Denim Dyeing
The skying passage is a length, but what the dye needs is a time. The sheet speed converts between them, and it is usually the speed that has to give.
— m
To fully oxidise between dips at the working speed
A negative shortfall means the range has skying to spare. Where it is positive the options are a slower sheet, a longer skying passage or forced-air oxidation — raising indigo concentration does not fix an oxidation shortfall, it deepens the ring while leaving the same unoxidised carry-over into the next box.
Slasher Indigo Dyeing Oxidation Distance Calculator — free, with the formula and a worked example, at Textile School.