3 REVIEW OF BEVA 371 APPLICATIONS ON SKIN AND LEATHER

3.1 ADHESIVE PROPERTIES SOUGHT

There are several requirements of an adhesive for use in skin and leather repair. Ideally, the adhesive should be compatible with the substrate in its physical and chemical properties. It should not stain or change the appearance of the skin or leather and should remain flexible to allow movement of the object. The bond should be strong enough to hold the repair and to withstand stresses to which the object will be subjected. Such stresses can be induced by the weight of the skin or leather or through the handling required during mounting, exhibition, or travel. In terms of chemical qualities, an adhesive used for backing repairs should not interact with the substrate. It should be chemically stable and have good aging properties, i.e., it should release no harmful vapors and should not weaken over time. Also, the adhesive should be easily removable without adverse effects on the skin or leather. Thus, a stable adhesive that can create a sufficiently strong, flexible nap-bond would be ideal, as this type of bond minimally penetrates the skin and is most easily removed.

BEVA 371 meets most of these requirements. It can create a nap-bond without saturating the substrate. If required, it can be adapted (e.g., used at higher temperatures) in order to flow and more readily impregnate the substrate. It adheres very well to most skins and leathers. As previously mentioned (Fenn 1984), this characteristic is especially true for skin artifacts treated with wax polishes or oily leather dressings to which most adhesives, particularly water-based ones, will not adhere. Concerning aging or stability, BEVA 371 solution has proved to be highly stable according to the testing accomplished at the CCI, and it is assumed that the same holds for BEVA film. Reversibility can be accomplished by exposure to hexane or heptanes for a short time, in either liquid or vapor form, or by mechanical means, with or without heat (Berger 1976).

Compared to BEVA 371 solution, BEVA 371 film is a more recent product, but it is used increasingly for skin and leather repairs. As was seen in section 2.2, the film has been shown to produce more consistent bonds than the solution and gives a higher degree of control (Forest 1997). In relation to these results are BEVA film's practical advantages over BEVA solution: it requires no preparation time, is easier to apply, and is more even and uniform than any film cast in-house. BEVA solution, on the other hand, may need its thickness built up in layers, a process that requires diluting the solution, warming it up, and applying it on a backing either by rolling, squeegeeing, spraying, or flocking. Sufficient drying time is required between each application.

Although BEVA 371 has been used successfully on numerous occasions for skin and leather treatments (see sec. 3.2), in some cases it will fail to meet the objectives of a treatment or simply not succeed as well as an alternative. It can be helpful to examine the reasons invoked in the literature for not selecting BEVA 371 after its initial testing for a skin or leather repair treatment. Dignard (1989, 1992) mentions that BEVA 371 solution was considered for the treatment of tears in a kayak, but was avoided due to the lack of proper solvent extraction. Also, heat-setting or solvent reactivation in this case was difficult because of limited access to the back. Fenn (1984) reports that BEVA 371 solution used at room temperature did not adhere well to gut artifacts such as Inuit parkas and bags made from caribou stomachs. It also altered the translucency of the thin skin (if heat-set, the adhesive is stronger and becomes transparent, but presumably heat was to be avoided for these vulnerable materials). In these instances, more success was found using water-soluble (Klucel G or Modocoll EK1200) or water-dispersed (Elvace 1874 or CM Bond M2) adhesives. Fenn also describes issues concerning the treatment of buckskin shirts with friable pigments rubbed into their surface. Tests suggested that if these were to be treated with BEVA 371 solution, later attempts at reversal with specific solvents could drive the adhesive into the pigments, thus altering the color through saturation. For these reasons, it was believed that the polyvinyl acetate (PVAC) emulsions may give better results. Kite (1991) reports that in the treatment of an alumtawed fur-skin, the film was tested, and, although it held well, it was visually disturbing and seemed to penetrate the skin, making it look translucent. She ended up using a wheat starch paste–sodium alginate mixture with a paper backing material. For the treatment of a sheepskin lining of a saddle, Selm (1989) found that BEVA 371 solution used in solution or cast as a film (and presumably heat-set) gave a weak join, saturated the leather causing staining, and had an unpleasant, lingering smell. Instead, Paraloid B-72 film cast from a 15% solution in acetone was employed. The acrylic film was applied to Reemay and either heat-set or solvent-reactivated with acetone.

3.2 TEAR REPAIRS

Besides its initial use on paintings and textiles, BEVA 371 solution was also used at a fairly early date on leather, specifically upholstery. Sheetz and Cochran (1978) describe backing an upholstery leather piece with BEVA 371 solution and Stabiltex (now called Tetex). They describe the process:“After the [fill] repairs were made, the leather was turned unfinished side up and lined with a reinforcing material, Stabiltex, a sheer synthetic [polyester] material, both flexible and strong. The Stabiltex was placed over the leather and attached to it with BEVA 371, which was melted over the Stabiltex a small area at a time with a warm tacking iron.” Following this publication, literature on BEVA 371 repairs covers a range of objects, such as leather upholstery, saddles, gilt leather, and wall coverings. Most frequently, the treatments involve a system of applying the adhesive to a backing, or carrier, and heat-setting this “band-age” as a repair or stabilization measure. However, there are many interesting variations. The following review attempts to give a different perspective to the published information on skin and leather tear repairs using BEVA 371 by presenting the treatment variables that researchers have quantified (see table 1) and that practitioners have used to achieve the bond they required. It should be recognized, however, that probably the most important variable in determining the strength of the adhesive bond is the object's surface and condition (Berger 1972).

3.2.1 Variables in Manipulating the Adhesive

The quantity of adhesive plays a major role in bond strength. Lining tests have shown that, when heat-setting at 65�C and 70�C, doubling the thickness of the 2.5 mil BEVA film can double or even triple the peel strength (Forest 1997). An increase in weight in the BEVA 371 solution used in a lining context also results in an increase in bond strength when used at temperatures above 60�C (Pullen 1991; Gayer 1992; Hardy 1992). Pullen (1991) also gives correlations between temperatures and thickness of adhesives; for example, a bond strength achieved at 70�C could be produced at 65�C by using 50% more adhesive. None of the treatment articles that were reviewed specified the coat weight when using BEVA 371 solution. For the film, though, not surprisingly, the thickness is usually specified (as given by the manufacturer).

The most common method of applying BEVA 371 solution to the backing material in skin and leather treatments is to spread it on with a brush, allow it to dry, and, if necessary, reapply the BEVA in successive layers. The adhesive can also be spread by using a roller or a squeegee, or it can be sprayed on. As can be expected, the coat weights achieved using these methods can vary greatly (Hardy 1992). Another method of applying the BEVA 371 solution is to flock it onto the carrier fabric. This method differs from spraying in that it produces cobweb-type filaments of adhesive. Flocking is said to help reduce or better control the amount of adhesive applied. This method has been used for the treatment of deteriorated upholstery leather from two 17th-century chairs, using Reemay as the carrier (Howard and Berry 1995). The prepared backing was heat-set in the form of sutures for tears as well as in the form of a full lining to the back of the upholstery. Dignard and Gordon (1999) also use this flocking technique to apply BEVA 371 solution onto Stabiltex (Tetex). The prepared backings were heat-set to individual tears as well as to the full backside of a degraded and powdery fur trim and collar.

Generally in a conservation repair involving a backing, the adhesive is applied to a carrier and not to the object's surface. However, just as the application of a slight sizing of BEVA 371 to the back of a painting has been found to improve adhesion (Berger 1975), some practitioners have applied BEVA 371 to both the skin/leather and the repair material in order to increase the bond strength. Calnan (1992) used BEVA 371 film on the carrier and BEVA 371 solution as a primer to repair tears in Spanish gilt leather. The prepared lining was heat-set in place. Similarly, Sturge (2000) used BEVA 371 solution on the leather as well as the carrier and heat-set the elements in place. Fenn (1984) describes repairs using BEVA 371 solution applied sparingly on the inside of a weakened buckskin artifact. When the adhesive had almost dried, an equally sparing layer of BEVA solution was spread on the lining material and pressed onto the artifact without heat. It is likely that the success of this practice is due in part to an increase in wetting or contact with the object's surface. Experimental tests to compare these various application practices were accomplished by Calnan et al. (1991). They found good bond strengths (as measured by peel tests) resulted when BEVA 371 solution was heat-set after having been applied to the backing fabric alone, to the new upholstery leather alone, and to both the fabric and the leather. A slightly stronger bond was achieved when the adhesive was applied to the leather alone. However, when the tests were repeated using a thinner, deteriorated leather, the most appropriate bond for BEVA 371 was formed by applying the adhesive to the support fabric only, rather than on the leather only (application to both was not tested in this case). These somewhat contradictory results suggest that the condition of the skin object, and perhaps other treatment variables, plays a large role.

BEVA 371 solution has also been tested and used in leather conservation as a discontinuous film applied to a carrier fabric and heat-set onto the object. The optimum arrangement was found to be a series of 2 mm diameter dots, 1 mm thick, 5 mm apart, in a staggered formation. A 1 mm thick aluminum sheet was used to produce the pattern (Calnan et al. 1991). The theory behind this preparation was to provide the backed leather with more freedom of movement when exposed to a fluctuating climate. Tests performed on this repair technique did show an increased flexibility as compared to a continuous film of adhesive, but, as could be expected, a weaker bond resulted. Such a discontinuous film of BEVA 371 solution was used with Reemay to fully support the front-seat cover of a 19th-century Panhard-Levassor automobile (Calnan 1991). This method of application appears to be rare, as there are few reports of its use in the literature.

The activation temperature used will affect the degree of penetration or impregnation of the adhesive into the substrate and the color change or staining of the substrate. BEVA 371 solution becomes tacky at 55�C, liquid with an aggressive tack at 65�C, and at 70�C produces an even stronger bond (Berger and Russell 2000). In the case of the BEVA 371 film, peel strengths on lined canvases were found to vary considerably within this 15�C range of temperature: at 55�C they were too weak to even be measurable using a tensometer; at 60�C they were too low to ensure acceptable lining for a canvas painting (average of 0.05 N/mm); at 65�C they were on average approximately 10 times higher than at 60�C, falling within what was established as an acceptable strength for linings; and at 70�C they were found to be quite strong, being approximately twice as high as at 65�C (Forest 1997). This direct relationship between bond strength and temperature was also measured for BEVA 371 solution (Pullen 1991; Gayer 1992; Hardy 1992). In view of these results, the performance of hot spatulas or other heating devices becomes very important, in terms of the accuracy of the temperature and of its precision (or variability of the heat delivered). Although such data are not outwardly presented in the treatment literature, it is reasonable to assume that treatment temperatures for skins and leathers have fallen within, or close to, this range, depending on the required results. For example, in the treatment of a very fragile, fragmented ermine fur lining of a cape, Kite (1990) mentions heat-setting BEVA 371 film at 70–75�C using nylon gossamer as the backing material. Similarly, Calnan (1991), in the treatment of elements of the previously mentioned 19th-century automobile upholstery, mentions heat-setting a discontinuous film (dots) of BEVA 371 solution at 70�C. Other reported heat-set treatments using the film include the treatment of torn stitchings within a saddle (Sturge 2000), a variety of skin materials including deerskin drums and lizard natural history specimens (Nieuwenhuizen 1998), a pair of fine suede gloves (Kite 1996), and a leather sedan chair (Selm and Bilson 1992). Heat-set examples employing the solution include the repair of Spanish gilt leather (Calnan 1992), repairs to parts of the above-mentioned 19th-century automobile upholstery (Calnan 1991), and the treatments by Howard and Berry (1995), Dignard and Gordon (1999), and Sheetz and Cochran (1978) mentioned earlier. One case study describes the use of very high temperatures to repair fragmented and weakened vegetable-tanned leather car upholstery. During this treatment, BEVA 371 solution was impregnated into the Reemay backing material and heat-set to the leather at 100�C using a short contact time (Sturge 2000). This is said to have allowed some reshaping of the leather while the BEVA was warm and soft, while producing a strong, secure bond once the adhesive was cool. In the vast majority of cases, such high temperatures are avoided for degraded skins and leathers.

BEVA 371 bonds quasi-instantaneously once the activation temperature is reached (over 60�C), but if the activation temperature is applied for a length of time, the bond strength is significantly affected. In a series of lining tests, Forest (1997) found that, with the hot table used, it took approximately 12, 14, and 18 minutes to reach the activation temperatures of 60, 65, and 70�C respectively, and about 20 minutes to cool back down to room temperature. If a holding time of 10 minutes was added when each activation temperature was reached, a bond of twice the peel strength resulted, as compared to the bond created with no holding time. In particular, it was found that at 65�C with no holding time, the bond was too weak for half of the samples. But, with the 10-minute holding time under the same conditions, the peel strengths ranged between moderate and too strong. Also, it is likely that the rate at which the BEVA 371 is heated and cooled may affect strength (Forest 1997). Holding time was not found to be reported in the treatments surveyed, but it would be expected to be in the range of seconds or tens of seconds rather than minutes. Rate is never mentioned either and most likely varies with the type, quality, and age of the equipment used.

Another treatment variable that affects bond strength but is not often quantified in skin and leather treatments includes the amount of pressure applied (Gayer 1992). In the case of the BEVA 371 solution, the amount of solvent retained or the length of time for solvent evaporation to occur prior to use will also have an impact on bond strength (Hardy 1992).

Conservators have often chosen to use BEVA 371 solution without any heat beyond room temperature to avoid potential heat damage. This technique results in a weaker bond than that achieved through heat-setting (Calnan et al. 1991). In some cases a weak bond is all that is required. Boulton (1986) describes repairs to a pair of Aleutian Islands boots using BEVA 371 solution diluted in toluene, brushed onto goldbeater's skin, and applied at room temperature. Fenn (1984) also mentions the use of BEVA 371 solution without heat with various nonsynthetic backing materials to repair Inuit clothing water-proofed with sea mammal oils and also native-tanned clothing. Kronthal (2001) experimented with BEVA 371 solution at room temperature for some repairs to rawhide shadow figures.

In painting conservation, if temperatures lower than the activation temperature are required, they can be obtained by spraying films of the BEVA 371 with solvents. According to the BEVA 371 film technical data sheet, spraying the film lightly with naphtha makes it tacky at about 38–43�C (methylene chloride is also mentioned, but it is highly toxic). For BEVA 371 solution, the activation temperature can be lowered to 40–45�C or less by lightly spraying with aromatic mineral spirits. It can also be lowered by using the adhesive about two hours after applying it to the backing material, while it still retains some of the solvents (Berger 1975). No references to using BEVA 371 in this way on skins and leathers were found in the literature.

Another method of using BEVA 371 that avoids the use of heat is solvent reactivation of the dry film. The solvent can be sprayed onto the film or, for small repairs, applied with a syringe or a fine brush. If the solvent is applied with a syringe, it becomes possible to position the adhesive-covered carrier behind the object while the adhesive is in a dry state, an advantage shared with the heat-setting technique. The amount of time that good contact must be held through the application of pressure before the bond is achieved can be short, but it depends on the type and quantity of solvent used. In comparison, heat-setting involves quasi-instantaneous bonding time. Solvent reactivation was one of several methods used by Kronthal (2001) in the treatment of shadow puppets. In this context, goldbeater's skin was brushed with the BEVA 371 solution. This solution was left to dry as a film, and the backing and adhesive were applied by reactivation with naphtha or petroleum benzine. Though this technique was sufficient in many cases, a stronger bond was achieved with other adhesives in combination with the gold-beater's skin.

3.2.2 Choice of Lining Material

Many carrier materials have been used for skin and leather repairs, depending on the nature of the substrate and the required results. Their composition, method of construction (woven or nonwoven), and nap can affect the bond strength (Calnan et al. 1991; Gayer 1992; Daly Hartin et al. 1993; Forest 1997; Berger and Russell 2000) as well as flexibility of the repair.

Both spun and woven synthetics have been used successfully, including Stabiltex (Tetex) woven polyester (Sheetz and Cochran 1978; Dignard and Gordon 1999), Hollytex spun polyester (Nieuwenhuizen 1998), Reemay spun polyester (Kaminitz and Levinson 1988; Calnan 1991; Howard and Berry 1995; Nieuwenhuizen 1998; Sturge 2000), Dacron woven polyester taffeta (Tsu et al. 1999), Arvex woven polyester or polyester sailcloth (Calnan 1991; Calnan 1992; Selm and Bilson 1992), and Cerex or nylon gossamer (Kaminitz and Levinson 1988; Kite 1990; Calnan 1991; Selm and Bilson 1992; Kite 1996).

Natural materials have also been used. Tsu et al. (1999) describe repairs to tears within an 18th-century gilt wall hanging using Japanese paper with BEVA 371 solution in combination with the BEVA gel. Fenn (1984) used both new oil-tanned skin and cotton fabric with the BEVA solution in treating native tanned skins (Fenn 1984; Tsu et al. 1999). Skin lining materials, such as goldbeater's skin and natural skin condoms, have also found useful applications. For example, Kronthal (2001) found that the translucent properties of goldbeater's skin matched those of a collection of Chinese shadow puppets while also producing a strong, flexible mend. Boulton (1986) has also used goldbeater's skin with BEVA solution at room temperature to treat Aleutian boots.

3.3 FILLS

One can utilize BEVA 371's thermoplastic properties and flexibility to create a fill material and to replicate textured surfaces. Calnan et al. (1991) investigated a series of possible polymeric fills, including the use of BEVA 371 film. They found that the heated film could be worked to create a flexible and extensible fill and had the following advantages: it requires virtually no drying time, it is easy to reverse with heat or solvents, and it is easily overpainted with acrylic emulsion paints. They also noted that the fill material needed reworking to ensure a smooth finish and uniform adhesion and had a tendency to spill over onto the immediate surrounding leather surface. In this case, the preferred application involved heat activation of small rolls of the film placed in the crevice and building up the fill material in layers to ensure that the adhesive was fully activated and bonded well to the leather. Kaminitz and Levinson (1988) used the solvent form of BEVA 371 mixed with dry pigments and glass microballoons to fill losses in untanned skin stretched over wooden drums and harps. The mixture was applied in a thin layer over an insert of Japanese tissue paper or synthetic web fabric. In this case, considerable strength was required to join the fill material to the very thin edges of the loss, and great flexibility was necessary to allow the skin to respond to environmental changes without separating from the fill. Nieuwenhuizen (1998) mixed warmed BEVA 371 film with dry pigments and glass microballoons and applied it as a fill material to replicate textured surfaces. In this case, the solution was avoided in an attempt to minimize shrinking. Sturge (2000) describes the use of colored BEVA “sticks,” made by mixing small amounts of dry pigment into BEVA 371 solution and allowing the solvents to evaporate after spreading the mixture onto silicone paper. Once dry, the solid BEVA 371 can be cut into strips and melted into cracks and splits using a heated spatula.