Aluminum amalgam is a chemical substance formed by a solution of aluminum with mercury. Aluminium amalgam may be prepared by either grinding aluminium pellets or wire in mercury, or by allowing aluminium wire to react with a solution of mercury(II) chloride in water.
In Breaking Bad, aluminum amalgam is shown to be usually forgotten in methamphetamine cook sessions ("Box Cutter"). The cook sessions for Blue Sky utilize reductive amination of phenylacetone in the presence of methylamine and aluminum amalgam to yield methamphetamine.
Aluminum amalgam is used as a chemical reagent to reduce compounds, such as the reduction of imines to amines. The aluminium is the ultimate electron donor, and the mercury serves to mediate the electron transfer.
The reaction itself and the waste from it contain mercury, so special safety precautions and disposal methods are needed. As an environmentally friendlier alternative, hydrides or other reducing agents can often be used to accomplish the same synthetic result. Another environmentally friendly alternative is an alloy of aluminium and gallium which similarly renders the aluminum more reactive by preventing it from forming an oxide layer.
Aluminium in air is ordinarily protected by a molecule-thin layer of its own oxide. The aluminium oxide layer serves as a protective barrier to the underlying aluminium itself and preventing chemical reactions with the metal. Mercury coming into contact with it does no harm. However, if any elemental aluminium is exposed (even by a recent scratch), the mercury may combine with it to form the amalgam. This process can continue well beyond the immediately exposed metal surface, potentially reacting with a large part of the bulk aluminium before it finally ends.