I’ve used the two methods recommended by Barger and White in their excellent book "The Daguerreotype".
The first method I have used on a lot of Dags suffering from general excessive tarnish extending into the image. The procedure is simple and every one I have done shows significant improvement.
The solution is made by dissolving a small amount of washing soda (NaCO3) in water, enough to get a slightly slippery feel to the solution. Take a piece of aluminum sheet or foil larger than the plate and put it at the bottom of a tray or dish. pour the carbonate solution in the tray and then place the plate face up on top of the aluminum. Within a few seconds you should see the lightest areas of tarnish start to disappear. A few minutes should be sufficient to remove most of the tarnish, but if heavy areas of tarnish still remain I suggest continuing to the second method.
A good indication as to weather it is working is a faint rotten egg smell from the sulfides coming off the plate.
The Second method is a bit more aggressive, but is required for plates that have been stored without cover glasses or cases.
The solution is made by diluting ammonia 1:2-1:3 with water enough to fill a tray or dish. DC current is supplied from a D cell with two wires with alligator clips. The Plate is attached to negative, and a silver anode clipped onto positive. A piece of silver wire from a jeweler’s supply works well, but a electroplated silver spoon could work. The anode is moved around above the plate, concentrating on the areas that are most corroded and tarnished.
I used this method to clean a plate that was completely blackened from exposure and discovered an amazing copy of a painting that is now completely visible.
For an extreme example of how well this works:
http://forum.treasurenet.com/index.php?PHPSESSID=09514a884313bece8221010c8b6c5184&topic=141557.msg1035981#msg1035981
After both methods of cleaning a good wash and drying by heat is all that is required. I have used both methods and I have not seen any sign of retarnishing or degradation.