Design of Decorative Wooden Wall Panels from Sliced Pedunculate Slavonian Oak (Quercus robur L.) from Veneer Production Residue

Abstract

The growing awareness of nature conservation encourages producers, engineers, and designers to contribute to the rational and innovative use of raw materials. Sustainable development is imperative, symbolizing the balance between meeting the needs of the present generations without compromising the needs of future ones. The aim of this paper is to investigate the production process of cut veneer of Pedunculate Slavonian oak (Quercus robur L.) and which parts of the logs are lost or thrown in the production of cut veneer, to analyze the increase in the use of veneers that currently represent residue in processing, and to propose directions of designing a new product from the analyzed residue—decorative wall panels. The research was conducted in the company Spačva Ltd. in Vinkovci, Vukovar-Srijem County, Croatia, in 2021/2022. The findings emphasize that the vast majority of unused veneers, considered technologically unacceptable in the production process, can be decoratively and visually desirable due to their natural appearance in the design of a new product. By analyzing and applying the design principles and elements, natural phenomena were applied to the veneers that represent the basic motif of the new decorative wall panels. The initial solutions of the decorative wall coverings made of veneer with a natural appearance and the solution from other wood residues from production are shown. The results of this study have potentially far-reaching implications for wood product manufacturers and demonstrate the importance of applying design in wood technology to reduce residue, demonstrating that residue from standard production can be designed into an innovative, sustainable product that reduces environmental damage.

1. Introduction

Wood is a natural material that has the ability to store CO₂ and, thus, reduces the negative impact on the environment [1,2]. The production of wood products has a significant positive impact on CO₂ emissions, so better utilization of wood raw materials should be strived for in order to reduce CO₂ emissions in the future. Therefore, it is necessary to use wood and wood-based residue more rationally in the production and design of new products for sustainable buildings and to minimize wood residue for heating use [3].

Oak has had a great and important cultural role among Europeans, but also among Croats, since ancient times. Valuable products and goods were made from the tree, the wood was used for heating and construction, acorns were used as food for cattle, and bark for tanning leather. From the Greeks to the Germans, Slavs and Celts, oak has a sacred status, so it is no wonder that it often appears on national and regional symbols [4]. Until this year, when Croatia accepted the Euro as its currency, the oak tree was also on the Croatian national coinage. Both Quercus robur and Quercus petraea are among the most economically important deciduous wood species in Europe as they are characterized as high-quality wood for construction and furniture production [5]. Quercus Robur is particularly valued for its flat grain and durability due to its strength and high tannin content, which also make it more resistant to insect and fungus attacks [6,7]. This paper is based on the research of the visual features of the unique Slavonian Oak from the largest European integrated white oak forest (Quercus robur slavonica), located in the area between the Danube and Sava rivers in eastern Croatia. Compared to autochthonous oak forest stand, Slavonian oak is characterized by its relatively low seed production and late budding, but also its accelerated growth, stem straightness, and a long clear bole. As a result of its exceptional quality, it was brought to Germany in the second half of the 19th century [8], and other authors state that the seeds of the Slavonian oak were spread throughout the Austro-Hungarian Monarchy [9,10], and that afforestation was also carried out in the Czech Republic and Slovakia [11]. Therefore, Quercus robur slavonica is the pride of Croatian forestry and a valuable raw material that must be used wisely in order to obtain high-value products.

Wood is one of the oldest building materials, and its use continues to increase even today as a construction and decorative material. Considering its environmental impact, the goal is to have greater utilization of raw wood, and new wood-based materials are continuously being developed and introduced to the market [12,13]. The long-term trend of wood utilization depends on the changes in the resource itself. Whilst the diameters of logs are decreasing, the prices are increasing, which forces the industry to use the raw material as best as possible. Manufacturing processes in which wood is broken into smaller pieces or fibers are much more adaptable to the continuous flow in production, standardization, automation, and large-scale operations. Such production processes are characterized by a better utilization of raw materials and lower labor costs, and the natural progression of wood is formed in boards made of particle wood [14,15]. Veneer production technology is generally divided into slicing, peeling and sawing. Slicing and peeling veneers are cut with a knife, and sawing veneers are cut with a saw blade [16]. Peeled veneers are produced from cheaper and softer diffuse porous wood species (poplar, linden, birch, beech) for the production of veneer panels. Due to the production technology and the cutting angle, vertical cracks (known as lathe checks) are caused, which makes the surface of peeled veneers rough, and they are not as decorative as cut veneers [17,18].

Veneer logs are classified on the market as goods of the best quality, and their quality exceeds sawmill logs and peeling logs [19]. These are logs with a minimum number of visible defects, with large scope, and there are fewer of them in nature because their growth lasts for over 100 years. Their procurement is not easy and often their purchase takes place by the method of the best bidder, i.e., by auction [20]. Nowadays, driven by the current crisis, oak prices are reaching increasing numbers [21,22]. There is a great need for the rational use of this rare and expensive raw material.

The role of sliced veneer is decorative, and gluing veneer to a substrate (most common wood-based board) gives a natural look to the board [18]. This makes production cheaper compared to the use of solid wood. Therefore, the goal in production is to obtain as thin a veneer sheet as possible in order to make greater use of expensive logs.

This paper deals with the study of oak veneer production and the determination of wood residue, as well as the use of that residue in the design of new innovative and aesthetically valuable products—wooden wall panels—in order to design healthy and sustainable interiors and increase the market competitiveness of the observed company, Spačva Ltd., Vinkovci, Vukovar-Srijem County, Croatia. The Vukovar-Srijem County is the easternmost Croatian county, where the forests of the Pedunculate Oak (Quercus robur L.), popularly known as the “Slavonian oak”, are particularly famous. Slavonian oak grows in the Spačvan basin in two protected forest areas. In recent years, we have witnessed a great demand for Croatian wood from the Spačvan basin, which has the status of one of the most valued raw materials in the world [23].

1.1. Cut Veneer Production Process

Cut veneer for decorative purposes is most often produced using the crown/flat cut, quarter cut and rift cut techniques (Figure 1).

Crown/flat cut is a widely used technique in which a log is cut in half. The veneer is obtained by moving the half towards the fixed knife, tangentially to the rings of growth and parallel to the line passing through the center of the log. A veneer with a strongly irregular texture is obtained, the growth rings of which resemble the outlines of the cathedral [16,18,24].

Quarter cut is the same cutting method as crown/flat cut, but only uses one-quarter of the log. Veneer is produced by cutting knives at right angles. A regular veneer texture with a linear arrangement of growth rings is obtained [16,18,24].

Rift cut is the technique specially designed for the rotary cutting of oak wood for greater efficiency, where the blade enters at a slight angle of 15 degrees to the radius of the log (Figure 1 and Figure 2). This technique produces a flat striped wood texture and avoids ray fleck, which are actually medullary ray cells [18,24].

During the production of crown cut and quarter cut veneer, the back board remains (Figure 1). The back board left from the crown cut and quarter cut techniques is usually 7/8 inch thick [19]. When the board is of that thickness, any further cutting is stopped because it can be further commercially used. During the production of veneers using the rift cut technique, the residue is a back board, a semicircular board that is partially used andand is the subject of further research in terms of maximum raw material utilization.

The production process of cut veneer begins with sawing logs into quarters or halves, followed by cooking, cutting, drying, clipping, packaging and grading [18]. The classification is conducted by trained workers who, often with additional supervision, classify and determine the places of clipping and extracting errors within the bundle. Thus, it is immediately determined in which aesthetic and dimensional class the individual veneer bundle will be placed. The aim of such a process is to obtain the highest possible class and value from the obtained veneer by trimming out the defects. There is a moment in the process when unacceptable wood defects in the veneer are separated from the valuable raw materials. The utilization in the production of cut veneer in the year 2022 was 42%, or 11.25 million m² (Official information from the company Spačva Ltd. Vinkovci, Vukovar-Srijem County, Croatia). The usable residue is 18.5%, or 5 million m², which is a large and valuable amount [25]. It is important to note that this amount of residue in the veneer production are veneers of appropriate dimensions, which nevertheless end up as an unacceptable production material.

These cut veneers and the back boards from cut veneers are the area of more detailed research in this paper. All the residues, with the exception of sawdust, were observed and analyzed, and new innovative ideas for wood wall panel designs based on aesthetical principles of design are given.

1.2. Aesthetic Principles and Product Design

Designing a new product derives from the development process that consists of constantly thinking and questioning every decision [26]. In this process, the creation of a new image of a wall panel was guided by the design methodology using visual (design) elements and principles.

Design elements are ubiquitous and are part of every object around us. They are fundamental parts or aspects embedded in the visual structure [27]. These are the building blocks needed to make a certain design [27,28]. Design elements are essential components in art and design when constructing a visual image. In addition, each element is a key part of the visual message that has an impact on the design perception. It can be used alone or in combination, depending on the effect we want to achieve. In art and design, the main elements of design are line, shape, form, color, value, texture and space [27,28].

Design elements can be formed in any way that then form a composition, and these compositions are called design principles [27,28]. Design principles describe the ways in which they can be applied to the layout and structure of a new visual design. The ways in which they are applied affect the end result and the overall message and perception that we perceive in that observing experience. It is important to understand the principles and know how they can work, support, reinforce, or even oppose, each other in creating the desired effect. In addition, not every product has to contain all the principles of design. The correct mutual combination is enough to obtain an aesthetically pleasing and functional new design [28]. In art and design, the main principles of design are balance, movement, rhythm, contrast, emphasis, pattern and unity [27,28].

Design elements and principles are often used for the design analysis and evaluation of a product [29]. Knowledge of how to use design elements and principles is the basic principle of creating a “good designed” structure (products).

1.3. The Aim of the Paper

The aim of this paper is to analyze the production processes in the company Spačva Ltd. in Vinkovci, Vukovar-Srijem County, Croatia, and observe what is left as residue, how it is produced and how to turn residue back into valuable raw material from which a new eco-friendly product could be created. This paper is written as part of a research and development project in which the goal of residue veneer is to create a new aesthetic design solution for innovative wall panels in order to increase the utilization of veneer logs of highly valued Pedunculate Slavonian oak (Quercus robur L) from the Spačvan basin in Vukovar-Srijem County. The idea is to find a new purpose for each residue element and to identify the possibilities and variants that can be used to design a new decorative wall panel by using design elements and principles. In this paper, all wooden remains are considered, whether they are discarded veneers or the remains after cutting the veneer (back panels). Various possibilities of creating a panel by stringing elements, gluing, stacking, folding or pressing are also taken into account, in order to place applicable, decorative and exclusive products.

2. Materials and Methods

2.1. Polygon Research

The research was conducted in the company Spačva Ltd. [30] in Vinkovci, Vukovar-Srijem County, Croatia, in the period between December 2021 and April 2022. The company is well-known for the production of floor coverings, panels, doors and veneers, mostly from the high-quality Pedunculate Slavonian oak (Quercus robur L.). In Spačva Ltd., an oak veneer is produced using veneer log cutting technology.

2.2. Materials

The oak logs come from the famous forests of the Spačva basin in Vukovar-srijem County, eastern Croatia, from which the company Spačva Ltd. takes its name. Logs are usually 4–5 m long and the most common diameter is 60 cm. First, they are sawn into quarters or halves, depending on which veneer cutting technique will be used to produce the veneer. After that, the halves and quarters go to the steam chambers, where the oak is cooked so that it can be cut more easily and obtain a smooth cut and, thus, the perfect processing quality [17,18]. Veneers obtained by the method of cutting (rift cut, crown cut and quarter cut) were used as the materials in this research. The produced cut veneer is 0.52 mm thick. All of the veneer and other wood residue from the veneer production were also analyzed, such as the back boards from the rift cut technique, which has an interesting shape and great potential in the design of a new product.

As previously mentioned, the utilization of raw materials in the veneer product is close to 42% [25]. This is about 11.25 million m² of veneer per year. The remaining 60% refers to back board, bark, short clippings, but also veneers that were rejected due to technical errors, too small dimensions and aesthetic properties. A large amount of the discarded material is sapwood. This realization set a new requirement for what would happen if the material was used only by birch: the utilization would increase by at least 5%, which is an annual amount of 1.2 million m². Therefore, sapwood exploitation was included as research material. These are significant quantities that have great aesthetic and, later, commercial potential.

When designing the initial material (wall panel, which can be achieved in many different ways and with different techniques), the focus was on obtaining a two-dimensional surface, and the easiest and most common method in the factory is to use a press. For the mold, 3 mm thick and 10 cm wide slats were used, which were placed in a press to obtain a relief. For the process of making the decorative panel, veneers with a thickness of 0.52 mm were used, which were folded into 5 layers, between which glue was applied. To prevent the adhesive from sticking to the wooden slats (mould) and the press, greaseproof paper was placed on the veneer surfaces as protection.

2.3. Research Methods

During the research on the polygon, methods of observing and photographing the production processes and new product design processes were used.

In order to collect information about the production processes and the rational use of wood residues, the company’s official data on the use of raw materials for the year 2022 were used [25].

When creating new products, knowledge of design elements and principles was applied in order to make aesthetically and visually harmonious innovative conceptual solutions and obtain so-called “good design” structures. All of the obtained solutions were photographed and analyzed in terms of the application of natural appearances that are considered as defects and unacceptable material in the production. The individual production processes were photographed, and the measured samples and individual wood (both veneer and massive) residue were systematically classified. The 3D visualization was made in Fusion 360—Autodesk software, and all photo processing was conducted in Adobe Photoshop.

3. Results

The obtained research results are divided into two main parts: (Section 3.1) the results of the analysis of the production process and design of the veneer residue from the production of cut veneer and (Section 3.2) the results of the analysis and design of the back board from the rift cut production technique. The visual effects that appeared on the veneers as natural phenomena were analyzed and the corresponding elements and principles were associated. This approach in the analysis of the appearance through elements and principles made it possible to understand the principles of harmony and “good design” on the obtained newly designed samples of panels.

3.1. Results of Analyzis and Design of Veneer Residue from the Production of Cut Veneer

3.1.1. Examples of Decorative Wall Coverings from Veneer Residue Joined by Sewing

As the observed research polygon produces a veneer at 0.52 mm thickness, all the veneer and veneer residue have an identical thickness. A large proportion of the residue consisted of veneers that had an unacceptable natural appearance, but from the technological point of view, do not pose a problem. These veneers can be used for decorative purposes, as wood panels, and as such they are analyzed and collected. The veneers are interconnected in a slip match way and in the vertical direction of the wood grain. The successive stringing of the sheets creates a repetitive texture. This way of joining veneer evens out the visual image of the wood because it creates a great contrast of color in the line where the veneer is joined, which is most often the case with these collected veneers with a natural appearance. The slip match veneer stacking method is mostly used for veneers produced with the quarter cut and rift cut techniques.

So-called book match and slip match are the most common ways of stacking veneers used in the production of panels, doors and large furniture products [19,31,32].

The selection between veneers to be used further in the production and the ones that are perceived as residue is based on their dimensions (maximum length or width), as well as technical and visual errors (defects). Wood residue is therefore classified into two dimensional categories (veneers 0–80 cm long and veneers 0–9 cm wide) and one category for the veneers with errors (defects). All of the veneers are shown in dimensions 90 × 70 cm.

I.

Veneers 0–80 cm long

These veneers do not necessarily contain defects, but are residue due to their size. In the examples are veneers with visual defects to show the natural appearance and visual effect they create. They are formed in a longitudinal clipping veneer bundle. All of the veneers are residue, regardless of the width, which can go up to 50 cm, and which are shorter than 80 cm. Veneers of such dimensions are used only in the AA and A quality classes. Veneers in other classes are residue unless there is a special order for them.

II.

Veneers 0–9 cm wide

These veneers also may or may not contain defects and, as in the previously mentioned group of veneers, are exclusively I due to their dimension. They are formed through the width trimming of veneer bundles in which visually undesirable sapwood is most often separated. In addition, the first bundles in veneer cutting are obtained by the crown/flat cut and rift cut techniques and are veneers that consist mostly of sapwood and, as such, the whole bundles are discarded. All of the veneers are residue, regardless of the length, which can go up to 400 cm, and which are more than 9 cm narrow.

III.

Veneers with defects

This category includes all of the veneers deemed unacceptable due to defects of an aesthetic and technical nature that are considered a defect (mistake) and which are residue regardless of their dimensions (they can be up to 400 × 50 cm). Some veneers clearly cannot be used as face veneers due to cracks, large holes or openness of the ray fleck, through which the glue passes to the decorative surface of the face when pressed. They can be used as veneers for backing or making multi-layer decorative panels and there is no need to throw them away. In addition, all veneers containing worm and insect holes, discoloration, sapwood, stains and wood rot, ingrown bark or knots are discarded.

The veneer patterns are specially composed according to rules (I), (II) and (III) and have formed new visual images that could be used as wall decorative panels. Each natural defect of the veneer that causes it to be considered residue is associated with certain design elements and principles. The design elements used to develop the new decorative wall panel are line, shape, value and texture. The design principles used to develop the new decorative wall panel design are: rhythm, contrast, pattern, unity and repetition (

Table 1. Samples of designed decorative panels from residue veneer: (A) heartwood/sapwood; (B) ray fleck/ingrown bark; (C) pin knots/discoloration; (D) sapwood; (E) sapwood/heartwood; (F) ingrown bark; (G) ingrown bark/knots/discoloration; (H) ray fleck/ingrown bark; and associated design elements and principles. (© Domagoj Mamić, 2021).

Category	Type	Decorative Wood Panel/New Design	Natural Appearance on the Veneer	Aesthetic/ Design Elements	Aesthetic/ Design Principles
I Veneer length 0–80 cm	A				balance
			prevailing heart wood	line	unity
			minority of sapwood	color	rhythm
			ingrown bark	texture	contrast
					pattern
	B				rhythm
			ray fleck	line	contrast
			ingrown bark	shape	pattern
				value	unity
II Veneer width 0–9 cm	C			line	movement
			pin knots	shape	rhythm
			pink discoloration	color	contrast
				texture	pattern
	D		pure sapwood	value texture	balance
					unity
					emphasis
	E			line	movement
			prevailing sapwood	color	rhythm
			minority of heartwood	texture	contrast
					unity
III Veneer with defects	F		ingrown bark	line	rhythm
				shape	movement
				value	pattern
	G		ingrown bark knots discoloration	line	movement
				color	rhythm
				texture	contrast
				value	pattern
	H				movement
			ray fleck	line	rhythm
			ingrown bark	value	contrast
					pattern

).

Figure 3 presents a 3D visual spatial simulation of the interior, in which the veneer sample in Table 1 (type A) was used.

3.1.2. Design of a Decorative Wall Panel from Veneer Residue Created by Multi-Layer Pressing and Gluing

Cracked residue veneer that has large holes from loose knots or veneer that has an open ray fleck cannot be used as a front layer due to the adhesive passing through the surface of the decorative side, but can be used to make decorative panels. Multi-layer folding of 0.52 mm thick veneer by pressing under the mold gives a surface texture. The module shown on Figure 4 has dimensions 90 × 55 × 0.5 cm.

Figure 5 presents a 3D visual spatial simulation of the interior, in which the decorative sample panel from Figure 4 was used.

3.2. Results of Analysis and Design of Back Board Residue after the Production of Cut Veneer

The production of rift cut veneer is obtained through the rotation of the flitch (Figure 1 and Figure 2). It is necessary to mill out two channels that serve to tighten the flitch on the machine, so the flitch is fixed during rotation. [19]. After the flitch is used to the maximum (by dimension) or when ray fleck appears, the back board is what remains (Figure 6). The dimensions of the back board are within the dimensions 3.5 × 23 × 400 cm.

Back board from rift cut veneer production is a semicircular plank with two channels, which is subsequently planed to obtain a flat surface in the middle part (Figure 7). The central part between the two channels is then sawn out and a 3 × 7.5 × 400 cm lath is obtained, which is used for further utilizations. Wood residue are external slats of the triangular cross-section that are later ground for wood pellet. Wood residue measuring 3.5 × 5.5 × 400 cm was determined. Due to a small cross section in relation to the length of the slats, for better utilization and dimensional stability, the optimal length of the slats as a building element is up to 1.0 m. Thus, it is possible to eliminate some of the errors and have better dimensional stability of the building element itself.

The obtained slats are trimmed and unified to the dimension of 3 × 5 × 65 cm. As the slats are wood residue from the best quality veneer logs, it is evident that the slats are also of impeccable first-class quality. By arranging these elements and combining different orientations, the shown module (Figure 8) is obtained, the width of which is defined by the number of elements. The height is recommended up to 100 cm.

As an example of 3D visual spatial simulation, the panel from Figure 8 was used, which functions as a decorative wall panel (Figure 9).

4. Discussion and Conclusions

This research presents some of the technological and design directions in which it is possible to use waste from cut Pedunculate Oak veneer (Quercus robur L.), also called Slavonian oak, and thus obtain better re-usage of this expensive material. This waste raw material can be turned into a more usable raw material, as well as into a final product with a higher added value and thus contributes to improving the market competitiveness of the company. A more rational use of raw materials is also very important, particularly when this material has a significant impact on reducing CO₂ emissions compared to other materials. The higher promotion of wood in construction and interior design and encouragement of the use of wood raw materials in production is needed because they contribute to environmental protection.

In the observed factory, Spačva Ltd., the wood residue from veneer production is unfortunately still largely used for the purposes of heating the factory or the production of pellets, and there are no alternative uses. Several studies have dealt with veneer residues from the production and panel production made of chipped veneer MDP (Medium Density Particleboard) [33], multi-layer, strand-type composite boards [34] or veneer corrugated core for composite board [35]. In addition, most research looks at the technological side of the woodworking process and does not mention the application of design and how visual design affects the acceptance of a new wood product by users.

When it comes to wood residue revision, the vast majority of oak cut veneer waste is selected on an aesthetic basis, due to which the raw material is eliminated. Wood does not have the same characteristic as a plastic, for example, so the visual defects in wood are a natural occurrence. Today’s trends encourage the use of wood that emphasizes the natural appearance and irregularities, in contrast to the recent desirable perfectly flat, uniform grain and wooden strips.

New research will certainly go in the direction of testing the quality of prototypes/products, meeting the requirements of technical correctness, dimensional stability and durability, as well as fire resistance for public-use products according to the European and world standards. It is also necessary to investigate the application of an ecological surface treatment that will enable acceptable visual solutions and alleviate the defects and other natural phenomena of wood that is visually unacceptable. Further research into users’ attitudes about the visual perception of the desirability of the natural appearance of wood is also needed in order to confirm the criteria for the visual acceptability of natural wood defects.

The use of wooden wall panels in the interior creates a serious problem in the fire resistance of the product, as wood is flammable raw material and there is always a risk of fire in interiors. Wood is widely used as a structural or decorative element in construction, and it is necessary to preventively protect it with flame retardants because they are susceptible to fire hazards [36]. Fire safety issues are of great importance and application requirements must be met. In order to meet the standards, wooden wall and ceiling panels in public spaces must meet two strict tests of reaction to fire (ignitability test) and a single burning item, or SBI, test, which are prescribed by the European standards [37,38].

The goal is to create sustainable decorative wood wall panels from wood residue that promote a sense of comfort and coziness in the interior and, at the same time, help the company to find its own brand.

Considering the current state of the market, inflation, ecological requirements of sustainability and environmental protection, as well as geopolitical circumstances, the authors hope that this research project will awaken a greater awareness among producers regarding wood residue and, with the application of design and further research, help to achieve the stated requirements through the maximum utilization of available wood and production resources. This work can serve to deepen the knowledge of improving rational wood processing by applying visual design elements. By including such products in the project documentation for the construction of new buildings, more sustainable products are obtained, and at the same time, more sustainable buildings that contain visual harmony and well-being.