I am glad I came across this! I have been trying to reform my chalk (with other powders of question, but 90% chalk) and my texture is just off or something. I have seen cement rocks set and used in buildings softer than my reforms came out!
Use purified water. I use a Zero water filter. If you try to reform it with hard water it comes out weird due to all the other minerals in the water. Even with purified water it can only be reformed twice.