Expanding the possibilities of ATR-FT-IR spectroscopy in determination of inorganic pigments

Abstract

The present work focused on expanding the possibilities and usefulness of micro-ATR-FT-IR spectroscopy in identification of different inorganic pigments in historic artefacts. Most of the pigments used historically in paintings and other artefacts are inorganic compounds. ATR-FT-IR has found limited use for determination of inorganic pigments, because the mid-IR (4000–400 cm–1) region of the IR spectrum of many of them is not characteristic enough and also there are many pigments that either do not absorb in that region at all (oxides, sulphides, etc) or have absorptions that are at the low wavenumber end of that region and are not characteristic enough for pigment identification. So, many pigment materials absorb IR radiation in the far-IR region (below 500 cm–1). In contrast to the mid-IR region the far-IR region has traditionally been significantly less accessible for routine IR spectroscopic measurements. However, recent advances in instrument design have brought the wavenumber range below 500 cm–1 well within reach of commercial FT-IR equipment. Nevertheless, this wavenumber region has up to now found almost no use in investigation of historic artefacts. In this dissertation 47 most widespread inorganic red, white, yellow, blue, green, brown and black pigments were examined. An analytical method for determination and identification of these inorganic pigments in paint layers by micro-ATR-FT-IR using the wavenumber region of 550–230 cm–1 was developed. The advantages and limitations of the ATR-FT-IR and developed analytical method of the analysis of inorganic pigments in the low wavenumber were discussed. This work provides a comprehensive overview of the inorganic pigment identification possibilities using ATR-FT-IR as well as a collection of reference spectra in the low wavenumber range (550-230 cm-1) and is expected to be a useful reference material for conservation practitioners and material scientists. The usefulness of ATR-FT-IR in the region of 550-230 cm-1 for identification of inorganic pigments is demonstrated by 5 case studies on art objects (several of them are important in Estonian history).Käesolevas doktoritöös keskenduti mikro-ATR-FT-IR spektroskoopia võimaluste hindamisele ja rakendusala laiendamisele erinevate anorgaaniliste pigmentide uurimisel. Sajandeid on erinevate maalide maalimisel ja teiste kunstiobjektide kaunistamisel kasutatud värvi koostises anorgaanilisi pigmente. FT-IR spektroskoopiat on piiratult kasutatud anorgaaniliste pigmentide analüüsimisel kuna IR spektris kesk-IR ala (4000–400 cm–1) pole mitmete pigmentide määramisel küllalt karakteristlik, samuti on palju selliseid pigmente, mis ei neela üldse selles alas (oksiidid, sulfiidid) või omavad neeldumisi, mis on keskmise IR ala lõpus ja need ei ole küllalt karakteristlikud pigmendi tuvastamisel seguproovis. On teada, et paljud pigmendid neelavad kaug-IR piirkonnas (allpool 500 cm-1). Varem, kasutades tavalisi FT-IR spektromeetreid, oli võrreldes kesk-IR alaga kaug-IR alas mõõtmisi palju raskem teostada. Tänapäeval uuemad FT-IR seadmed võimaldavad kasutada lainearvude piirkonda allpool 500 cm-1. Hoolimata sellest seda piirkonda kunstiobjektide uurimisel pole praktiliselt üldse kasutatud. Käesolevas doktoritöös uuriti 47 valget, kollast, punast, sinist, rohelist, pruuni ja musta pigmenti. Töötati välja metoodika värvikihtides pigmentide analüüsimiseks mikro-ATR-FTIR meetodil kasutades lainearvude vahemikku 550–230 cm–1. Toodi välja ATR-FT-IR spektroskoopia ja väljatöötatud metoodika eelised ja puudused anorgaaniliste pigmentide uurimisel madalatel lainearvudel. Doktoritöö annab ülevaate anorgaaniliste pigmentide uurimise võimalustest ATR-FT-IR-iga, samuti võrdlusspektrite kogumiku pigmentidest, mis neelavad madalatel lainearvudel (550–230 cm–1). Viie rakendusnäite põhjal demonstreeriti ATR-FTIR meetodi kasulikkust tuvastada värviproovides pigmenti kasutades lainearvude piirkonda 550-230 cm-1. Rakendusnäidete värviproovid võeti kunstiobjektidelt, millest mitmed on Eesti kunstiajaloos olulisel kohal.