Characterization of cinematographic films by Laser Induced Breakdown Spectroscopy

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Abstract

The emulsion-coated transparent plastic-base film has been the main carrier for production and preservation of motion picture contents since the 19th century. The knowledge of the composition of black and white silver gelatine cinematographic films is of great importance for the characterization of the photographic process and for identifying the optimum conditions for conservation. A cinematographic film is a multi-component system that consists of a layer of photographic emulsion overcoating a polymeric support (plasticized cellulose triacetate) and a protective transparent cross-linked gelatine layer coating the emulsion. In the present work, Laser Induced Breakdown Spectroscopy (LIBS) is used to characterize the composition of the materials of cinematographic films. LIB spectra of film samples and of different individual film components, polymeric support and reference gelatines, were acquired in vacuum by excitation at 266 nm (Q-switched Nd:YAG laser, 6 ns, 10 Hz). In the cinematographic film, silver lines from the light-sensitive silver halide salts of the photographic emulsion are accompanied by iron, lead, chrome and phosphorus lines. Iron and lead are constituents of film developers, chrome is included in the composition of the hardening agents and phosphorus has its origin in the plasticizer used in the polymeric support. By applying successive pulses on the same spot of the film sample, it was possible to observe through stratigraphic analysis the different layers composition. Additionally, the results obtained reveal the analytical capacity of LIBS for the study and classification of the different gelatine types and qualities used for the protecting layer and the photographic emulsion.

Introduction

The concept of silver based black and white photography was introduced in 1727 by J.H. Schulze who observed that a mixture of silver nitrate and chalk darkened on exposure to light. Originally, the silver salts were held on glass using egg white as binder. This provided relative sharp images although they were easily damaged. By 1871, the problem had been solved by R.L. Maddox, who discovered a way to prepare gelatine dispersions of silver salts on glass plates. In 1887, G. Eastman introduced the system in which a silver halide in gelatine dispersion was coated on a cellulose nitrate base and loaded into a camera [1], [2].

Since the advent of motion pictures in the 19th century until the coming of professional video in the 1970s, the use of emulsion-coated transparent plastic-base film has been the main carrier for production, dissemination and preservation of motion picture contents. The conservation of materials pertaining to the photographic and cinematographic heritage has been a constant pre-occupation and has motivated diverse works [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], to study the degradation and deterioration processes, as well as to optimize the conditions of conservation. The composition of the cinematographic materials surely influences the stability of the films exposed as demonstrated in the case of biodegradation by microorganisms [11], [12]. Nevertheless, the multi-component nature and the complexity of the formulations used make difficult the composition studies.

Fig. 1 shows a scheme of a cross-section of a cinematographic film that describes the complex multi-component system, consisting typically of a protective layer, the photographic emulsion layer, the subbing layer, the polymeric support and an anti-curl backing layer.

The protective layer consists of a transparent superficial cross-linked gelatine coating of about 1 μm thickness. It covers the surface of the emulsion to protect the film. The photographic emulsion, of about 20 μm thickness, is composed basically of gelatine (60–70%) where the light-sensitive silver halide salts, the film developers and the hardening agents are in suspension. The subbing layer is a thin coating that allows the emulsion to better adhere to the support. In general, it is a mixture of cellulose acetate, cellulose nitrate and gelatine. The polymeric support, of about 120 μm thickness, consists of cellulose triacetate and triphenylphosphite added as plasticizer. Cellulose triacetate was introduced around 1950, as this material met all the technical and safety requirements for professional motion picture films due to its low inflammability and slow burning characteristics. Fire retardant substances known as plasticizers (10–15% by weight) are added. The backing layer, applied on the non-emulsion side of the film is usually made of gelatine. It counteracts any curl caused by a change in dimensions of the emulsion due to temperature and humidity variations [1].

Gelatine is a natural protein derived from collagen, which consists of amino acids chained together by a peptide bond. Gelatine is a useful biological material with both complex structure and special functions. In general gelatines may be classified according to purpose as edible, pharmaceutical or photographic [13]. The application of gelatine revolutionized photography due to its favorable physical and chemical properties which have caused gelatine to remain the dominating vehicle for silver halide photographic materials for more than a century [14]. The photographic gelatine is mainly type-B, obtained from collagen [15] of cattle bone by a liming process (alkaline pre-treatment).

The knowledge of past artistic, commercial and experimental cinematographic processes and technologies and the design of adequate conservation strategies require the development of advanced methodologies. Along the history of photography and motion pictures, a variety of materials and processes have been used and the identification of the compounds applied during the film processing provides historical information on the cinematographer's technique, as well as a starting point for any necessary restoration or design of conservation strategies [16], [17].

Techniques used to elucidate information on the chemical composition of historical photographic and cinematographic materials include among others X-ray fluorescence spectroscopy (XRF), inductively coupled plasmas mass spectrometry (ICP-MS), infrared and Raman spectroscopy and scanning electron microscopy (SEM) [18], [19], [20], [21], [22], [23]. ICP-MS analysis provides extremely low detection limits and high accuracy but requires sample preparation. XRF analysis, a non-destructive technique requiring neither sampling nor direct contact with the photograph or film, provides qualitative and quantitative information about inorganic elements in the photographic material. However the lateral spatial resolution of XRF is poor (about few mm) and the depth profile capability is very limited. FTIR and Raman spectroscopies allow the identification of molecular compounds present in the sample surface but are insufficient to analyze the composition of the layers underneath. For characterization of the multilayer structure and composition, methods have been developed for the selective removal of the individual emulsion layers [24].

Nowadays the Laser Induced Breakdown Spectroscopy (LIBS) technique is frequently used for the characterization of the elemental composition of different types of materials as for laser cleaning diagnostics and for the determination of stratigraphies [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35]. Although laser techniques have been applied on many types of complex organic materials, their application to photographic or cinematographic heritage are scarce [36]. The characteristic features of LIBS, supplying instantaneous compositional information by consuming a microscopic amount of sample material, not requiring preparation of the sample and featuring high lateral resolution and depth profiling capabilities, are initially advantageous for characterization of the complex multilayer cinematographic film substrates. The objective of this research was to explore the applicability and potential of LIBS for the analysis of black and white silver gelatine cinematographic films which represent an important part of historical collections held in film archives and in particular of Filmoteca Española (National Spanish Film Archives).

First, we determined the elemental composition of the different materials used in cinematographic films, i.e. of cellulose triacetate, of polymeric support and of different types of gelatines. Then, we carried out the analysis of the stratigraphy of a cinematographic film sample by applying successive laser pulses on the same spot, allowing the analysis of composition of the different layers of the film. The results gathered contribute to the effort of preservation of the films by identifying the gelatine type (Bloom and degree of cross-linking) used in the emulsion and the trace elements related with the compounds used as developers, hardeners and plasticizers. This information serves to identify the specific photographic process employed for the film making and to assess the adequate conservation strategies in the case of archival films.

Section snippets

Materials

Gelatine films were prepared to assess the capability of LIBS to characterize the different types and qualities of this material. The gelatine of the photographic emulsion, of high bloom value, is cross-linked in order to obtain a suitable dimensional stability of the image support. The bloom value is defined as the value of the force in grams necessary to apply in a standard plunger to deform 4 mm the surface of the gelatine gel [37]. This parameter characterizes the gel strength resistance

Results and discussion

In the results presented below, we describe the analysis performed in pure gelatine films, on the polymeric support and finally in samples of cinematographic film.

Conclusions

The results reported here show the potential of LIBS for the chemical and structural characterization of the different layers of the cinematographic films (protective and emulsion layers and polymeric support) as well as for extracting information on the chemical composition of the developers, hardening and plasticizer agents used. LIBS allows the identification of the analytical signatures of specific photographic process variant. Additionally the results obtained reveal the analytical

Acknowledgements

Work funded by MEC Projects CTQ2007-60177 and MAT 2006-05979, Spain. SG thanks the EU 6th FP for a Marie Curie EST Fellowship (MESTCT-2004-513915), MO and ER thank CSIC I3P program for a contract and a fellowship respectively.

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    Present address: Institute of Applied Physics, Johannes-Kepler-University, Altenbergerstr. 69, A-4040 Linz, Austria.

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