We have been annealing these shells between 500C and 650C, with a warm up time of 30 minutes and 2 hours at peak temperature. We have also been opening the oven door fully at peak temperature.
By adjusting the annealing temperature you can respectively change the patina and the hardness of the steel. Mostly it goes that the higher the temperature, the harder the material. Lower temperatures will rather give a golden like color and higher dark purple. The temperature you will choose to anneal your instruments with will also depend very much on your own shaping and tuning process. Between the different batches you will also want to vary your temperature as the hardness differs as well as the possible colors to be achieved.
When you oil the shell the color changes a lot! We use phoenix oil. See below a photo of four different shells unoiled:
As there is so much to explore in temperatures per batch, this is a great place to all share our experiences.