When a fabric is woven with two different yarns–as union cloths are– the dying process is particularly difficult, as yarns take and absorb dyes at different rates and intensities.
When linen and cotton mixes are piece dyed they show a subtle tonal shading, but the differences are more marked with, say, a wool and cotton mix.
Traditionally, the separate fibres of any mix were dyed separately, i.e. before weaving, as so that the dyeing process could be adapted to suit each fibre.
Today, if uniformity is a pre-requisite, woven cloth can be piece dyed using a new technique that reverses the chemical charge of one to match the other, i.e. it changes the cotton from negative to positive before dying; wool is already positive.
For more info- www.agclassroom.org- William Marmer and Jeanette Cardamone