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Blackdust is a term used to refer to Thread that has been killed by cold air.
Though Thread is notorious for being able to devour organics, it is unable to survive heat, cold, or water. In very cold air, Thread freezes, turns black, and breaks into tiny pieces. The dust-size pieces then fall to the ground, doing no damage. Blackdust can appear as an early sign of an upcoming pass.
It was noted at Sadrid Stake, mixed in with rain.
Sweepriders brought buckets to show S'nan during the 'dud' fall in Second Pass. G'don and other riders also watched. One piece of frozen Thread was brought but the sink as it unfroze, they got rid of it.
F'nor runs into Blackdust while performing weather checks at Tillek Hold. Dust storms were also noted to be taking placeun upper Crom Hold. When warning his riders about the signs of Thread, F'lar realized that he had forgotten to mention the possibility of Blackdust, until Lessa quoted a passage from The Ballad of Moreta's Ride  He would note that the only thing preventing Pern from danger was the extended cold season taking place that Turn.
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- Red Star Rising
- Moreta: Dragonlady of Pern