Historic Lighthouse Preservation: IRON - National Park Historic Lighthouse Preservation Handbook ... Cast-iron-and-steel skeletal 191 ... • Uniform attack is where the iron component

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  • Historic Lighthouse Preservation Handbook Part IV. B, Page 1

    IRON

    Iron was also used for the production ofarchitectural trim features such as gallerydeck brackets, entryway pilasters andpediments, doors, and prefabricated lanterncomponents. These iron features were usedon masonry and wood as well as ironlighthouses. Other iron alloys such as steel,galvanized iron and steel, and stainless steelare mostly found in modern additions suchas handrails, equipment brackets, securitydoors, etc.

    This section will discuss the preservation ofiron alloys used in lighthouse towerconstruction and decoration. Because oftheir similar properties, the various ironalloys will be discussed together; specialtreatments concerning a specific alloy will

    Second to masonry, iron was the mostcommon lighthouse construction material.For lighthouse construction, iron was usedin a variety of its commerciallymanufactured alloys: wrought iron, castiron, steel, galvanized iron and steel, andstainless steel. In historic lighthouses themost widely used alloy was cast iron. Theuse of cast iron in lighthouse constructionranged from simple prefabricated lanternsto caisson-style foundations to 190-foot-tallfirst-order coastal towers. For more on thevariety of iron lighthouse construction typesrefer to Part II., History of the LighthouseService and Lighthouse ConstructionTypes.

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    Figure 1. Cast-iron-and-steel skeletal 191-foot-tall tower atCape Charles, Virginia

    Historic Lighthouse Preservation:

  • Part IV. B, Page 2 IRON

    be discussed accordingly. The use of ironin the construction of lanterns and thespecial considerations associated with ironin the presence of unlike metal corrosion(galvanic corrosion) will be discussed in theLantern section. Other metals such as brassand bronze will also be discussed in theLantern section.

    Iron Alloys Found in HistoricLighthouses

    Of the iron alloys, cast iron was a perfectchoice for lighthouse construction for twoprincipal reasons. First, cast iron isrelatively resistant to corrosion because ofits microstructure component compoundsgraphite and phosphide eutectic. Thesecompounds are not present in steel, whichexplains why the two materials corrode in

    different manners. Second, cast iron can becast into virtually any shape that is requiredfor structural or decorative purposes. Toform complex shapes and structuralsystems, these castings were designed withflanges that made it possible to bolt thecomponent parts together. Thisprefabricated style of constructionfacilitated the erection of lighthouses in atimely, economical manner. This methodalso allowed for the dismantling andrelocation of a lighthouse if site conditionswere compromised by encroaching erosion.

    The various steel alloys were usedthroughout the structure of a historiclighthouse, but to a lesser degree than castiron. Most mild steel, stainless steel, andgalvanized steel components have beenused in modern additions or repairs. Thesecomponents appear mostly as pre-

    Figure 2. Example of a keeper's quarters fitted with aprefabricated cast-iron-and-steel lantern.

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    Figure 3. Example of an all-cast-iron construction"sparkplug" caisson-style lighthouse.

    Figure 4. Double-wall, cast-iron, first-order 163-foot-tallcoastal tower at Cape Henry, Virginia.

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  • Historic Lighthouse Preservation Handbook Part IV. B, Page 3

    Figure 5. A view of decorative-rope nautical-style cast-iron window surrounds on Cape Canaveral Lighthouse.

    Iron Alloys Commonly Found inHistoric Lighthouses

    Wrought iron is relatively soft, malleable,tough, fatigue-resistant, and easilyworked by forging, bending, rolling, anddrawing. Until steel was available,wrought iron was used structurally forbeams and girders as it had strength inboth tension and compression. Duringthe late 19th and early 20th centuries, itwas not unusual to find a mixture of cast-iron columns and wrought iron or steelbeams in the same lighthouse. Currently,very little wrought iron is being produced.

    Cast iron is an iron-carbon alloy with ahigh carbon content. It is easily pouredwhile molten into molds, making possiblenumerous decorative and structural uses.Cast iron is too hard and brittle, however,to be shaped by hammering, rolling, orpressing. Cast iron contains in itsmicrostructure several relativelycorrosion resistant components which aremostly absent from the microstructure ofsteel. Because of this, the two materialscorrode in different manners. It is morerigid (highly resistant to buckling) thanother forms of iron and can withstand

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    manufactured items such as structural Ibeams, replacement handrails, equipmentbrackets, and items that can be fabricatedinto functional parts of the lighthouse.

    Figure 6. A view of the inner cavity and skeletal structureof the Cape Henry Lighthouse.

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    Figure 7. Shop drawings used for the production of cast-ironlighthouse parts for a screwpile lighthouse.

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  • Part IV. B, Page 4 IRON

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    Figure 9. A modern range finder attached to thegallery deck using modern steel members.

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    Figure 10. Close-up of a lantern glass frame: notethe use of stainless steel bolts; the lower clamps arealso stainless steel that have been painted black.

    great compressive loads, which helps account for itsubiquitous use for lighthouse tower structurecomponents such as wall plates, columns, sockets,struts, deck plates, etc. Cast iron does have somedrawbacks. There is the potential for inherent flaws incast pieces such as trapped air pockets or foreignmaterial such as casting sand or slag trapped in the ironduring the casting process. These flaws can be avoidedif the castings are thoroughly inspected and the castingprocess is performed to accepted industry tolerances.

    Steel is an alloy of iron and carbon that contains notmore than 2% carbon, and is malleable in block oringot form. Steel may include phosphorus, sulfur,oxygen, manganese, silicon, aluminum, copper,titanium, molybendum, and nickel. The properties ofsteel can vary greatly in relation to the chemicalcomposition and the type of heat treatment andmechanical working used during manufacture.Characteristics affected by these differences includestrength, hardness, ductility, resistance to abrasion,weldability, machinability, and resistance to corrosion.A grade of medium carbon steel is used for mostlighthouse applications today such as handrails,equipment brackets, new light support structures, etc.

    Galvanized steel and iron consist of steel or iron with azinc coating, which makes it highly resistant tocorrosion. As in the past, zinc is still widely used as aprotective coating for iron and steel. A majoradvantage of zinc coating on iron is that if the zinc isworn away or broken and the iron is exposed to theatmosphere, galvanic corrosion of the more base zincoccurs, protecting the more noble iron. (The termsbase and noble refer to the relative reactivity of the zincand iron. A metal that is considered a base is morereactive than a metal that is considered noble. Theseproperties are directly related to the number of freeelectrons that exist in the molecular structure of themetal.)

    Stainless steel is defined as a steel containing sufficientchromium, or chromium and nickel, to render it highlyresistant to corrosion. Stainless steel is malleable,hardened by cold working, and resistant to oxidation,corrosion, and heat. It has characteristics of highthermal expansion and low heat conductivity, and canbe forged, soldered, brazed, and welded. Because of itsrelatively inert properties, stainless steel componentsare mostly found in replacement parts such as boltswhere the possibility of galvanic corrosion could occur.

    Stainless steel is available in various grades. Given thecomplexity of the issues and potential application, theselection of the proper grade of stainless steel for use ina marine environment requires careful evaluation by anengineer.

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    Figure 8. An example of the level of detailachieved with cast-iron construction.

  • Historic Lighthouse Preservation Handbook Part IV. B, Page 5

    Causes of Iron Deterioration andFailure

    Iron lighthouse components are subjected toa host of forces associated with a marineenvironment. How successfully a lighthouseresists these pressures depends on how wellit is designed and maintained. Ironlighthouses that are poorly maintained willdeteriorate rapidly.

    In scientific terms, deterioration is generallydefined as a decrease in the ability of thematerial to fulfill the function for which itwas intended. It usually refers to thebreakdown of a material because of naturalcauses, although deterioration can also beeither directly or indirectly caused by man.Deterioration can also be defined as thechanging of a material from a higher to alower energy state. Although deteriorationusually implies a chemical change, undersome conditions the change can be physical.There are five possible forces that can act onan iron lighthouse component and cause itsfailure: corrosion, inherent flaws,mechanical breakdown, weathering, andconnection failure.

    Corrosion

    Corrosion, in one form or another, is the major cause of the deterioration of iron lighthousecomponents. Often called oxidation, it is the chemical reaction of a metal with oxygen orother substances. The deterioration of iron lighthouse components is a complex processbecause the type and degree of corrosion is affected by minor variations in environment,contact with other metals and mat