I made a batch of studio clay last week. Some were using it that same night and it was great, by Saturday when someone went to use it it was the same color on the outside (beige) but just under the outer layer it's as charcoal grey as can be and stinks! It was the same today. It doesn't stink until you open it up and expose the dark clay. I used the same as we always do, one glug of muriatic acid, 60% fire clay, 40% ball clay, a scoop of grog and about a gallon of slake (studio scraps). No one has ever seen this happen before and we make a lot of studio clay. I made two other batches that day with a different recipe and my own slake but no problems with it. I'm wondering if it could be an organic in the slake? But how odd that it's solid through except for where the air hits it. OH and it eventually turns the beige color if it sits out in the air long enough.