Tap Cinder

Iron-rich slag raked from a puddling hearth. Roasted and ground it fettles the next heat - the trade called it bull dog.

Tap Cinder is the iron-rich remainder raked out when a puddling bed is cleaned, a fixed three per heat regardless of how the heat itself went. Grid-crafting it back into Puddling Fettle is exactly what the next heat's three rows want, so let it accumulate no further than one heat's worth.