I also was researching that not long ago. I have not tried yet, but apparently the tarnishing to purplish is because of oxydation to a0ambient air and b) our finger oils. Oils are the most commom causes. So there are a few ways to prevent it (again I haven't tried but if you do, please post results). You can soak your inclusions in a light acid like vinegar and then dry it wearing gloves before including. You can also use a metal brush to "revive" the metal; it would manually remove the oils and oxydation. Some say they do both and get good results. Lastly, and what I will deo, is to take new untouched copper and cover it with a light coat of transparent enamel like unique glass and then dry and fuse. Please let us know if it works. I have googled and researched for so long, and have yet to try any of the tricks. Good luck.