Henkel's "Sil" was largely sodium perborate. It was claimed using product in first rinse helped prevent greying, yellowing, removed marks not shifted during wash...
Much of the reason behind Henkel's German wash trinity (Henko, Persil, Sil) was that Persil still was soap based product by late 1950's. Meanwhile Unilever came out with Sunil which was a detergent (Henkel had Dixan).
Because Sunil was a detergent much of the need to pre-soak, add various additives and use something in first rinse was eliminated.
https://www.automaticwasher.org/cgi-bin/TD/TD-VIEWTHREAD.cgi?68413
When soap is used on wash day first (and maybe second) rinse must be hot to boiling or at least warm water. If cold water is used right after a hot or warm water wash soap will solidify back into fabrics trapping dirt and schmutz with it. This residue leads to "tattle-tale grey" wash.
Detergents OTOH don't have this issue which is one of the reasons why they largely displaced soap on wash day.
Much of the reason behind Henkel's German wash trinity (Henko, Persil, Sil) was that Persil still was soap based product by late 1950's. Meanwhile Unilever came out with Sunil which was a detergent (Henkel had Dixan).
Because Sunil was a detergent much of the need to pre-soak, add various additives and use something in first rinse was eliminated.
https://www.automaticwasher.org/cgi-bin/TD/TD-VIEWTHREAD.cgi?68413
When soap is used on wash day first (and maybe second) rinse must be hot to boiling or at least warm water. If cold water is used right after a hot or warm water wash soap will solidify back into fabrics trapping dirt and schmutz with it. This residue leads to "tattle-tale grey" wash.
Detergents OTOH don't have this issue which is one of the reasons why they largely displaced soap on wash day.
