Historic Lighthouse Preservation Handbook Part IV. B, Page 1

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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

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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 compounds�graphite 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 �I�beams, 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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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 materials, and the composition of the component itself.

Upon exposure to the atmosphere, almost all new or newly cleaned metals become coatedwith a thin film of metallic oxide, which is a result of the reaction of the metal with oxygen.This film may modify the properties of the metal and make it less susceptible to furthercorrosion. In the case of rusting iron, however, the oxide does not form a protective coatingbut rather promotes the continued corrosion of the metal. The three most common types ofcorrosion experienced by iron lighthouse components are as follows:

· Oxidation or rusting occurs rapidly when the iron component is exposed to moisture and air. Theminimum relative humidity necessary to promote rusting is 65%, but this figure can be lower in thepresence of corrosive agents, such as sea water, salt air, acids, acid precipitation, soils, and somesulfur compounds present in the atmosphere, which act as catalysts in the oxidation process.Rusting is accelerated in situations where the shape of the iron details provide pockets or crevicesto trap and hold liquid corrosive agents. Furthermore, once a rust film forms, its porous surfaceacts as a reservoir for liquids, which in turn causes further corrosion. If this process is not arrested,it will continue until the iron is entirely consumed by corrosion, leaving nothing but rust.

· Galvanic corrosion is an electrochemical action that results when two dissimilar metals reacttogether in the presence of an electrolyte, such as water containing salts or hydrogen ions. The

Figure 11. Detail view of a steel ladder that has beenuniformly attacked by corrosion.

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Inherent Flaws

Castings may also be fractured or flawed as aresult of imperfections in the originalmanufacturing process, such as air holes,cracks, and cinders, or cold shuts (caused bythe “freezing” of the surface of the molteniron during casting because of improper orinterrupted pouring). Brittleness is anotherproblem occasionally found in old cast-iron

elements. It may be a result of excessive phosphorus in the iron, or of chilling during thecasting process.

The corrosion of iron lighthouse components takes several forms:

• Uniform attack is where the iron component corrodes evenly where exposed to corrosive agents.

• Pitting is the localized corrosive attack on the iron component.

• Selective Attack can occur where an iron component’s composition is not homogeneous and certainareas are attacked more than others.

Figure 12. Close-up of localized corrosion or pittingwhere the corrosion has eaten through the cast iron.

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Figure 13. View of a steel railing where corrosion hasoccurred in distinct locations either because ofvariations in composition or localized failure of thecoating system.

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severity of the galvanic corrosion dependson the difference in potential between thetwo metals, their relative surface areas,and span of time. If the more noble metal(higher position in electrochemical series)is much larger in area than the baser, orless noble, metal, the deterioration of thebaser metal will be more rapid and severe.If the more noble metal is much smaller inarea than the baser metal, the deteriorationof the baser metal will be much lesssignificant. Iron lighthouse componentswill be attacked and corroded when theyare adjacent to more noble metals such aslead or copper. For more on galvaniccorrosion refer to the Lantern .

• Graphitization of cast iron, a less commonproblem, occurs in the presence of acidprecipitation or seawater. As the ironcorrodes, the porous graphite (soft carbon)corrosion residue is impregnated withinsoluble corrosion products. As a result,the cast-iron element retains its appearanceand shape but is weaker structurally.Graphitization occurs where cast iron isleft unpainted for long periods or wherecaulked joints have failed and acidicrainwater has corroded pieces from thebackside. Testing and identification ofgraphitization is accomplished by scrapingthrough the surface with a knife to revealthe crumbling of the iron beneath. Whereextensive graphitization occurs, usuallythe only solution is replacement of thedamaged element.

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• Stress corrosion cracking can occur wherestresses were induced into the iron componentin the pulling or bending process ofmetalworking and the component latersubjected to a corrosive environment. Forexample, stainless steels can crack inenvironments containing chloride, and carbonsteels in nitrate, cyanide, or strong causticsolutions.

• Erosion occurs when the corrosion-resistantfilm or oxide or layer of protective corrosionproduct is removed by abrasion, exposing freshmetal to the corrosive agents.

Mechanical Breakdown

Iron lighthouse components can also failfrom purely physical causes such as abrasion,or a combination of physical and chemicalattack, such as weathering and stresscorrosion cracking.

• Abrasion is the erosion of the iron componentcaused by moving dirt, dust, sand, grit, sleet,and hail, or rubbing by another lighthousecomponent or human element. Abrasives can

Figure 15. As corrosion attacked this steel handrail,wind and airborne sand eroded the loose and flakingsurface rust.

Figure 16. As corrosion attacked this steel turnbuckle,wind and airborne sand eroded away the flaking rust.

Figure 14. Nearly 30% of this ventilation shroud hasbeen lost to two forms of abrasion: first, sand or gritblasting abraded away a majority of the material;second, human touch has smoothed the once roughsurface.

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also encourage corrosion by removing theprotective coating (paint) from the ironlighthouse component.

• Fatigue is failure of an iron component by therepeated application of cyclic stresses belowthe elastic limit—the greatest stress a materialcan withstand without permanent deformationafter removal of the load. It results from agradual or progressive fracture of the crystals.

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• Overloading is the stressing of an ironlighthouse component beyond its yield pointso that permanent deformation, fracturing, orfailure occurs. It can fail through theapplication of static loads, dynamic loads,thermal stresses, and settlement stresseseither singly or in combination. “Buckling”is a form of permanent deformation fromoverloading which is usually caused byexcessive weight but can also be caused bythermal stresses. Members can also beoverloaded if their support is removed andloads are redistributed to other memberswhich can become overstressed anddeformed. An iron lighthouse componentcan fail or become permanently deformed bythe phenomenon known as rust-jacking. Thefailure or deformation is the result of theexpansion of the iron component as itoxidizes. This expansion “jacks” the twomembers apart.

Weathering

An iron lighthouse component subjected tothe weather is exposed to various chemicaland physical agents singly and incombinations of several at one time. Theresult is a kind of synergism where the totaleffect is greater than the sum of theindividual effects taken separately. Forexample, the rate of corrosion accelerateswith increases of temperature, humidity,and surface deposits of salts, dirt, andpollution.

Connection Failure

The failure of the connections of ironlighthouse components, especiallystructural members, can also be caused bya combination of physical and/or chemicalagents. The most common type ofconnections used for iron structuralelements of historic lighthouses includebolting, riveting, pinning, and welding.These connections can fail through theoverloading, fatiguing, or corrosion of theconnectors. Common examples of this

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Figure 17. The cyclic pressure of the different ratesof expansion and contraction of the exterior cast-ironplates and the interior brick lining of the caisson-stylelighthouse has caused the cast-iron plates to fatigueand crack.

type of failure include the corrosion, usually by concentration cells (or battery affect causedby dissimilar metals), of bolt heads, rivets, and areas covered by fastening plates. Theeffective cross-sectional area of the connectors is often reduced by corrosion, making theconnectors more susceptible to stress failure.

Figure 19. This handrail has been damaged by rust-jacking; rusting began between the two pieces of flatbar stock that formed the rail.

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Figure 18. The internal structural skeleton of thislighthouse is cracked because of overloading possiblyduring assembly.

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Inspecting for Possible Problems

In order to develop an effective treatment plan for iron lighthouse problems, an in-depthinspection must be made of the iron lighthouse and its immediate surroundings. Thefollowing chart is a listing of locations that should be inspected regularly. Associated withthese locations are the possible problems to look for during the inspection.

Inspection Chart for Iron Lighthouses

THE SITE

Look For: Possible Problems:

EnvironmentGeneral climatic conditions, including averagetemperatures, wind speeds and directions,humidity levels, and average snow accumulation

Severe conditions can lead to deterioration of themasonry foundation, which in turn could lead todifferential settlement that could ultimatelydamage the iron lighthouse structure.

Number of freeze-thaw cycles Severe cycles can cause damage to iron lighthousecomponents from frost action.

Location near sea Salt (chloride) in the air can lead to acceleratedcorrosion of exposed iron surfaces.

Acid rain in the region or from nearby industry Acid rain can lead to accelerated corrosion ofexposed iron lighthouse components.

Proximity to a major road highway or railroad

Location in the flood plain of a river, lake, or sea

Vibrations are harmful to masonry foundationmortar joints as well as iron lighthouse parts.Repetitive vibration can cause premature failure iniron components if the oscillation cycles fatiguethe metal to the point of failure.

Floodwaters can bring damaging moisture tofoundations and walls; such damage can result indifferential settlement that could ultimatelydamage the iron lighthouse structure.

Exposed or sheltered sections of a lighthouse Exposure to the sun and elements affects moistureevaporation and rain penetration into the jointsbetween iron members. Sheltered areas such asthe underside of an iron gallery deck are highlysusceptible to corrosion and rust pitting because ofa tendency to accumulate moisture and the slowdrying rate without direct sunlight.

Terrain

Soil type�clay, sand, rock The type of soil influences water drainage aroundthe structure. Excessive water in the soil couldlead to differential settlement that could ultimatelydamage the iron structure. This is a minimalconcern for most iron lighthouses. Most ironlighthouses were constructed on sites that hadbeen chosen for their soil and/or underlying stratastability.

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Look For: Possible Problems:

Slope away from lighthouse on all sides If no slope exists, puddles will form at the base ofthe lighthouse walls during heavy rains, causingwater penetration and possible damage tofoundation systems that could lead to differentialsettlement and ultimately to damage of the ironstructure.

Earth covering part of a brick or stone wall orfoundation

Moisture accumulation or penetration is possibleand could lead to differential settlement andultimately to damage of the iron structure.

Concrete or other impervious paving touchingwalls

Water accumulation and rain back-splash onto thewalls which could lead to accelerated corrosion ofthe iron wall structure.

Trees and Vegetation

Species of trees within 50 feet Elms and some poplars dry up clay soil, leading tofoundation failure and differential settlement thatcould ultimately damage the iron lighthousestructure.

Branches rubbing against a wall Branches abrade surfaces, possibly exposing bareiron surfaces to the elements and acceleratingcorrosion of the iron lighthouse structure.

Ivy or creepers on walls Leaves prevent proper drying of the painted ironsurface resulting in possible accelerated corrosionof the iron surface. Tendrils from some speciescan penetrate joints and can literally break the ironlighthouse members.

THE LIGHTHOUSE

Overall Condition

General state of maintenance and repair A well maintained lighthouse should require fewermajor repairs.

Evidence of previous fire or flooding Such damage may have weakened the lighthousestructural members or caused the introduction ofexcessive moisture.

Signs of settlement Uneven settlement can crack foundations and leadto differential settlement that could ultimatelydamage the iron lighthouse.

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Look For: Possible Problems:

Lantern

General condition A well maintained lantern should require fewermajor repairs. A leaking lantern may leave stainsunder the gallery deck on the exterior of thelighthouse as well as streaks on the interior wallsof the tower spaces below. This condition canintroduce excessive moisture into the interior ofthe lighthouse and possibly cause accelerateddeterioration of interior features and structure.

Roof drains (usually associated with larger first-order lights) and covering

Clogged roof drains can hold water in the built-inguttering system and accelerate deterioration ofthe roof covering. Small holes in the roof coveringcan be moisture infiltration points.

Gallery decks, copings, and structural seams Gaps in gallery decking can allow water topenetrate in the interior cavities of an iron towerwall.

Condition of storm panels Cracks and holes in storm panel glazing canprovide an infiltration point for moisture into thelantern.

Humidity level within the lantern Non-functioning lantern vents can inhibit therelease of humid air from within the tower. Thewater vapor will ultimately condense on thesurfaces inside the tower and lantern and possiblycause accelerated corrosion of iron lanterncomponents.

Windows and Doors

Straight and square openings Deformed openings in the lighthouse structuremay be a sign of structure settlement.

Door and window sills sloped to shed water; dripsunder sills to prevent water from running backunderneath; caulking

If any of these is inadequate, water can penetrateinto the wall and start corrosion from the insideout.

Foundation

Composition of foundation walls Stone or brick is more likely than concrete toallow water to infiltrate.

Water condensation or other signs of moisture Wood joists or iron structural members resting onmasonry foundation walls may begin to rot orcorrode at the ends. Termites or algae, mold,mildew, or moss may be present, causing damageto the wood or masonry.

Damp proof course This can impede rising damp, lesseningdeterioration of the masonry wall.

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Look for: Possible Problems:

Interior

Damp walls, stains on walls, rotting wood These indicate water infiltration.

Walls

Construction method�iron plate, sheet irondouble wall, iron plate with masonry infill, woodframe interior walls, etc.

Knowing how a tower wall is constructed willhelp in analyzing problems and selectingappropriate treatments.

Masonry-lined iron lighthouses Rust-jacking of iron members captured by masonryinfill may cause cracking of the infill. If themasonry infill becomes wet, the different rates ofexpansion and contraction of the masonry infilland the iron sheathing during a freeze-thaw cyclecan cause the iron and masonry both to crack.

Sheet iron cavity walls Water infiltration will show as rust forms on theinterior of the cavity and appears as blistering onthe exterior of the panels. Rust streaks known as�rust weep� or �rust bleed� appearing on interiorwall surface plate seams may indicate waterinfiltration has occurred.

Iron Components

Materials

Type of iron�wrought, cast, steel, galvanizedsteel, or stainless steel

Types of materials indicate the susceptibility orresistance to damage and proper repair method.

Areas of intricate castings or moldings These sections may need special attention orprotection during treatment.

Missing or broken iron components Missing material may allow water penetration.

Evidence of sandblasting, such as a pitted surface;evidence of erosion, flaking, scaling, or other formof corrosion.

Surface deterioration is not only aestheticallydispleasing but can lead ultimately to the completedeterioration of the lighthouse.

Dirt or stains Surface stains usually cause few problems otherthan being unpleasant to look at. Accumulateddirt or debris in built-in gutters or other �pockets�,however, may trap water and cause acceleratedcorrosion.

Moisture

Water penetration through joints between ironcomponents and between iron and otherlighthouse components

Moisture can lead to deterioration of the iron andother parts of the lighthouse structure throughcorrosion and rot. Water that has entered a cavitymay go unnoticed until extensive corrosion hasoccurred.

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Look for: Possible Problems:

Location and type of corrosion on surface The type of corrosion may indicate the source ofthe deterioration; refer to the following section oncorrosion for more information.

Rust streaking or �rust weep� present on interior orexterior wall surfaces near seams or constructionjoints in the iron structure

This condition indicates that moisture haspenetrated the joint or interior cavity of the ironwall. The water entry point should be identifiedand sealed or the damaged area repaired.

Coatings

Paint; type of paint Various paint types require different treatmentmethods and safety precautions, i.e., lead-basedpaint hazards, etc.

Blistering, flaking, and peeling paint These conditions indicate the paint is at or nearthe end of its effective life span.

Rust streaks or rust weep This indicates localized failure of the coatingsystem which has caused the exposed iron tobegin to rust. The rust scale should be removedand the area spot painted in the interim until thenext repainting of the lighthouse.

Construction Joints

Joints between iron lighthouse components weretypically sealed with white lead mixed withlinseed oil

The white lead/linseed oil mixture hardens andbecomes brittle over time and eventually falls out,thus allowing open joints for water infiltration.

Concrete or mortar used as a seam or cavity filler The concrete and mortar are very hard and caneasily break and thus allow for water infiltration;cavities in an iron lighthouse that have been filledwith concrete or mortar are susceptible tocorrosion because of the alkalis present in theconcrete and mortar and the possible trapping ofwater between the filler and the iron.

Iron copings over masonry portions of thelighthouse such as watertables and window anddoor surrounds.

The alkali nature of the mortar used in themasonry may cause the iron to prematurely rust.These areas are prone to rust weep and should bethoroughly cleaned of rust scale and paintedduring the scheduled lighthouse repainting.