LOS stands for Liver of Sulfur which chemically tarnishes silver (called a patina.) It's original form was in lumps, but currently on the market is a liquid LOS and a LOS gel. There are other chemicals like Black Max that also chemically darken silver. Other kinds of metals have their own patinas.
The lump forms break downs very easily if it is exposed to light or dampness. It is very upsetting to open a can of LOS and find it all stuck together. (Can you tell I'm speaking from experience?)
Cool Tools has LOS in a gel form. I used their first gel LOS and it worked great. The newest version is more stabilized and isn't as picky about light and moisture.
LOS is mixed with hot water (and sometimes other chemicals to control it's color). The mixture looks like "pee" and smells like rotten eggs. (The stinky smell is the one constant among all the kinds of LOS.) The silver piece is then dipped into the mixture (but not left laying in the mixture) and watched until it is the color or darkness desired. The silver piece is then neutralized with water or a mixture of baking soda to stop the tarnishing action. The piece is dried and then the LOS is polished off, exposing the high spots. The recessed areas stay dark.
Not all pieces need LOS. Some pieces look better with a matte silver finish. Some look better with a high shine finish and some look better with an antique finish (hence the use of LOS.)
Some artists would never dream of using LOS on any of their pieces and some use it on everything. (I'm somewhere in the middle. I look at the piece and make my decision after everything is done.)
Sometimes I make a bad decision and add a patina, thinking the piece needs it. Whoa..... what a mistake! Not to worry, it can be fired off the piece or the piece can be dipped in silver cleaner. The only problem with the silver cleaner is that the pieces seem to tarnish faster once they have been cleaned.
The students in my last dichroic class all opted to add a patina to their pieces. Good choice as the patina brings out the details of their designs.