As the key pigments in temporary and semipermanent color, direct dyes have been around for a long time, but today's sophisticated color formulators are using the technology to achieve effects far more dramatic than your mom's "rinse."

Direct dyes are pre-synthesized, so the color is already fully formed, requiring no oxidation, no developer.

According to Martin Uellner, Goldwell's category manager, R & D Color & Perm, due to their often-larger molecular size, it's difficult for direct dyes to penetrate the hairshaft.

"Most of them act more on the outside of the hair, leading to low durability," he notes. "Their benefit is they are very bright and intense."

Uellner says the dye's tendency to fade can be suppressed by a special penetration agent, like the one used in his company's Elumen line. Direct dyes in the classic semipermanent color are normally cationic, but Goldwell uses anionic (acidic) direct dyes in Elumen which, says Uellner, don't cluster so they can penetrate easily. An electrostatic attraction also anchors them to the hair, he says.

In the fabric-dying world, negatively charged direct dyes are somewhat repelled by negatively charged fabric, until salt or an ionic agent is used to create a positive charge, allowing an ionic bond to form between the two. The difference with hair color is that the pH of the base formulation is creating positive-charged hair, which attracts the negative-charged dyes.

Direct dyes, whether used as the central pigment as in Elumen and Keune's Color Contrast or as part of a combination oxidative and direct technology as in Wella's Magma or Schwarzkopf's Igora Vibrance, are for creating intense results. Pick a conventional permanent or demi formula when you want more natural shades.

Send your chemistry questions to matherton@vancepublishing.com.