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Solid zinc sheet is a popular architectural metal with designers and builders because of its natural atmospheric corrosion resistance and the aesthetic warmth of the natural gray patina that forms over time with exposure to the environment.  From a metallurgical standpoint the gray patina that develops in most temperate environments is zinc carbonate, ZnCO₃.

When exposed to air solid zinc will react with oxygen to produce white powdery zinc oxide, ZnO, on the surface.  With time the zinc oxide will react with moisture, either from humidity or more effectively from rain, to produce zinc hydroxide, Zn(OH)₂.  Zinc hydroxide will then react with carbon dioxide in the air to form an insoluble dense zinc carbonate layer.

In coastal regions with the presence of sodium chloride (salt) in the air, a different patina forms that has a moderately lighter gray appearance.  This product is chemically characterized as zinc oxychloride, Zn₂OCl₂.  As is the case with zinc carbonate, zinc oxychloride will provide enhanced corrosion resistance compared to its preceding zinc oxidation products.

The development of zinc carbonate or zinc oxychloride depends on how quickly the metal surface dries after a wet cycle.  If moisture sits on the surface for a period of time the underlying zinc hydroxide does not have access to atmospheric elements necessary to the formation of the more robust zinc oxidation products.  In fact perpetually wet surfaces will continue to react with available zinc in the base metal and accelerate localized corrosion.  It is for this reason back protected products are available for installations with inadequate ventilation to remedy trapped condensation (see Contrarian Metal Resources document Ventilation of Zinc Panel Systems).

Solid zinc as produced in the mill has a highly reflective, silvery metallic appearance.  The time required to develop a uniform gray patina will vary with the environment.  In arid regions this process can take many years and in seacoast environments it is a matter of months.  For this reason most manufacturers of solid zinc sheet offer pre-weathered products that have an out of the box zinc carbonate "look".  The appearance of these patina finishes will vary with the nature of the chemical treatments employed in production.  In the case of InvariTique® , Contrarian Metal Resources and Alltrista Zinc Products Company have engineered the pre-weathering process to produce a finish that most closely replicates the appearance of naturally weathered zinc.  Regardless of the producer, all pre-weathered patina finishes are superficial and therefore temporary.  The process of forming a true patina will begin as soon as the metal is exposed to the environment.  In some cases, typically in temperate environments, water-soluble coatings are applied to pre-weathered products to prolong the durability of the artificial patina.  Be advised these products will weather evenly only if the coating deteriorates uniformly.

The manner in which solid zinc sheet is pre-weathered has no impact on the long-term appearance of the natural patina.  Zinc wants to be zinc.  One of the advantages in selecting solid zinc for phased construction projects is that with time the most recent installation will age to the appearance of the existing elements.  Product from different producers can even be blended on the same project at different site elevations and they will eventually age to the same patina provided they were melted to the same chemistry specifications.  However, it is not recommended that different pre-weathered products be installed adjacent to one another for two opposing reasons, namely 1) if a contrasting effect is desired, it will only be temporary and 2) if homogeneity is desired, it will only happen with time.

With an understanding of the natural weathering behavior of zinc, solid zinc sheet products can be employed by designers and engineers to produce enduring, aesthetically pleasing architectural elements in construction projects.