What is the mechanism of silver staining?

Silver staining works by selective reduction of silver to insoluble metallic silver. This takes place at specific sites that are in close proximity to the protein molecule.  

The first step in the silver staining technique is fixation. It involves immobilizing the proteins and removing any interfering compounds. In the second step, the gel is treated with compounds to either accelerate the silver reduction or make the protein more reactive to silver. In the third step, the silver is impregnated using either plain silver nitrate or ammoniacal silver. The last step involves rinsing the gel to obtain the silver metal image. The gel displays different shades depending on the amount of silver attached to the protein bands.