A mordant is a chemical that fixes or intensifies a dye or stain. In fabric dyeing, a mordant forms a coordination complex with the dye, helping the dye attach to the fabric. This polyvalent coordination complex is called a lake. Mordants also intensify stains in microbiological slides or tissue preparations.
The word “mordant” comes from the Latin word mordere, which means “to bite.” This refers to the way a mordant helps a dye bite onto fibers to retain their color during washing.
How a Mordant Works
Most mordants are polyvalent metal ions that react with a dye or stain, forming a colloidal coordination complex. A covalent chemical bond involving a hydroxyl oxygen forms between the mordant and dye. Also, a coordinate bond forms with another oxygen atom. The coordination complex may be either acidic or basic. The coordination complex or lake attaches to fabric or tissue via covalent and coordinate bond formation (the same way the mordant and dye connect). The electrical charge of the lake helps the complex bind to fibers or cell features. Also, the complex has a higher molecular weight than the dye alone, making it harder to wash away. While the dye might be water-soluble, the coordination complex is insoluble.
The coordination complex may have a different color than the original dye. For example, cochineal scarlet or Dutch scarlet is a bright orange-red from the reaction between cochineal (used to make carmine red) and a tin mordant.
The end result also depends on when a mordant is added in the dyeing or staining process:
- Pre-mordanting or onchrome: First the substrates is treated with the mordant, followed by the dye. The coordination complex forms on the fiber.
- Meta-mordanting or metachrome: The mordant is added to the dye bath. The coordination complex forms in the dye bath. While treating the substrate in one step is simpler, meta-mordanting only works with a few dyes.
- Post-mordanting or afterchrome: The material is dyed and then treated with a mordant. As with pre-mordanting, the coordination complex forms on the fiber.
Whether the mordant is added before, during, or after dyeing not only affects the color and its stability, but also impacts the fibers. For example, adding an acidic mordant to an acidic dye damages fabric more than pre-mordanting or post-mordanting. In other cases, a mordant affects the texture of a substrate. For example, chromium-based mordant softens wool.
Examples of Common Mordants
Common mordants used in dyeing and tanning include aluminum, chromium, iron, copper, iodine, potassium, sodium, tin, and tungsten salts (usually oxides); sodium chloride; alum, tannic acid, and chrome alum. Important histotechnology mordants include alum hematoxylin and Harris’ hematoxylin in the hematoxylin and eosin (H&E) stain.
Mordant in the Gram Stain
Iodine is referred to as a mordant in the Gram stain in microbiology. However, it’s actually a trapping agent. A trapping agent inhibits dye removal during the decolorizing step after staining. Similarly, sodium chloride in the Salt Gram technique and picric acid in the Gram-Weigert method are trapping agents rather than true mordants. While mordants are metals with a valency of at least two, trapping agents are usually nonmetals, typically used with basic dyes, and are applied after the dye. Processes involving trapping agents stain perfectly well if the step is omitted. Processes requiring a mordant don’t stain unless the mordant is used.
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