Hi mrjin
My First 15K had the same problem. Other stones in the series had different problems. I contacted Taidea about it and they told me they had problems with temperature and moisture during the hardening process in the past. These problems were addressed and should be solved with the newer stones. They sent me a complete new set for free that I'm testing right now and so far everything is ok, the stones are great. You should contact Taidea about it, their customer service is very good. Kind regards, Peter.