Carter et al. Herit Sci (2020) 8:51 https://doi.org/10.1186/s40494-020-00395-y RESEARCH ARTICLE Open Access Rapid identifcation of cellulose nitrate and cellulose acetate flm in historic photograph collections Elizabeth A. Carter1,2* , Brad Swarbrick2, Thérèse M. Harrison1 and Lucilla Ronai3 Abstract Cellulose nitrate transparent plastic flm was used by photographers and movie flmmakers from its release in the 1880s to the 1950s. The storage of this material is a challenge for cultural institutions because of its instability and haz- ardous nature, as nitrate is highly fammable and deteriorates over time. Historically, cellulose acetate gradually began to replace cellulose nitrate as it is not fammable. Despite its non-fammable properties, leading to cellulose acetate being called ‘safety’ flm, over time it became clear that it also deteriorates in hazardous ways. Identifcation of cellu- lose nitrate and cellulose acetate in collections is necessary for preservation and risk management to collections and humans. Both cellulose nitrate and cellulose acetate are found in gallery, library, archive and museum photographic collections. As a result, identifcation and management of this material might be the responsibility of curators, librar- ians, archivists, collection managers, registrars or conservators, depending on the institution stafng and structure. Currently, there is no single identifcation method readily available to heritage institutions that meets the require- ments to identify and distinguish between cellulose nitrate and cellulose acetate reliably, non-destructively and rapidly. In this study samples from both the Sydney University Museums and Australian National Maritime Museum collections have been analysed using infrared total refectance spectroscopy, a novel approach which demonstrates great potential for rapid and non-invasive identifcation particularly when combined with multivariate statistical analysis methods. Principal component analysis (PCA) allowed for non-subjective data mining and the development of a model which is hoped to be used in the future for predicting sample identifcation. After calculation of the PCA model, the novel application of the Parsimax orthogonal rotation method potentially provides a means of determin- ing the level of chemical deterioration in the cellulose nitrate flm. Keywords: Infrared spectroscopy, Total refectance, Cellulose nitrate, Cellulose acetate, Deterioration, Parsimax rotation, Multivariate analysis Introduction acetate flm base. With its images of people, landscape Te Historic Photograph Collection (HPC), part of the and town views, family albums, images from travels and Macleay Collections of the Chau Chak Wing Museum1 expeditions, scenes of university life, scientifc work and of the University of Sydney, includes over 70,000 pho- ethnographic photographs, it is a rich historical resource. tographic objects with a wide variety of formats dating Te Australian National Maritime Museum (ANMM) from the mid-nineteenth to mid-twentieth centuries, collection has over 24,000 negatives and positives on a tenth of these are on a cellulose nitrate or cellulose transparent plastic flm bases. Te collection spans from *Correspondence: [email protected] 1 Sydney Analytical, Vibrational Spectroscopy Facility, The University 1 In 2020 the former Macleay Museum, Nicholson Museum and University of Sydney, Madsen Building, Level 2, Camperdown, NSW 2006, Australia Art Collection will be amalgamated in the Chau Chak Wing Museum of the Full list of author information is available at the end of the article University of Sydney. © The Author(s) 2020. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat​iveco​ mmons​.org/licen​ses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creat​iveco​mmons​.org/publi​cdoma​in/ zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Carter et al. Herit Sci (2020) 8:51 Page 2 of 13 the 1890s to the early 2000s with photographs by many Film composition and structure notable maritime photographers. A reliable and rapid Te image of a black and white still or sheet photograph method for the identifcation of the plastic flm base for on a plastic support, known as the emulsion layer, is com- the items within these collections is needed to manage prised of fne silver particles embedded in a thin trans- this material due to the unstable nature of the plastic flm parent binder which is typically gelatin, see Fig. 1a [7, 8]. bases. A colour photograph image on a plastic support contains yellow, magenta and cyan dyes suspended in three dis- Plastic flm bases tinct layers of gelatin that contain silver halide crystals, Cellulose nitrate was used by photographers and movie see Fig. 1b. Black and white photographs on plastic sup- flmmakers as a fexible plastic flm base for their images ports were either a cellulose nitrate, cellulose acetate or and movies from its release in the 1880s to the 1950s. polyester base. Colour photographs were only on cellu- However, it was possibly still used some years after man- lose acetate or polyester base [9]. ufacture ceased by photographers and flmmakers who Cellulose nitrate flm bases were made by reacting cel- had a stockpile of flm [1]. Te use of cellulose nitrate as lulose polymers with nitric acid in the presence of sul- a flm base was driven by its fexibility, transparency, low phuric acid to produce nitrate esters, which were then weight and strength. Te production of a thin, light and combined with a plasticiser (for example phthalates, aryl fexible cellulose nitrate flm in the 1880s was praised for phosphates, or camphor) to provide fexibility and allow the resulting luminous, high-contrast images, that were for the casting of thin flms and sheets [7, 10]. Between attributed to a silver rich emulsion and the excellent the image layer and plastic flm base is a coating of diluted transparent nature of the flm [2]. It allowed the industry nitrocellulose, called subbing, used to enhance the adhe- to move away from the heavy glass plates being used and sion of the layers. An anti-curl layer of gelatin with anti- made the process more accessible [1, 3]. During the 20th halation dyes was also applied to the non-image side of century, amateur and professional photographers and cellulose nitrate bases to prevent light refecting from the flmmakers used cellulose nitrate more frequently than back of the support and exposing the silver salts. Early any other flm support [1]. cellulose nitrate flm occasionally had varnishes applied Initially used in the manufacture of military explosives, including a shellac containing borax and glycerine or a cellulose nitrate was stabilised with camphor to produce dammar varnish [7]. celluloid. However, the instability and highly famma- Cellulose acetate is a generic term used for cellulose ble nature of the plastic flm material resulted in many diacetate, mixed esters of cellulose acetate including fres and subsequent deaths where cellulose nitrate was butyrate and propionate, and cellulose triacetate bases. housed and where motion picture flms were screened [4, Te composition of these vary, however, generally cellu- 5]. As a result it was gradually replaced from the 1920s, lose polymers undergo an acetylation reaction with ace- and production ceased in the 1950s [6]. From the 1920s tic acid and an excess of acetic anhydride in the presence cellulose acetate was a new transparent plastic flm base of sulphuric acid. A plasticiser (triphenyl phosphate or in use for photography and flmmaking. Over the twen- dimethoxyethyl phthalate) was then added to the poly- tieth century multiple forms were chemically developed mer to enable fexibility [7, 10]. As with cellulose nitrate, including cellulose diacetate, cellulose acetate propion- a subbing of diluted thin cellulose nitrate was used, a gel- ate, cellulose acetate butyrate and cellulose triacetate atin anti-curl layer with anti-halation dyes applied to the (collectively referred to as cellulose acetate) [6]. It was non-image side of the base and occasionally it had a shel- dubbed ‘safety’ flm due to its comparatively non-fam- lac or dammar varnish [7]. mable properties and began to gradually replace cellu- lose nitrate. However, it was also found to have stability Deterioration issues. Some forms of cellulose acetate are still used as a Te preservation of cellulose nitrate and cellulose acetate flm base today. flm presents numerous challenges for institutions due Polyester, a synthetic polymer, was used as a plastic to their hazardous and inevitable deterioration. Acidic base for photographic material from the 1950s onwards and hazardous vapours that of-gas from cellulose nitrate due to its superior dimensional stability and mechani- (nitric acid) and cellulose acetate (acetic acid) are a by- cal strength. During the 1960s and 1970s it gradually product of their deterioration. Tese vapours pose a replaced cellulose acetate as it was found to be more risk to humans and to other material that is stored with chemically and physically stable than cellulose nitrate it. Table 1 lists the diferent levels of physical and visual and cellulose acetate [7]. More than 50 years after it frst deterioration of cellulose nitrate and cellulose acetate began to be used in the photography industry it is still flm which in some cases can be characteristic and aid demonstrating little to no deterioration.