Previously the need for color etching on the microstructure was conditioned by, among other things, the qualities of the equipment of that time. Sometimes it was quite difficult to recognize the features of the phase structure and determine the composition of the phases at magnifications up to 1000 times in the absence of scanning microscopy tools.

Thin films of oxides or deposits of a complex chemical composition are formed on the surface of the microsection during color etching. Films formed at different phases have different thicknesses (Fig. 1). The thickness of the film depends on the chemical composition, physical properties, orientation, etc. (Fig. 2). The color coloration of the phase is formed as a result of the interference of rays reflected from the outer and inner surfaces of the film (Fig. 3). The interference color depends on the thickness of the film and the ratio of the refractive indices of the film and the medium.

Figure 1. Scheme of the formation of an oxide film during etching, depending on the phase composition.

Figure 2. Scheme of the formation of an oxide film during etching, depending on the orientation of the crystal.

Figure 3. The path of light rays reflected from the oxide film.

Film thickness and interference color are related (table). (X unit is 1.002 • 10−13 m; KX = 1.002 • 10−10 m = 1.002 • 10−1 nm.)
Oxide films are produced by chemical, electrolytic or thermal etching.
Etching was followed by the process of photography, which involved photographing on film (or photographic plate), developing and printing the photographs. It took about 1 hour and 20 minutes to develop color film, and about 1 hour to develop photographic paper. Taking into account that the process of photographing itself was challenging and included the selection of lighting, the choice of color filters, etc., it appears that overall time required to obtain an image was considerable.
Comparing to the recent times metallography has reached a new level. Both microscopes and imaging devices have changed; digital cameras and camcorders had been created. We also offer numerous image processing programs that allow obtaining images of maximum information content. Therefore, now we can record tens and hundreds of images of the structure a day, whereas earlier it was possible to get only 10-15 frames.
Therefore, at present, the need for color etching is significantly less, if any. Nevertheless, the issue of color in metallography is relevant, but on a different level and by other means.
Any, even traditional etching, is colored and the task of microscopy is to adequately reproduce these colors. This is achieved by combining the technical capabilities of modern optics and recording devices, as well as image editing.
The simplest is the case of tarnishing, oxidation, etc. (fig. 4).

Figure 4. Oxidation of the lead surface during etching with acetic acid

The phases of doped silumin (Fig. 5, a) after etching with Keller's reagent have different shades. In the Al-4% Cu alloy, etching reveals the effects of dendritic segregation (Fig. 5, b).

a	b

Figure 5. Structure of doped silumin AK21 (a) and alloy Al-4% Cu (b).

Cast iron is a classic example of color etching using a standard reagent. Etching is standard: 4% nitric acid solution in ethyl alcohol. Figure 6 shows the structure of a cast iron welding on steel, photographed with a digital camera. It can be seen that the phases of cast iron have different colors. The color scheme is also determined by the light source. In this case, yellow prevails.

a	b

Figure 6. The structure of cast iron (surfacing on steel): a - as obtained, b - after the selection of brightness and contrast.

With a smaller proportion of the yellow component, more colors or shades can be seen and it is easier to see the characteristic color of the phase components. Inclusions in the structure of cast iron (without etching) have their own characteristic color (Fig. 7). Graphite is dark gray. Matrix without etching appears light yellow. Light gray inclusions are present, their composition was not determined .After etching, the cast iron structure exhibits a wide variety of colors. There are various shades of pearlite, from brown to blue, gray graphite, white ferrite (Fig. 8, a). The structure of the same cast iron after quenching looks even more colorful (Fig. 8, b). Martensite has a color of blue, green and blue (it is also shown in the article "What does martensite look like?").

Figure 7. Phases of gray cast iron (no etching).

a	b

Figure 8. The structure of cast iron: a - cast, b - hardened (conduction cooling).

The concept of "Non-ferrous metallography" should include methods of color visualization of the structure using special devices. These are:
1. Polarized light;
2. Differential interference contrast (DIC);
3. Dark-field illumination.
Staining with polarized light and differential interference contrast creates conventional colors. Fig. 9 shows an example of the formation of a conventional color using polarized light. The object of study is the structure of the iron-stone meteorite.

a	b

Figure 9. The structure of the meteorite: a - bright field illumination, b - polarized light.

Polarized Light Interference (DIC) allows you to increase the depth of field. In Fig. 10 а, the lower part of the image is indistinct, because the silicon plate is not perpendicular to the lens axis. When using DIC, the depth of field is increased and the surface is painted in different colors according to the position of the Nomarski prism.

a	b	c

Figure 10. Carbon coating on silicon: a - bright field, b, c - differential interference contrast at different positions of the Nomarski prism.

When using dark field illumination, objects have natural colors (see article "Darkfield microscopy"). For example, a plastic coating on a zinc layer in a bright field illumination looks gray (Fig. 11, a). In a dark field illumination, the coating has its own natural color (Fig. 11, b)

a	b

Figure 11. Polymer coating: a - bright field illumination, b - dark field illumination.

Well, finally, let's show the most colorful example! These are traces of red marker on the metal surface (Fig. 12). The color is natural.

 Figure 12. Dried drops of red marker on the surface of a steel section; bright field illumination