A Second Set of Experiments Using Hydrolyzable Polymers to Preserve Waterlogged Wood, 2002

In this study, methyltrimethoxysilane (MTMS), a hydrolyzable, multi-functional alkoxysilane polymer and Q9-1315, a MTMS alkoxysilane polymer diluted with methanol, were used in conjunction with acetone dehydration to conserve waterlogged wood. Three groups of wood were used as samples for study. The Group 1 samples, waterlogged tongue depressors, were treated using acetone dehydration followed by immersion in MTMS. After using the acetone/MTMS displacement method, images of the treated wood were obtained using an environmental scanning electron microscopic, and were compared to similar images of non-waterlogged samples of the same wood samples, providing data on the ability of the polymer to preserve the micro-structure of the wood. A waterlogged plank from a submerged 17th-century architectural feature, from Port Royal, Jamaica, was divided into four sections to form the Group 2 samples. Two sections of this plank were treated by using MTMS on one section and Q91315 on another. The remaining two sections of wood were used to calculate the water content of the wood. The Group 3 samples consisted of 18 waterlogged treenails extracted from the frames of La Salle’s vessel, La Belle, which sank off the coast of Texas in 1686. Eight of the Group 3 treenails were treated using MTMS immersion after dehydration in acetone. Eight other Group 3 treenails were treated using acetone dehydration followed by immersion in Q9-1315. The remaining two were used as controls for comparison. The nuclear magnetic resonance spectra of the samples from these Groups indicate that when waterlogged timbers are immersed in MTMS, resins are formed through self-condensation. The hypothesis of these experiments is that alkoxysilane polymers are sufficient to preserve the diagnostic attributes of wooden artifacts. Smith: A Second Set of Experiments Using Hydrolyzable Polymers to Preserve Waterlogged Wood 2002 WAG Postprints—Miami, Florida Per Hoffman conducted some invaluable studies observing that wood structures do not degrade at uniform rates, which led to his development of a highly effective, two-phase PEG-treatment strategy (Hoffmann, 1984). ARC-Nucleart has advanced studies in the preservation of waterlogged wood by impregnation with resins, which are then hardened using radiation (Ginier-Gillet, 1984). Additionally, they have worked successfully in treating larger artifacts using PEG impregnation followed by freeze-drying. Although contributions to the discipline of waterlogged wood conservation continue, some of the long-term problems of treatments using PEG are being realized. In his address at the Ottawa ICOM Conference, Dr. Allen Bronstein, a senior chemist at Union Carbide Company, addressed the complexities of wood conservation and many factors related to the degradation of PEG (Bronstein, 1981). During the discussion, Cliff McCawley touched on the topic of the effects of metal salts on the degradation of PEG. In retrospect, this has become a topic of great concern. In recent years, the problem of PEG decomposition and the formation of chemical complexes, including aldehydes and ferrous, ferric and cupric salts, has become a pressing issue. Indeed, some of our finest examples of conserved waterlogged wood are on the verge of disintegration due to our inability to control oxidation, miscibility and chemical reactivity of PEG with oxides and compounds found naturally in waterlogged timbers. This experiment investigates the use of trifunctional polyols to both stabilize and maintain the physical attributes of waterlogged wood samples, as suggested by Bronstein. (Bronstein, 1981) However, instead of directing research at creating “very hard and durable finishes,” as he suggests, this experiment focuses on impregnating a variety of waterlogged wood samples with a self-condensing polymer, in such a way as to cause formation of resins throughout the pore structure of the wood. There are benefits to this type of resin-forming chemical reaction. Displacement of water with the self-condensing polymer methyltrimethoxysilane (MTMS) does not appear to distress waterlogged wood. The low viscosity of MTMS allows for the thorough impregnation of waterlogged wood using either ambient or low vacuum processes. Using trace amounts of water, the alkoxysilane condenses, forming complex polymers that do not cause cell wall distortion or appreciable shrinkage. Post-treatment microscopic and NMR evaluation of the treated wood indicates that complex resins are formed throughout the wood. These resins are bound to the cell wall structures, giving the wood both strength and durability. Additionally, the wood is aesthetically pleasing without the waxy, dark coloration associated with PEG-treated wood. Notably, the wood treatment strategies outlined in this experiment are not reversible. Considering the chemical structure of wood, however, few conservation materials used as bulking agents are truly reversible. This, in part, is due to the polymer nature of wood. Lignin, for instance, is a natural polymer composed of coniferyl alcohol and other compounds. Cellulose is a linear polymer with notably high concentrations of hydroxyl groups which form microfibrils. Even the starches found in wood consist of glucose polymers which contain amylose and amylopectin (Mills, 1994). This, in part, explains the non-reversible nature of PEG. PEG itself is a polymer composed of poly(ethylene oxides) polymerized from ethylene oxide. When introduced into wood as a bulking agent, PEG chemically bonds to cell walls, making complete reversal impossible. The polymer nature of wood also explains why alkoxysilane polymers are effective bulking and stabilizing agents. When introduced into an organic substrate, silanes form Si-OH groups that condense to form cross-links that are more stable than bonded PEG in a similar substrate. As prescribed by the ICOM activities committee in 1978, experimentation using alkoxysilane polymers and other organic polymers was, and still is, an essential phase of development in the discipline of organic artifact conservation.