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The information presented on the Paintings Conservation Wiki is the opinion of the contributors and does not imply endorsement or approval, or recommendation of any treatments, methods, or techniques described.
Author: Christine Romano
Editors: Anne Schaffer, Christine Gostowski
- 1 Introduction
- 2 Causes of Media Failure
- 3 Considerations for Choosing a Consolidant
- 4 Materials and Tools
- 5 Natural Consolidation Materials
- 6 Synthetic Consolidation Materials
- 7 Other Techniques
- 8 References
- 9 Further Reading
Consolidation refers to the stabilization of degraded or weakened areas by introducing new materials capable of holding them together (AAT 2002). As a treatment intervention, paintings conservators generally use an adhesive, known as a consolidant, to secure detached, friable, lifting, or otherwise insecure media in order to stabilize and preserve original material.
Consolidants are usually made of two parts: an adhesive and a carrier solvent. In most cases, the carrier solvent will evaporate, leaving an adhesive film behind. Other materials, such as bulking or plasticizing agents, may also be added to modify the properties of the consolidant.
The treatment of isolated areas of lifted paint is commonly referred to as local consolidation. In cases when the entire paint film is badly delaminated, consolidation can be carried out overall, typically through lining or other structural treatments.
Causes of Media Failure
Consolidation is necessary when the paint or media is in danger of separating from the support. Media failure can be caused by any number of external and internal factors, including drastic changes to the painting’s environment, damage from previous treatment, heat or impact, and the inherent vice of the materials. Paintings requiring consolidation can exhibit deterioration in a number of manifestations, such as paint cleaving, cupping, or flaking.
Identifying the type of paint deterioration and its potential underlying causes can help to determine subsequent treatment steps.
As paintings are composite objects, their constituent materials will react to environmental fluctuations in different ways. Abrupt changes in temperature and/or relative humidity (RH) can cause a painting’s support and glue sizing layers to swell and expand, resulting in the formation or worsening of cracks when the climate returns to normal levels (Michalski 1991, Mecklenburg 1991, 1994, 2004). Alternatively, if the support contracts to a greater degree than the paint film, such as when the RH drops very low, tenting paint and blind cleavage can occur. Tenting can also arise when paintings with certain cotton or linen canvases (known as “shrinkers”) are exposed to high RH or direct moisture. In these cases, the shrinkage occurs when swollen, enlarged weft yarns cause the crimped warp yarns of the canvas to become accentuated and drawn more closely together, which in turn forces the paint upward (Pocobene & Hodkinson 1992).
The storage or display environment in which a painting is kept can also have an effect on its condition beyond movement of the support. Poor storage or unforeseen disasters may result in water exposure, mold growth, and insect damage, which can lead to compromised paint layers. Paintings in an environment of 70% RH can grow mold within 24 to 48 hours (CCAHA 2013, Erhardt et al. 2007). Mold and other biological activity can result in the increased powdering and sensitivity of the paint due to the weakening of the binder.
The response of previous conservation materials to the painting’s environment must also be considered. If the strength and rigidity of the introduced consolidant exceeds that of the surrounding paint film, new patterns of deterioration can develop at the site of the repair during the painting’s natural movements in response to its environment.
Excessive exposure to heat, whether from a fire or from previous restoration, can result in the disintegration and flaking of paint, scorching, alligatoring, and the formation of blisters. Blisters are extremely fragile air pockets that can occur between paint layers after the painting is exposed to excessive heat.
Impact and Handling
Accidents, poor handling, and inadequate storage can result in punctures, tears, or indentations in the canvas (Shelley 1987). Damage from impact or handling can cause the paint to lift or detach. In many cases, the support must be humidified, flattened, mended, or otherwise stabilized prior to or alongside the consolidation of the paint layer.
Less easily perceived causes of instability in the paint and ground may occur due to inherent vice, which includes not only the artist’s use of incompatible materials (e.g. acrylic paint layered over oil paint), mixed media, or poor quality materials, but also poor painting techniques (e.g. not following the “fat over lean” rule). In such cases, the paint may delaminate from its support or other paint layers, requiring consolidation.
Detachment due to poor adhesion between different layers is known as interlayer cleavage. This common type of delamination can occur between either two distinct paint layers, such as between the paint and ground, or between the ground and canvas. Interlayer cleavage is caused by the chemical and physical incompatibility of layers, which can be exacerbated by an unstable climate where temperature and RH fluctuates.
Poor cohesion of the pigment and binder can result in intralayer cleavage, in which the detachment occurs within a single layer of paint. Cohesive failure can sometimes be caused by high pigment volume concentration, which results in the chalking or powdering of the layer (Hansen and Lowinger 1990). Another cause of intralayer failure is the presence of metal soaps in the paint layer. Metal soaps primarily occur in lead- or zinc-containing oil paints, where metal cations from the pigment interact with free fatty acids from the oil binder. In some modern paintings containing zinc white paint, delamination occurs within a single layer, with the zinc white present on both the support and the underside of the detaching flake (Rogala et al. 2010). The soaps can accumulate at the interfaces between paint layers and cause delamination (van Loon 2016).
Considerations for Choosing a Consolidant
Because consolidants are often impossible to fully remove from a painting, it is essential to select an adhesive with properties appropriate to the task. Among the considerations in selecting and applying a consolidant are:
- the solubility and composition of the painting’s materials
- compatibility of the consolidant to the painting’s materials
- the strength, appearance, and aging properties of the consolidant
- the type of media failure being consolidated
- storage and display requirements
- the impact of the chosen adhesive in future treatments
- whether the work requires local or overall consolidation
Type of Media Failure
It is important to identify areas of instability and determine the underlying causes for the condition, as they may influence consolidant choice. For example, it will likely be ineffective to consolidate powdering paint by brush, as loose particles will become dislodged. Instead, the adhesive might be applied using a non-contact method, such as by spraying diluted consolidant onto the area with an atomizer. Conversely, in a situation where a thick, impastoed passage of paint is lifted, a bulked or high molecular weight adhesive may be the best choice for securing the paint in place. The type of consolidant and carrier should be determined based on the considerations listed above, the properties of the particular adhesive, as well as any material limitations that the work of art presents (Hansen et al. 1996).
Concurrent Condition Problems
In some instances, paintings requiring consolidation exhibit additional condition problems that must be addressed concurrently. For example, if an area of flaking paint is also torn and distorted, the support must be brought back into plane and mended prior to or alongside consolidation efforts. Adhering paint without addressing the underlying structural problems can result in the improper alignment of paint flakes on the surface. Likewise, a cupping paint film often cannot be consolidated in a single step. Because cupped paint is concave in morphology, the support and paint must first be relaxed, typically with a local application of water or solvent vapors. In an emergency scenario, loose paint layers may require securing with a paper facing tissue in order to address larger problems, such as the stabilization or removal of a critically damaged support (Levenson 2012).
If the painting requires cleaning, the conservator should assess whether the work can be safely cleaned prior to consolidation. This may be important to consider if the grime layer covers light-colored passages, as the consolidant can trap loose dirt in the cracks, increasing their visibility. In any case, loose debris should be brushed away from the area to be consolidated, if possible, so as to avoid adhering fibers, dirt, and other airborne pollutants to the painting’s surface.
The selection of a consolidant will be contingent on the paint film’s sensitivity to certain solvents. Before proceeding, the paint and ground layers should be tested to determine their sensitivity to any carrier solvents that may be used for consolidation and the subsequent removal of excess consolidant (Hansen & Volent 1994). The speed or rate at which the solvent evaporates is another important factor that may influence the choice of consolidant, as certain adhesives can only be dissolved in certain solvents. A slower evaporating solvent may allow the consolidant to flow more deeply through cracks or between layers, while a faster evaporating solvent may be purposefully used to limit the movement of the adhesive.
Working Properties of Consolidant
The working properties of a consolidant includes its behavior and appearance once it is introduced into the paint system. The chosen adhesive should be stable, reversible, and slightly weaker than the existing adhesive forces within the object to which it is applied. In order to be effective, it should have a glass transition temperature (Tg) above room temperature (Horie 1987). Consolidants with a higher than room temperature Tg exhibit greater tensile strength and are less likely to undergo adhesive failure, cold flow, or creep in normal storage or display conditions.
Working properties to consider include but are not limited to:
- shrinkage on evaporation
- rate of solvent evaporation
- glass transition temperature
- physical and chemical compatibility with original and pre-existing restoration materials
- visual alteration of paint (such as saturation, staining, or change in gloss)
- aging effects, such as yellowing and irreversibility
- compatibility of consolidant with subsequent treatments
- possible methods of application
- surface tension and capillary action (i.e. wicking ability)
Application and Technique
The manner in which a consolidant is applied to a paint layer should be determined based on the needs of the painting. The location, magnitude, and type of deterioration present, as well as the chemical and optical properties of the paint film, are all factors to be considered when deciding how best to apply a consolidant.
Examples of consolidation techniques include but are not limited to:
- By brush
- By injection with syringe
- From the front or back
- By ultrasonic mister, or other spray technique
- Through facing tissue
- Over a suction platen or table
- Re-cohesion with solvent
- Humidifying areas prior to consolidating
- Using a “leader solvent” such as ethanol or ethanol/water solution to reduce surface tension prior to consolidating
- Warming the adhesive (can decrease viscosity and allow for better penetration)
- Warming the paint layer from front or back with a radiant heat tool, and setting down with light pressure
- Warming with a tacking iron after applying consolidant
- Consolidating over suction, or allowing the paint to dry under suction following application
- Placing under weights after consolidation
- On a warmed hot table
- Overall infusing with adhesive, or lining to a secondary support with adhesive
Gloss and Visual Alteration
During the process of consolidation, the conservator should avoid altering the appearance of the paint wherever possible. Porous paint can quickly darken when the voids and air spaces surrounding the underbound pigment are filled by a consolidant (Hansen 1996). Similarly, a glossy adhesive applied to matte paint can result in darkened, glossy passages, and the inverse is also possible—matte adhesives can cause dulling or blanching of glossy surfaces. In some cases, the change in gloss is irreversible, so it is crucial to determine an adhesive and application method most appropriate for the situation at hand.
The creation of tidelines, especially in matte films, can be caused by the consolidant’s solubilization and redistribution of dirt and other low molecular weight particles within the paint (Hansen 1996). Changes in appearance related to aging should also be avoided, such as the yellowing or darkening of the adhesive. This visual alteration is especially disfiguring in light-colored passages.
Complete removal of a consolidant from a painting is nearly impossible (Horie 1987). Paintings lined in the past with wax-resin adhesives are extremely difficult to re-treat using aqueous and emulsion-based consolidants due to the incompatibility of the materials. Today, conservators consider consolidation materials to be compatible if they do not harm or worsen the painting’s appearance or structure over the long term, and do not preclude the possibility of future treatment with different materials.
Short-term consolidation efforts should be readily reversible without adversely affecting the painting. The application of facing, or the temporary securing of loose paint with an adhesive applied through a thin paper, is one example of a consolidation step in which easy reversibility is critical. Facings are sometimes necessary if a painting that exhibits flaking is to undergo laborious structural stabilization or be subject to vibration and handling. Weak adhesives that will not penetrate the paint layer, such as starch-based paste mixtures, are useful on paintings that are not sensitive to water. Wet-strength paper and machine-made Japanese papers are good choices for facing papers, as they exhibit minimal shrinkage (von der Goltz et al. 2012).
Infusions and linings are overall consolidation interventions wherein a layer of adhesive infuses the canvas, ground, and paint layers, typically with heat and vacuum pressure. In linings, a secondary material is simultaneously attached to the reverse. The adhesive flows through the cracks and secures lifted layers in place once cooled or set. It is very difficult, if not impossible, to fully remove all of the adhesive from an infused or hot-melt lined painting. These techniques have a long history of use and debate in the field of conservation, particularly as new studies and approaches offer safer alternatives (Stoner 1994, Ackroyd et al. 2002, Villers 2003). While linings are sometimes necessary for stabilizing a painting, the materials and methods should be carefully considered to minimize harm to the painting and allow for future re-treatability.
Storage and Display
The storage and display of the work may have an impact on consolidation approaches. Prior to treatment, the conservator should determine whether the painting will be placed into a controlled or changeable environment. If the artwork is to travel, the strength and flexibility of the adhesive will be important to consider, as fragile structures are vulnerable to damage from vibrations (Green 1991).
Materials and Tools
Below is a list of commonly use materials and tools for the consolidation of paintings:
- Spray equipment, e.g. ultrasonic mister, pneumatic spray gun, air brush
- Hand tools for lifting flakes and manipulating paint film, e.g. micro tweezers, microspatulas, scalpel, metal or tungsten probe, bamboo skewers, vacuum tweezers
- Heated spatulas, tacking irons, radiant heat tools
- Stirring hot plate for mixing and warming adhesive solutions
- Glassware and lidded jars
- Ventilation sources, such as a fume hood, ventilation trunks, or portable extraction units
- Magnification source, such as a stereomicroscope or Optivisor
- Suction table or variable suction platen, such as the Mitka suction disk (to be used with or without heat)
- Humidification materials, such as Dahlia sprayer, blotter, polyester web, Plexi or glass weights, Gore-tex, polyester film and structural frames for creating chambers, etc.
- Non-stick membranes, such as silicone-release paper or polyester film
- Facing tissues, such as wet strength tissue or machine-made Japanese paper
- Vacuum hot table
- Appropriate solvent for removing excess adhesive
Natural Consolidation Materials
The following are a few common examples of mammal, fish, and plant-derived adhesives used by conservators in the past and present.
Protein-based glues have a long history of use in artistic and artisanal fields as well as in conservation. Some of the more commonly used glues in conservation are derived from parchment clippings, rabbit skins, cow hides or bones, and the swim bladders of sturgeons. The natural polymer responsible for the adhesive properties of proteinaceous glues is collagen. Collagen is composed of covalently bonded amino acid protein chains that form a triple helix structure through hydrogen bonding. The triple helix bonds must be broken, or denatured, using heat or other treatments in order to change the raw material into the soluble gelatin used by conservators (Schellman 2009). Protein-based glues can vary in their adhesive properties, but all will shrink and embrittle as they age, and are extremely reactive to changes in relative humidity (Mecklenburg et al. 1991, 1994).
Gelatin is a glue traditionally made of animal hides. Gelatin can vary widely in quality and purity by source, with food-grade and photographic-grade being the most consistently pure. Gelatin and mammalian glues tend to have a higher glass transition temperature than glues derived from marine species, therefore requiring more heat for use and preparation, while their tack is slightly lower than sturgeon glue. Freshly prepared gelatin has a relatively short shelf life, and excessive heating will denature the protein completely, rendering it ineffective (Schellman 2009). Hide glue can be purchased bottled and ready-to-use, though it should be noted that these products include a number of preservatives and modifying agents in their ingredients that may be undesirable for the consolidation of paint.
Sturgeon glue, also referred to as isinglass, is made from the swim bladder of the sturgeon fish. Sturgeon glue became popular in Western conservation toward the end of the twentieth century, though it has been used by Russian conservators for many decades. Sturgeon glue has greater tack, lower viscosity, and a lower gelation temperature than mammalian glues, requiring less heat for preparation and use (Petrukhova et al. 1993). It is also reported to retain its adhesive properties better with age, and undergo less change in dimension compared to gelatin (Michel et al. 2005). However, like all collagen-based adhesives, its shelf-life is short, and continued heating will denature it completely. Sturgeon glue is not not easily re-solubilized with water once dried (von der Goltz et al. 2012).
Funori is a comparatively weak adhesive prepared from any of the three red funoran seaweeds found along the Japanese coasts. Funori is washed and soaked to remove salts and impurities, and then sold as dried mats (Swider and Smith 2005). Funori has a long history of use in Japan by scroll makers, and has been employed in recent decades for the consolidation of friable, matte paint by Western conservators. It is flexible, has a low viscosity, and dries matte, making it a good choice for matte, underbound paint (BPG Wiki 2018). A more highly purified product, Junfunori® , has been developed to give conservators a more consistent product (von der Goltz et al. 2012). In addition to its use alone, Junfunori® has been combined with sturgeon glue to make a strong, matte consolidant (Geiger and Michel 2005).
Synthetic Consolidation Materials
The following is an incomplete list offering examples of some common synthetic adhesives used by conservators in past and present years.
Paraloid® B-72 (previously known as Acryloid® B-72) is an ethyl methacrylate/methyl acrylate copolymer thermoplastic resin produced by the Dow Chemical Company. It can be purchased by weight as solid, transparent pellets. Paraloid® B-72 is expected to remain clear, colorless, and soluble in the solvents in which it was initially dissolved for at least 200 years, making it one of the most stable resins available to conservators (Feller et al. 1981). It is soluble in xylene, toluene, acetone, and 100% aromatic hydrocarbons (Cameo Materials Database 2015). The high viscosity of B-72 makes it a good choice for readhering materials, but less well-suited for penetrating paint films or consolidating underbound paint. Its glass transition temperature is above room temperature (40°C or 104°F). As it is thermoplastic, B-72 can be activated with heat to flow and set lifting paint back into plane (PSG Wiki 2014).
Acrylic resin predecessors to B-72 include Acryloid® B-67, a poly(isobutyl methacrylate), also known also as Paraloid® B-67 or Elvacite® 2045, Paraloid® F-10, a poly(n-butyl methacrylate), also known as Elvacite® 2044, and Plexisol® 550 (PSG Wiki 2014). Conservators have largely stopped using these resins due to concerns of their yellowing and decreasing solubility with age.
Lascaux 4176 (Medium for Consolidation)
Lascaux 4176, or Medium for Consolidation, is an aqueous dispersion of an acrylic copolymer based on acrylic ester, styrol, and methacrylic ester. It was developed by the Swiss company Lascaux Colors and Restauro with funding from the Swedish Heritage Board in 2005 for the consolidation of polychrome sculpture and painted surfaces (Hedlund & Johansson 2005, Pataki-Hundt 2018). Lascaux Medium for Consolidation has a pH of 8.5, and can be thinned with water during application. Its low viscosity allows for good penetration and consolidating of loose, underbound paint. Once dry, it is soluble in esters, aromatic hydrocarbons, xylene, toluene, acetone, and MEK (Lascaux 2017, Kremer 2018). Hedlund & Johansson (2005) report that the consolidant is a stable, flexible resin that retains its appearance over time without yellowing.
It should be noted that recent research by Pataki-Hundt (2018) into the properties of several consolidants refutes the claim that Medium for Consolidation will not yellow, and moreover notes that the adhesive releases acrylic acids that cause corrosion on the lead coupon in an Oddy Test.
Modified cellulose-derived adhesives, such as methylcellulose and hydroxypropylcellulose (HPC) (e.g. Klucel-G), have been used by paper conservators since the mid to late twentieth century. These adhesives are dissolved in water, ethanol, or polar solvents and typically used as a weak adhesive to consolidate underbound paint. Klucel-G dries matte and tends not to saturate paint as much as other consolidants—for this reason, it is often employed when dark, underbound colors require consolidation (BPG Wiki 2018). Please refer to the BPG Wiki on Consolidation for a more in-depth view of this material and links to recipes.
Klucel HPC adhesives are produced in several viscosity grades:
- Klucel EL: MW=40,000 g/mol
- Klucel E: MW=80,000 g/mol
- Klucel L: MW=95,000 g/mol
- Klucel J: MW=140,000 g/mol
- Klucel G: MW=370,000 g/mol
- Klucel M: MW=850,000 g/mol
- Klucel H: MW=1,150,000 g/mol
Conservators have frequently used Klucel G for the consolidation of matte paint, though other grades have been used in poultices, inpainting media, and
Klucel HPCs are noted by the manufacturer to become cloudy prior to precipitating out of solution when heated to 40 degrees C (104 F). Higher viscosity Klucel HPCs can agglomerate and become stringy when heated. Dissolving the Klucel HPC in solvents will change its behavior (Ashland 2018).
Feller and Wilt’s 1993 study found the higher molecular weight grades of Klucels (such as M and H) to be less stable than lower molecular weight grades (G and L). They also concluded that cellulose ethers are unsuitable to long-term applications, as they will discolor and lose flexibility under moderate aging conditions. Similar findings were published in a 2005 study by Geiger and Michel, where Klucel E’s low adhesive strength and darkening following consolidation was noted. Accelerated aging of the consolidant resulted in a shiny, milky film that contracted and cracked. Conservators should be aware of these potential aging effects, and continue to be judicious in the selection and use of any consolidation materials.
PVA adhesives can refer to poly(vinyl acetate) and/or poly(vinyl alcohol), which were used either separately or mixed together from the 1930s until recently (Bria 1986, von der Goltz et al. 2012). The properties of PVAs are largely dependant on the solvent in which they are dissolved, but they tend to have high pHs (Hansen et al. 1991). PVAs are thermoplastic and soluble in acetone, alcohols, and toluene. Various grades with differing properties have been produced and discontinued by Union Carbide (AYAA, AYAC, AYAF, AYAT). As a result of their uncertain availability and further developments in acrylic polymers, PVA resins in solvent are not as commonly used today (von der Goltz et al. 2012). PVA emulsions, however, such as Jade 403 (manufactured by Jade Adhesives, Chicago), remain popular with conservators for small structural repairs and various other interventions, occasionally including consolidation.
BEVA 371 is an ethylene vinyl acetate (EVA)-based adhesive mixture created by Gustav Berger in 1969 as an adhesive for vacuum hot table linings. BEVA 371b, a new version of the adhesive dating from 2010, substitutes Laropal A-81, an aldehyde resin, for Laropal K-80 (which replaced the Ketone Resin “N” used in the original formula) (Ploeger et al. 2014). It can be purchased as a thin cast film (Beva 371 film), or as a concentrated resin mixture in toluene. BEVA is also available as an aqueous dispersion (BEVA D-8) and as a gel mixture of EVA, acrylic resin, and cellulosic materials (BEVA Gel).
BEVA adhesive is strong, flexible, can be diluted and used hot or cold, and dries with a matte, waxy finish. It is thermoplastic and can be activated by heat at 60° C (140° F). BEVA is soluble in aromatic hydrocarbons (Berger 1975). The instructions that come with the adhesive recommend taking one pint of BEVA and one pint of either naphtha, xylene, or a mixture of the two in order to make a stock solution of 37% resin to solvent (Talas 2016). It is best used warm, as it is better able to flow into cracks. BEVA is known to yellow over time, and its long-term stability continues to be investigated (Ploeger et. al 2014, Karsten & Kerr 2013).
Aquazol is a poly(2-ethyl-2-oxazoline) resin available in a number of molecular grades, each with different handling properties. The following molecular weights are most frequently used by conservators:
- Aquazol 50: MW=50,000 g/mol
- Aquazol 200: MW=200,000 g/mol
- Aquazol 500: MW=500,000 g/mol
Aquazol resins are soluble in a number of polar solvents, including water, MEK, acetone, ethanol, methanol, methylene chloride, and are slightly soluble in toluene and n-pentane, among others (Polymer Chemistry Innovations 2018). Depending on the solvent chosen, Aquazol has good bond strength, minimal shrinkage, a matte sheen, and can be used to penetrate paint layers and aid in plasticizing and relaxing cupped paint (Arslanoglu 2003, 2004). Different molecular weights of the resin can be combined to achieve the desired viscosity and adhesive strength. Conservators have reported that certain batches of Aquazol will turn yellow in color, either in solution or while stored in dry pellet form, and it has also been noted to undergo cold flow while stored in environments of 50-70% RH and 70-75°F (Arslanoglu 2004). The greatest concern, however, is Aquazol’s response to higher RH, where films of the resin were found to become tacky at around 60-65% RH (Muros 2012). In such cases, the adhesive was seen to swell and, in a few instances, lose some of its adhesive strength.
It is sometimes possible to regenerate or reactivate glue sizing layers or previous consolidation adhesives. For paint that appears to be cupping due to desiccation of the glue sizing layer, humidification or the introduction of warm water to cracks can sometimes return the paint to plane and either fully or partially readhere it (BPG Wiki 2018). If a flaking paint layer had previously been consolidated with a known synthetic adhesive and is to be re-treated, an appropriate slow-evaporating solvent can be introduced to reactivate the old consolidant. In many cases, additional adhesive is necessary.
In their paper on the consolidation of acrylics, Gridley and Cranmer (2007) describe a method of alternating brush applications of gelatin and alcohol into fine, lifted cracks. It is presumed that the alcohol is plasticizing the paint, “allowing it to coalesce and re-form in plane while drying under weights” (Gridley & Cranmer 2007). In another case study, the treatment of Théorème de Gödel, 1957, by Georges Mathieu, involved the successful consolidation of paint in part by solvent regeneration. Over low suction, ethanol (chosen for its low surface tension and good wetting abilities) was injected by syringe into lifted impastos and cracks and weighted with gently-conforming weights until dry. The cohesion was regenerated in several passages, though it was necessary to use an adhesive in others (de Ségogne 2014).
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