Sounds kinda dirty, somehow, or maybe that’s just me. Anyway, when you fire Oribe greens, you usually get an oxide layer over the surface that dulls the color, or in extreme cases changes the color completely to a sort of what I always imagined the color of the Great Grey Green Greasy Limpopo River to be. (always loved Rudyard Kipling)
The traditional method for cleaning this up is chestnut husks soaked in water, which creates an acidic, extremely smelly brown concoction that you soak your dishes in for a few days. Nowadays, people use a 3% solution of hydrochloric acid. Now, this is nasty stuff so if you are going to use it, use it in a very well ventilated area, with gloves, eye protection, etc… When you open the bottle, white mist rises, just like in the movies. Breathing that is not a good idea.
Here is a before after pic of a dish I cleaned up. These dishes did not have a heavy oxide layer, but the cleaned one is noticeably brighter.