Everyone has their own thoughts and methods on this subject, so I'm just going to throw in my two cents. I have always preferred backing soda as an electrolyte. It is mild, works well, no chance of chemical burns, and no environmental hazards with disposal. I use plastic containers, usually 5-gallon buckets, but for larger items like 12-inch Coast Artillery mortar rounds, or those from Naval main batteries, I've used 55-gallon drums and even a coffin vault liner. For my waste electrode I've found nothing beats CRES (stainless steel). I get the pieces I need from scrap yards. You can often find a piece of discarded pipe that is just the right size to line your container. I find with the combination of stainless steel electrode and baking soda electrolyte, it is possible to do items of disimilar metals al at once without damaging lead, zinc, or copper parts. The stainless electrodes last an extemely long time, are very inexpensive if purchased at scrap weight compared to copper, and you don't run the risk of copper-plating your iron parts. Post processing is extremely important, too. Your shell has absorbed salts from the ground and backing soda is also a salt. These have to be removed, and boiling in distilled water will do the trick. Keep changing the water until it is clear after boiling. To make sure all the salts are gone, add a few drops of silver chloride. If it turns milky, you need to change the water and boil again. When all the salts are gone, then you need to drive out all the moisture. After the piece is cool to the touch and OUTSIDE, immerse in acetone and leave it for several hours. Then, remove it and allow any residual acetone to evaporate from the piece. You can now brush your piece clean. All the salts and moisture have been removed, and the molecular structure has been stabilyzed. You are now ready to fill in the pores so no moisture is absorbed from the atmosphere. There are a number of thoughts on this subject, but in my experience the best all involve gentle heat and thick petroleum distilates.