Stability of paper in the presence of pigment verdigris

Verdigris sodi med najpomembnejše in pogosto uporabljene pigmente zelene oziroma modrozelene barve od antike do 19. stoletja. Njegova barva se spreminja s časom in v odvisnosti od atmosferskih pogojev. Kljub slabi obstojnosti je bil med umetniki zelo priljubljen. Ker je semitransparenten in ne prekriva tiskarskega črnila, ga najdemo na iluminacijah, ilustracijah knjig in zemljevidih. Poznamo dve vrsti verdigrisa, in sicer nevtralni in bazični verdigris. Verdigris ne spreminja zgolj svoje barve, ampak povzroča tudi razgradnjo nosilca, na katerega je nanesen. Ker so ogrožena tudi številna zgodovinska umetniška dela, so študije stabilizacije pigmenta in razvoj konservatorsko-restavratorskih postopkov izjemnega pomena. V magistrskem delu sem raziskovala razgradnjo in možnosti za stabilizacijo različnih modelnih papirjev z nanesenim pigmentom verdigris. Študirala sem tri različne modelne papirje, in sicer Whatman No. 1 filtrirni papir kot referenčni papir in dva različna ročno izdelana papirja, ki sta podobna zgodovinskim papirjem. Vzorce papirja s pigmentom verdigris sem izpostavila pogojem termično pospešene razgradnje pri 50 °C in 65 % relativne vlažnosti, ki poteka primerljivo z razgradnjo pri sobnih pogojih. Za študije stabilizacije papirja sem uporabila dva antioksidanta, ki sta se do sedaj izkazala za najbolj učinkovita, in sicer 1(H)-benzotriazol (BTA) in tetrabutilamonijev bromid (TBABr). Z atomsko absorpcijsko spektroskopijo sem določila vsebnost bakra v modelnih vzorcih papirja in ugotovila, da lahko z uporabo ročne pršilke pripravimo vzorce z enakomernim nanosom pigmenta na papir. S spremljanjem pH vrednosti vzorcev papirja sem ugotovila, da do pretvorbe nevtralnega v bazični verdigris pride v prvem mesecu termično pospešene razgradnje. pH vrednosti papirja so pokazale, da za stabilizacijo papirja zadostuje zgolj uporaba ustreznega antioksidanta, brez dodatka alkalije oziroma sredstva za razkislinjenje. Tudi meritve spremembe barve vzorcev papirja nakazujejo na dejstvo, da se nevtralni verdigris sčasoma pretvarja v različne zelene oblike bazičnega verdigrisa. Izmed obeh antioksidantov je z vidika barvne obstojnosti takoj po nanosu najbolj učinkovit TBABr, saj so bile spremembe barve minimalne, vendar so bile po treh mesecih termično pospešene razgradnje bolj izrazite kot pri vzorcih, obdelanih z BTA. Poleg tega so na nekaterih vzorcih nastale črne pike, kar ni zaželeno. S spremljanjem stopnje polimerizacije (DP) celuloze sem ugotovila, da je najbolj učinkovit antioksidant za stabilizacijo TBABr, saj je bila razgradnja papirja, obdelanega s TBABr, po treh mesecih termično pospešene razgradnje v povprečju manjša za 33 % v primerjavi z neobdelanimi papirji. V nasprotju pa BTA ni izkazal pozitivnih učinkov na stabilizacijo papirja. Primerjava DP vrednosti, določenih z velikostno izključitveno kromatografijo in z viskozimetrijo, kaže, da bi lahko bila viskozimetrija primerna za določanje DP vrednosti za papir z verdigrisom. Meritve mehanskih lastnosti so pokazale, da pri neobdelanih vzorcih papirja po treh mesecih termično pospešene razgradnje ni prišlo do tako izrazite izgube mehanskih lastnosti, kot bi lahko pričakovali glede na določene vrednosti stopnje polimerizacije celuloze. Z ramansko spektroskopijo smo potrdili, da smo za pripravo barve uporabili nevtralni verdigris, ki se s časom spreminja v različne oblike bazičnega verdigrisa, ne glede na to, ali je papir obdelan z antioksidantom ali ne. Za stabilizacijo papirja z nanesenim pigmentom verdigris je na osnovi rezultatov najbolj primeren antioksidant TBABr, saj je bil pri zaviranju razgradnje papirja najbolj učinkovit. Poleg tega ni imel negativnega vpliva na pH vrednost papirja in ni povzročil izrazitih sprememb barve takoj po nanosu. Negativni učinek se je pokazal zgolj po treh mesecih termično pospešene razgradnje, saj so na nekaterih vzorcih pojavile črne pike. Vzrok za to je potrebno pred uporabo postopka na originalnih dokumentih natančno raziskati. Verdigris is one of the most important and frequently used green or blue-green pigments from antiquity until the 19th century. Its colour changes with time and depending on atmospheric conditions. Despite its poor durability, it was extremely popular among artists. Because it is semi-transparent and does not obscure the printed ink, it can be found on illuminations, book illustrations and maps. There are two types of Verdigris, namely neutral and basic Verdigris. Verdigris does not only change its colour, but also causes the degradation of the substrate on which it is applied. Since many historical works of art are also at risk, studies of pigment stabilization and the development of conservation-restoration procedures are of utmost importance. In my master’s thesis I investigated the degradation and possibilities for stabilization of various model papers with applied pigment Verdigris. I studied three different model papers, namely Whatman No. 1 filter paper as a reference paper and two different handmade papers that resemble historical papers. Paper samples with pigment Verdigris were exposed to conditions of thermal accelerated degradation at 50 °C and 65 % relative humidity, which are comparable to the degradation of paper with pigment Verdigris at ambient conditions. For paper stabilization studies, two antioxidants that have proven to be the most effective so far, 1(H)-benzotriazole (BTA) and tetrabutylammonium bromide (TBABr), were used. The copper content in model paper samples was determined using atomic absorption spectroscopy. The results show that an even application of the pigment on paper samples can be obtained using a spray gun. Monitoring the pH value of paper samples showed that the alteration of neutral to basic Verdigris occurs in the first month of thermal accelerated degradation. The pH values of paper indicate that for the stabilization of paper the use of an appropriate antioxidant, without the addition of alkali or deacidification agent, is sufficient. Measurements of the colour change of paper samples also indicate the fact that the neutral Verdigris eventually transforms into various forms of basic Verdigris. From the point of view of colour persistence immediately after application, TBABr is the most effective of the two antioxidants, as colour changes were minimal, but after three months of thermal accelerated degradation, the degradation was more pronounced than in samples treated with BTA. In addition, black spots appeared on some samples, which is not desirable. By monitoring the degree of polymerisation (DP) of cellulose, I found that the most suitable antioxidant for inhibiting cellulose degradation is TBABr, as the degradation of paper treated with TBABr was on average 33 % lower after three months of thermal accelerated degradation compared to untreated papers. In contrast, BTA did not show significant stabilization. Comparison of DP values, determined by size exclusion chromatography and by viscometry, indicates that viscometry may be suitable for determining DP values for Verdigris paper. Measurements of the mechanical properties showed that the untreated paper samples did not show as pronounced loss of mechanical properties after three months of thermal accelerated degradation as might be inferred from the determined values of the degree of polymerisation of cellulose. Using Raman spectroscopy, we confirmed that neutral Verdigris was used to prepare the colour, which changes over time into different forms of basic Verdigris, regardless of whether the paper is treated with an antioxidant or not. Based on the obtained results, the antioxidant TBABr is the most suitable for stabilizing paper with applied pigment Verdigris, as it was the most effective in inhibiting paper degradation. In addition, it had no negative effect on the pH value of paper and did not cause significant colour changes immediately after application. The negative effect became apparent only after three months of thermal accelerated degradation, as black dots appeared on some samples. The cause of the appearance of these black dots must be carefully studied before the treatment is used on original documents.