To keep the copper from going red, try this:
1. Prepare your copper shape/inclusion by pickling it in lemon juice; it works faster and better if the pickle is warm. The pickling will clean the copper of oils. Fifteen minutes works best for me. I usually have a small crockpot of the pickle solution going. (There are commercially available pickles, but lemon juice does work for the small bits and pieces I use, it’s not as caustic, it’s cheap and it’s at the grocery.)
2. Remove items from pickle using tweezers. Dry. Paper towels work fine– a hair dryer if you’re in a hurry. Use tweezers in handling the copper to prevent putting oils back on the cleaned copper.
3. Coat with Super Spray. I give it at least two coats on both sides. Paint it on in one direction, then paint perpendicular to the first set of paint strokes. Or you can spray the solution– you might be able to get away with one coat. Take precautions if you do spray.
4. Place your dried inclusions within your design and fire away.
This has worked for me pretty regularly. I’ve sometimes put items into the kiln and discovered that I hadn’t done a great job of coating something and have gotten red streaks. Sometimes it looks very cool — duotoned copper! Sometimes not so much.
There’s a recipe for devit spray on this site that might work with copper, but I haven’t tried it.
If you do pickle more than one thing at a time, I’ve found that it works better if they aren’t touching one another in the pot. And if you don’t want to invest in a small crock pot (I got two at the Goodwill for a buck each), then just set the container with the lemon juice inside another container with hot water.