Black chain, (noun) [work gangs]; ostensibly free, but often 'conscripted' or 'drafted'. They are free labor, but not slave. These chains may be other colors, as well, depending on their purpose
"Another for the black chain of Ionicus," said one of my master's men.
Ionicus was a master of work chains. He had several, the "red chain," the "green china," "the yellow chain," and so on, each of which boasted several hundred men. Supposedly these were free work chains, "free" in the sense of not utilizing slaves. Goreans generally do not employ slaves for such labors as road construction, siege works, raising walls, and so on.
Book 22, Dancer of Gor: Page 301