Pigments are finely grounded inorganic and organic insoluble and dispersible particles that provide essential properties such as color, opacity, durability, hardness, and resistance to corrosion of paints. They are different from dyes because they aren’t intended to dissolve in the host matrix system. Even after adding into the matrix, they still maintain their properties as they are inert. In the upcoming sections, we will describe the analysis of paint pigments.
Paint Pigments give colors when applied to a surface of an object. There can be single or multiple layers of paint, each having its pigments giving different colors. Macroscopically, through the process of visualization, we can have a brief idea about the pigment type, either inorganic, organic, color shifting, metallic and pearlescent, or fillers and extenders. Its microscopic analysis can be accomplished by polarized light to check for minute details.
Along with the analysis of paint pigments, we will also brief about the types of pigments that are used before applying paints on an object surface to give a good and protective finish. Let’s start!
Types of Paint Pigments
1. Inorganic Pigments
It includes metal oxide, complex metal oxide, metal salts, etc. Chemical properties handle stability and other pigment properties. However, the light absorption property is not as good as organic.
- Metal Oxides: Iron is commonly used as metal oxide. Some are synthetic or natural. According to the earth tones with different sizes and a wide range in opacity or transparency, color ranges from black to red, tint strength, and hues.
- Complex Metal Oxides: It is similar to Metal oxide. A crystal with a metal oxide lattice contains more than one metal. Some crystal lattice of complex metal oxide is the same as metal oxide. Both of them are inert, excellent in chemical stability, handles heat, light, and weather.
2. Metallic and Pearlescent
Metallic pigments include aluminum pigment that consists of leafing and non-leafing pigment. In leafing, the pigment float on the surface while coating produces a silvery metallic surface, while in non-leafing, it disperses throughout the film.
Pearlescent pigments or the interference pigment includes titanium oxide coating on mica that gives complementary color on both sides, i.e., green and red when light reflects and transmits.
Iron oxides coating on mica gives Copper-red bronze color, giving a lustrous look to the finishing top. Natural pearlescent comes from sea products, e.g., fish skin, pearl by synthesizing crystals used as pigments.
3. Color Changing Pigment
The technology has made paint color change when viewed from different angles. Thermo-chromic pigment changes conformation to color and coating through heat liquid crystal. Thermometer strips and tapes used in aquaria and photochromic paints and coating contain dye.
4. Organic Pigments
It is vibrant, intense in color, and has high tinting strength. It includes carbon with other elements such as madders, carmine, crimson lake, indigo, etc., from coal tar. Few are light and stable still fade; light and moisture together are still more potent.
5. Fillers and Extenders
It improves the final film finish, e.g., Calcium carbonate increases gloss and fire-retardant properties, barium sulfate and gypsum are cost-effective, soft to grind, and low water solubility.
Magnesium silicate, potassium, and aluminum silicate are grinded and pulverized to the extent that their size is smaller than the film thickness.
Analysis of Paint Pigment
- A critical review of the chain of custody for an initial evaluation is done.
- Package sealing, identification marking of the sample, and potential cross-contamination between the samples are done.
- The first step for analysis is the visual and macroscopic evaluation, description, and documentation of the original condition of the sample.
- It involves the general conditions, weathering characteristics, exterior colors if multi-layered, the color of each layer, etc. This description can be accomplished by examining each item using a stereomicroscope.
- Polarized light is the detailed technique of layer structure examination and the comparison and identification of particles present in a paint film, such as pigments, extenders, contaminants, and additives.
- The forensic coater examiner should always use the specific and least destructive technique.
- Samples that are not constrained by the amount should be subjected to determine the color and texture of the paint in the single-layer pint and the number, order, color, and texture of the layer in the multi-layered sample.
- Matches should be documented with a descriptive note, photography, photo transparency, or other appropriate imaging techniques.
These techniques are helpful for the analysis of paint pigments from the sample connected with the investigation. Paint also has a great Forensic significance.