Q.K. TRAN ARC-Nucléart, Grenoble
Email: quoc-khoi.tran@cea.fr J.L. BOUTAINE
Centre de recherche et de restauration des musées de France, Paris
France
4.1. INTRODUCTION
The consolidation of porous artefacts is a second application of ionizing radiation in the field of cultural heritage preservation. It is derived from studies dating from the 1960s in which the aim was to improve the mechanical properties of porous material — wood and concrete in particular [4.1–4.4]. The method uses vacuum impregnation with a liquid resin followed by polymerization under gamma irradiation, called radiation curing. Even if it is less commonly used than disinfestation, this method is useful because it fully consolidates porous parts of the artefact. After the item has been impregnated, the resin filling the micro-pores is polymerized (cured, i.e. solidified) by radiation. This technique is called
‘densification’ (or the ‘Nucléart process’) in opposition to traditional consolidation techniques that use a solvent to convey the resin into the material, which only forms a film of solid resin after the solvent has evaporated. The resin traditionally used for wood is a styrene unsaturated polyester formulation [4.5–4.7].
The mechanical properties of artefacts are indubitably much better after densification than after any other conventional form of consolidation. The appearance of the object remains unchanged, or at least any changes that do occur are no greater than those that can be observed with any other type of impregnation.
However, it is obvious that the material and its physicochemical properties have been transformed (enhanced in density and in mechanical strength) and that these changes are irreversible. That is why this practice is deliberately limited to justified cases in which the mechanical properties must be greatly reinforced. In the case of polychrome wood, preliminary tests must be carried out to determine
whether or not there is any interaction (swelling, dissolution) between the liquid resin and the polychrome layers on the artefact.
Derived applications concern waterlogged archaeological wood, and make use of complex impregnation techniques. In addition to very strong consolidation, the technique provides excellent results in terms of conservation of the initial volume, as well as a surface appearance that is also very satisfactory. But the main advantage is that it can be used as a stabilizing treatment for composite wood and metal objects, while conventional treatments with water soluble polymers tend to accelerate corrosion.
The first publications concerning waterlogged wood artefacts were those by de Guichen [4.8] relating to fragments from lakeside towns Switzerland, those by Munnikendam [4.9] in the Netherlands and those by de Tassigny and Ginier-Gillet [4.10] of ARC-Nuléart relating to 11th century artefacts from Lake Paladru near Charavines, France.
The first operations on dry wood artefacts carried out at ARC-Nucléart concerned the parquet floor of the main room of the Stendhal Museum in Grenoble (1970) and the statue of the Virgin of Flavigny (1970) [4.11].
A similar process is applied to enhance the properties of wood as a building or flooring material. The first research programme was initiated in 1956, at the initiative of the Division of Isotopes Development of the former United States Atomic Energy Commission, under the management of the Division’s head, E.E. Fowler. The participants in this programme included the Brookhaven National Laboratory, West Virginia University and several industrial companies.
The programme was called ‘Wood Plastic Composites’. The initial objectives were to improve the qualities of wood such as hardness, compression resistance, dimensional stability, abrasion resistance, toughness, insect repellent properties, low water sorption and attractive appearance. The first public application was for the floor of the United States Pavilion at the New York World Fair in 1965.
The industrial coordinator was the Georgia Nuclear Aircraft Laboratory and the parquet was made of yellow pine (Pinus rigida) impregnated with the monomer methyl methacrylate (MMA).
More recently, in France, a programme has been dedicated to enhancing the value of wood species such as those in the beech (Fagus), hornbeam (Carpinus), birch (Betula), poplar (Populus) and ash (Fraxinus) genera, which are of low commercial value but common in western European forests. In this way it is possible to produce high quality parquets that can compete with floors made of oak or tropical species so they can be laid in places where there is intense pedestrian traffic. Some museums in France, such as the Musée de la musique, Museum national d’histoire naturelle and Musée de La Poste, and the Seoul Incheon Airport were equipped with such densified parquets (through technology
REFERENCES TO CHAPTER 4
[4.1] KENT, J., WINSTON, A., BOYLE, W., Preparation of Wood-plastic Combinations Using Gamma Radiation to Induce Polymerization: Effects of Gamma Radiation on Wood, Rep. No. ORO 600, West Virginia University, Morgantown, WV (1963).
[4.2] KENT, J., WINSTON, A., BOYLE, W., Preparation of Wood-plastic Combinations Using Gamma Radiation to Induce Polymerization, Rep. No. ORO 612, West Virginia University, Morgantown, WV (1963).
[4.3] KENT, J., WINSTON, A., BOYLE, W., LOOS, W.E., AYRES, J.E., Preparation of Wood-plastic Combinations Using Gamma Radiation to Induce Polymerization, Rep. No. ORO 628, West Virginia University, Morgantown, WV (1965).
[4.4] KENT, J., WINSTON, A., BOYLE, W., TAYLOR, G.B., Preparation of Wood-plastic Combinations Using Gamma Radiation to Induce Polymerization, Rep.
No. ORO 2945–7, West Virginia University, Morgantown, WV (1967).
[4.5] DETANGER, B., RAMIERE, R., DE TASSIGNY, C., EYMERY, R., DE NADAILLAC, L., “Application des techniques de polymerisation au traitement des objets en bois”, Proc. Congresso internazionale: Applicazione dei metodi nucleari nel campo delle opera d’arte Roma-Venezia, 1973, Accademia nazionale dei Lincei, Roma (1976) 661.
[4.6] CORNUET, R., RAMIERE, R., DE TASSIGNY, C., Application des Techniques Nucléaires à la Conservation des Œuvres d’Art, Bureau Eurisotop, Bruxelles (1975).
[4.7] BROUQUI, M., CORNUET, R., DE TASSIGNY, C., Traitement par rayonnements gamma, de la momie de Ramsès II, Rev. Gén. Nucl. 1 (1978) 10.
[4.8] DE GUICHEN, G., Méthode de conservation des bois provenant de cités lacustres par diffusion d’un monomère et polymérisation par rayonnement gamma, Thesis, Ecole Polytechnique Fédérale de Lausanne (1966).
[4.9] MUNNIKENDAM, R., Conservation of waterlogged wood using radiation polymerization, Stud. Conserv. 12 2 (1967) 70–75.
[4.10] DE TASSIGNY, C., GINIER-GILLET, A., La méthode par imprégnation et irradation gamma, Sonderdruck aus Zeitschrift für Schweizerische Archäologie und Kunstgeschichte 36 2 (1979) 138–141.
[4.11] EYMERY, R., DE NADAILLAC, L., ‘Utilisation du rayonnement-gamma pour la conservation des objets en bois’, CEA, Saclay (1972).