In this study, Fourier transform infrared spectroscopy (FTIR) and luminescence imaging were used to identify the coatings of seven Persian lacquer papier mache penboxes, of which two were contemporary, one was from the Pahlavi era, and four belonged to the Qajar era. First, FTIR was used to identify the nature of the coating. Then, visible-induced luminescence imaging at the spectral ranges of 420–680 nm (UVL), 425–495 nm (UVIBL), and 615–645 nm (UVIRL) was performed for further examination. The FTIR results showed that the coatings were made of alkyd resin, oil-resin varnish (Kaman oil), and shellac. In visible-induced luminescence images, synthetic alkyd resin showed no fluorescence, which made it distinguishable from the natural organic coatings. While it is slightly challenging to differentiate Kaman oil from shellac based on FTIR results, these two coatings can be easily distinguished by their fluorescence in UVL and UVIBL images. The results suggest that the combined use of spectroscopy and spectral imaging methods can provide substantial information about the organic coatings of historical objects.

Making varnishes from diterpenoid resins with the several orders had been common in Iran from 12th Century and it has also been reported in Europe from medieval ages. Identification of these resins in old samples and investigation of their degradations through ageing process have performed by various methods. FTIR spectrometry has always been of interest to researchers because it is cheap, available and it requires little sample material. Ditepenoid resins are classified into two classes: mixture of abietane and pimarane structures resins that include colophony and venetian turpentine, and resins that are formed from labdane and pimarane structures which involve sandarac and copal types. Abietane structures do not polymerized in the time but labdanes and pimaranes polymeriz at a very short time. For this reason, they mostly are used in oil-resin varnishes. These characteristics have led the two classes of resins to operate differently during ageing process and production of degradation products. This differentiation helps to identify to characterize the original structure of resin in the varnish compound in FTIR spectrometry. This article reviews the identification characteristics of diterpenoid resins in FTIR spectrometry and it tries to investigate and introduce every resin characteristics according to their structure from several references. Moreover, identification characteristics of linseed oil is introduced because of its application in oil-resin varnishes. Also, degradation process and its production are also reviewed. Differentiation in chemical structures of diterpenoid resins causes the differentiation of ageing process such the polymerisation of labdanes and pimaranes and cross linking in ageing process produce some new functional groups. Also some functional groups are destroyed during ageing. The changes observed in FTIR spectrometry are inclined oxidation and polymerization of varnishes that in turn changes bands intensity related to OH region, especially in 3000 and 3450 cm-1 and carbonyl region bands. Degradation process of resins in solvent and oil-resin varnishes is same but identification of resin characteristics in oil-resin varnishes is difficult because of effects of oil oxidation on varnish spectra after aging process. Also cross links in varnish structure during aging process cause the formation of some new bands in aged varnishes spectra. Degradation products in resinits and ambers are to some extent like that of old resins and varnishes because their aging process is same.