The knowledge of which pigments are present in a paint can be very helpful in assessing the evidential value of matching paint samples. As different paint suppliers often use different pigment mixtures for obtaining the same color shade, the identification of the pigment composition can enhance the discriminating power. The knowledge of which pigments are present can also be of paramount importance in hit-and-run cases. If their dates of introduction and/or composition are known, their identification can provide a guide to the make, model, and year of a vehicle.

In the last few years, some schemes have been developed for identifying "one-dimensional" organic and inorganic pigments. The protonation technique using trifluoroacetic acid must be mentioned.1 Applying this technique many organic pigments can be solubilized after which they can be separated by thin layer chromatography (TLC) and identified by microspectrophotometry (MSP). IR-spectroscopy2-5, Raman-spectroscopy6,7 and MSP8, when used on thin sections, can also be applied successfully. Microscopy (incident brightfield at a magnification of 200-500x) proved to be the easiest way to establish the presence of interference pigments and to measure their dimensions.9

Mica-based pigments are characterized by their non-uniform surface color of the plate-like pigments due to the scattering effects caused by surface irregularities (steps, edges, etc.) of the mica platelets.

This feature makes determination of a specific interference color more difficult. The exact identification of a pearlescent pigment type is only possible by applying a combination of analytical methods.

Following microscopy it is essential to use SEM/EDX which, for example, allows high-chroma mica platelets coated with titanium dioxide and a layer of cobalt titanate to be distinguished from other high chroma interference pigments based on mica.

Analyzing thin sections cut parallel to the surface using MSP (transmission mode) should be the next step. Alternatively MSP using bright field specular reflectance can be applied successfully. It is important to analyze only flat flakes laying parallel to the surface.

The identification of many types of luster pigments can also be carried out using XRD, but it is not possible to distinguish between different pigments based only on varying thickness of the same metal oxide. Transmission electron microscopy, in combination with EDX-analysis, is the best technique for analyzing complex mixtures such as iron oxides in the presence of an iron oxide coated mica pigment.

Bright field microscopy with incident light is also the easiest way to recognize OV luster pigments.

In contrast to mica pigments, the platelets of most OV pigments have either sharp edges or show uniform colors across the surface due to controlled thin film deposition techniques such as chemical or physical vapor deposition and wet chemical coating.

The best method of identifying the different pigment types/systems (Fabry-P?t types and inner reflector types) is SEM/EDX.

Microspectral analysis with brightfield specular reflectance is an ideal tool for the discrimination of OV pigments based on the same type.

Other plate-like pigments not belonging to the OV pigment group or the mica pigment group like platelet and/or micaceous iron oxides (e.g., BASF Paliocrom Copper L 3000 and L3101), plate-like phthalocyanine (BASF Paliocrom Blue Gold L 5000) colored aluminum pigments and iron oxide coated aluminum pigments (BASF Paliocrom Gold L 2000 and L2020) are easily identified by brightfield microscopy with incident light (200-500x magnification).

More difficult is the recognition and identification of plate-like pigments like platelet molybdenum sulfide (Ciba Geigy No. 7700) and platelet Graphitan (Ciba Geigy No. 7500 and 7525). Their identification is proved by microscopy of thin sections cut parallel to the surface in combination with SEM/EDX analysis.

Microscopy in combination with SEM/EDX is also the best method for identification of Graphitan-based TG pearl pigments.

Engelhard Corp. has boosted the light stability of bismuth oxychloride pigments to such a degree that they can now be used in automotive coatings.10 As a result, Chrysler has offered a dark slate BiOCl pearl in its 1998 line. BiOCl can easily be identified by microscopy in combination with SEM/EDX.

It is not an easy task to identify micro titanium dioxide (or ultra fine TiO2) in coatings. The most suitable techniques for their identification are elemental-mappings on 20Km thin sections in the SEM or TEM/EDX analysis after ashing of the sample.

The results of XRD analysis are only unequivocal if the samples do not contain both rutile and rutile coated mica pigments.

Conclusion

The first step in classifying and identifying plate-like pigments is microscopy (200- 500 x).

Maintain a library of prepared slides of various plate-like pigments.

Exact identification of plate-like pigments is only possible by applying a combination of analytical methods: SEM/EDX, MSP and XRD.

References

1 Massonnet G, Stoecklein W. Identification of organic pigments in coatings: applications to red automotive topcoats. Part I: Thin layer chromatography with direct visible microspectrophotometric detection. Science & Justice. 1999;39:128-134.
2 Suzuki EM, Marshall WP. Infrared Spectra of U.S. Automobile Original Topcoats (1974-1989): IV. Identification of Some Organic Pigments Used in Red and Brown Nonmetallic and Metallic Monocoats - Quinacridones. J. Forensic Sciences. 1998;43:514-542.
3 Suzuki EM. Infrared Spectra of U.S. Automobile Original Topcoats (1974-1989): V. Identification of Organic Pigments Used in Red Nonmetallic and Brown Nonmetallic and Metallic Monocoats - DPP Red BO and Thioindigo Bordeaux. J. Forensic Sciences. 1999;44:297-313.
4 Suzuki EM. Infrared Spectra of U.S. Automobile Original Topcoats (1974-1989): VI. Identification and Analysis of yellow Organic Automotive Paint Pigments - Isoindolinone Yellow 3R, Isoindoline Yellow, Anthrapyrimidine Yellow , and Miscellaneous Yellows. J. Forensic Sciences. 1999;44:1151-1175.
5 Massonnet G, Stoecklein W. Identification of organic pigments in coatings: applications to red automotive topcoats. Part II: Infrared spectroscopy. Science & Justice. 1999;39:135-140.
6 Massonnet G, Stoecklein W. Identification of organic pigments in coatings: applications to red automotive topcoats. Part III: Raman spectroscopy (NIR FT-Raman). Science & Justice. 1999; 39:181-187.
7 Suzuki EM, Carrabba M. In Situ Identification and Analysis of Automotive Paint Pigments Using Line Segment Excitation Raman Spectroscopy: I. Inorganic Topcoat Pigments. Presented at the 52nd Annual Meeting of the American Academy of Forensic Sciences. Reno NV; 2000:1-60.
8 Stoecklein W. The Role of Colour in the Characterisation of Paint Fragments. Chapter 9 In: Caddy B, editor: Trace Evidence Analysis and Interpretation. Taylor & Francis; 2001 in press.
9 Stoecklein W, Tuente J. Using the light microscope for analytical aids for solving cases involving hit-and-run offenses. Zeiss Information with Jena Review. 1994;3:19-22.
10 Novinski SJ, Nowak PJ, Venturini MT. Pearlescent Pigments in High Performance Coatings. Presented at Intertech Conference; 1997 Oct. 27-29, Chicago.