I have lately done some dying of wool with some of the milling dyes in the Pro Washfast Acid Dyes series. I have used the conditions recommended by Pro (ammonium sulphate 1g/L or vinegar 12mL/L, and NaCl 1g/L). I have problems with the yellow called 119 sun yellow (acid yellow 19). I does not exhaust properly. For mixtures, the other colors strikes first and the yellow remains in the solution for a much longer time, making it difficult to get a level result.
I therefore made an experiment where I measured pH and observed the solutions visually for different conditions. For all experiments, I used wool yarn and a 0,5% dye mixture of sun yellow and 413 navy (acid blue 113). Both of these dyes are (as I can see) milling dyes, and the molecular weight are quite similar (601 and 681). I tried three different acid conditions: (1) ammonium sulphate, (2) vinegar added in the beginning and (3) vinegar added at the boiling point. Before heating, the navy had almost disappeared from the solutions, and the pH was 6, 3 and 6. At the boiling point, all solutions was clearly yellow, but nr 3 was most intense and nr 2 was less intense. After 10 minutes of boiling, the pH was 5, 4 and 4, and nr 3 was now as intense as nr 2. After 30 minutes, all solutions was still yellow. I added more vinegar (12 mL/L) to all solutions. The pH was now 4.5, 3.5 and 3.5. After 50 minutes, nr 2 and 3 was almost clear, but nr 1 had still a yellow color.
I also did parallell experiments without the NaCl, but there were no difference that I could see.
From my experiments, I conclude that the navy strikes easily without adding acid, but the yellow need a pH at 3.5 to (almost) completely exhaust. This is not what I would expect for a milling dye, but maybe the sun yellow is not a distinct milling type? I have seen the same effect when I have tried to mix the sun yellow with other milling dyes (490 brilliant blue and 338 magenta). Is there any way I can dye with mixtures including the sun yellow and get a more level result? I guess that I have to either make the yellow strike faster, or (preferably) delay the other dyes. The addition of NaCl did not make any difference, at least not at the concentration I used. Glauber's salt is not recommended for milling dyes. I may exchange the yellow for another one, but I can not see that any of the other yellows are expected to be better.