Found the research article.  Postepisiotomy Pain: Warm Versus Cold Sitz
Bath, LaFoy & Geden, 1989, JOGNN, pp.399-403.
They were kinder about lowering the temp of the water than I, added ice
water to sitz via an administration bag and tubing.  Has some good info on
edema. I  gave this article to student nurses in maternity.

 "Nursing Implications: Since both warm and cold sitz baths are comparable
with respect to hematoma, sensation, and distress, and cold sitz baths are
more effective in decreasing edema, both modalities should be considered as
treatments for the postpartum patient.  Some patients prefer the cold
treatment, so perhaps the patient should be given the choice of either
therapy when minimal or no perineal edema is present.  Those patients who
have a greater degree  of perineal edema probably should be managed with
cold sitz baths as opposed to the traditional warm bath.

Edema is the initial response to injury and decreases over time.
Therefore, nurses may recommend that the patient use cold sitz baths
initially and for 24 to 48 hours following episiotomy, and then the patient
should switch to warm   sitz baths.  The warm baths would increase
nutrition in the tissues and promote healing.  This sequencing of therapies
would be similar to that used in acute soft tissue and musculokeletal
injuries.

Sincerely, Pat in SNJ  P.S. I don't think  this is off topic, as how the
perineum feels directly relates to mother's comfort, BF positioning and
sore nipples.