Historic Wrought Iron is Not What You Think. Development of ferrous metals technology and consequent evolution of the usages of the term “wrought iron” has created a frequently dangerous set of assumptions about the equivalence of today’s commercially available architectural iron, known as mild steel, and the true historic “wrought iron” found in historic buildings. Mild steel is routinely substituted for this material in restoration projects. Replacements are fated to a short, unhappy career unless it is understood that mild steel is composed of a high carbon steel impatient to recombine with oxygen and to return to its natural state, red rust, as rapidly as possible. True historic “wrought iron” is a stable very low carbon iron that is quite happy as it is and capable of standing centuries to the elements as long as its surfaces are well drained and ventilated. Understanding the distinction between these quite dissimilar metals is important so that measures sufficient to protect the original work are not assumed to be adequate protection for replicated elements. To understand the magnitude of difference between these seemingly identical materials, it is necessary to understand what, exactly, true historic “wrought iron” is. Confusion has been created both by appropriation of the term by manufacturers producing inexpensive cold formed ornamental work and by the efforts of the artisans to counter this, in their eyes, misuse of the term by insisting that their hand forged work is the only legitimate “wrought iron”. True historic “wrought iron”, however, earns its name not on the blacksmith’s anvil at all but in the smelting process itself. In general, true historic “wrought iron” was smelted at relatively low, sub-molten temperatures and then “wrought” (the archaic past tense of “work”), hammered while yet a red hot solid to beat out impurities. These low temperatures result in a carbon content of about one twentieth that of mild steel. The high heat of the modern smelting process results in the incorporation of more carbon into the steel. This higher carbon content increases the strength of the material but also increases the rate at which the metal will oxidize. While these metals can both be “wrought” by the smith with equal artistry, the true historic “wrought iron” is by far the more stable for use in exterior applications. It rusts, as all iron will, but without the reckless enthusiasm of mild steel. With all its virtues, however, true historic “wrought iron” has one very serious drawback and that is availability or, rather, a virtual lack of it. It is an archaic material that is simply not being produced anywhere on any scale. The prevalent manufacturing technology from the dawn of the Iron Age through the eighteenth century, the nineteenth century saw it gradually supplanted until the last “puddle furnace” went cold around the middle of the twentieth. Salvaged material can still be located and there is some commercial remanufacturing of salvage being conducted in Europe. Specification of salvaged or imported material is not totally unfeasible but it will seriously impact construction budgets and schedules and will therefore not be acceptable for many projects. For good or ill, the world has moved on and we must consider all of the alternatives it leaves. To discover what effective measures can be taken to extend the life of ornamental iron work created in the materials available, we talked to Ed Mack, an award winning architectural blacksmith and lead artisan at Fine Architectural Metalsmiths in Chester, NY. Mr. Mack assured us that between strategic design measures and the application of the latest coating technologies that we can have full confidence in the performance of new work. One of the most effective and long established measures is hot dipped galvanizing. This is not always feasible either because the work is too big for the tank, because of concerns that the work would distort because of the heat or because the zinc coating would obscure the fine detail of the work. In these cases, Mr. Mack has had particular success in sealing traditional type iron finishes under a clear or tinted durable “automotive” type finish. Hot applied finishes such as gun bluing, stove polish, peanut oil or graphite all lend themselves to this application. These are traditional iron finishes normally not considered durable in exterior applications but which can now be enjoyed outside as well as indoors. Metal pastes and mixtures designed to produce patinas, such as a mixture he uses of hydrogen peroxide, salt and vinegar, can also be applied and then given a long life under such coatings. The coatings are themselves extremely versatile and can be built up to create color and an illusion of depth. For example, he recently created a very effective faux bronze finish for an iron fence and gate system. Beyond these “automotive” type finishes, he notes that powder coating and metalizing coating systems have begun to offer a wider range of colors and options as well. This exciting and growing new palette of finish options was totally unimaginable just a short time ago. Mr. Mack cautions, however, against making it all sound too easy. These finishes must be carefully researched and tested before application. Poor planning can result in disaster if a failed coating has to be removed from every surface. In general, these high tech coatings must be applied in carefully controlled environments previous to installation. In every case, the success of the application will depend on the most diligent and scrupulous preparation of the surface. All fire scale (slag) and mill scale must be removed from the surface by mechanical means such as sandblasting or wire brushing. Mill oils need to be removed with an appropriate solvent. Depending on the coating, it may be necessary to etch the surface with acid taking care to neutralize the surface afterwards. If mechanical assembly is to take place before coating, care must be taken to prepare and pre-prime all parts of the mated surfaces so rust won’t have a chance to originate in these inaccessible areas. The artisan is always prepared to design details which will shed the elements most efficiently or to select a material to perform a specialized function. Ironwork is the darkest of the building arts with ancient origins in magic and shamanism. No longer a magician, perhaps, but the skilled architectural blacksmith remains a diligent student of arcane and diverse technologies, antique and modern, and can help to design the most durability available into the work. As a recent example, Mr. Mack salvaged a section of a severely damaged true historic “wrought iron” fence (it had been run over by a dump truck) to build latches and hinges for the replacement work. These are the areas least coatable and most vulnerable to rust because of their hidden abrading surfaces and thus best served by selective incorporation of the historic low carbon material. Mr. Mack believes that when addressing the needs of historic installations, it’s wise to incorporate intimate expertise with the materials into the design process at the very beginning
Posted on: Tue, 23 Jul 2013 07:49:59 +0000
Trending Topics
Recently Viewed Topics
© 2015