Luminsign

A non-verbal sign language used by the clans of the Sea of Black Ice to communicate when hunting on the iceflows. It entails the use of specially formulated bioluminescent inks painted upon bodies and hands of the kin. These inks act in two ways - first they shine in the dark, but also create heat, which the Shadowkin can see.
The kin will then flash signs with their hands and arm movements to created complex and rapid sequences of light and heat to convey meaning.
It was developed to enable easier communication in the harsh environs of the Sea of Black Ice. The darkness, sleet and constant howl of the winds makes verbal communication impossible, and the use of luminsign, with its light and heat signatures overcomes these issues.
The clans of Shadowkin also have verbal language, and they use this to converse with other they trade with, especially those from the Twilight and Sunside lands.
The ink used needs to be constantly replenished and so the shadowkin will revert to verbal communciation when not out upon the ice. Even then though, their verbal language is accenuated by hand signs as they talk. A conversation amongst clans is a quiet, whispered affair, and as muchb is said by their body movements and subtle twitches of their hands as the speak as anything else.

Luminsign inks

The bioluminescent ink used is farmed from the lichen veins found throughout the iceflows. It is prepared by the clan's Shadowseer and imbued with magics that also heat the ink. The paint is applied to the clothing of hunters before they set out, and will usually last a week before the lichen has died and the ink loses its potency.
The ink can be kept in storage before application though, and exbiditions that last longer than a week will need to take with them stores of the ink to replenish their signs.