The Tod Head lighthouse lantern. The conservation-restoration of a technical object that has been continuously modified over the years

Marie Grima

doi:10.37190/arc200102

Abstract

This paper presents the conservation and reassembly of the Tod Head lighthouse lantern in the storage area of the National Museums Scotland. The Tod Head lighthouse was located on the Scottish east coast, north of Edinburgh. The lantern was dismantled in 2011 and sent to the National Museums Collection Centre. Firstly we look at a technical explanation of a functioning lighthouse. The lighting device – which is called a lantern – uses Fresnel lenses, also called stepped lenses. The design of the lantern was a collaboration between the Scottish engineer David Alan Stevenson and the horologist Augustin Henry-Lepaute, partner of Augustin Fresnel. Secondly, a short history of the object is presented. Throughout the 20th century, the lantern has undergone many changes. The light has been electrified and the mechanism has been motorised. Finally we detail the conservation operations and the reassembly.

Keywords:
lantern
lighthouse
stevenson
fresnel

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2020

1(61)

DOI: 10.37190/arc200102

Introduction

In March 2019 the National Museums Scotland started

the conservation and reassembly of the Tod Head light-

house lantern. The museum wished to add a lighthouse

lantern to its collection, which already has two lighthous-

es on its premises (the Sule Skerry lighthouse and the In-

chkeith lighthouse) and one on loan to the Science Muse-

um in London (the Eilean Glas lighthouse).

The interest in these objects comes rstly from their

complexity as technical objects. These are complex light-

ing devices whose technology has been gradually devel-

oped since ancient times (an ancient example being the

Lighthouse of Alexandria) [1]. An acceleration happened

in the middle of the 18

century, with the abandonment of

various fuels (wood, coal, oils) in favour of groups of can-

dles, the development of reectors, and the introduction of

eclipse res.

Lighthouse lanterns are the products of an entire or-

ganization: the rotation of the keepers and their constant

presence; their supply of water, food and fuel; the boats

providing the link; the building tax allowing the mainte-

nance of the lighthouses and the organization attached to

their management.

Marie Grima*

The Tod Head lighthouse lantern.

The conservation-restoration of a technical object

that has been continuously modified over the years

Latarnia morska Tod Head.

Konserwacja i restauracja obiektu techniki

przez lata poddawanego modernizacji

After being electried and then automated, some of

these objects were gradually deactivated. Electronic means

of geolocation – by which we mean satellite positioning

systems – are very ecient and make some light towers

redundant. It was not always viable to recover the lens

and mechanisms from remote areas, as is the case with the

Sule Skerry lighthouse mechanism where only the lens

was repatriated to land from its isolated rock. The rest

would have been too complex and costly to collect. The

Tod Head lighthouse was fortunate enough to nd refuge

in the reserves of the National Museum Scotland.

The project began in March 2019. Its completion is

scheduled for Christmas 2019. The four permanent mem-

bers of the engineering conservation section and their in-

tern participated in the adventure.

How does a lighthouse work?

A brief general explanation

First of all, what is a lighthouse?

A lighthouse is rst and foremost a signalling system.

Depending on their position and light intensity, lighthouses

are divided into four groups: rst, second, third and fourth

order [2]. The presence of approaching coastlines is the

most important information to transmit to navigators, and

from as far away as possible. This information is provided

by rst order lighthouses located at the most prominent

* ORCID: 0000-0002-3328-0497. National Museums Scotland,

Scotland. Institut National du Patrimoine, Paris, France.

10 Marie Grima

points along the coast. They are placed in such a way that

it is impossible to approach the coast without seeing one.

This is the case for the Tod Head Lighthouse, which was

located on the east coast of Scotland some 100 miles north

of Edinburgh and which had a signalling range of 21 miles.

Second and third order lighthouses indicate sandbanks,

reefs and small islands. Port entrances are signalled by

fourth-order lighthouses.

It is necessary for the navigator to dierentiate between

each of these lights, and especially to be able to identify

those of the rst order, in order to know their position.

The lantern is located at the highest point of the lighthouse

building and is both the light source and the signature of

the headlight. Each headlight has its own identication

signal, obtained through the eclipse system, a complex

system explained below.

Description of the Tod Head lantern

The Tod Head lantern is more than 6 m high, its larg-

est diameter is about 2 m and it weighs about 5.5 tonnes

(Fig. 1). It is divided into three parts: the lowest part, xed

to the ground, contains the mechanism; the second part

contains the mechanical roller bearing and structural ele-

ments; the third part consists of the lenses.

The clock type mechanism consists of a set of brass

wheels and gears, held by steel plates and protected by

a box, also made of steel. The movement produced by the

mechanism is transmitted to the lenses by the roller bear-

ing, which is also made of steel. The lenses are glass

prisms held together by brass frames.

The lenses are divided into two hemispherical parts.

The part on the left in Figure 2 corresponds to the part

called the “mirrors”. They are actually glass prisms whose

particular angle reects light. The right side of the dia-

gram is composed of six panels as seen in elevation in Fig-

ure 1. The upper part is called the “beehive”, the middle

part the “bull eye” and the lower part the “bottom”.

These six panels correspond to six ashes. The mirrors

correspond to a period of darkness. The lantern makes

a revolution in 30 s. This is the identication signal from

Tod Head Lighthouse.

Explanation of the light device function

To understand the lenticular system of a headlight, it

is rst necessary to know that the light intensity does not

increase proportionally with the power of the light source.

The unit of measurement for light intensity is the cande-

la (cd): 1 cd corresponds to the ame of a candle with

a range of 2 km. These values are not proportional. To

illuminate at 4 km, it will take 6 cd and to illuminate at

36 km, 100 000 cd will be necessary.

This phenomenon is explained as follows: the light

emitted by a light source propagates in a straight line but

widens (Fig. 3). In other words, light propagates in the

shape of a cone. A receiver close to the source (position 1)

will receive many rays while a receiver far from the source

(position 2) will receive few rays.

Fig. 1. Technical drawing of the Tod Head lighthouse lantern

(author: M. Grima)

Il. 1. Latarnia morska Tod Head – rysunek techniczny

(autor: M. Grima)

Fig. 3. Light propagation diagram

(author: M. Grima)

Il. 3. Schemat propagacji światła

(autor: M. Grima)

Fig. 2. Top view section and identification signal

(author: M. Grima)

Il. 2. Przekrój górnej części z widocznym sygnałem identyfikacyjnym

(autor: M. Grima)

 TheTodHeadlighthouselantern / LatarniamorskaTodHead 11

We call the lantern the light device of the lighthouse.

The purpose of the lantern is to collect the lost rays (which

go up, back and down) and gather them into a beam of

very high intensity (Fig. 4). To do this, the lantern at Tod

Head Lighthouse has three types of lenses.

This system was developed after the work of Augustin

Fresnel from 1822 [3]. The system used until then was

composed of hemispherical or parabolic metal mirrors.

It was not very eective because the metal, even polished,

absorbs light strongly and tarnishes easily.

Augustin Fresnel’s work was to develop a lens shape

that could be used for lighthouses [4]. His invention, called

a “stepped lens”, produces the same result as a conventional

Fig. 5. Explanatory diagram of the stepped lens

(author: M. Grima)

Il. 5. Schemat objaśniający działanie soczewki schodkowej

(autor: M. Grima)

Fig. 4. Diagram of the light displacement in the Tod Head lantern

(author: M. Grima)

Il. 4. Schemat rozchodzenia się światła latarni Tod Head

(autor: M. Grima)

Fig. 6. Detail of the mechanism, technical drawing, 1897 (source: image courtesy of the Northern Lighthouse Board)

Il. 6. Szczegół mechanizmu, rysunek techniczny, 1897 (źródło: zdjęcie dzięki uprzejmości Northern Lighthouse Board)

lens. It is nevertheless lighter, less expensive and above all

feasible in the scales we are interested in, in other words

around 1 m high. Fresnel realized that it is not the thickness

of the lens that causes the refraction of light but its external

curvature. He therefore removed all unnecessary material and

then brought the fragments obtained to the same level (Fig. 5).

The three types of lenses used in the Tod Head lantern

are as follows: lens no. 1 is the equivalent of the bull eye

(Fig. 1); lens no. 2 is the equivalent of the beehive and

bottom and lens no. 3 is the equivalent of the mirror. The

glass prisms of these three lenses have a dierent angle in

each case. This allows them, although in dierent areas, to

always reect light in the same direction.

12 Marie Grima

Fig. 7. Details of the mechanism, technical drawing, 1973 (source: image courtesy of the Northern Lighthouse Board)

Il. 7. Szczegóły mechanizmu, rysunek techniczny, 1973 (źródło: zdjęcie dzięki uprzejmości Northern Lighthouse Board)

 TheTodHeadlighthouselantern / LatarniamorskaTodHead 13

This type of device makes it possible to recover most

of the light rays and gather them into a beam with a range

greater than 21 miles.

Some chronological landmarks

The Tod Headlight was rst switched on in 1897. Its

design is the result of a technical collaboration between

a Scottish engineer, David Alan Stevenson, and a French

watch maker, Augustin Henry-Lepaute. David Alan Ste-

ven son came from a family of lighthouse engineers, whose

work began in 1811 with the construction of the Bell Rock

lighthouse by Ro bert Stevenson [5]. Augustin Henry-Le -

paute [6], a student of Gustave Eiel and a partner from

1823 of Augustin Fresnel, was the watch maker of Louis

Philipe (1830–1848) and Napoleon (1848–1852). He built

many light houses in Europe (including the one in Valsö-

rarna, Finland).

Around 1950, a major upgrade was completed to re-

place the rotating lantern system at Tod Head Lighthouse.

The original system is known from the construction draw-

ings and its similarity to the Inchkeith Lighthouse. The

Inchkeith Lantern is displayed in the large gallery of the

National Museum of Scotland: it has circular elements

xed to the central axis and pivots around this axis by

means of a rail.

The entire structure of Tod Head, from its rotating rail,

was lifted in order to replace the previous system with

a mechanical roller bearing. This new device stayed in use

throughout the rest of Tod Head’s life. This modication

is the rst in a long series. Only major events are listed here.

In 1964, the paran vapour burner was removed and

replaced by a light bulb. The central pillar that provided

the fuel supply was removed and replaced by an alu-

Fig. 8. Tod Head lighthouse identification signal after 1977

(author: M. Grima)

Il. 8. Sygnał identyfikacyjny latarni morskiej Tod Head po 1977 r.

(autor: M. Grima)

Fig. 9. The lantern in spare parts

at the National Museums Collection Centre

(source: image courtesy of National Museums Scotland)

Il. 9. Latarnia w częściach w magazynach National Museums Scotland

(źródło: zdjęcie dzięki uprzejmości National Museums Scotland)

minium column. In 1973, the mechanism was motorized

(Figs. 6, 7). On this occasion, openings for ventilation

and cooling of the motors were added to the doors of the

mechanism box.

In 1977, the Northern Lighthouse Board – the admin-

istration responsible for managing Scottish lighthouses

– requested the removal of two lens panels, to change the

identication signal of the lighthouse. Tod Head no longer

had six ashes in 15 s followed by 15 s of darkness, but

4 ashes in 10 s followed by 20 s of darkness (Fig. 8).

In 2007, the lighthouse was deactivated and the building

was sold as private property. It was dismantled in 2011 by

the Northern Lighthouse Board and stored in crates at the

National Museums Collection Centre. Around 2015, the

clockwork mechanism was cleaned by a former member

of the conservation-restoration engineering section spe -

cialising in horology.

The cleaning and the reassembly of the Tod Head Light -

house began in March 2019. The project will be com -

pleted by the end of 2019.

The reassembly

The objective

How to reassemble the Tod Head lighthouse lantern?

What elements need to be reassembled (Fig. 9)?

The National Museums Scotland already has a lantern

on display in the museum’s exhibition galleries in Edin-

burgh. This lantern comes from the Inchkeith Lighthouse

and was reassembled to be as it was when it was built in

1804. The addition of the Tod Head lantern to the col-

lection will illustrate the complexity of the modications

made to it over time, and will therefore involve all the

elements held by the museum. This will also help pre-

vent loss or dissociation of elements of the lantern from

some being on display and others being in storage.

In order to design the conservation project, we relied

on the study of the original technical plans, the observa-

14 Marie Grima

Fig. 13. Parts after cleaning

(source: image courtesy of National Museums Scotland)

Il. 13. Części po czyszczeniu

(źródło: zdjęcie dzięki uprzejmości National Museums Scotland)

Fig. 14. Parts after cleaning

(source: image courtesy of National Museums Scotland)

Il. 14. Części po czyszczeniu

(źródło: zdjęcie dzięki uprzejmości National Museums Scotland)

Fig. 10. Cleaning product residue

(source: image courtesy of National Museums Scotland)

Il. 10. Pozostałości środków czyszczących

(źródło: zdjęcie dzięki uprzejmości National Museums Scotland)

Fig. 11. Element of the mechanism before intervention

(source: image courtesy of National Museums Scotland)

Il. 11. Element mechanizmu przed naprawą

(źródło: zdjęcie dzięki uprzejmości National Museums Scotland)

Fig. 12. Mechanical cleaning of the ball bearing structure

(source: image courtesy of National Museums Scotland)

Il. 12. Mechaniczne czyszczenie konstrukcji łożyska kulkowego

(źródło: zdjęcie dzięki uprzejmości National Museums Scotland)

tion of the photographs of the dismantling and the study

of other lanterns.

Conservation operations

From a general point of view, the object is in good con-

dition, particularly the metal which is in very good con-

dition. In hard-to-reach areas (corners, screws), whitish

deposits from old cleaning products are present (Fig. 10).

In addition to damaging the protective coating, these resi-

dues have initiated a corrosion process, as shown by the

green colouring of these areas.

There is a strong general fouling accompanied by oily

deposits on all the elements (Fig. 11).

Residues of cleaning products, fouling and oily deposits

pose a risk to the conservation of the object because they

promote corrosion. Fouling was sometimes so severe that it

prevented the various components from being reassembled.

Cleaning was therefore carried out to remove this fouling.

The cleaning was carried out mechanically (scalpel,

steel wool, Garyflex

) and chemically (white spirit and

acet one in the case of larger deposits) (Fig. 12).

 TheTodHeadlighthouselantern / LatarniamorskaTodHead 15

Fig. 15. The lantern being reassembled

(source: image courtesy of National Museums Scotland)

Il. 15. Latarnia ponownie zmontowana

(źródło: zdjęcie dzięki uprzejmości National Museums Scotland)

The cleaning operations also involved cleaning 35 kilos

of screws. We were dealing with an object dating from the

end of the 19

century which had been modied through-

out the 20

century. The dierences in the standardization

systems of the dierent elements testify to this: the ori

ginal elements are in British Standard Whitworth, the ball

bearing is in United National Fine, some elements on the

platform are in metric and the lens screws do not belong

to any known system.

The two lens panels dismantled in 1977 were separat-

ed from the rest of the lantern. Thanks to the collaboration

of the Northern Lighthouse Board, we were able to nd

their location in the reserves of the Lighthouse Museum in

Fraser burgh, a town in the Grampian Mountains, north of

Aberdeen. Thanks to their collaboration, we were able to

recover these panels. Nevertheless, of the 18 lens panels,

one is missing. It is the bull eye (middle part) numbered 1/2.

After cleaning the parts (Figs. 13, 14), reassembly began.

Once the mechanism was completed, the machine case

was closed and the mechanical roller bearing installed.

The object then weighed nearly two tonnes. Before being

completed, the mechanism had to be moved to its nal

location because the capacity of the lifting tools did not

exceed two tonnes.

For safety reasons, the mechanism was xed to the

ground with 15 cm long dowels. It is important to note that

not everything above the ball bearing was xed since it

must be free to rotate. Therefore, the three tonnes of lenses

were simply placed on the ball bearing.

The wheel that transmits the movement of the mecha-

nism to the lenses was the rst element to be added. Next

came the central pillar, as well as the white paint ed arms

supporting the inner platform (xed) and the green paint-

ed arms supporting the lenses (rotating) (Fig. 15). Final-

ly, the “bottom”, which is the rst level of lenses, was

raised; then the structural elements to support the mirrors.

This is how the lighthouse looked in the storage area

of the National Museums Scotland (National Museums

Collection Centre) at the 1

of September 2019.

Conclusion

Before continuing this work, the team must face two is-

sues. The rst is that while all the lens panels have been

found, this is not the case for all the screw elements that

supported them. About 60% of the lens screws are missing.

In addition, these are screws that do not belong to a stand-

References /Bibliografia

[1] Lighthouse of Alexandria, [in:] Encyclopaedia Britannica, https://

www.britannica.com/topic/lighthouse-of-Alexandria, [n.d.], [ac-

cessed: 26.11.2019].

[2] Figuier L., LesMerveilles de laScience ou Descriptionpopulaire

des Inventions humaines, T. 4, Furne, Jouvet et Cie, Paris 1870,

419–470.

[3] Fresnel A., Mémoiresurunnouveausystèmed’éclairagedesphares

des Ponts et Chaussées, 1822.

[4] Brewster D., On the Construction of Polygonal Lenses, and their

Combination with Plain Mirrors, for thepurposes of Illumination

in Lighthouses, Transactions of the Royal Society of Edinburgh,

Vol. 11, Edinburgh, published by Charles Tait, Bell and Bradpute

and T. Cadell, London 1831, 53.

[5] Morrison-Low A.D., NorthernLights, National Museums Scotland

and the Royal Scottish Society of Arts, Edinburgh 2010, 239.

[6] Beillard A., Recherches sur l’horlogerie: ses inventions et ses

célébrités:noticehistoriqueetbiographique,d’aprèslesdiversdoc-

umentsdelacollectiondel’écoled’horlogeried’Anet, E. Bernard,

1895, 168.

ardized threading system. They must therefore be manufac-

tured by an external company specically for this project.

The second thing that will allow the site to resume will

be the installation of a scaolding for the second and third

lens levels.

16 Marie Grima

Abstract

This paper presents the conservation and reassembly of the Tod Head lighthouse lantern in the storage area of the National Museums Scotland. The

Tod Head lighthouse was located on the Scottish east coast, north of Edinburgh. The lantern was dismantled in 2011 and sent to the National Muse-

ums Collection Centre. Firstly we look at a technical explanation of a functioning lighthouse. The lighting device – which is called a lantern – uses

Fresnel lenses, also called stepped lenses. The design of the lantern was a collaboration between the Scottish engineer David Alan Stevenson and the

horologist Augustin Henry-Lepaute, partner of Augustin Fresnel. Secondly, a short history of the object is presented. Throughout the 20

century,

the lantern has undergone many changes. The light has been electried and the mechanism has been motorised. Finally we detail the conservation

operations and the reassembly.

Key words: lantern, lighthouse, Stevenson, Fresnel

Streszczenie

Tematem artykułu jest proces konserwacji latarni morskiej Tod Head przeprowadzony na terenie magazynu National Museums Scotland. Latarnia

Tod Head znajdowała się na wschodnim wybrzeżu Szkocji, na północ od Edynburga. W 2011 r. rozebrano ją i wysłano do National Museums

Collection Centre. Na początku sprawdzono dokumentację techniczną funkcjonującej latarni. Urządzenie oświetlające – które nazywa się latarnią

– wykorzystuje soczewki Fresnela, zwane również soczewkami schodkowymi. Projekt latarni powstał dzięki współpracy szkockiego inżyniera Da-

wida Alana Stevensona oraz gnomologa Augustina HenryLepaute’a, współpracownika Augustina Fresnela. W pracy przedstawiono krótką historię

obiektu. W XX w. latarnia przeszła wiele zmian. Źródło światła (latarni) zostało zelektrykowane, a mechanizm (obrotowy) zautomatyzowany.

W artykule szczegółowo opisano operacje konserwatorskie i ponowne złożenie obiektu.

Słowa kluczowe: latarnia morska, Stevenson, Fresnel