1 Published in Proc SPIE 12/2013; 9065. DOI:10.1117/12.2052820 Laser removal of mold and foxing stains from paper artifacts: preliminary investigation D. Ciofini 1 , I. Osticioli 1 , S. Micheli 2 , L. Montalbano 2 and S. Siano 1 1 Istituto di Fisica Applicata “N. Carrara”, IFAC-CNR, Sesto Fiorentino (FI), Italy, 2 Opificio Delle Pietre Dure, Firenze, Italy ABSTRACT In this work we focused on the laser removal of fungi growths and foxing stains from old paper artifacts. Irradiation tests have been carried out using Nd:YAG laser’s second harmonic and characterized through morphological analysis by means of optical microscopy and UV-VIS light fluorescence, along with SEM-EDX microscopy. In addition, FTIR spectroscopy and VIS diffuse reflectance spectroscopy have provided, respectively, very useful information on chemical processes of the paper ageing and on the removal effectiveness of the laser treatment. The optimized laser setup with fiber coupling has allowed to achieve significant results in the removal of the foxing and bio-deterioration spots from aged paper. In particular, high selectivity and degree of control were observed in the treatment of metal-induced foxing (Fe and Pb), likely arising from the contact with metal objects. Moreover, similarly good results were pointed out for the removal of fungal colonies without inducing detectable structural damage to the cellulose fibers. The best operating conditions determined have been finally used in order to approach a concrete conservation problem of an original engraving by G. B. Piranesi entitled Veduta del Ponte e Castello Sant’Angelo (18th century). Further research will be carried out on both topics in order to evaluate the range of applicability and possible long-term effects of this approach. Keywords: Foxing, mold, engraving, cellulose, paper, bleaching, Nd:YAG laser 1. INTRODUCTION 1.1 Main causes of foxing stains and mold damage Books, prints, drawings, watercolours, engravings as well as all collectable objects based on paper can undergo over time to degradation processes, especially if not stored under controlled environmental conditions. Foxing spots, or rusty-red stains, can be considered as a widespread damage of all fiber based-materials. The causes of and detailed descriptions for some types of fox spots have been reviewed in the literature 1-5 . Metal contamination, microorganisms (fungi and bacteria), as well as moisture condensation and capillary localized evaporation processes at wet-dry interface, are suggested as possible causes of foxing. Metal and microorganisms contamination may come either from the papermaking and from airborne dust. It is noteworthy that airborne dust can contain iron measuring up to 15% of the inorganic component. Additional sources of metal ions may be from contact with metal objects or related to the iron gall inks. Research on ink corrosion showed that most iron gall inks contain an excess of iron which is not bound in the complex forming compound of the ink and can migrate on a small distance into the surrounding area. Summarizing, transition metals ions, such as iron (II) and copper (I) can act as catalysts for free radical cellulose oxidation bringing to depolymerization of cellulose chains and increasing discoloration of paper containing cellulose. In addition, carboxyl and carbonyl groups formed from cellulose oxidation can enter an amino carbonyl reaction with nitrogen-containing compounds inherent in paper (gelatin sizing, fungal amino acids) or present in pollution (dust, spores, starch grains). The result of amino carbonyl reaction, also named as the Maillard reaction, is the formation of melanoidin type brown colored compounds which are called age pigments 6-8 . It is worth noting how fungi and related metabolic products are often involved in the formation of melanoidin compounds. Within the context of foxing generated from microorganisms, it was stated that in old paper, fox spots are mainly formed from fungal structures in a limited stage of growth. Tipically, fungal structures found include germinated spores which have died before vegetative growth and a limited amount of hyphae or developed mycelia 9-10 . Fungus-induced foxing results in the formations of spots in various colors. The