Here is a brief description of the non-chemical aluminization process: Aluminum wire is inserted into tungsten filaments. The wrokpiece is then introduced with the finish surface facing the filaments. Then a high vacuum is achieved, thus removing almost all oxygen. The filaments are electrified, and heat up in excess of 2000 deg. F. The aluminum is ionized and radiated onto the workpiece. The presence of oxygen in the atmosphere will allow the filaments to burn/smoke, which usually causes a yellowing of the reflective surface. This can be caused by foriegn materials, or vacuum leaks. If the finish surface is too close to the coils, a cloudyness occurs, and is sometimes blue in color. Not enough aluminum will cause the surface to look "dark". If your mirror was aluminized using this type of process, I would say that oxygen was present, and smoke caused a yellow or "bronze" tint. Possibly form outgassing as you said.