Blackened steel refers to a number of techniques which can be used to produce a blackened finish on steels. Among the processes include chemical baths and finishes, cold-coatings, hot-coatings with various levels of success. In many architectural uses, blackened steel may refer to galvanized steel with a blackened zinc patina oxide.
THE BASE METAL FOR STEEL
The iron alloys are not commonly used in the uncoated state. Left to nature's devices, these alloys will develop a rust oxide which will eventually corrode in entirety (the exception to the rule is weathering steel, which contains copper as an alloying constituent and will develop a protective rust oxide layer). The natural color of the iron alloys, when cleaned of scale and mill oil, is a dark blue-gray.
The steels have a dark blue-gray color, while the heavier carbon alloys of cast iron are almost black. Cast iron has a dark carbon-gray surface, which will weather to black. The wrought iron forms are dark blue-gray in tone. Heating blackens the surface, and subsequent shaping drives the dark tones into the surface. Wrought iron is almost always coated with a thick paint to inhibit corrosion.
Steel can be blackened or given colors ranging from dark brown to black. One such method to achieve these tones is by dipping it in a heated chemical bath. This process can develop a deep black oxide over the surface of the steel. Oxides produced by this alkali method are weak and prone to abrasion if not protected with waxes or oils. Protection against corrosion is enhanced slightly, but the base metal characteristics may cause a blackened finish on steel to fail if not continuously maintained.
ALTERNATIVES TO BLACKENED STEEL
With all metals you have the ability to adjust the reflective nature of the surface. The ability changes with exposure time; and with some metal surfaces, the choices are limited. However, if desired, you can achieve a dull, flat, black appearance, devoid of the slightest visual sheen of any kind. Blackened by oxide, copper, zinc, and aluminum can have grainy, black, mottled surfaces. The mottling has degrees of black, some with a reddish tint, others with a gray tint.