1) Finely-sifted soil.
Charcoal burners covered the wood that was to be ‘coaled’ in a way that excluded the air and prevented combustion. This might be done with sods, grass, bracken and the like but one component was finely-sifted soil, called ‘dust’ in wood leases: 1672 libertye to … get sods Turfes & dust in the said wood,/i>, Tong
1763 to make Charcoal pitts & Burn the said cordwood & to get Dust & Cover for the said Charcoal pitts in the most proper part of the said Woods, Esholt.
2) The small particles separated from grain in the milling process.
1320-1 Pro xvij bus. de dust’ emptis de bovaria, Bolton Priory
1574 an arke called a dustarke, Hipperholme
1621 One olde duste arke 6s 8d
in duste 3s 4d, Slaidburn
1684 one trough to put dust in ... one dustin sieve, Whitley
1739 dusting sieves, Riddlesden.