The following step is important:
Before placing your copper plate in the Edinburgh Etch, the 'Sting must be taken out of it', to prevent uncontrolled biting. This can be done by placing a small copper blank in the bath and waiting until this has been completely dissolved. Or, when the mordant has been exhausted, you may use a small amount of this exhausted solution and add it to the fresh mix to take the sting out of it.
When the Edinburgh turns very dark brown and acquires a less liquid consistency, this indicates that the solution should be refreshed.
I have made use of the recipy for Edinburgh Etch as described by Keith Howard who has figured out the actual quantities to use. He gives the following example: 6 litres saturated ferric chloride + 1.2 litres water and 400 grams citric acid dissolved in water.
I took a workshop with Barbara Seidenath on electro-etching, using a saturated salt solution. While it's a bit difficult to get everything set up, I love that there's no use of chemicals, only salt and batteries. Although the solution doesn't etch silver, you can etch copper, brass, bronze, steel... I etched a large piece of brass and then used it to rollerprint on silver and copper for beautifully textured pieces, deep enough for champlevé.
Etching for Copper:
Ferric Chloride is inexpensive and is available from Chemical Companies straight from the bottle. It can also be mixed from crystals: 13 oz. of ferric chloride crystals to 16oz. of water.
Ferric Chloride will etch copper but not silver. A mustard residue is left on the surface of copper when etching with ferric chloride and it should always be neutralized and scrubbed off the metal with ammonia or ammonia and water after etching.
Metal: Traditionally, l4 or 16 gauge metal is used to etch a piece for enameling, especially a large piece because the heavier metal will still have structural strength, even after it is etched. If you want to have plenty of room to inlay color, you should plan to etch about halfway through the metal, but this isn't always necessary. On large vessel forms use 14 or 16 gauge metal, but on smaller jewelry pieces, especially if they are formed, 18 - 20 gauge metal work well, depending on the function of the piece.
Etching for Silver:
Ferric Nitrate is the acid used for etching silver. It etches cleanly and works well with oil based paint pens and heat transfer paper. It will also etch copper, but Ferric Chloride is much less expensive and lasts longer. Ferric Nitrate is basically nitric acid, which contains iron and it is neutralized with baking soda.
Recipe: 500 grams of Ferric Nitrate crystals are mixed with 1 pint of hot water until they dissolve - If you are using an etching machine - 3136 grams to a gallon of water.