NOTES ON A METHOD FOR CONSOLIDATING LEATHER
Morgan W. Phillips
ABSTRACT—An experimental method of consolidating leather by the precipitation within the leather of polymethyl acrylate is described.
ALTHOUGH LEATHER IN FAIR or good condition may benefit from lubrication or the adjustment of moisture content, crumbly (often “red rotted”) leather may instead require consolidation—the introduction of a resinous binder. The subject of consolidating old leather has received relatively little attention. In 1972 Waterer reported successful consolidations of leather using dilute solutions of a polyacrylate resin; in some cases repeated applications of the solution were used.1 In 1978 Nikitina described the impregnation of fur clothes with an aqueous dispersion of a copolymer of vinyl acetate and 2-ethyl hexyl acrylate.2 In the same year Thompson noted that BEVA 371 had a consolidating effect on leather.3 Also in 1978 Hallebeek and van Soest suggested methods for using a flexible aliphatic epoxy consolidant, which had been formulated as a wood consolidant by Munnikendam.4, 5 Hallebeek reported in 1980 that tests of this system were in progress.6 In 1984, van Soest, Stambolov, and Hallebeek stated that improved properties in the cured product had been obtained by using a revised formulation based on a higher-molecular-weight homolog of the aliphatic epoxy.7
Cains has used hydroxypropylcellulose for some years as a consolidant for bookbinding leathers at Trinity College Library, Dublin.8 A polyurethane has recently been recommended to the British Library for this purpose.9
1 AN ACRYLIC PRECIPITATION METHOD
IN VERY PRELIMINARY TRIALS, I have observed that an acrylic precipitation consolidant can work well for strengthening some leathers. The acrylic preciptation method was described in some detail in 1982, in respect to test applications on wood, stone, and brick.10 Essentially, an acrylic monomer is mixed with a solvent and intiating materials and soaked into a porous substrate. Polymer is formed after impregnation; at present a thermally activated initiator is used which requires the impregnated object to be heated to about 40°C or higher. Evaporation of the consolidant during polymerization is prevented by wrapping the treated object in foil. The object is unwrapped after polymerization to allow the solvent to evaporate. A solvent is chosen in which the polymer to be formed will be insoluble: thus, the newly formed polymer is not drawn out toward the surface with the evaporating solvent.
The precipitation method may sometimes circumvent limitations inherent in other methods of using acrylics as consolidants: the higher viscosities of polymer solutions and their tendency to consolidate the surface of an object more than the core; and stresses that may occur when undiluted monomer is polymerized within an object. In addition, the precipitation method often seems to darken an object less than other polymeric treatments that impart similar strength, perhaps because the resin is deposited as a light-scattering gel or as particulates. The amount of monomer used, in relation to the solvent, can be varied over a wide range.
In earlier work on wood and masonry materials I used methyl methacrylate as monomer: the current trials on leather have been done with methyl acrylate, which forms a flexible polymer. The brand of monomer used contained 15 p.p.m. of p-methoxy phenol inhibitor. A “starting” formulation that seems promising is, by weight,
100Exxon IsoparRG solvent
50Noury PercadoxR 16N iitiator
PercadoxR16N is bis [4-t-butyl cyclohexyl] peroxydicarbonate.11 Exxon IsoparRG is an industrial grade of mixed isomers of mostly C-10 and C-11 aliphatic hydrocarbons. Laboratory grades of decane or perhaps dodecane should work similarly. In previous work on masonry materials and wood, heptane was used, but there may be some advantage in using solvents more viscous than heptane, as the polymerization rate and the molecular weight of the polymer formed might be enhanced by higher viscosity.12
2 TEST APPLICATIONS
SMALL SAMPLES OF DETERIORATED LEATHER were soaked in the consolidant solution, wrapped in aluminum foil, and embedded in sand that had been preheated to about 45°C–50°C. The sand, with the samples, was placed in an oven and maintained at roughly this temperature for four hours. The purpose of the sand was to buffer the temperature changes of the poorly controlled oven, and to draw off any heat of exothermic reaction from the samples. The samples were then withdrawn, unwrapped, and dried to remove solvent and residual monomer. The samples were dried by overnight warming followed by a week's airing at room temperature. Most of the samples continued to stiffen slightly over the following weeks. The following describes the uneven but encouraging results.
An early-nineteenth-century American calfskin, brittle and rather crumbly, was greatly strengthened, though also darkened. The strength increase was easily sensed by abrading the leather or by breaking narrow strips in tension. The treatment, however, made the leather more subject to cracking on being folded sharply. Another, similar, piece seemed completely unaffected by the treatment—neither strengthened nor darkened.
A red-dyed and gold-striped late-nineteenth-century calfskin, thoroughly red-rotted and intolerant of bending, became remarkably strong in tension, and was darkened only on the reverse side. Though stiffened, the treated leather became tolerant of repeated gentle flexing; it still cracked on being folded sharply. The gilding was unaffected.
A late-nineteenth-century suede, partially rotted, was made much stronger in tension and much more resistant to abrasion. It was darkened more than would be desirable, but mostly on one side.
Two late-nineteenth-century embossed leathers from wall coverings, both completely rotted, were considerably strengthened. The consolidant attacked the leafed-and-painted finishes on one of the leathers, but did not disrupt the metal leaf or paint on the other. Neither example was darkened at all—either on the finished surfaces, on the reverse, or in spots where the finishes had broken away.
The strengthening of these wall covering leathers was somewhat uneven. Pre-treatment of one small sample with a dilute solution of the polymer (polymethyl acrylate in toluene) followed by the precipitation consolidation produced a more uniform, though darkened, product. Perhaps, besides contributing some strength on its own, polymer deposited on the fibers from solution enhanced subsequent deposition of precipitated polymer.
3 FURTHER CONSIDERATIONS
WHILE POLYMETHYL ACRYLATE alone is flexible, and rather soft, leather impregnated with the resin is a composite material of which the hardness and other properties may not be easy to predict. The surface hardness of the composite would determine whether there is a tendency to pick up dust, in which case a surface coating might be needed. The composite's comparative strengths in tension versus compression would affect its resistance to cracking when folded sharply: if the inside surface of a fold does not compress easily, the outside may break in tension. Though only one formulation was tried—rather “rich” in monomer—the properties of composites made by treating different leathers varied widely.
Outline tracings of the samples indicated little or no shrinkage during treatment. Shrinkage might be observed in some cases, caused either by contraction within the polymer as it formed or dried, or as a direct effect on the leather of the heating needed to activate the polymerization initiator. Room-temperature initiation systems, such as those activated by ultraviolet light, might be useful.
An important consideration is reversibility of treatment. Polymethyl acrylate is a linear acrylic resistant to cross-linking and thus, theoretically, removable. The actual practicality of removal is another question, however, involving the molecular weight of the polymer, the effect of solvents on the leather, and stresses exerted by the polymer when swollen by solvent during removal.
VERY GOOD RESULTS ACHIEVED in treating some of the samples indicate that the method deserves further study. A water-white thermoplastic polymer can be uniformly deposited throughout the thickness of leather, in substantial quantity but with relatively little darkening effect. The strengthening of the leather can be dramatic. Various linear and cross-linked acrylic copolymers could be tried, including very soft ones made with n-butyl acrylate.
Though offered as a subject for further study, the method is by no means recom mended as a treatment. Systematic testing and modification would be needed to make the process safer and more predictable.
THIS RESEARCH WAS FUNDED BY the Preservation Society of Newport County, Newport, Rhode Island. I would like to thank Sharon Blank, Toby Raphael, and Robin Chamberlin for providing literature references. Earlier development of the acrylic precipitation process was funded by The Smithsonian Institution through the National Museum Act.
John W.Waterer, “A Novel Method for the Conservation of Fragile Leather,” Studies in Conservation17 (1972), pp. 126–130.
K.F.Nikitina, “Conservation and Restoration of Fur Clothes from the Burial Place of Oglakhty,” ICOM Committee for Conservation, 5th Meeting, Zagreb (1978), section 78/19/3.
Jack C.Thompson, “An Application of BEVA 371 to Book Restoration,” Contribution to the 3rd Pacific Region Conservator's Meeting (1978), pp. 1–2.
P.Hallebeek and H.A.B.vanSoest, “Gilded Leather,” ICOM Committee for Conservation, 5th Meeting, Zagreb (1978), section 78/19/2.
R.A.Munnikendam, “Low Molecular Weight Epoxy Resins for the Consolidation of Decayed Wooden Objects,” Studies in Conservation17 (1972), pp. 202–204
P.B.Hallebeek, “The Restoration and Conservation of Gilt Leather: Part II,” Conservation Within Historic Buildings, International Institute for Conservation (1980), pp. 164–165.
H.A.B.vanSoest, T.Stambolov, and P.B.Hallebeek, “Conservation of Leather,” Studies in Conservation29 (1984), pp. 21–31.
AbigailQuandt, “Klucel-G,” note in Leather Conservation News2 (1983), p. 8.
The Conservation of Bookbinding Leather, A Report Prepared by the British Leather Manufacturers' Research Association for the British Library, London, The British Library (1984), pp. 35–42.
Morgan W.Phillips, “Acrylic Precipitation Consolidants,” Science and Technology in the Service of Conservation, International Institute for Conservation (1982), pp. 56–61.
“Percadox 16N,” Bulletin No. 7–150, Noury Chemical Corporation, MAY 1979.
Paul J.Flory, Principles of Polymer Chemistry, Cornell University Press (1953), pp. 124–129.