Dyeing Kinetics
A recipe is unlevel before it is unlevel to the eye. The gap between the two exhaustion curves at cut-off is where the shade drift is already committed.
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Difference between the two dyes when the bath is dropped
A compatibility index near 1 means the dyes strike together. First-order kinetics assumes constant temperature — on a ramped cycle the effective half-times shorten as the bath heats, so measure them at the plateau temperature you actually hold.
Dye Compatibility, Strike Rate & Half-Dyeing Time Tool — free, with the formula and a worked example, at Textile School.