Historic Lighthouse Preservation Handbook Part IV. G, Page 1

Historic Lighthouse Preservation:

LANTERN

The primary purpose of the lantern is toprovide a weathertight shelter for thelighting apparatus. The lantern alsofunctions as the �roof� for the tower.Lighthouse lanterns come in a wide varietyof shapes and sizes; most lanterns havesimilar components and therefore, sharesimilar problems. A typical lantern consistsof a frame that supports the lantern glassand roof, in some cases a masonry or woodparapet wall, a lens apparatus, and interiorand exterior hatches.

Despite the inherent durability of thelantern design and construction,deterioration caused by environment is stilla constant threat. Improper maintenanceor repair techniques can also acceleratedeterioration; therefore, all treatmentshould be executed using the gentlestmeans possible. Character-definingfeatures such as material type, size, profile;decorative brackets; lantern glass (almostalways clear or red glass); decorativerailing standards; etc., should all beexamined. Whether the plannedpreservation treatment is mothballing orrepair, a proper inspection and diagnosis

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Figure 2. Original construction drawing of the first-orderCape Fear Lighthouse showing the lantern above theservice/watch room.

Figure 1. Lantern housing hyper-radiant lens atMakapuu Lighthouse near Waimanalo on theIsland of Oahu in Hawaii.

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Figure 3. Illustration of the four standard lantern sizes used by the U.S. Lighthouse Establishment in the first half of the20th century. Drawing assembled by WPTC from images dated 1898 and 1903 courtesy of USCG CEU Oakland.

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should be performed in order to determinethe most effective treatment solution.

For more information on replacing missingor severely deteriorated lantern componentsrefer to Part V., Beyond Basic Preservation.

Lantern Construction

In order to withstand harsh weatherconditions, the components of a typicallantern are made from a variety of materialsand metals. The main support structure(including the floor), certain types ofparapet walls, and the lens pedestal aretypically made of cast iron for strength. Theroof, ventilation ball, and lightening rod aretypically made of copper, which canwithstand severe weathering (and in thecase of the lightning rod, can conductelectricity). The astragals and clamps thathold the lantern glass in place as well asgrab handles are typically made of bronzewhich resists corrosion and is durable.Brass screws compatible with the bronzeare typically used to attach the astragals tothe lantern frame. If these parts corrode,damage to the lantern glass can result.

The variety of metals used in lanternconstruction creates the potential forgalvanic corrosion. Various techniqueswere employed to prevent corrosion. Anelectrolyte such as water must be presentfor galvanic corrosion to occur, so joints

were caulked with litharge to keep areas ofcontact between dissimilar metals dry.Litharge was used to protect iron lanternframes from bronze astragals. In otherlocations such as where the cast-ironlantern roof �rafters� came in contact withthe copper roof covering, an insulatingbarrier was used. When preservationtreatments are performed on historiclanterns, these details should bemaintained.

Interior features such as vent dampers andlens frame parts are typically made of brassfor its durability, stability, and decorativequalities. Other interior finishes includebeaded tongue-and-groove wood panelingor sheet iron on the parapet walls, andwood tongue-and-groove flooring. Thesefinishes are typical in the smaller fourth-through sixth-order lanterns.

Special Conditions Associated withHistoric Lantern Systems

A variety of special maintenance conditionscan occur in a historic lantern system butmay not occur in any other part of thelighthouse. (The treatment and preventionof these conditions are addressed under therepair treatment in this chapter.)

Figure 5. Close-up of the first-order lantern at CapeCanaveral in Florida.

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Figure 4. Close-up of the fourth-order lantern atCoquille River Lighthouse in Oregon.

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Figure 6. Diagram of a typical first-order lantern; the parts are similar to second- and third-order lights. (Diagram based onUSLHS drawing in the National Archives)

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Source: USCG Fixed Aids to Navigation Maintenance, Commander, Ninth Coast Guard District, Cleveland Ohio,CCGDNINEINST MI6500.2. For more information on galvanic corrosion of metals, refer to Metals in America'sBuildings: Uses and Treatments (U.S. Dept. of the Interior) or Corrosion Handbook by Herbert H. Uhlig (ed.), 11th ed.1969.

metal (higher position in electrochemicalseries) is much larger in area than the baser(or less noble) metal, the deterioration ofthe baser metal will be more rapid andsevere. If the more noble metal is muchsmaller in area than the baser metal, thedeterioration of the baser metal will bemuch less significant.

An example of an undesirable situation thatpermits galvanic attack is the use of steel oraluminum fasteners to hold together acopper-covered lantern roof. Since themore noble metal is in contact with a smallarea of a more base metal, galvanic attackwould corrode away the fastener withnothing to hold the copper cover to thelantern should the coating system fail andallow water (the electrolyte) to facilitate thegalvanic corrosion.

Galvanic Corrosion

Galvanic corrosion is an electrochemicalaction that results when two dissimilarmetals react together in the presence of anelectrolyte, such as water containing salts orhydrogen ions. This type of corrosion isnormally significant only between groupsseparated by lines in the Galvanic Series inWater table below; the effect is smallbetween members of the same group.Galvanic corrosion is the result of aspontaneous flow of positive electriccurrent from the more �noble� metal to themore �base� metal. The more �base� metalthen dissolves. The severity of the galvaniccorrosion depends on the differencebetween the two metals, their relativesurface areas, and time. If the more noble

GALVANIC SERIES IN WATER, 20oC

MORE NOBLE

MORE BASE

GROUP I Titanium Alloys

Nickel Alloys

Stainless Steels

Silver

GROUP II Copper Alloys (Bronze/Brass)

Lead Alloys

Tin Alloys

GROUP III Cast Iron

Structural Steels

GROUP IV Zinc Alloys

GROUP V Aluminum Alloys

GROUP VI Magnesium Alloys

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Sea water is especially corrosive. Marineatmospheres and sea water contain severalcorrosive agents including chlorides andother salt particles which can be depositedon the surface of the metal. Thesecorrosive agents can affect metals as far as60 to 70 miles from the sea (depending onweather patterns). Metals immersed inwater are also subject to corrosion bydissolved solids and gases, especiallyoxygen.

If this condition occurs all screws, bolts,nuts, welds, and fastenings of any kindshould always be made from a more noblematerial than the remainder of the structure.(For more information concerningprevention of galvanic corrosion refer to therepair treatment in this section of thehandbook.)

Rust-Jacking

Rust-jacking threatens any iron or steelcomponent. In lantern glass, the conditioncan cause severe damage. When moistureenters the channel that retains the glass, theiron frame may begin to rust. As the ironrusts, it expands and in turn cracks theglass. This phenomenon can occuranywhere a ferrous metal (iron or steel, etc.)is in direct contact with another material.The pressure created by the exfoliating rust(iron oxide) may damage the adjacentmaterial. (For more information concerningprevention of rust-jacking refer to the repairtreatment in this section of the handbook.)

Ventilation

Nearly all, if not all, lanterns have aventilation-ball vent located at the apex oftheir roofs which served as the primary ventfor the fumes and smoke created by the oil-fired illuminant. Secondary vent locationsvaried by the size of the lantern: first- andsecond-order lanterns typically had ventslocated in the watch room area below thelantern and in the sill and head areas of thelantern glass; smaller third- through sixth-order lanterns had vents typically located inevery other panel of the parapet wall.

All vents were baffled to prevent strongwinds from blowing directly into thelantern and extinguishing the light. Airflow through the vents was also controlledby a variety of sliding registers and/orrotating dampers. While the illuminant waslit, vents located in the lantern opened toallow fresh air into the lantern; it would beheated by the flame and then rise outthrough the ventilation ball. This actioncreated a draft that helped keep the lanternglass clear of condensation and maintain anambient humidity level within the lantern.This ventilation was essential for theoperation of the lantern as well as itspreservation. (For more information on

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Figure 7. The white chalky residue around the steel boltheads on this aluminum deck is the by-product of thecorrosion occurring at these connections.

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Figure 9. An example of a parapet-wall-mounted vent on afourth-order lantern.

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Figure 10. Close-up of missing clamp bolts; these boltsshould be replaced to keep this area weather tight.

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lantern and lighthouse ventilation refer tothe repair treatment in this section as wellas the Windows section.)

Lantern Glass

Lantern glass is typically 3/8 inch thick.The glass plays two important roles in thelantern system. First, the glass should beclean and clear to allow the greatestamount of light transmission. Second, theglass has to withstand high winds, drivingrain, and airborne material (i.e., sand,wave-tossed rocks, and birds). Properinstallation care and replacement ensuresthat these demands are met. Refer to therepair treatment in this section for moreinformation on lantern glass care andreplacement.

Figure 8. Despite the fact that this first-order lanternventilation ball has been repaired several times, it stillprovides adequate lantern ventilation.

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Why Do Lanterns Deteriorate?

Lanterns are made from a variety of metals and materials. These materials are subject to a host ofsevere weather conditions. How successfully a lantern resists these pressures depends on howwell it is designed and maintained. A well-built, well-maintained structure may withstand theseforces indefinitely.

The leading causes of decay are:

• excessive moisture from leaking roofs and lantern glass, and condensation because of poor ventilationwithin the lantern itself all corrode iron components and provide the electrolyte that facilitates galvaniccorrosion between dissimilar metals;

• corrosion of iron lantern glass frames which results in ‘rust-jacking’ that causes the glass to crack, thusproviding a moisture infiltration point; or

• failed coating systems that no longer protect lantern components.

Secondary factors causing decay:

• abrasion by the wind and wind-born solids that accelerates deterioration by rapidly removing corrodingor exfoliating material;

• mechanical damage due to ice, impact, or wind;

• damage caused by vandalism;

• chemical disintegration caused by pollutants in the atmosphere; or

Figure 14. Wind and airborne sand have accelerated thedeterioration of this lantern gallery deck handrail

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Figure 13. The paint coating that once protected thislantern parapet wall has failed.

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Figure 11. The water that has collected in this lanternsill is causing the iron lantern glass frame (the verticalmembers in the center of the photo) to rust and corrode.

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Figure 12. The glass in this lantern has been crackedby the rust-jacking that is occurring along the ironframe.

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Figure 16. This lantern glass has been damaged by rock-throwingvandals. The resulting hole is allowing rain and insects to enter thelantern.

• damage caused by deterioration of the tower structure that supports the lantern.

Inspecting for Lantern Problems

In order to develop an effective treatment plan for lantern problems, an in-depth inspectionshould be made of the lantern and its immediate surroundings. The following chart is alisting of locations that should be inspected regularly. Associated with these locations arethe possible problems to look for during the inspection.

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Figure 15. Close-up of ice on St. JosephNorth Pier Lighthouse causing damage togallery deck and handrail.

Inspection Chart for Lighthouse Lanterns

THE SITE

Environment

Look For: Possible Problems:

General climatic conditions, including averagetemperatures, wind speeds and directions,humidity levels, and average snow and iceaccumulation

Severe conditions can lead to lantern deteriorationcaused by excessive heat build-up, moisturecondensation, or snow or ice load that couldliterally tear exterior decks off of the lantern.

Number of freeze-thaw cycles Severe cycles can produce damage from frostaction within masonry parapet walls.

Location near sea Salt in the air can lead to accelerated corrosion ofmetal components.

Acid rain in the region or from nearby industry Acid rain can act as an electrolyte, which mayfacilitate galvanic corrosion between dissimilarmetals.

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Look For: Possible Problems:

Proximity to a major road highway or railroad Vibrations are harmful to mortar joints in masonryparapet walls; cyclic vibrations may cause failurecaused by fatigue in metal components.

Location in the flood plain of a river, lake, or sea Floodwaters can bring damaging moisture tofoundations and walls. If differential settlementresults, the lantern may be damaged by themechanical action.

Exposed or sheltered sections of a lighthouse Exposure to the sun and elements affects moistureevaporation and rain penetration.

THE LIGHTHOUSE

Overall Condition

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

Evidence of previous fire or flooding Such damage may have weakened structuremembers or caused excessive moisture within thelighthouse tower and lantern, thus causing oraccelerating corrosion.

Signs of settlement Uneven settlement can crack foundations or wallsor result in sloped or wavy mortar joints. Ifdifferential settlement results, the lantern may bedamaged by the mechanical action.

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 moisture canaccelerate corrosion of lantern components.

Roof drains (usually associated with larger first-order lights) and roof 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. This moisturecan accelerate corrosion of lantern components.

Gallery decks, copings, and structural seams Gaps in gallery decking (cast-iron plate, flat-seammetal, stone, concrete, etc.) and tower wallcopings (stone, metal, concrete) can allow water topenetrate into the interior cavities of the towerwall, thus accelerating the deterioration of thetower.

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Look For: Possible Problems:

Lantern vents and humidity level within thelantern

Non-functioning lantern vents can prohibit therelease of humid air from within the tower. Thewater vapor will ultimately condense on thesurfaces inside the tower and lantern. Theexcessive moisture will promote mold and mildewgrowth and accelerate corrosion.

Condition of storm panels Cracks and holes in the storm panels can providean infiltration point for moisture into the lantern.

Condition of storm panel glazing compound If the glazing compound is cracked or missing,water can enter the frame channel and causepossible rust-jacking to occur along the perimeterof the storm panel. The bottom edge of the stormpanels is especially susceptible to this condition.

Lantern Coatings

Paint; type of paint Various paint types require different treatmentmethods and safety precautions.

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

Lantern Parapet Walls

Construction method�iron, masonry, wood�solid or cavity

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

Condition of seams between wall constructionmaterials

In wood parapet walls the seams betweensheathing boards must be watertight and all end-grain must be protected from moisture contact. Iniron parapet walls the seams between panels mustbe completely watertight to prohibit water fromentering the interior wall cavity and causing theiron to corrode from the inside out.

Evidence that parts of the parapet wall wererepaired or modified

Inappropriate repairs may be the source ofdeterioration.

Tower Interior

Cracked plaster, signs of patching, floors orlandings askew

These are signs of lighthouse settlement.Differential settlement can cause mechanicaldamage to the lantern.

Interior moisture levels High interior moisture levels may causeaccelerated deterioration of lantern components.