The Relationship between Carving Work and Timber Features: A Database for the Italian Wooden Statuary

Abstract

Wood has a wide range of applications in the artistic field, most of the time used as a support for works of art, for example, in panel paintings. Compared with stones and metals, wood is by far the material most utilised for sculptures. Through the identification of wood species, a study of timbers used in statuary in Europe, with a deeper exploration of the works of art preserved in Italy, can provide further insights for historical–artistic studies. Indeed, in reality, generic wood does not exist, but there are different timbers. This is a notion that the wood fine carving artists/craftsmen knew well. The initial idea of this study was to verify whether timber features could influence the formal results in figurative carving; therefore, a specific database that had been developed to catalogue the carving works was used for which the identification of wood species was carried out through reliable methodologies. The aim of this research was to analyse the relationship between the timber species used in statuary, the formal results of carving, and the thickness of the preparatory layer, through the data stored in a specific database, called ArISStArt. It contains a sort of registry of artistic carving and inlay works: For each artefact, a record was compiled consisting of different fields, which contains the historical–artistic information available, including the images of the artefact and also types of timber/s used for its creation, with a link to the relative field. The number of the recorded artefacts is, at present, 480. On most of the artefacts, more than one sample was taken; therefore, the total number of identifications carried out corresponds to 691. Based on the results, the vast majority of wooden statuary is carved from hardwoods, more specifically hardwoods, with diffuse porosity and small variations in density within the growth ring. The timber most present is poplar, followed by lime and walnut. With the listed three timbers, almost 80% of the artefacts recorded in ArISStArt were created.

1. Introduction

Wood has a wide range of applications in the artistic field, most of the time as a support for works of art, for example, in panel paintings. Compared with stones and metals, wood is by far the material most utilised for sculptures.

It is a common belief that the widespread use of wood, for carving in general, and in statuary more specifically, derives from its availability, easy workability, and evident lightness, especially when compared with that of other materials used in the artistic field, and surprising in relation to the high mechanical performance typical of this material. Compared with marble (or other stones) and bronze, wood was, and still is, undoubtedly cheaper and lighter.

There are other composite materials widely available that are easy to work with and are lightweight. In statuary, there are several examples of light multi-material works, moulded and finished with extreme care, which are made with easily available materials such as terracotta [1] or even waste materials such as papier-mâché (an example from southern Italy is presented in [2]). These are materials for which there are examples of use that are not rare but have not had the same wide diffusion as wood. Wood is less fragile, and wooden artworks can easily and cheaply be painted and modified to adapt them to new fashions or, for sacred works, to new liturgical needs.

Through the identification of wood species, a study of timbers used in statuary in Europe, with a deeper exploration of works of art preserved in Italy, can provide further insights for historical–artistic studies. Indeed, in reality, generic wood does not exist, but there are different timbers. This is a notion that the wood fine carving artists/craftsmen knew well.

The scarce but significant treatises found on this topic (the precepts on the use of some wood species for statues and carvings), from Vitruvius to Renaissance treatises, to Filippo Baldinucci, reveal a notable knowledge of wood technology related to its treatment for artistic purposes.

In De Architectura (1st century AD, [3]), Vitruvius argued that the types of wood originating ‘from the black and white poplars, as well as the willow, the lime […] not being hard to the touch […] due to their non-compactness, they are white and guarantee a comfortable ease to be worked in the sculptures’. It is, therefore, evident that for Vitruvius, the density of the wood was especially important, meaning that it was important for the wood to be soft because it is easier to work with; no reference was made to the formal result allowed by the different wood types.

In the De re aedificatoria [4], Leon Battista Alberti highlighted some wood types that were used to create images of the gods, among which were boxwood, cedar, cypress, and the larger roots of domestic olive trees. ‘In the same way, black and white poplar, willow, hornbeam, rowan, elder, fig have not been neglected to create statues and tables for paintings. The wood of these trees is in fact not only very dry and uniform, which makes it suitable for receiving glues and pastes from painters, but also extremely ductile and such that it can be modelled in various forms. However, it is known that the softest of all is lime wood. Jujube also finds approval for the creation of statues’. The information collected by Alberti on this aspect of artistic woodworking indicates a favourable view towards types of wood with low degrees of hardness, uniformity, fine texture, and levels of finish obtainable. Poplar, elder, fig, and lime are appreciated for being soft and easy to work with. Other wood types (rowan, hornbeam, and jujube), which are less soft, allow for excellent levels of surface finish but are less easy to work with.

Francesco di Giorgio Martini [5] only noted that ‘walnut and pear are very suitable for carving and other figurative works’.

Pietro Cataneo [6] showed greater interest in the subject when he began by quoting Vitruvius: ‘the white and black poplar, the willow, the lime […] become candid, light and very easy to work with, and therefore Vitruvius praises them for making the statues ’. Therefore, this first series of timbers was only appraised for being very tender and easy to work with. A separate discussion was made for walnut wood, defined as ‘most beautiful and best wood […]. And his works are carved with foliage, animals, masks, figures, and other varied designs: that in all ways, whether by carving or not, his works offer great majesty and are very grateful to the eye’. Very appropriately, Cataneo highlighted the final result and the effect of the walnut, worked and polished. Then, follows the mention of a series of fine-textured timbers: ‘pear, apple, rowan […], boxwood, but most of all ebony, are of very dense or tight wood […]. They all serve many different and varied works, but they hardly keep glue; and ebony and boxwood are the tightest and heaviest of all: and therefore they do not float in the water. The pear, rowan, and boxwood, as well as many other works, are very apt to carve them to make prints’. With the terms ‘dense or tight wood’, the author indicated both the density and the fine texture; with regard to ebony and boxwood, he added that in addition to being ‘the tightest’, they are also the heaviest, thus indicating a further technological feature—density. Finally, for pear, rowan, and boxwood, he underlined their enhanced fine texture, stating that they are suitable for xylography. High density and fine texture allow for the finest carvings required for quality woodcuts.

Vasari’s [7] references to the types of wood used in sculpture in the technical introduction to the ‘Lives’ mainly concerned lime wood. Already in the title of the chapter was indicated an interest in wood preferably used for statuary: ‘How the wooden figures are conducted, and what wood is good for making them’. Vasari immediately relegated wood to the last place in the hierarchy of materials for three-dimensional figuration, that is, after metal, marble, stucco, wax, earth. However, he did not exempt himself from pointing out ‘the best […] of all the woods that are used in sculpture’, which is the limewood, ‘because it has the same pores on each side, and obeys more easily the file and the chisel’. In this way, Vasari highlighted three characteristics of lime wood—its great homogeneity, easy workability, and the possibility of having a good quality finish.

Boxwood and walnut were also mentioned: ‘and of the artifices of this craft are also seen highly praised boxwood works and beautiful walnut ornaments; which when they are of beautiful walnut that is black, appear almost bronze’. Vasari, like Cataneo, also highlighted the great effect of the exposed surface of walnut, for example, by making it appear as bronze, thus demonstrating that wood should only be appreciated as a substitute for a more noble material.

Filippo Baldinucci, in his Tuscan Vocabulary of the art of drawing [8], also dealt with a large number of woods intended for artistic processing. Among these, we can mention the item relating to ebony, a type of wood known for its high density and high levels of finish it allows the artwork to have, as well as walnut, which is also exalted for the high-quality finishes obtained by its use.

The same importance for the knowledge of wood and the differences between the various timbers, unfortunately, is not shown by modern scholars who study the works of wooden statuary from a historical–artistic point of view. Additionally, for visitors to museums, information on timber is rarely available, because timber has rarely been identified. Art historians themselves point out that ‘there is no systematic research for the identification of wood species in sculpture’ [9].

In the relevant literature, papers reporting the scientific identification of wood in statuary or, more generally, in carving works are rare, certainly numerically inferior to those dedicated to the study of pigments applied on polychrome wooden statues.

There are interesting studies on statuary in the Far East (e.g., [10,11,12]), while on wooden statues in Europe, there are studies that describe the scientific analyses made on the individual artworks accompanying the restoration work. For instance, the authors of [13] report the conservation procedure applied to a wooden sculpture, including among the other analyses, also wood identification. It is interesting that the whole procedure, from the set of analyses (i.e., from surface pigment to wood identification and quality) to the execution of artworks is described [14] on a single statue, to show the working process of a German medieval workshop. More numerous are papers just describing the wood identification of single artworks, with few connections to other aspects (such as [15] or [16]).

Very rare are studies probing the connection between carving works and wood species through analyses conducted on a specific corpus of artworks [17], those at a regional level, or in connection with a school of carving or sculptor workshops.

Vuola [18] studied 134 wood carvings in the diocese of Turku (Finland) in the 14th century. In this case, the study of wood aimed at evaluating the possible origin of the artworks analysed. It is interesting to note that traditionally, there was a tendency to consider artworks made of birchwood native, even though the results of this study showed that birch was totally absent. Instead of birch, local artists preferred alder or limewood. A point that must be considered is that the previous analyses, carried out by personnel lacking the necessary skills in wood anatomy, stated that artworks not made of oakwood, which are easily identifiable by the naked eye, were a priori made of birch but without any scientific evidence.

However, is the identification of wood in statuary works a simple curiosity, or can it open up new fields of study? Vuola [18], again, noted that ‘the analysis of wood species alone is insufficient unless it is combined with analysis of the form, style, and iconography of the studied sculpture’.

Actually, from the mid-1990s of the 20th century (in particular, consequent to the functional research of the exhibition ‘Wooden sculpture, Lucca: 1200–1425’ [19]), some art historians have become aware of the importance of material and technical data as essential elements for the historical and stylistic analyses of polychrome wooden statues. Subsequently, some research groups of the University of Perugia and Rome ‘Tor Vergata’, in collaboration with the IVALSA-CNR (today IBE-CNR), have initiated a series of investigations [20], useful to legitimise wood identification as a practical critical tool, through initiatives and publications of a national and international nature in which it was possible to validate this innovative line of research, which, for the first time, addressed the data relating to wood texture and grain, the thickness of the preparatory layer, and the formal results achieved by the sculptor.

Without prejudice to the primacy of the artist’s creative and productive path, the developments in the field of research on wooden artefacts of historical–artistic interest have made it possible to acknowledge the importance of the technological characteristics of the material, both for three-dimensional figures (statues and carvings) and for two-dimensional ones (inlays). The relationship between art historians and wood technologists has meant that the features of each timber can be considered in terms of the formal results that it can allow.

The quality of the formal result could in fact also be determined by manual woodworking tools, but on this aspect, it is not possible to make any evaluation. Each workshop certainly had its own equipment (made to order by an unknown local blacksmith) and performed its own maintenance. It is not possible today to obtain any information on these aspects, because nothing has been recorded, and therefore, it is not possible to find reliable information.

The initial idea of this study was to verify whether timber features could influence the formal results in figurative carving; therefore, a specific database that had been developed to catalogue the carving works was used for which the identification of wood species was carried out through reliable methodologies.

By studying the records included in the database, it was possible to analyse the results on the timber used in the art of statuary and to focus on the relationships between different timbers and the aspects related to the technical–artistic realisation of artistic works. Among the most important features were the formal results of carving, the thickness of the preparatory layer, and the geographic–cultural environment of the artistic creation.

The aim of this study was to analyse the relationship between the timber species used in statuary, the formal results of carving, and the thickness of the preparatory layer, through the data stored in a specific database.

2. Materials and Methods

The information material included in the database derives mainly from identifications carried out within the Laboratory of Anatomical Characterisation of Wood of CNR-IBE (in the past, first called CNR-IRL and then CNR-IVALSA) on samples derived mainly from polychrome wooden sculptures, from medieval to early modern age; it should be stressed that analyses were only carried out on artworks that still have their original polychromy. The aforementioned artefacts imply the presence of a preparatory layer (generally a stucco consisting of gypsum and protein glue) between the wooden surface and the paint film, which serves—primarily—for the adhesion of the pigments. In this study, only the original preparatory layer was taken into account, the thickness of which was graded as thin (less than 1 mm), average (1–2 mm), and thick (more than 2 mm).

The identifications were partly requested by restorers on individual artefacts and partly derived from more extensive research on the characterisation of artworks in permanent collections, such as those of the Palazzo Venezia Museum in Rome [21,22] and the Galleria Nazionale dell’Umbria in Perugia, or other held exhibitions. Among the latter, we can mention ‘The sculpted Renaissance, masters of wood between Marche and Umbria’ [23] or ‘Il Teatro del Sacro, wooden sculpture of the seventeenth and eighteenth centuries in the Asti area’ [24,25].

Recently, an interdisciplinary group studied the work of the Flemish XVIII century sculptor Jan Geernaert between the dioceses of Parma and Pontremoli [26], and the results of this large study were also included in the database.

An important part of the database concerns the long and fruitful collaboration with the School of Restoration of the University of Urbino, in particular through the graduation works carried out by the students ([27,28]).

Not all the identifications related to the almost 500 artefacts entered so far have been carried out by the staff of CNR-IBE. In fact, the data relating to restorations carried out in their laboratories were entered by the ICR in Rome [29], and data traceable from catalogues of exhibitions for which the identification work had been carried out with scientific methods, were reported with reference to bibliographic sources. Two important examples are [30], which concerns the exhibition held in Matera in 2004, and the identification works performed at the Diocesan Museum of Palermo [31].

The database is called the Archive of the Identification of Wood Species of Historic-Artistic Heritage (ArISStArt, see [32]). It is a relational database developed in FileMaker^®, and therefore, it is an archive with a series of correlated tables that are interconnected (Figure 1). These correspond to the authors of the artefacts, the description of the individual artefacts, the results of wood identification, the photo gallery, and finally, the characteristics of the identified timbers and the trees that produced them. Among the data on sculpture, those regarding the geographical–cultural context were widely used in the course of the analyses. It is a datum that derives from the historical–artistic analysis. Most of the recorded works were sculpted by unknown authors, but the artistic and morphological characteristics of the sculpture allow specialists to identify the artist’s cultural context and, therefore, to define the provenance also geographically.

The data entered were subsequently processed based, in particular, on the identified timbers, which were put in relation to the geographical–cultural area of origin of the artwork, with the historical period in which the artefact was created, and with the thickness of the preparatory layer.

3. Results

The recording of new artworks is still ongoing; the number of recorded artefacts is, at present, 480. On most of the artefacts, more than one sample was taken; therefore, the total number of identifications carried out corresponds to 691.

With respect to the historical period of production, in Figure 2, it can be seen how the listed artworks are distributed between the 5th century (a few cases corresponding to the carvings of the door of the paleo Christian church of Santa Sabina in Rome), up to the century. The great majority were created between the 15th and 18th centuries. Among the artefacts of the 15th century, those made in central Italy prevail, while for those of the 18th century, the clear prevalence is for artworks from northern Italy.

In total, 29 different timbers were identified; in most cases, it is possible to identify the genus (such as Populus sp. or Tilia sp.) and not the species. Softwoods were identified in only 65 cases, while the other 626 identifications correspond to hardwoods.

Figure 3 shows that almost half of the artefacts (48.9%) were made using poplar (Populus sp.) and willow (Salix sp.). Poplar is absolutely the most carved wood, and only a few artworks are made with willow. They are two very similar types of wood, belonging to the same botanical family, that of Salicaceae. In very few cases, the analysed samples were so small, such that it was impossible to have a reliable way to investigate the anatomical features that differ between Populus and Salix, that is the homogeneity (poplar) or heterogeneity (willow) of parenchymatic rays.

A quarter (25.5%) of the artworks are made of limewood (Tilia sp.) and almost 7% of walnut (Juglans regia).

With just three timbers, more than 80% of the artefacts recorded on ArISStArt were created. Adding the two most present coniferous wood types, Swiss stone pine (Pinus cembra, 4%) and cypress (Cupressus sempervirens, 2.5%), we found that almost 9 out of 10 artefacts are made with just 5 different timbers.

3.1. Wood Anatomy

Examining in detail the anatomical aspects of the 20 hardwoods found, then dividing them according to the distribution of the vessels within the growth rings (Figure 4), we can see that the most widely used hardwoods are the so-called ‘diffuse-porous’ (Figure 3, 88.1%), while a very low percentage (4.2%) belongs to the category of ‘ring-porous’ timbers. The remaining 7.7% corresponds to the semi-ring-porous timber, such as walnut.

The technical, material artist’s choice of wood would, therefore, seem to be closely suggested by wood anatomical features. Wood must preferably be a diffuse-porous hardwood, and the identified timbers largely correspond to those listed by the Renaissance masters. In the case of softwood, only timbers with very thin layers of latewood were considered.

3.2. Geographic Distribution

As suggested in the previous section, the anatomical features are key to understanding timber choices by artists, but can a correlation with the geographic distribution of the wood-producing trees be excluded?

The recorded artefacts are from all European countries but with a very large majority from Italy, which are distributed between northern (227 identifications), central (262), and southern Italy (94). However, artworks produced by artists from German (54) and Alpine (22) cultural environments also provided important contributions. Less represented are French (12), Dutch/Flemish (9), and Spanish (7) artworks.

Considering the distribution of most of the timbers identified, it can be said that they are found almost everywhere in the various environments of central and southern Europe.

An exception with respect to the ubiquitous distribution should be made for the two most identified conifers: the Swiss stone pine (Pinus cembra), typically alpine, where it grows between 1400 and 2400 m a.s.l., and for the cypress, which is an exclusively Mediterranean species.

The various species that make up the genus Populus (or even Salix) grow in fact throughout Europe in environments rich in water content of the soil. Populus alba, P. nigra, and P. tremula are the three species endemic in Italy, and the three are diffused in central and southern Europe. Populus tremula has a larger area, spread on mountains from north Africa to the Scandinavian peninsula [33].

Similarly, the genus Tilia can be found in mixed broad-leaved types of wood between the plains and low mountains almost everywhere in Europe. The most diffuse species are Tilia platiphyllos and T. cordata. The former is found in central and southern Europe mainly in the mountains, in mixed forests with beech, silver fir, and sycamore maple. T. cordata has instead a more extensive range, extending to Scandinavia, but less expanded in the southern areas.

Among the most utilised timbers, walnut also is, and has been, cultivated as a fruit and oil plant all over Europe, despite its origin from the eastern Mediterranean region.

Therefore, with the exception of the two conifers, the timbers mainly used have a very wide distribution range and are present throughout central and southern Europe. It, therefore, seems possible to exclude the hypothesis that the choice of wood was caused by availability due to the region in which the artist worked.

Nevertheless, by analysing the distribution of wood in the three geographical areas of Italy, it can be seen that it changes from north to south. However, the results revealed that the type of wood used in various geographical–cultural areas can only be partially justified by the ecology of the plants. In Figure 5 and Figure 6, it is clearly seen how the clear prevalence of poplar in the works carried out in northern Italy becomes less important in central Italy, to drastically decrease in the works of the central–southern cultural sphere, where it is the limewood that becomes prevalent.

Figure 7 shows the distributions of the identifications made on carved artworks outside the Italian cultural sphere.

Here, it is noted how the presence of poplar wood becomes almost negligible. Of the almost 50 Germanic artefacts analysed, not even one is made of poplar wood, although poplar is present in the German flora, while the lime tree assumes a prevalence that becomes almost exclusive.

The works carried out by artists of the Alpine cultural field are mainly made of stone pinewood, a species that in Europe, grows at the vegetational limit only in the Alps. It is a type of softwood with little latewood inside the growth ring (Figure 8 left) and, therefore, very suitable for carving, with a very characteristic scent. The artists who were born and raised in the Alps were, therefore, trained mainly in working with this wood.

3.3. Preparatory Layer

From the point of view of the execution of an artwork, possible relationships between the different timbers and the thickness of the preparatory layer were also analysed, that is to say, the layer generally composed of plaster and animal glue, often also called ‘primer’, constitutes a link between the wood and its decorative layer, both to favour the adhesion between pigments and the wood and to create a cushioning layer between the rigid, thin pigments and the typical deformations in the wood that occur due to environmental thermo-hygrometric variations.

The data on the thickness of the preparatory layer were not always available, so it was statistically treated in 389 cases out of the 691 for which data were available. Of these, 57 had no preparatory layer, meaning that the carving work shows the wood’s bare texture, although not infrequently the wood had been made visible again by restoration work by eliminating the paint residues.

As can be seen from the graph in Figure 9, the thickness of the preparatory layer for most of the artefacts ranges between 1 and 2 mm, while less than 100 have thin and thick preparatory layers, respectively.

On the other hand, Figure 10 shows the relationship between the thickness of the preparatory layer and the type of wood carved. The pie charts show all wood types, but it is probably prudent to focus on the relationships between the two timbers most present—lime and poplar—because the high numbers that represent them give greater solidity to the reading and examination of the results.

An inversely proportional relationship can be noted; that is to say, the thinness of the preparatory layer corresponds to more artefacts carved in limewood, while at a high thickness (thicker than 2 mm), poplar returns to be clearly prevalent, compared with limewood. With a thin thickness, the presence of limewood is well above the 25% representation of the total works, conversely, poplar is used in 70% of the artworks having a thick preparatory layer, well over 44% of the average total.

As regards other timbers, it is worth noting that species well known for high carving workability are more present in the thin preparatory layer grade: walnut, cherry, and pear. On the other hand, ring-porous timbers (oak and chestnut) are only used in artworks having average and thick layers.

4. Discussion

The number of artworks currently included in the database, and the relative identifications, constitute a sufficiently large numerical base to be able to consider the evaluations in progress representative of the corpus of figurative carving works in Italy.

Based on the results shown, the vast majority of wooden statuary is carved from hardwoods, more specifically, hardwoods with diffuse porosity and small variations in density within the growth ring.

It is worth noting that very similar results were obtained by scholars who analysed timber species used in traditional Chinese and Japanese sculptures [11,12]: statues were made only with diffuse-porous hardwoods in China, while in Japan, some of the carvings were made on peculiar, very uniform softwoods such as Torreya and Chamaecyparis.

Observations of the cross-sections in Figure 4 and Figure 8 reveal an understanding of the anatomical differences between softwoods and hardwoods, as well as within hardwoods.

In Figure 8, on the left, and in Figure 11, it can be appreciated how softwood is characterised by almost a single type of cells (called tracheids), with its cell wall varying in thickness between earlywood, with a thin wall and, therefore, a lower density, and latewood, which is thicker and denser (earlywood is the one produced by the tree in the first part of the growing season; it is generally lighter in colour. Latewood, by contrast, is produced in the second part of the growing season and is generally denser and darker in colour. The greater the difference in density between early and latewood, the greater the visibility of the growth rings.). In hardwoods, on the other hand, there are several types of cells, mainly vessels—with a large lumen and thin walls—and fibres—with a more or less thick wall and very narrow lumen.

Hardwoods with diffuse porosity have vessels of a rather uniform diameter along the whole ring and are, therefore, characterised by a fine texture to the touch; conversely, ring-porous hardwoods are characterised by a series of 1–3 very large diameter vessels in earlywood, while the final part of the growth consists of small vessels and numerous fibres, which gives rise to a coarse texture; in-between these two conditions, there are hardwoods with a semi-ring-porous distribution, such as walnut, for which the diameter of the vessels gradually decreases along the growth ring.

For gouge and file work, the ideal situation is that the wood density remains as constant as possible along the growth ring because a uniform amount of material is removed for the same applied force. By contrast, when the density of the wood changes considerably along the ring between early and latewood (as occurs in Larchwood, Figure 11 right), with the same applied force, more earlywood material will be removed and less latewood. Diffuse porosity results in a uniform density within the growth ring.

In softwoods, the growth rings are generally very visible precisely due to the strong difference between early and latewood density, while a characteristic of wood types of Swiss stone pine and cypress is having a slightly or very little marked latewood, always very thin. They are, therefore, among the most suitable softwoods for carving.

Carving work is, therefore, favoured by uniform densities within the annual ring. However, this is not enough. Maple (Acer sp.) or pear (Pyrus sp.) wood are very uniform in density, even more so than poplar wood. In the list of wood types identified in this project, both are present, but in a way that we can define them as sporadic. Moreover, there are many maples in the European flora, with very wide diffusion. However, maple wood has a much higher density than poplar and limewood. Higher density (about 680 kg m⁻³ for maple and 480 for white poplar) means greater difficulty in carving and greater weight of the final work, important factors to take into account, given that many of the statues studied are for processional purposes.

Poplar and limewood are both soft hardwoods with uniform porosity. In Europe, their geographical distribution does not place significant limits on availability. Based on the results, however, their use seems to be conditioned by the geographic–cultural context of the origin of the artworks. On the one hand, tradition certainly contributes to the choice of wood used in artworks, which in the Germanic context seems to clearly prevail: it is rare that works made in central–southern Germany are not made of limewood, while in southern Germany, close to the Alps, most of the artworks were carved in Swiss stone pine.

In the part of Europe corresponding to the mouths of the great European rivers, such as the Rhine, Moselle, or Elbe—a region corresponding to Flanders, the Netherlands, and northern Germany, where the great ports of northern trade led to the transport of enormous amounts of oakwood—the artists had no choice: their guilds were forced to make their own carving works on oakwood [34].

Compared with what has been described and explained above, oakwood does not correspond to the characteristics of soft hardwood and uniform density along the ring. On the contrary, it has ring porosity and high density. In this regard, it can be commented that oakwood was principally marketed for use in naval structures and in building construction. The very low-growing oaks have narrow growth rings characterised by the almost exclusive presence of light earlywood porous rings and almost no dense latewood, in fact anatomically more uniform than the ‘normal’ oakwood [35]. Such a light oakwood was less suitable for structural purposes, for which it was discarded, generating the lots that the guilds of artists had to use for their works. It is no coincidence that this is the only situation in which the use of a specific wood became mandatory, while in other European regions, artists and clients could afford the wood they considered most suitable.

It is known that the availability of poplar wood could have been much easier and cheaper in central–northern Italy, but this could clash in the execution of the work, with the equally well-known ease of chipping of this timber. By the ease of chipping, it is meant that this type of wood has a tendency to suddenly produce larger splinters than intended under the gouge.

The ease of chipping could make it impossible to carve the finest details on this material. Ultimately, limewood had a better reputation and, therefore, could fetch higher prices, first as raw material and then as an artwork.

As shown in Figure 12, it is possible to note the different anatomical features of limewood and willow wood (poplar wood, for comparison, is in Figure 8 in the centre). Poplar and willow have vessels slightly larger than those of lime, but especially lime has larger rays that widen at ring boundary, providing an important mechanical contribution in bracing the growth rings.

With lime, it is possible to obtain anatomical details more minutely than with poplar. This may be an explanation of the stronger thicknesses of the preparatory layer found on the poplar: the details are not extracted from the wood by the gouge but modelled on the added preparatory layer.

Sculptors knew the wood they worked with—if it was easy to chip or had a coarse texture; therefore, they used the gouge by balancing the potential of the chosen wood species with the modelling of the preparatory layer. The knowledge of wood potentials and the information related to the thickness of the stucco layer below the original polychromy (data that can only emerge from an examination by the restorer) allow the art historians to grasp the formal vocation of the wood species and to add to this body of knowledge by seeking an additional (and valuable) critical tool.

5. Conclusions

The data contained in the ArISStArt database allow us to state that the timbers used in the artistic carving works were selected from a very small group. From the anatomical point of view, the majority of these are light hardwoods with diffuse porosity, in which the differences in density between early and latewood are very narrow. They are, therefore, types of wood characterised by intra-annular uniformity. The few softwoods are equally characterised by a very reduced latewood layer.

The two most represented timbers are those produced by the various species of poplar and lime trees. The study of the thickness of the preparatory layer allows us to see how, generally, artworks using limewood are characterised by a thinner preparatory layer than those made with poplar. The typical anatomical features of limewood, therefore, seem to allow a level of greater detail, while in artworks made with poplar wood, the finer details were often performed through the modelling of the preparatory layer, instead of through carving.

Taken together, results demonstrate that when there were no problems related to economic factors or procurement, the choice of timber was never random but linked to the formal results the artist wanted to achieve, in terms of the directly proportional relationship between texture and preparatory layer thickness.

The results derived from the ArISStArt database suggest that the critical activity of an art historian who studies a wooden statue cannot ignore the knowledge of the timber in which it is made, as well as its technological characteristics.

This is, in fact, an essential aspect for reconstructing the sculptor’s production path, which is strongly influenced by wood texture in terms of the surface finish quality and the amount of preparation layer used to obtain the final image of the figure.

In this way, we can establish that, at least in the field of three-dimensional wooden figuration, artists must comply with the ‘rules’ imposed by the chosen material (or those provided by the client), but they can use the aesthetic potential and compatibility with other materials, such as the preparation layer.