I have been sculpting in porcelain for a while now and had the same problem. I make sure to scrub out all the clay with a nail brush and then I use A+D ointment.
http://www.amazon.com/Original-Ointment-All-Purpose-Skincare-Formula/dp/B000R37O18/ref=sr_1_2?ie=UTF8&qid=1349447414&sr=8-2&keywords=a+d+ointment
A manicurist told me about it years ago for my toenails. I use it in the winter too, for my feet, slather on A+D and then put some socks on before bed - it works wonders! I think the main ingredients are petroleum jelly and lanolin. I think I will give the vinegar a try
