I got a call today from a lab that was seeing a green tint to their sintered zirconia crowns. I have seen yellow tint due to contamination and kind of an oatmeal color due to under sintering but never green. I told him with his next batch of zirconia place an uncolored piece of unsintered zirconia in with his crowns and see if it is bone whit or has a yellow or oatmeal tint to it. Especially the edges. He will run the test and get back to me. Hopefully we can find out if its his oven, coloring liquid, zirconia pucks or some other outside contamination.