Peter Paul Biro
Fine Art Conservation & Forensic Studies in Art


Methodology


Fingerprints - Justification, Protocol and Viability


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Justification

1.    The use of fingerprints to identify a specific individual has a history well in excess of a century and the uniqueness of fingerprints is a fully accepted principle in fingerprint identification that has been extensively tested, both scientifically and in legal systems around the world and always withstood such tests. 

2.    Fingerprint examiners recognize several types of prints, including so-called 'latent' prints, where a mark is left on a surface from skin secretions, 'visible' or 'stamped' prints, where the mark has been plainly made with transferred material, and 'impressed' or 'plastic' prints, where an impression is made into a soft surface. Latent prints have a relatively short lifespan, whereas stamped and plastic prints can potentially survive indefinitely. 

3.    Identification of fingerprints in paintings does not consider latent prints, only stamped and plastic prints that have been made in wet or soft paint that has subsequently dried. The presence of such fingerprints in paintings is well attested, with numerous known examples covering all ages and styles. The importance of this is that the print was incontrovertibly impressed when the painting was still fresh – hence connecting it with the artist with a very high degree of probability.

4.    It has never been an issue within the fingerprint community what objects a fingerprint is left on (the substrate) and with what substance (the matrix). As long as both the sample fingerprint and the exemplar fingerprints provide sufficient information there is a clear justification to attempt a comparison to determine if a match can be demonstrated. 

5.    Fingerprint details are generally described in a hierarchical order at three different levels, namely: Level 1 (the basic pattern), Level 2 (minutiae such as islands, bifurcations and ending ridges) and Level 3 (the position of skin pores and ridge contours). It is possible to show that fingerprints in paint can in principle retain information up to this third level of detail, providing the strongest comparisons. 

6.    While in legal circles there is access to fingerprint records of known individuals, this does not typically occur with artists' fingerprints. Therefore, it is necessary to develop reference material based on uncontested sources. The most secure kinds of reference prints come from works and artifacts associated with an artist where it is unlikely that anybody else would have come into contact with them. For example, early works where the artist is known not to have had assistants are suitable for this purpose, as are artifacts such as paint tubes, brushes and so forth, personal to the artist. Identical prints where they have been identified on a wide range of works by an artist can also be used since it is unlikely that more than one individual could be involved. 

7.    Fingerprints can also be used to show associations between works, even without the identification of a specific individual. 

8.    Partial prints can be used if there are a sufficient number of characteristics available to demonstrate a match. Partial prints can also be built up into more complete prints for reference purposes. 

9.    Prints may not be immediately evident on an artifact and may require special imaging techniques, and/or digital image processing, to make them sufficiently visible for the comparison process. Such techniques are well recognized and include standard imaging processes such as the use of special illumination techniques, UV fluorescence and infrared photography, and X-radiography. Digital filtering techniques are also used; these methods are explicitly accepted in the legal community as a valid approach. However, invasive chemical enhancement techniques commonly used for latent print detection are not used and we only employ non-invasive methods of imaging fingerprints. 

10. As a result of the extensive history of fingerprint examination, there are well-developed protocols for performing such studies. While artworks are a special case, many of the general principles and specific procedures are still valid and applied in the processes undertaken. For example the practice of checking matches with another expert examiner is a standard practice also used by us for fingerprints in paintings. 

11. While there are different standards used to declare a match in different jurisdictions, the methodology used by us for fingerprint work on paintings employs the so-called 'holistic criterion'. This uses a combined assessment of quantitative and qualitative aspects of corresponding features in prints, which stands in contrast to the 'empirical criterion', which seeks a quantitative threshold of comparison points. Following the outcome of a commission set up by the International Association for Identification (IAI), a resolution was adopted in 1973 favoring the holistic approach. It further permitted its practitioners to determine the number of points sufficient for a match. This position is now accepted by fingerprint experts and courts in many countries including the US, Canada, the UK and Norway.  In 2010, the International Association for Identification rescinded its 30-year ban on probabilistic evidence permitting the use of statistical models and expert opinion.

12. However, because of the special nature of artworks, additional expert knowledge is essential to the process of determining whether it is part of the artwork’s original structure. Moreover, significant experience is required to find and read prints in a context where the object’s surface can interfere with ridge patterns. The detailed examination of the object is an integral part of the process, ranging from study of the objects, known conservation history up to and including materials analysis, multispectral imaging, to demonstrate contiguity of the fingerprint with the original strata of the object as well as its context and relationship to the artwork. 

13. We have worked with a number of museums and collections examining and documenting fingerprints on paintings and other works of art. We therefore have considerable experience in issues that typically concern such institutions, such as safe handling of objects and security.  

14. Frequently, the need for fingerprint examination stems from lack of verifiable historical documentation of the object. Provenance, when available, can further buttress the strength of a comparison. Similarly, full chemical testing of the object should rule out anachronistic materials not available to the artist at the proposed date of its creation.

Protocol 

1. Prior to commencing a fingerprint study, the painting is examined and documented in detail to establish whether and how aspects such as condition could impact the subsequent interpretation of fingerprint data. Pre-existing documentation is consulted where relevant and available.

2. Close examination of the painting is made using visual means, as well as with the aid of low-power magnification and other imaging techniques that may aid in the location of stamped and plastic prints.  Prints may not always be visible to the naked eye, hence the use of imaging ranging from the ultraviolet to the near-infrared.

3. Where appropriate, other imaging techniques such as X-radiography may be applied, or the results of such examinations studied. 

4. Once prints have been located, they are documented in relation to the object as a whole and then recorded photographically at a ratio of 1:1 or better. Photographs are made using high-resolution digital imaging techniques capable of providing well over the required spatial resolution for recording level 3 detail, which is considered to be ~400 pixels per centimeter (1000 ppi). 

5. Where results from other imaging systems such as X-radiography are considered, similar spatial resolution criteria are assumed. However, this may be relaxed to include only up to Level 2 detail, for which ~200 pixels per centimeter is considered sufficient due to the inherently lower resolution of x-ray imagers.

6. Illumination of fingerprints for photography uses localized diffuse or directional light sufficient for proper acquisition of the image. The spectral distribution is normally confined to the visible region unless images are specifically being collected for UV, UV fluorescence or near-IR. Exposure to illumination and heat is minimized to the period of setting up and image acquisition. 

7. Images are taken and stored in native digital format and converted to uncompressed file formats to avoid introduction of compression artifacts.  Image processing is carried out on copies only and the original files are kept intact.

8. Where image processing is required, this is carried out using standard software and algorithms. All steps are recorded to metadata files so that modifications are explicitly retained. Results are critically reviewed to ensure that visual artifacts have not been introduced by the processing. 

9. Comparison prints are established by the fingerprint examiner, either from relevant archive prints or from sets specifically collected for the study.

10. Comparative features are identified in each print or partial print found according to recognized standards of documentation. These are then related to features in the reference prints. 

11. A match will only be declared if it meets the holistic criterion. 'No match' or 'undetermined' may also be declared. 

12.Comparisons are then checked by an independent fingerprint examiner. 

13. Results of fingerprint examinations are documented in a report, including all relevant contextual information, steps involved in the discovery and assessment of prints.

Viability

Before attempting a large-scale survey, we perform an assessment as to the likelihood of finding prints. Experience shows that with certain artists the probability is high as they used their fingertips as part of their painting technique. With others, the occurrence of prints is typically a result of handling the still wet painting.

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