Liver of Sulphur (LoS)
According to Wikipedia Liver of Sulphur is a poorly defined mixture of potassium sulphide, potassium polysulphide, potassium thiosulphate, and probably potassium bisulphide. Synonyms include hepar sulphuris, sulphur, sulphurated potash and sulphurated potassa.
I know LoS as a smelly chemical, a powder, liquid or gel, which is used to darken metals that contain copper including brass, bronze and silver.
The final patina can be anything from a light grey to brown to complete black depending on how quickly the colour has been developed, this depends on both the strength and temperature of the patina solution, it can also contain a wonderful iridescent affect.
Using LoS is a simple process, you simply dilute the LoS with warm water, about 60deg C seems to work best, and either dip the item that you want the patina on into the solution or apply selectively to the areas that you want to have a patina. In reality, as with all processes, you may need to practise a little. Starting off with a weak solution which takes a little longer to work will give you more control over the speed of the reaction and the final colour achieved.
I currently use and sell 3 different types of LoS
Yellow LoS is a high quality mix that is easy to control and use which gives a consistent colour
Red Los which is a mix of the high quality yellow LoS and some additional chemicals that help to give the final colour a softer hue that includes more of the iridescent colours
Black LoS, this is a much stronger thicker version of the yellow LoS which has a longer shelf life. It requires far less of the original solution compared to the yellow LoS liquid to produce the same amount of patina solution.