2.2.1. Animal Glues
Animal glues were among the first adhesives used in construction, manufacture, and conservation (their history dates back at least to ancient Egypt) and they are still in use to present times. Derived mainly from animal collagen—the most abundant mammalian protein present in bones, tendon, skin, connective tissue, and cartilage—they have different chemical, physical, and mechanical properties depending on their origin and a method of preparation. Nevertheless, they have proven to be extremely useful as adhesives and binding media for various purposes, and under certain conditions, they can be highly durable as evidenced by centuries-old fills or joints found in historical artefacts [
17,
30,
54,
55].
Animal glues are natural and water-soluble, which reflects in their properties. They are non-toxic, thermoplastic, as well as reversible and retreatable (unlike synthetic resins) and have great adhesion to the wood surface, which suits the conservation purposes. On the other hand, as a good source of nutrients, they are prone to biodegradation, and their hygroscopicity makes them susceptible to moisture changes, which results in their shrinkage, brittleness, and a tendency to fracture when they dry, or in their softening and bleeding when they undergo plastic deformation under wet conditions. These drawbacks predispose animal glues to serve mainly as interior adhesives [
30,
33,
55,
56,
57,
58]. However, their modification with additives such as inert fillers, plasticisers or pigments can significantly improve their performance (by limiting their dimensional changes upon moisture fluctuations and altering their mechanical properties) and give them the properties necessary for gap fillers for specific outdoor conservation purposes [
57]. Animal glues do not stain the wooden surface nor cause wood discolouration and can be easily removed from places where they are undesired. Under optimal external conditions (without significant changes in moisture and temperature and limited risk of biodegradation), they can be stable and durable [
30,
33,
55].
Besides the glues based on mammalian collagen, there is a variety of different animal glues, including casein glue, albumin glue or fish glue, which have been applied as paint media, adhesives for architectural, manufactural, and conservation purposes, as well as binding media for bole and gesso in gilding [
33,
55,
59].
2.2.2. Waxes, Oils, and Natural Resins
Waxes are some of the oldest materials used in monuments craft and conservation–beeswax was used by ancient Egyptian craftsmen or applied by the ancient Romans to fix Greek marble statues damaged during transportation [
60]. Apart from beeswax, other natural waxes have been used since the beginning of the 19th century, including those of plant (e.g., carnauba, candelilla, esparto, ouricuri) or animal origin (Chinese insect wax, lanolin, shellac wax, spermaceti); nowadays, mineral waxes originated from natural deposits (ozokerite or montan wax) or extracted from crude oil are also in use in the form of paraffin (larger crystalline size) or microcrystalline wax (smaller crystalline size) [
17,
61].
Waxes are used as coatings, binders or consolidation agents for degraded wood and can be suitable as gap fillers for wooden objects which are not subjected to high temperatures (close to or exceeding the melting point of waxes) [
17,
31,
55,
60]. They are non-toxic, inert, chemically stable, resistant to organic solvents, and hydrophobic. They are the traditional choice in conservation since they do not shrink upon setting, can be colour matched to the surrounding surface, and easily removed (due to their low bonding strength and lack of tension between the fill and the surrounding wood). On the other hand, the hydrophobic nature prevents them from expanding or contracting along with wood movements caused by humidity fluctuations, which can lead to falling out of wax fills. Unfortunately, waxes melt and leak out under higher temperatures and significantly increase the weight of a conserved artefact; they are prone to dust accumulation and can be easily scratched because they have poor mechanical resistance. However, their filling performance can be improved when they are mixed with fillers and adhesives [
17,
55,
56,
62,
63].
Linseed oil (so-called flax oil or flaxseed oil) has also been widely used in wood craft and conservation. It is non-toxic and water-resistant, ubiquitous, easily available and relatively cheap, and as a drying oil that can polymerise into a solid form when exposed to oxygen from the air, it is perfect for various purposes: for finishing wood surfaces, for wood impregnation when blended with resins, other oils or solvents, as a plasticiser and hardener in putties and other filling masses, or as a pigment binder in paints and gildings. However, the development of synthetic resins with better performance significantly limited its application in the past few decades [
55,
56,
62].
Natural tree resins were also utilised in the past for art craft and conservation. Although relatively thermo–chemically stable, they have insufficient flexibility and adhesion to wood to serve as components of gap-fillers in wooden objects [
31].
2.2.3. Cellulose Ethers and Esters
Cellulose, as mentioned above, can serve as a filler, but its soluble derivatives–ethers and esters–are also used as adhesives or consolidants in wood conservation.
Cellulose ethers are water-soluble polymers produced by chemical modification of cellulose. The modification process includes extraction of cellulose from natural fibres (cotton, wood), treatment with an alkaline solution, and then reaction with various etherification reagents. The process disrupts hydrogen bonds in cellulose, which allows other compounds to bond with glucose units, but also results in a significant reduction of cellulose crystallinity. The resulting products are hygroscopic white powders applied as thickeners, stabilisers, and viscosity modifiers in a variety of industries, and their properties are similar to some synthetic or natural water-soluble polymers (polyurethanes, polyacrylates, polyvinyl alcohol or carrageenan, xanthan gum, locust bean gum) [
18,
58,
64,
65].
In wood conservation, mainly methyl and hydroxypropyl cellulose are used as consolidation agents or adhesives; however, other ethers have been recently tested [
18,
23,
24,
36,
55,
66,
67]. The most common compound is Klucel
® (hydroxypropyl cellulose), available in a wide range of grades and viscosity; however, because high-molecular-weight Klucel
® (types H and M) was found unstable, mainly the low molecular G type is in use. Klucel
® is soluble in water or ethanol; it has been applied as an adhesive in combination with paper pulp, microballoons, and cellulose powder, or a mixture thereof, for filling loses in wooden objects exposed mainly in museum interiors [
18,
68,
69,
70]. It was shown to be susceptible to ageing/weathering, similar to cellulose or other cellulose ethers [
18].
Studies on Klucel
® G mixed with ground chalk revealed that such a mass is prone to slump and shrink upon drying; therefore, is not suitable as a gap-filler [
13]. On the other hand, as shown by Fulcher [
18], a blend of Klucel
® G with paper pulp meets almost all the criteria set for gap fillers for wooden artefacts. The fill is compatible with wood and responds to moisture fluctuations similarly to wood. Therefore, potentially, it could be used outdoors. Its hardness and strength can be adjusted to the requirements of the object by changing the concentration of an adhesive, and it can be prepared using a variety of solvents suitable for individual artefacts. The important thing is that the fill should be considered a system, and its properties are not a sum of the properties of its components because individual ingredients–a bulking agent, an adhesive, and a solvent–affect each other, modifying the resulting material and its reactions to the surrounding environment. Kryg et al. [
23] studied fills composed of Klucel
® G and wood powder or glass microballoons as bulking agents, using acetone or water as solvents. The results show that the properties of the examined fills differed significantly depending on the solvent and the filler/adhesive ratio. Because all the fills were susceptible to water, none of them was suitable for external application.
Among other cellulose derivatives sometimes used in conservation are cellulose esters: nitrate and acetate. They dry quickly and degrade easily by discolouring and releasing acidic vapours, become brittle in time and prone to cracks, and their binding properties are relatively weak. To improve its filling performance, cellulose nitrate was sometimes plasticised with other materials; mixed with wood dust served as a filling mass called “Plastic Wood”. Today, however, there are plenty of better adhesives. Therefore, cellulose esters are not a primary choice as adhesives for wood conservation [
17,
56,
58].
2.2.4. Synthetic Resins
Synthetic resins are liquid, semi-liquid or soft solid viscous substances industrially produced in polymerisation, polycondensation or polyaddition reactions. They are mixtures of prepolymers (oligomers and short polymers) that contain highly reactive functional groups, which enable their further polymerisation and cross-linking in the process called curing. It leads to the formation of rigid polymers as final products. Depending on the chemical structure and the production method, there are various classes of synthetic resins, including acrylic, epoxy, vinyl, polyurethane, etc. Considering their reversibility, there are two different types of resins: thermoplastic, which softens upon heating and hardens again–they are easily moulded, and therefore are suitable for filling purposes–and thermosetting, which undergo irreversible hardening by curing. Synthetic resins are applied widely as adhesives, coatings, cross-linkers, etc. They are also used in art conservation as consolidants, adhesives or protectives and are considered as more resistant to degradation and better performing compared to natural adhesives [
30,
31,
53,
58,
71]. Some of the most common synthetic resins in wood conservation are described below.
Acrylic resins are thermoplastic or thermosetting polymers commonly used in artwork conservation. They are colourless, transparent, inert, reversible, insoluble in water, but easily soluble in organic solvents including fast-drying acetone or safe for finishes or paints xylene, which are frequently used in wooden artwork conservation. They have good adhesion and mechanical properties. Although highly resistant to temperature, acids and alkalis, in the long term they are prone to microbial degradation, UV radiation, and mechanical damage to some extent, which can change their physical and mechanical characteristics [
20,
22,
30,
61,
72,
73].
Paraloid B72 (methyl-acrylate/ethylene methyl-acrylate copolymer) is the most popular acrylic resin applied in conservation and restoration owing to its flexibility, good optical properties, and removability combined with higher photo-thermal oxidation stability and lower water sorption than other members of this resin family. It performs well as a consolidant for degraded wood, a barrier coating or protective layer for different surfaces, but is also useful as an adhesive, including its application for gap-filling masses in combination with different bulking agents [
20,
21,
22,
31,
72,
74]. When mixed with microcrystalline cellulose, it can form an innovative thermoplastic composite with enhanced thermal stability and mechanical performance (including an increase of the elastic modulus and storage modulus, as well as improved creep stability) proportional to the concentration of MCC. Although MCC increased the moisture content of the composite, it also had a stabilising effect on it. The results show that the addition of MCC is an efficient way to improve the performance of acrylic resins used for conservation purposes where dimensional stability and energy absorption capability under external loads are necessary, which relates to some of the requirements for gap-fillers for wooden artefacts exposed outdoors [
20,
22]. A study by Kryg et al. [
23] revealed that a blend of higher concentrations of Paraloid B72 and glass microballoons, although not highly flexible, was relatively dimensionally stable upon drying and exposure to water vapour and liquid water (despite high absorption of moisture) and was quite easy to finish; these features make it potentially useful as a gap-filler for external use. On the other hand, the blends of Paraloid B72 and wood powder turned out to be soft and fragile, difficult in finishing, and highly sensitive to moisture. Only those with a binder concentration of 50% were relatively compact and stable under humid conditions [
23].
There is also a variety of commercial fillers based on acrylic polymers that are potentially useful in filling losses in wooden objects. These ready-to-use fillers contain various binders, bulking agents, and additional substances. The problem is that their composition is a trade secret; therefore, it is hard to predict their properties under certain conditions. Although they are usually recommended by manufacturers for particular applications, there are limited references for their use in wood conservation [
15].
Epoxy resins are reactive polymers or pre-polymers that contain epoxy groups. They are soluble in organic solvents, and their solubility depends on their molar mass. Low-molecular-weight liquid resins are soluble in toluene, xylene, benzene, acetone, cyclohexanone, etc. Higher-molecular-weight solid-state resins are soluble in acetone and cyclohexanone, and partially soluble in aromatic hydrocarbons. Due to reactive epoxy groups, epoxy resins can cross-link with themselves or with several different chemicals (e.g., acids, alcohols, amines, phenols, thiols, etc.) in the process called curing or hardening, forming a thermosetting polymer as a final product. Epoxy polymers are insoluble, relatively resistant to weathering, moisture, and UV radiation, with enhanced mechanical properties and thermo-chemical resistance. Therefore, they are employed widely in different industries, i.e., as adhesives, coatings, and components of composite materials [
61,
75].
Epoxy resins are also useful in wood conservation and applied as adhesives (e.g., for conservation of panel paintings), consolidants or fillers. Different formulations can make them compatible with concrete, glass, stone, wood, or other materials. Since they are thermosetting polymers, they should be considered irreversible, which is against conservation rules. Nevertheless, in some cases, where the strengthening of an artefact is a primary aim, the principle of irreversibility becomes secondary, and the application of epoxies can be approved. Irreversibility can be sometimes bypassed by coating the surface of a hole in wood with a reversible coating which enables the fill to be easily removed if necessary. The only problem that remains then is the proper adhesive strength between the coating and wood and between the coating and a fill [
11,
12,
13,
14,
33,
63,
72].
Epoxy resins should not be used alone as fills for wooden objects because they can penetrate wooden tissue, thus irreversibly change the properties of wood (they are not flexible enough when they polymerise, making wood rigid, brittle, and prone to rupture). However, when combined with various fillers and additives that increase their viscosity, they can make a gap-filling paste ideal for filling voids, holes, checks, and splits of different origin in wooden objects. Epoxy-based masses adhere well to a wooden surface and are relatively durable and long-lasting, which makes them a good choice for external applications. They are easy to work with (sanding, carving, etc.), leaving a nice finished surface; they can also be coloured by adding pigments to resins or painting the surface of an already cured mass [
11,
13,
16,
61,
76,
77]. Unfortunately, they are not free from drawbacks. First of all, even when mixed with fillers (e.g., microballoons), they can still have high compression modulus. Under humid conditions, the surrounding wood can be stressed because it cannot expand while absorbing moisture, which may result in its warping or crushing. On the other hand, when such wood dries, tensile stress can cause check formation between the fill and wood [
13,
14]. Additionally, due to the different mechanical characteristics of microballoons and epoxy matrix, the stresses localised in various parts of the composite itself can cause cracks and voids in the fill [
51]. Other concerns are the previously mentioned irreversibility, yellowing when exposed to heat or light (important for art or historical artefacts where epoxies are visible on the surface), and the heat released during the curing process [
13,
33,
77]. Nevertheless, epoxy-based masses have been applied frequently for gap-filling in wooden artefacts exposed outdoors, and, in some cases (e.g., in applications where wooden artefact remains in a moderately stable relative humidity or when wood does not undergo significant dimensional changes upon changes in moisture conditions), they can be the best choice [
13,
14,
77,
78].
Polyurethanes are synthetic resins produced via polyaddition reaction of dialcohols or polyols and polyisocyanates. Depending on the chemical composition, both thermosetting plastics and elastomers are available with different properties. Therefore, they can be used in a wide range of industrial applications. Generally, they have medium viscosity, which makes them easy to mix and process, but also to cast and shape. They can be combined with various additives, including inhibitors, pigments or fillers, to give them properties required for specific applications. They have a short curing time and do not shrink much upon curing; however, their adhesive properties are not as good as those of epoxy resins [
55,
79,
80,
81,
82,
83].
Polyurethane resins can be used to produce polyurethane foams, which are commonly used as insulating materials in construction. In accordance with their mechanical characteristics, particularly their rigidity, stiffness, and compressive and tensile properties, they can be categorised as flexible, semi-rigid or rigid foams [
82,
84]. Their good dimensional stability, high durability, low weight, low thermic coefficient, good moisture and compression resistance, and easy workability make them also suitable for filling gaps in wooden artefacts. Polyurethane foams adhere well to wood and are resistant to high temperatures, UV radiation, and chemicals. However, their moisture properties are not compatible with wood. They should be applied layer by layer to the dry wood surface to limit temperature rise and ensure the proper foaming process. Cured foams are easily workable with all tools typically used for wood and can be painted. They can also be mechanically removed from an object when necessary [
84,
85].
The term “vinyl resin” refers to different compounds, usually vinyl acetate, vinyl chloride, vinyl ether, vinyl butyral or copolymer blends of these. They can be used in dispersions, emulsions, and solutions and also mixed with different additives (e.g., pigments) to tailor their properties to specific applications. They polymerise easily at ambient or elevated temperatures, and once cured, they are flexible and resistant to water and various chemicals. They adhere well to different surfaces and therefore are commonly employed as adhesives and coatings for various purposes [
73,
86].
Vinyl resins have found application as consolidants and fillers in wood conservation since the early 1950s. Some examples include a mixture of polyvinyl acetate with sawdust and gypsum applied for filling cracks and holes made by wood-destroying insects, or a blend of polyvinyl butyral, glass microballoons, and pigments applied for filling loses in ancient dry wooden furniture [
31,
33]. There are also different commercial ready-mixed vinyl filling masses intended for patching plaster or wallboard in the construction industry, which can be adapted for wood conservation. These materials are flexible, have good working properties, and great adhesion to wood. However, they are irreversible, susceptible to UV and heat ageing (they become yellow and brittle), and may be swelled by water, thus damaging paints or gildings and swelling the adjacent wood layers [
33]. They may also shrink significantly upon curing and become much harder than low-density wood. All these make them less desirable as gap-fillers [
15,
30,
76].
Owing to exceptionally stable silicon-carbon and silicon-oxygen bonds, semi-organic silicone resins are particularly resistant to oxidation and thermal decomposition. Depending on the type of functional groups attached to the silicon atom, various polymers with different properties can be obtained and further modified via their reactive groups (alkyd, polyester, acrylic, and epoxy) to enhance their durability. They have high thermo-chemical stability and are resistant to external factors (humidity, UV radiation). These versatile materials are applied in almost all industry branches, as well as in art conservation [
73,
87,
88,
89].
Among silicon resins, particularly RTV (room-temperature-vulcanising silicone) silicones are suitable for filling losses in wooden objects. These dimethyl siloxanes are commonly available as proprietary formulations in most hardware stores. They are easy to apply, water-resistant, with long-term flexibility, and relatively stable when used outdoors. They do not penetrate wood tissue and can be easily removed by pulling them out of the voids. Their compression modulus is lower than that of most wood, and they can form a perfect, inert, durable, and elastic fill. However, they have poor working and finishing properties. To improve these, they can be blended with various additives, including pigments and fillers. The addition of fillers (e.g., powdered fibreglass or glass microballoons) increases their viscosity. This provides the surface with enough roughness to enable its finishing (sanding, carving, etc.) and painting with acrylic paints [
13,
16,
30,
63].