STAINING OF GLASS CONTAINERS IN CONTACT WITH IRON*

A wet glass surface brought in contact with metallic iron acquires a brown stain which cannot be removed easily. This phenomenon was observed by the industry interested in the automatic washing of glass containers. Iron in contact with wet glass surfaces will produce a chemically detectable contamination in one minute and a visible stain in ten minutes. This reaction of the glass surface with iron is galvanic in nature. Iron immersed in water sends positively charged ferrous ions into solution and leaves the metal with a negative charge. In the neighborhood of the iron, a negative electrical potential is established in the glass surface which helps to attract the positively charged ions from the solution. The stain is a result of the interaction of the glass surface and the iron ions, thus leading through oxidation to the formation of an insoluble, colored ferric hydrosilicate. The staining can be prevented by reagents which (1) prohibit the corrosion of the iron, (2) block the active centers of the silica gel, and (3) produce anionic iron complexes.