Liver of sulfur is mainly used in metalworking to form a brown or black patina on copper and silver as well as many (though not all) copper alloys and silver alloys (brass, for example— a copper alloy— does not react with sulfur compounds).  It is sold as a yellow brittle solid (a "lump" which must be mixed with water before use) as well as a pre-mixed liquid and a gel form.  The solid is believed to have the longest shelf life, though all liver of sulfur tends to decompose with time.  Modern gel forms contain stabilizers that allow the reactivity to last much longer.  Liver of sulfur that is kept dry, sealed from air, out of the light, and in a freezer will last many times longer than that kept in any other condition.
The highest quality liver of sulfur in solid form is a dark yellow, almost "liver" colored substance.  As it ages and is exposed to air, its potency decreases, it will turn lighter yellow and finally white, at which point its reactivity is negligible.  Liver of sulfur decomposes to sulfate of potash and carbonate of potash, neither of which has any value as an oxidizer of metal.
The reactivity of liver of sulfur with silver and copper quickly creates a dark or colored patina on the metal.  This is done by immersing the metal object in a solution of liver of sulfur and water.  When treating silver, the solution must be hot, though if the bath is brought to its boiling point the liver of sulfur will quickly decompose and become ineffective. Also, if the concentration of the solution is too strong, the oxidation process will proceed too quickly and the layer of patina thus created will tend to flake away.  The best results are usually obtained by using more dilute solutions and allowing the patina to build more slowly but more securely, and, for silver, keeping the solution just under its boiling point.  Lastly, it is critical that the metal surface be extremely clean, as clean as would be necessary to electroplate the same surface.  Even small amounts of oil on the metal such as that produced by handling without gloves will be sufficient to protect the metal surface from oxidation.