I'd recommend practice on scrap metal. It is tricky to get the finish right. Gil(redryder) was kind enough to post his method in another thread. Here it is:
There were several hours of preparation involved before the actual bluing process begins.
1. The copper must be completely and absolutely clean.
2. Final cleaning steps with extra fine steel wool, with latex gloves on in well vented area or outdoors.
3. Wipe down the whole thing again with a degreasing solvent (Loctite ODC-Free Cleaner & Degreaser)
4. Wipe it down yet again with denatured alcohol.
5. Next a cold water rinse and dry.
6. Birchwood-Casey Super Blue... sold in gun stores, diluted up to 10 to 1 with distilled water.
I used solution at around 7/1. The chemical name - Selenium Dioxide.
7. If it is turning dark too quickly you want to dilute it a little more with distilled water.
Can be applied to small pieces with an artists brush.
On the copper boiler I used a soft cloth and a non-abrasive 3M type scrubbing pad that is continually dipped
in a bowl of diluted Super Blue. It will seem like some areas won't take the blue so be patient. Keep rubbing on an ample amount. It will require some trial and error to get the feel of proper pressure to apply while rubbing in the blue.
When bluing looks as good as you think it will get, rinse immediately in cold water and wipe dry gently.
If you see areas that are too dark or or too dull, rub with a soft cloth the way you might burnish paint.
8. After drying and burnishing I coated the whole boiler with Remoil (gun oil) or similar rust protectant.
9. I may try clear lacquer to replace the gun oil at some point.
Here is a link to an article by a jewelry maker
with a similar product:
Good luck! Don't give up. If it looks really bad just take it down to bare shiny metal and try again.
My youtube channel: http://www.youtube.com/user/reddrryder/videos
Benny- AKA Dr. Reverend Delmar MacReady, High Epocht-Church of the SubGenius, Doktor of the Forbidden Sciences.