Comes with question that the bridge on Paces Ferry Road at Vinings, is somehow, well…blue. First a little corrective background.
Bids for a bridge across the Chattahoochee to replace the “real” Pace’s Ferry were let October 1903, and awarded to Cotton States Bridge Company of Atlanta in January 04′. Cost was to be approximately $10,000, borne 2/3’s by Fulton County and 1/3 by Cobb County. In March 04′ work was announced to begin May 1st and completed in about 6 weeks (not built in 1903 as some references mention).
Then the technical stuff started. The bridge was to be a 2-span, 280-ft.-long, pin-connected, Pratt, through-truss bridge, with timber cross planks, bearing single directional traffic (one direction at a time). This was a favored construction for medium river-crossing bridges in early 1900’s, for which 100’s were built and only a few remain.
So far so good.
Steel blue is more than just an eye color; literally a tempering process applied to steel to achieve a certain strength-pliability ratio, one level of which is called “blue annealed “ in the industry, or tempering.
If interested, steel is notably responsive to tempering, and makers of tools, weapons, armor, and other articles of steel have long had great skill in the process. Tempering is not necessary for such products as razors and files, in which hardness is sought but brittleness is not a serious disadvantage. Other products, e.g., swords, weapons, and industrial use requires tempering for toughness. In the handicraft process of tempering, the condition of the steel during heating is judged by its color, caused by an oxide film. A desired hardness can be achieved by plunging the steel into a bath when it has cooled to the right shade of yellow or brown or blue. To secure a bath of the right temperature, various liquids are used, e.g., pure water, saltwater, oil, and molten metal. The process of softening steel that is harder than desired is called annealing. In modern mass production the processes of tempering are guided by scientific tests in place of the artisan’s skill in the past. Comparable to tempering is the process of hastening the cooling of a surface of a casting to increase the hardness of the part so “chilled.”
Annealing is a heat or thermal treatment process by which a previously cold-rolled steel coil is made more suitable for forming and bending, and the term “blue” annealing is the incidental formation of a bluish oxide observed on the surface as it reaches the desired tempering ratio – and such color is retained. But…
A resulting color is more of a midnight blue, like you see in the “bluing” of gun metal, and that’s the only way the bridge iron would be, albeit a rather weathered blue at that.
Actually, the Vining Bridge “blue” color is not a tempered-processing color, nor a decorating event, depression over being replaced, a painting, an Elvis movie, or having a blue Christmas without you, – it’s just an oxidized primer before a more permanent (and better) layer is applied…
Source: Atlanta Constitution 1903-04; various period steel industry references via Google.