**1. Introduction**

Wood that is exposed outdoors is subject to surface degradation by solar radiation (U.V. and visible light), water, heat, environmental pollutants, and mould [1]. The most obvious manifestation of such degradation is the greying of wood exposed outdoors [2]. Another highly visible effect of "weathering" is the surface cracking (checking) of wood [3]. Checks develop when moisture-induced surface stresses exceed the tensile strength of wood perpendicular to the grain [4]. The severity of checking depends, in part, on the orientation of growth rings to wood surfaces exposed to the weather. For example, checking is less severe in coniferous wood (softwood) whose growth ring are perpendicular to the exposed surface (radial or quarter-sawn boards) when compared to wood whose growth rings are mainly parallel to the surface (tangential or flat-sawn boards) [5]. Checks in quarter-sawn softwoods often develop at the interface between denser bands of summer wood (latewood) and lower density springwood (earlywood) [6]. In flat-sawn softwoods, checks develop in and propagate radially into wood via rays [4]. Flat-sawn wooden boards can be classified into ones where growth rings either curve away from, or curve towards, their upper wide surface [7] (Figure 1). These boards are commonly known as bark-side-up and pith-side-up boards, respectively [7] (Figure 1). The checking of wood during exposure to the weather is less pronounced in wooden boards that are oriented pith-side-up [5,8].

**Figure 1.** Orientation of growth rings to the upper exposed surface of flat-sawn (tangential) western larch deckboards. The darker bands within growth rings are latewood and the lighter bands are earlywood: (**a**) flat board with growth rings that curve towards the exposed upper surface. These are known as pith-side-up boards. Growth rings can make shallow (α) or high (β) angles to the exposed upper surface; (**b**) flat board with growth rings that curve away from the exposed upper surface. These are known as bark-side-up boards; (**c**) profiled pith-side-up board; and, (**d**) profiled bark-side-up board.

In addition to checking, another less commonly known defect, shelling or loosened grain, develops at wood surfaces, especially those that are exposed to the weather [7,9–11]. Shelling occurs when mechanical or moisture-induced stresses at the surface of flat-sawn boards are sufficiently large to cause delamination of growth rings at the boundary between latewood and earlywood [9,10]. Such delamination of growth rings results in the projection of latewood above the surface of adjacent earlywood. Two forms of shelling have been observed at flat-sawn (tangential) surfaces that result from either the separation of tips or edges of latewood from earlywood, respectively [9,10]. Shelling mainly develops at the surface of boards oriented pith-side-up. It can also occur beneath opaque coatings and disrupt paint films on siding (cladding), and this is one of the reasons why wooden siding is fixed to building with growth rings oriented bark-side facing outwards [10,11]. Shelling, particularly shelled tips of latewood, can create sharp splinters that protrude from the surface of wood. These splinters are objectionable and dangerous according to Koehler [9], particularly when they occur at the surface of deckboards that people walk on. Shelling also makes refinishing and cleaning of wood surfaces difficult [9,10]. Shelling is most pronounced in softwoods with a pronounced difference in cell wall thickness between latewood and earlywood, for example, species, such as southern pine (*Pinus* spp.), larch (*Larix* spp.), and Douglas fir (*Pseudotsuga menziesii* (Mirb.) Franco) [9,10,12], but it has also been observed in hardwood with relatively uniform structure, such as yellow poplar (*Liriodendron tulipfera* L.) [10]. These wood species are very important commercially. Furthermore, the wood products that are negatively affected by shelling such as decking and siding are especially important. For example, the market for wooden decking and siding in the USA had a combined value in 2018 of ~US 2.8 billion, and each year over 300 million square feet (~28 million m2) of wood siding is installed on houses in the USA [13,14]. However, wooden siding and decking are losing market share, because they require more maintenance particularly refinishing and cleaning than products made from alternative materials, such as plastic and composites (wood-plastic and wood-fibre cement composites) [13,14]. Hence, there is a need to better understand and develop solutions to problems such as shelling that reduce coating durability and increase the frequency of refinishing of wood products used for siding and decking.

Our understanding of shelling is based on a small number of studies [7,9,10], but, unlike grain raising, which was until recently a similarly neglected surface phenomenon that affects the performance of coatings on wood, no method has been developed to quantify shelling at weathered wood surfaces. Grain raising was first quantified using stylus profilometry [15], and recent studies have used non-contact profilometry to quantify and image grain raising [16,17], and examine the shape recovery of earlywood and latewood beneath machined and painted radiata pine (*Pinus radiata* D. Don) panels soaked in water [18,19]. The latter research suggests that the current generation of non-contact profilometers will be able to quantify shelling at weathered wood surfaces and provide insights into its origins. We test this hypothesis in this paper. We use confocal profilometry and macro-photography to

quantify and image shelling of flat-faced western larch (*Larix occidentalis* Nutt.) deckboards (hereafter called flat deckboards) exposed to natural weathering for seven months. Profilometry and scanning electron microscopy were also used to image shelling of flat alkaline copper quaternary (ACQ) treated Douglas fir, western hemlock (*Tsuga heterophylla* (Raf.) Sarg.), and white spruce (*Picea glauca* (Moench) Voss) deckboards exposed to the weather for 18 months. We extend our observations of shelling of flat deckboards to include profiled-faced larch deckboards (hereafter called profiled deckboards) that were exposed to the weather because there are no reports in the literature on the shelling of profiled deckboards, even though profiling is widely used in Australia, Europe, Japan, and New Zealand to reduce the negative effects of checking on the appearance of deckboards exposed outdoors [20,21].

#### **2. Materials and Methods**

#### *2.1. Preparation and Weathering of Larch Deckboards*

Six flat-sawn, J-grade, kiln-dried, and dressed western larch boards cut from trees growing in the interior of British Columbia, BC, Canada, and measuring 38 × 140 × 3658 mm3, were donated by Tolko's Lavington Planer Mill Ltd. (6200 Jeffers Drive, Lavington, British Columbia, BC, Canada). Each parent board was cross-cut using a pendulum saw (Stromab ps 50/f, Campagnola Emilia, Italy) to produce six samples, each 600 mm in length. Samples were then randomly assigned to one of five deckboard profiles or the flat control, Table 1. Sample boards were then profiled on both sides using a moulding machine (Weinig Powermat 700, Michael Weinig Inc., Mooresville, NC, USA) equipped with a 125 mm diameter, two-wing, cylindrical rotary cutter head (Great-Loc SG Positive Clamping Universal Tool System, Great Lakes Custom Tool Mfg. Inc., Pestigo, WI, USA). After double-sided profiling, each sample was cut into two using a pendulum saw, as above. The sub-samples were oriented either pith-side-up or bark-side-up during the subsequent natural weathering trial. A total of seventy-two deckboard samples each with a final dimension of 32 × 130 × 292 mm<sup>3</sup> were manufactured. The ends of deckboard samples were coated with an epoxy sealer (Intergard® 740, International Paint Singapore Pty Ltd., Jurong, Singapore) to prevent end checking of boards during conditioning and weathering. The decking samples were stored in a conditioning room at 20 ± 1 ◦C and 65 ± 5% r.h. for at least three months before the outdoor weathering trial.


**Table 1.** Geometry of profiles in western larch deckboards.

1 Number of peaks per 150 mm was 20 for all profiles except flat samples.

Profiled and flat deckboard sub-samples from the same parent western larch board (board 1, 2, 3, 4, 5, or 6) were screwed onto separate sub-frames made from preservative-treated 2 × 4-dimensional lumber. Each deckboard was spaced 10 mm apart from adjacent boards and fastened at its four corners onto the weathering rack using hidden galvanized screws (CAMO® fastening system, National Nail Corp. Grand Rapids, MI, USA, http://www.camofasteners.com/). A total of six mini-decks were constructed, each measuring 30 cm long, 177 cm wide, and 61 cm high. In addition, end boards made from preservative-treated lumber measuring ~40 × 90 × 300 mm<sup>3</sup> were screwed on to the two ends of each rack to prevent the edges of samples at the ends of racks from weathering. The test decks were then placed outdoors in Vancouver for seven months from March to October 2019. The deckboard samples were removed from racks and stored in a conditioning room at 20 ± 1 ◦C and 65 ± 5% r.h. for two weeks before profilometry measurements. After the weathering trial, samples measuring 15 × 15 × 38 mm<sup>3</sup> were cut from each of the six parent western larch boards. The basic density of these samples was calculated using their oven dry weight (obtained by oven drying samples at 105 ◦C to a constant weight) divided by their water-saturated volume (obtained by Archimedean displacement).

#### *2.2. Measurement and Imaging of Shelling of Weathered Deckboards*

The shelling of the flat western larch deckboard samples was characterized using a non-contact surface confocal profilometer equipped with a 3 mm probe (Altisurf 500 ®, Altimet, 298 Allée du Larry, 74200, Marin, France) [22]. The profilometer measured the heights of each deckboard sample along a 100 mm line at three locations; each line was o ffset approximately 16 mm from the edges of samples. Height data were extracted from line scans using the software ProfilmOnline (Filmetrics, KLA Co., San Diego, CA, USA). Heights of shelled growth rings were measured from the tip of the latewood projecting from the wood surface to the adjacent earlywood. In addition, a 625 mm<sup>2</sup> area on samples was scanned using the confocal profilometer and reconstructed using Altimet Premium (version 6.2) to provide three-dimensional (3D) images of growth rings. Profilometry was also used to image shelling of latewood tips occurring on the peaks of profiled western larch deckboard samples. The number of detached or raised latewood tips at the surface of profiled deckboard samples was counted, and the surfaces of flat and profiled boards were photographed using a Canon EOS 80D digital single lens reflex camera equipped with a Canon EF-S 35 mm f/2.8 macro lens. End-grain of flat deckboards with shelling were scanned using a desktop scanner (Hewlett-Packard O ffice Jet 6700, Palo Alto, CA, USA) at 600 dpi resolution, which was su fficient for subsequent image analysis. Scanned images were then examined with the image analysis software ImageJ (version 1.51q) (https://imagej.nih.gov/ij/) to measure growth ring angle to the surface of the corresponding shelled growth ring (Figure 1a). The number of growth rings per cm on the end grain of deckboard samples was measured using a transparent plastic ruler.

We also imaged shelling in flat Douglas fir, western hemlock, and white spruce deckboards that had been treated with a 1.8% alkaline copper quaternary (ACQ) preservative and exposed to natural weathering for 18 months. These boards were part of an experimental trial that examined e ffects of three factors, profile type, wood species (Douglas fir, western hemlock and white spruce) and growth ring orientation (boards with growth rings oriented either bark-side-up or pith-side-up) on the cupping and checking of deckboards during weathering. The materials and methods that were used to prepare and weather these deckboards are described in detail in a recent publication [8]. Flat weathered deckboards with growth rings oriented pith-side-up were selected, and shelled growth rings were imaged using confocal profilometry and macro-photography, as described above.

Scanning electron microscopy was used to examine the interface between latewood and earlywood in shelled growth rings in flat Douglas fir, western hemlock and white spruce deckboards. Samples measuring 15 × 15 × 15 mm<sup>3</sup> and each containing a shelled growth ring were cut from deckboard samples using a small razor saw (Lee Valley Tools Ltd., Vancouver, BC, Canada, ultra-thin razor saw 60F0310). Samples were dried over silica gel at 20 ± 1 ◦C for 24 h and reduced in size to ~5 × 5 × 8 mm<sup>3</sup> using single-edged razor blades (gem surgical carbon steel blades, American Safety Razor Co. Cedar Knolls, NJ, USA) [23]. Samples with either transverse or tangential (weathered) surfaces facing uppermost were glued to separate aluminium stubs using Nylon nail polish as an adhesive. The stubs were sputter coated with an 8 nm layer of gold and they were then examined using a Zeiss Ultra Plus field emission scanning electron microscope (Carl Zeiss AG, Oberkochen, Germany) at an accelerating voltage of 5 kV, working distances of 14.8 to 15.9 mm, and a high vacuum (1.3 × 10−<sup>4</sup> Pa). Secondary electron images of samples were obtained and saved as TIF files.

#### *2.3. Experimental Design and Statistical Analysis of Data*

Our experiment was designed to examine the e ffects of surface profiling and growth ring orientation on the checking and cupping of western larch deckboards that were exposed to natural weathering. The experiment was a randomized block design with two fixed factors: (a) Boards with different surface profiles, as defined and described previously, Table 1 [24], including three rib profiles (rib, short rib, and tall rib), and two wavy profiles (ribble and ripple): flat boards acted as a control; (b) growth ring orientation (concave, pith-side-up; and convex, bark-side-up). Six replicate blocks of twelve samples (6 profiles × 2 orientations × 6 replicates = 72 samples) were prepared and exposed to natural weathering. Linear regression was used to examine the relationship between growth ring angle and height (mm) of shelled growth rings in flat western larch boards oriented pith-side-up. Analysis of variance was used to examine the effects of growth ring orientation and profile type on numbers of shelled growth rings in profiled boards. The latter count data was transformed (square root) before final analysis. Statistical computation used the software, Genstat (v. 19). The results are presented graphically and Fisher's least significant difference (LSD) bar (*p* < 0.05) can be used to determine whether differences between individual means are statistically significant [25].
