By Jen Frey I have found French Green Clay to be the best solution for a Poison Ivy Rash. It dries up the rash quickly, especially if applied as soon as the rash appears.
Mix a small amount of the clay with just enough apple cider vinegar to make a thin paste. (If you don’t have apple cider vinegar, you can use water.) Apply this paste to the rash. Allow it to dry completely. When dry, very gently, being sure not to rub, wash the paste off with cold water. I usually allow the water to run right over the clay. If it is really stuck, you can put a wet wash cloth on it. Repeat this as needed.
To be even more effective, you can infuse the apple cider vinegar with jewelweed or garden sage.