How is eosin different from watercolor?
From prehistoric times, people knew how to use various coloring agents for their needs. The first and the most accessible dying compounds were powdered minerals of different colors. Such powders are called pigments. They do not dissolve in water but instead form a suspension, similar to smoke (forming aerosol) in the air. After applying such coloring agents, water evaporates while pigment particles remain on the surface of an object being colored. Watercolor acts in exactly the same way. One of its advantages is resistance to air and sunlight.
Another type of coloring agents is dyes. They dissolve in water or in another suitable solvent. One example of such dyes is eosin solution. Dyes provide a wider spectrum of colors, and they are easy to work with. Only two centuries ago, colored fabrics were very rare because natural dyes are hard to obtain and synthetic dyes have only been discovered recently.
Dyes are irreplaceable for coloring fabrics because their molecules in solution easily interact with the fibers. Pigments cannot be used for this purpose because their particles can barely penetrate inside the material.
What is eosin?
Eosin is a maroon-red dye. It is a salt of a complex organic acid. Similar to regular cooking salt NaCl, it readily dissolves in water and forms nice red-orange solution.
Why does the fabric stained with eosin feature such a persistent coloring?
Regardless of chemical compound, each its molecule is constantly choosing between a total freedom of movement and a relative coziness of strong bonding with its surroundings. Eosin anions are facing the same dilemma. On the one hand, they may "swim" freely through the solution and twirl at any part of this huge volume. On the other hand, when absorbed by fabrics, eosin anions find themselves in a very advantageous position, like a tired man who settles himself in a comfortable armchair in the evening and doesn't want to get up. Similarly, eosin that penetrated into fabric doesn't want to leave it, and in our experiment we see it as a persistent coloring.
Why cannot the triangular fabric pieces be colored with eosin?
Just like people have their tastes and preferences, molecules choose their surrounding according to their nature. For instance, eosin prefers wool fabric as it contains special amide –NH–CO– groups. Wool reacts with eosin best of all the fabrics and therefore holds it the strongest. Eosin feels cozy and comfortable inside a wool fabric. Cotton or polyester materials do not have such groups, and therefore, eosin prefers liberty in an aqueous solution to questionable (from its point of view) coziness inside such fabrics.
May eosin be used to color something else?
Yes. However, it is important to remember that eosin preferences are quite specific: it needs amide –NH–CO– groups to bond to. These groups can also be found in other materials besides wool: for instance, silk or nylon.