market demands greater overall consistency of appearance since panels will be placed side-by-side around a room so that many square feet are visible. Lower quality veneer markets permit smaller, blemish-free clear areas, less color consistency, lower growth-ring density, and occasionally, sound defects/blemishes of various types. Rotary veneer often allows some mineral and sound defects but cannot be produced from logs with seams or multiple frost cracks.
In addition to internal and external log-quality attribute requirements, the higher value veneer markets (architectural, door, and panel) have diameter and length requirements. The range of veneer lengths accepted by the architectural veneer markets is from 8 feet 6 inches to 17 feet. Panel veneer lengths span a similar range, but 8- to 9-foot lengths are usually preferred. Door veneer typically is 7 to 8 feet long. The minimum acceptable length of veneer for the domestic furniture/dimension industry is 30 inches (clipped).
The Relationship Between Slicing Methods and Veneer Characteristics Quality
High-quality veneer logs are typically manufactured into high-value veneer products using either vertical or staylog slicing methods (Table 2). Vertical slicing is performed by passing a log section (or flitch) that is mounted (or chucked) on a table, up and down against a stationary knife. Flat- and quarter-sliced veneer patterns are produced using the vertical slicing method.
Stay-log slicing is performed by passing a log section against a stationary knife in an arc or circular motion rather than a straight line. Rift and half-round veneer patterns are produced using the stay-log slicing method. Horizontal slicing is performed by repetitively feeding a board (rather than a flitch) lengthwise over a stationary knife to produce high-quality veneer, but this slicing method is rarely used and considerably slower than the other production methods. This veneer processing method is referred to as rotary cutting, slicing, or peeling.
There are a few high-value veneer markets that utilize rotary peeled veneer. One such market is veneer for woven baskets. Another is the birds-eye maple market. This is probably the highest value veneer produced in North America and rotary slicing is required to produce the most desirable birdseye figure.Lower quality veneer logs are used for lower value products, such as cores, backing, and flooring.
Veneer Log Quality Evaluation
Appearance-grade veneer logs are evaluated at least two times prior to entering a veneer plant’s sawmill operation. The first evaluation is by the buyer in the field who commits to the price that will be paid for the tree or log. The second evaluation is by the grader in the veneer plant’s log yard who usually is evaluating the log to decide how it should be sawn and sliced.
While the buyer often needs to be skilled in recognizing defects in standing timber, the log grader at the plant has more information (e.g., log ends are visible and ends can be freshly cut to reveal wood character) and time for making his/her quality judgment. However, the log grader must understand fully the veneer log processing options (sawing and slicing) and customer/market requirements to determine the log breakdown approach that will optimize the quality and value of the veneer.
An experienced veneer log grader/buyer who possesses a keen eye and good knowledge of veneer processing and markets is “worth his weight in gold”, or perhaps “worth his weight in cherry veneer.” Proficient log buyers can make amazingly accurate inferences about the internal characteristics of a log since features seen on log surfaces are usually indicators of what can be found on the inside. Some of these features, such as limbs, forks, holes, bulges, bumps, burls, seams, and scars, are obvious; others are very subtle (e.g., bark patterns that indicate overgrown defects). Some of the subtle defect indicators can be missed or inaccurately evaluated by even an experienced grader.
Some veneer log defects are more evident when the bark is on the log (in the winter) while others are easier to spot on bark-free logs. For example, pin knots in cherry and walnut and worm track in ash can be easier to spot when the bark is off the log, but overgrown knots and bird peck are easier to evaluate when the bark is on the log.
External evaluation of logs includes not only bark or outer-wood evaluation, but also evaluation of log ends. Some log defects, such as gum in cherry, sugar streak in maple, and worm holes in several species, are seen only on the freshly cut ends of logs. Log buyers will spray log ends with water to enhance the visibility of the hardest to see defects (e.g., sugar streak and mineral stains/streaks). Growth ring consistency (or texture), density, and wood color also are judged by viewing log ends. The location of the pith (centered or off-centered) and evidence of tension wood (which can buckle and tear in the manufacturing process) are evaluated here too. Finally, the heartwood and sapwood content of logs is assessed by looking at the two log-end cross-sections.
The hardest-to-see veneer log defects, according to the veneer log buyers we spoke with, are bird peck, T-shaped scars, pin knots, ingrown bark (especially in cherry), and insect induced localized defects, such as glassworm in ash and sugar streaks in maple. Most log buyers surveyed indicated that sugar maple is the most difficult species for which to predict veneer log quality.
White oak also was cited by multiple sources as being particularly difficult to evaluate. Red oak, ash, and walnut are considered easy species in which to judge quality. Cherry is easy in many regards except that pin knots and gum pockets are important defects in this species that can be difficult to detect. Also, some cherry logs have extremely flaky bark, which is much more difficult to read.
Logs with excellent size, shape, and clear-wood characteristics sometimes yield poor quality veneer. In white ash, for example, it is common for heartwood to be large on both ends of a log but to be small between the two ends (hourglass shaped). This characteristic creates problems for the log grader since color is critical but uncertain for this species. This tendency for the size and shape of the heartwood to vary dramatically along the length of the tree also is seen in hackberry.
Several Forest Service research publications pictorially document the relationships between difficult-to-evaluate external indicators and their associated internal defects in red oak, white oak, black cherry, black walnut, yellow birch, yellow poplar, and sugar maple. These publications show external defect indicators, such as epicormic branch scars and bird peck, followed by a series of pictures of the wood veneers cut from the same log section, to demonstrate how the defect plays out as veneer slices are taken at greater depths in the log. Defect descriptions accompany the pictures.
Two other publications specifically address defects affecting the quality of appearance grade veneer. These publications are unique in that they discuss and show pictures of log-end defects as well as log face defects. Though research has been conducted to determine if internal defects can be detected using scanning technology while a log is still intact, scanning systems using x-rays, gamma rays, microwaves, nuclear magnetic resonance, or ultrasound are expensive and currently are used almost exclusively in research. Since these technologies are not readily moved from site to site, their most feasible future application would be for sorting/merchandizing logs at a large log yard (e.g., a log broker’s yard). Consequently, tree and log graders/buyers continue to rely on external attributes that distinguish veneer logs from high-grade sawlogs.
Veneer Log Sourcing
While most veneer logs are purchased directly from sawmills, some are obtained from log brokers who can provide sorted and graded logs, and some are bought at the log landings of harvest operations. A few are purchased as standing timber and occasionally veneer logs will be traded between veneer mills. Veneer procurement methods may be quite variable between mills. For instance, some veneer mills buy up to 20 percent of their raw material as standing timber, but many mills buy no standing timber. The veneer manufacturers we spoke with reported procuring between 20 and 90 percent of their logs from sawmills, 5 to 25 percent from concentration yards/log brokers, 0 to 25 percent as standing timber, and 5 to 50 percent from independent loggers (either at the landing or at the mill’s gate).
Other, less important log sources that were cited included veneer company-owned timberland and other veneer companies. A disadvantage of purchasing standing trees is that many of the most important log attributes may not be seen by the veneer log buyer if he/she is not present during the harvesting operation. These attributes include growth rate, wood color, and other log characteristics that can only be judged by looking at log ends.
A problem with purchasing logs from sawmills or concentration yards, however, is that they may have suffered storage degrade in the form of end splits, insect attack, or fungal stain. One advantage of purchasing standing timber is the 100 percent certainty of the source of veneer logs. It can be difficult to determine the origin of logs purchased from sawmills and concentration yards.
Veneer Log Pricing
There is general agreement about the most important veneer log characteristics (discussed in the next section), but there is no broadly accepted system for grading and pricing veneer logs. Each veneer mill establishes grading guidelines based on specific product markets and customers.
A clearly defined log-price structure based on industry-accepted log grade specifications does not suit the veneer industry with its highly variable markets. For unique uses, such as office paneling, irreproducible figure may be highly desirable. Other uses, such as a line of office furniture, may require large quantities with consistent appearance. While no unique grading system exists, knowledge of the effects of log attributes on quality and yield can help determine the prices that veneer producers can afford to pay.
The large range in prices paid, especially for cherry, hard maple, and white oak, reflect the wide range in quality attributes and markets for these species. The degree to which buyers are willing to adjust their quality guidelines and prices depends on market conditions.
In one company’s procurement system, maple and black cherry logs are bought in three grades: A, B, and C. Grade A is the commonly accepted veneer log, whereas grades B and C are accepted only when demand is high, as reflected in the price of grade A logs. Another survey respondent indicated that when a species is in “hot” demand, more log grade classes will be used (e.g., nine log grades for sugar maple).In some cases, bark distortions indicate underlying wood with attractive figure that increases veneer value.
Generalized Veneer Log Quality Requirements
The veneer log quality attributes that are common (but not absolute) among species and uses are:
•free of knots, bark distortions, decay, seams, worm holes, and bird peck on each of the four log faces
•uniformly spaced rings
•free of metal contamination
These generalized quality standards for veneer logs may be more or less important depending on the veneer market segment, species, and manufacturing system. For instance, logs above the butt log are acceptable when large trees with high crowns produce second logs of large diameter that do not have any branches. However, due to pin knots, the closeness to the surface of overgrown knots and smaller bole diameter, the majority of high quality veneer logs are butt logs – from 60 to 98 percent depending on species.
Species that grow straighter with less taper, such as cherry and yellow-poplar, will yield more veneer logs from upper portions of the tree than will white oak, walnut, and hard maple.
Veneer logs must be sliced soon after harvest to minimize discolorations caused by fungal and bacterial infections. This is especially important during the hotter months of the year. It is more important with lighter colored species, such as maple, white oak, birch, and ash, than it is with darker colored woods since color variations in the lighter woods are easier to discern.
Tapered or elliptically shaped logs can be a problem for some veneer products, but can be an asset if veneer with cathedral pattern is desired. Even severe butt taper is usually not a serious problem because it is removed with “butt reducers” during log preparation prior to peeling or slicing. A butt reducer grinds off buttresses and swollen bases and usually is accompanied by debarking. However, grain deviations associated with taper may be a problem if the veneer will be bent for furniture parts or woven into baskets.
Sweep is a defect that reduces the usable volume of a veneer log and affects the grain pattern of the veneer. The altered grain pattern can decrease the grade and value of the veneer. In addition, sometimes the change in grain direction characteristic of sweep causes buckling in flat-sliced or half-round veneers. Logs with significant sweep often will be sawn into three sections before slicing with one of the cuts running from the log’s pith to the concave surface of the log. Crook is similar to sweep but usually is caused by deflection of the main stem caused by a major branch. Thus, in sectioning logs with crook prior to slicing, the position of the branch knot also must be considered.
sawmill - the log is flitched so that the edge of the log sections goes through the pith. By doing this, the pith is contained in the part of the flitch that is not recoverable as veneer (called the backing board). Nonuniform rings (or double texture) are less of a problem in the furniture-grade veneer market since this market segment generally cuts the veneer into smaller face sections than do manufacturers of doors and panels.
Sapwood is desirable in maple, ash, yellow-poplar, hackberry, sycamore and sometimes in birch and hickory, whereas heartwood is desired in walnut and cherry. Sapwood color may be critical, such as in architectural panels and high-value furniture, or it may be less important when used in strips that will be dyed or given clear but darkening stains, such as basket-weaving strips. In white oak, red oak, hickory, and sometimes yellow-poplar, the heartwood-sapwood distinction is less important due to the color consistency between the two wood regions and less intense product color requirements.
Company-Specific Veneer LogQuality Requirements
Grades are related to the particular species characteristics that affect veneer quality. For example, the two highest grades of black oak specify butt logs only, whereas red oak does not impose this restriction for any grade. The highest grades of black oak must be butt logs because of the abundance of limbs in this species; red oak, in contrast, will have fewer buried branches and therefore second logs are acceptable in all grades. As previously discussed, there are no standard grading rules for veneer logs. This illustrates one company’s attempt at providing detailed guidelines for its log buyers based on current markets and customers.
For this company, heartwood size is very important in all grades of maple and ash, important in the highest grade of birch, but unimportant in oak. Because heartwood may be wound initiated in maple, ash, and birch, and is therefore not necessarily centered on the pith, off centered heartwood can affect more of the higher quality outer wood. Therefore, centeredness is important in the company’s highest grade (Prime Plus) of maple and birch.
Heartwood centeredness also is required in Prime Plus red oak, but in this case its importance is as an indicator of the location of the pith within the log. The requirement that the Prime and Prime Plus logs of all species have large diameters reflects the fact that wood near the center of a tree is of lower quality than the outer wood of large stems. However, relatively small diameter (10 inches) maple and birch are acceptable (in the lowest grades) because they are likely to have small heartwood.
Diameter requirements sometimes are eased slightly for logs bought from northern sources. Since northern timber typically grows more slowly, larger diameters are harder to obtain but the smaller diameters are offset by tighter growth rings (a desirable veneer trait) and smaller diameter pith-affected zones (juvenile wood regions).
Only two grades of ash and yellow-poplar are listed. It is common for more grades to be defined for species that are in high demand in multiple-product markets. Small diameter ash is acceptable if enough white wood can be produced from the logs, but yellow-poplar requires somewhat larger diameters to meet yield-per-log requirements.
The company also shared its veneer log pricing guidelines with us. Price ranges for different grades within a species are a good indicator of the variability of the species’ markets. In this instance, Prime Plus hard maple and birch logs commanded a price that was 80 percent higher than Select grade logs of the same species.For red oak and white oak, the price paid for the highest quality veneer logs was only 45 percent higher than for Select grade logs.
Log Buyers’ Observations on RegionalVariations in Veneer Tree Quality
The geographic source of logs frequently affects their quality. Not only is growth rate affected, but other characteristics, such as color and insect damage, vary with soil type, climate, length of growing season, topography, and silvicultural practices. Our survey respondents offered a plethora of anecdotal information on regional variations in veneer tree quality. Many reported that maple and birch from colder climates with slower growth have larger hearts but whiter sapwood.
One buyer reports that hard maple from Maine can be counted on to have both good color and minimal incidence of the blemish known as sugar track whereas hard maple from neighboring states may have good color. Although centered heart and uniformly spaced rings are important for production of sliced veneer with symmetrical appearance, some cutting methods can avoid this requirement.
Species Related Requirements and Characteristics for Furniture Grade and Better Veneer
The information contained in the following section was gleaned from discussions with several veneer log buyers during 2000 and 2001.
In general, the tree must be true white oak (Quercus alba L.), though bur oak and chestnut oak are occasionally accepted (they tend to have more variation in wood color). The minimum small-end diameter is typically in the 14- to 16-inch range and some users only are interested in 9- or 10- foot lengths or multiples thereof. Door length white oak veneer can be cut from 7-foot long logs. Growth rings should be at least 1/8-inch wide and concentric and the wood should have a light, uniform color without mineral stain (mineral is generally not a problem in white oak). Pin knots and bird peck are sometimes a problem. Though knots would ordinarily be unacceptable, at least one company is marketing knotty white oak veneer for use in “character-marked” furniture.
White oak tends to have more taper than red oak. Variations in the width and color of growth rings seem to be less of a problem (due to a more consistent rate of growth) with white oak than for other species such as maple and cherry. White oak from timber stands that have been released (having received an intermediate or thinning cut) will exhibit variable texture however.White oak bark is relatively easy to read for indications of internal defects, such as overgrown knots and bird peck. Approximately 80 to 90 percent of white oak veneer logs are butt logs.
Northern red oak (Quercus rubra L.) produces the best veneer logs, but shumard oak is sometimes acceptable. Because the tree typically has good form, 15 to 30 percent of the veneer logs are second logs. Diameter must be at least 14 inches at the small end. Usable lengths of 8 to 12 feet are typical. Growth rings should be at least 1/8 inch wide (under current market pressures 1/6-inch growth-ring widths are sometimes accepted), and ring width should be uniform around the log. Light colored heartwood is best, however, the heartwood/sapwood distinction is less important for red oak than for some other species. The wood should be relatively free of mineral stain . Defects that are difficult to identify on logs are adventitious buds, insect borer damage, ingrown bark, and deeply buried knots and wounds. In general, red oak is considered an easy-to-read species. Red oak veneer can be prone to buckling.
Minimum log diameter requirements for white ash (Fraxinus americana L.) range from 12 to 14 inches at the small end. Usable lengths must be multiples of 8, 9, or 10 feet for some companies. Other companies accept log lengths ranging from 8 to 12 feet. Color is particularly important in ash since the wound-initiated heartwood is of no value as veneer. Heartwood should be no more than 30 percent of the diameter. Heartwood proportions are difficult to estimate in ash because heartwood diameter is not uniform through the log like it is in most other species.
Even more serious is a feathered or irregular heart where darker heartwood flares into the sapwood. These color irregularities prevent longstrips of clear veneer from being clipped from a sheet. Stress cracks also are important in this species since ash is brittle (brash). The grain must be straight and logs should be free of the insect damage known as glassworm tracks caused by a cambium miner (Phytobia sp.). Defect indicators are relatively easy to read on ash bark, but since glassworm does not cause bark distortions, some log buyers rate ash as one of the most difficult species to evaluate.
In addition to sliced and half-round veneer for architectural, panel, door, and furniture manufacture, sugar maple (Acer saccharum Marsh.) also is peeled for basket weaving strips, plywood flooring, or specialty items such as laminated skateboards and handles. Thus, there is a very large range in acceptable log quality attributes. The highest grades require butt logs that are at least 13 inches in diameter at the small end; only 2 to 5 percent of the sliced or half-round production comes from second logs. Log lengths for sliced or half-round production must be in multiples of 8 to 12 feet to satisfy the high-end veneer markets (architectural paneling and doors). Like ash, maple heartwood is wound-initiated and of limited value as veneer.
The sapwood exhibits a range in color, from the most valuable pure white (required for architectural grade veneer) to various shades of yellow. High-value logs usually are sliced using the half-round pattern on the stay-log slicer in order to isolate the heartwood, which must be less than one-third of the log diameter. Sapwood thickness must be at least 5 inches. Veneer log buyers believe that regional differences in the color of sugar maple sapwood may be attributed to variations in the sugar content of the sap. The less highly demanded creamy yellow color becomes obvious after the logs are “cooked” in hot water vats prior to slicing.
Trees from northern parts of the species’ range usually have less sugar than those from farther south. The terms sugar streak, sugar fleck, sugar track, and worm track all refer to what is more generally referred to as pith fleck, which is caused by a cambium miner. This insect-induced defect can be a serious problem, especially since it is often undetectable until after the veneer is dried. It can sometimes be detected in logs by cutting a 3-inch disk from a log end and splitting the disk tangentially to reveal the insect tracks. Other hidden defects include ingrown bark, bird peck, T-shaped scars, and the grain deviations known as flares and bars.
Moisture pockets also can be difficult to detect in sugar maple logs. They are more common in trees from the southern part of the range and cause the veneer to buckle upon drying. Growth rate must be uniform (even textured), with an optimum growth rate of six to eight rings per inch. Growth must be uniform around the circumference of the log, since nonuniform growth indicates the presence of tension wood that will cause splitting or buckling of the veneer.
Buckling also can be caused by cross-grain and even minor deviations. For this reason, grain deviations are particularly troublesome when they occur in raw material to be used in basket-weaving strips. Because there is often little contrast between wood color, growth rings, and defects, it is necessary to make fresh cuts and to wet log ends to evaluate a log. Defect indicators are more difficult to read on the bark of sugar maple compared to many other species.
Some veneer log buyers stated that white birch (Betula papyrifera Marsh.) is worth more than black birch (Betula lenta L.) or yellow birch (Betula alleghaniensis Britton). Others indicate that white birch is more valuable than darker or more variably colored birch, but they do not distinguish between species in their log pricing. Birch is frequently rotary cut, sometimes as short bolts for the production of specialty items. Birch heartwood is wound-initiated, but is less of a defect than in ash or hard maple.
Nevertheless, the best veneer logs contain less than one-third heartwood. Recently, a market has developed for “natural color” birch veneer. Veneer mills (typically rotary mills) cutting this grade of veneer buy logs containing approximately 50 percent heartwood. Worm track is a defect that is more common in white birch than in black or yellow birch. One company, however, has successfully marketed specialty products that include worm track as “character.”
Black cherry (Prunus serotina Ehrh.) is probably the most valuable domestic veneer species and most cherry veneer production is in sliced veneer. Minimum log diameter specifications (small end, inside bark) for veneer quality cherry logs range between 14 and 17 inches for the companies we surveyed. Log lengths for cherry must be at least 9 feet for panel grade and up to 16 feet for architectural grade. Second logs comprise 10 to 20 percent of the supply of top-grade veneer logs. Color must be uniform since variable color is a defect in this species.
Fast-grown cherry is thought to have more within-tree color variation. The preferred color for cherry veneer is pink rather than olive or dark red. Bands of color, referred to as target when seen on the log ends,are serious defects.Other serious veneer-log defects in black cherry are the presence of pin knots (which often occur in clusters),ingrown bark, and bird peck. The most notorious defects in black cherry veneer logs are gum spots and gum rings, which are caused predominantly by bark and engraver beetles (perhaps the peach bark beetle Phloeotribus liminaris)
When gum is heavy, it is readily visible on log ends, but pin knots, ingrown bark, and bird peck may be difficult to find. Heavy ingrown bark can reduce veneer grade, whereas mild ingrown bark (known as rice pattern) is a less serious defect. Bird peck, if recent, may be detectable on log surfaces. The width of the annual growth rings is not as critical in cherry as in maple.
Cherry is considered a difficult species to buy because so many of the more critical cherry defects are ones that may only be detected on the freshly cut log ends. Cherry logs with flaky bark are more difficult to read for defects than are cherry logs with tighter scaly bark.
Although walnut (Juglans nigra L.) used to be the most valuable North American veneer species, this is no longer the case. The highest quality walnut comes from butt logs; only about 1 to 7 percent of the sliced production comes from second logs. Defects that are sometimes difficult to identify include overgrown knots, pin knots, worm holes, and bird peck. Sapwood is of no value, and wide sapwood is often an unwanted consequence of the vigorous growth that usually produces desirable characteristics (e.g., greater wood volume).
Uneven color (streaks) greatly reduces the value of walnut veneer; color problems may be the result of poor growing site or insect attack. There are large regional variations in walnut color. Walnut veneer logs must have at least 10 inches of dark heartwood. Moderate growth rate (eight rings per inch) is better than slow or fast growth; slow growth indicates lack of vigor, and fast growth results in wide sapwood and coarse texture.
Walnut slices easily but is typically soaked in the steam vat two to three times longer than most other species to reduce heartwood color variability. Defect indicators are comparatively easy to read on walnut bark.
Because yellow-poplar (Liriodendron tulipifera L.) veneer is usually used as core stock, the standards by which the species is judged are not as rigorous. The primary defect is dark colored (blue-stained) heartwood because color can show through thin face veneers if these are of light colored species. Some veneer manufacturers specify that heartwood should be less than one-third of the log diameter. There is some demand for yellow-poplar whitewood.
Other color problems, such as dark “tobacco-stain”, are common and can seriously devalue veneer, especially when the tobacco goes the whole length of a log. Mineral stain is an important characteristic in yellow-poplar logs selected for slicing. Other important defects are related to grain deviation, since these can cause warping. Yellow-poplar bark is relatively easy to evaluate for the presence of underlying defects.
Summary and Conclusions
Based on knowledge we gained from a series of visits with veneer log procurement and production personnel, it is evident that subtle appearance factors are critically important determinants of veneer value in both domestic and foreign high-end veneer markets (architectural, door, and panel). Therefore, these subtle factors are of keen importance to log buyers/graders when purchasing and sorting/selecting logs for processing to supply these highend markets. The basic color (shade) of the wood is important for all species, but especially so for maple, walnut, ash, and white oak in today’s veneer markets.
Minor color variations and blemishes are considered defects in these three high-end veneer market sectors since these sectors require large veneer sections of consistent appearance. Thus the color variations caused by insect pests are a concern. These include gum pockets and rings in cherry, sugar flecks in maple, and glassworm in ash. Mineral streaks in oak and maple also are important since they, too, cause color variations that are defects in high-grade veneer.
Next to color consistency,grain pattern consistency is almost as important. The texture of the growth-ring pattern must be uniform (i.e., consistent rate of growth) and the grain must be relatively straight. Veneer having wild grain or figure can command a high price for some species in the architectural woodworking market only if it is consistently wild along the length and throughout the thickness of the veneer flitch.