Conservation project of the sextant collection belonging to the Naval History and Culture Unit: Secretaría de Marina-Armada de México

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Photograph from the astronomical navigation exhibit.  Image courtesy of the author.

By Diana Citlalli Martínez Jiménez

The Naval History and Culture Unit (UNHICUN) which belongs to Secretaría de Marina-Armada de México (SEMAR) is structured as two collections: The Naval Museum Mexico-Veracruz and The Naval Museum Mexico-Vallarta. Their mission is to conserve, to exhibit, and to disseminate naval heritage for the purpose of magnifying national sovereignty and fortifying Mexico’s identity. The collections are playful and interactive; the permanent collection is formed by diverse objects such as armament, artillery, uniforms, insignia, cartographic material, scale models, and pre-Hispanic material among others.

It was during a building inspection season, while cataloging and diagnosing the Naval Museum Mexico-Vallarta’s collection, that I was able to study the “Astronomical Navigation” exhibit; the visitor is first introduced into a unique environment, a dark-walled space in which the constellations are made up of white LED light luminaires that create a cosmic and hypnotic appearance. The exhibition includes objects that have been used throughout history as essential parts of navigation, their purpose being to determine one’s position, course, and speed when onboard a boat. Such is the case for the astrolabes, slidings, probes, timers, nautical almanacs, crossbows, compasses, quadrants, octants, sextants, and nautical charts.

Within all these examples of navigation instruments, the one that most fully caught my eye—and that is one of the most representative devices for the museum and for SEMAR’s history—was the sextant. This is an instrument that has historical, scientific, educational, and museological relevance, a testament of the technological evolution of the maritime sciences and also the central axis within navigation. Although technological advances have replaced the sextant, at Heroica Escuela Naval Militar is still used for teaching the General Corps,* and is also still used as a secondary navigation device aboard active ships.

The sextants included in this project are no longer used for navigation but are now part of the Naval Museum’s heritage collection, where the mission focuses on the exhibition, conservation, and preservation of the cultural objects belonging to the institution. Even though no documentation on the sextants’ origins has been found, it is known that they were donated by inactive SEMAR ships. The sextants were not donated for failing to fulfill their function, but because the ships they belonged to were decommissioned. Until now, only one sextant belonging to the “ARM Nezahualcoyotl” ship (a Mexican naval destroyer, decommissioned in 2014) has been registered.

The sextants conservation project—which also constitutes my dissertation project—started with the examination of the sextant Tamaya 6100 at the Naval Museum Mexico-Vallarta. The piece was unstable and in poor condition. So, we decided to move it to the conservation department and disassemble it. The intervention was assigned to me, but when I was carrying out the research on its mechanism, historicity, materiality, and current context, I realized there was very little information about sexton conservation, as most of the articles relating to navigation instruments’ conservation are focused on subaquatic archaeological pieces. We subsequently decided to create a conservation methodology for the sextant that would involve both the material stabilization and the mechanisms’ successful operation. But, before explaining the technical conservation issues, I will briefly explain the development and main characteristics of a sextant.

According to Quinde and Tomala (2012) the development of the navigation instruments can be classified in different generations. The first generation (early 16th century) includes the nautical astrolabe and quadrant; both instruments were already used on land, but they were adapted to sea navigation. The second generation (late 16th to early 17th century), including the crossbow and the Davis quadrant, improved the observation precision and the location certainty. The third generation (mid-17th century) is marked by a great technological leap, achieved due to the application of optical instruments that improved the vision range. It was during the third period that the octant was proposed by John Hadley. The octant is an instrument that takes the shape of a 45° angle (an eighth of a circle) with an arc divided into 90 equal parts or half degrees.

The sextant, invented by John Bird, appeared contemporaneously with the octant; its name comes from the shape and scale, as it spans an angle of 60° (a sixth of a circle). In addition, it has a graduated arc—or limbus—in sixths of a degree. Sextants are used to measure the angle between the stars and the horizon. The sextant’s principle is based on light’s reflection over flat mirrors. Each one of the parts that make up the sextant are fundamental for the realization of the reflection’s principle. Although there are many variations among sextant models, in general terms a sextant is composed of the frame, nonius, ally, drum, horizon mirror, index mirror, telescope, filters, and handle.

Sextants represent a great challenge for the conservation discipline because of their complexity. Several factors converge to create this one object; factors such as the metallic composition (mostly made up of aluminum or bronze), the mechanisms (which allow accurate operation), the optical parts (which facilitate the observation through lenses and filters), and finally, the coating or paint-layer (which is responsible for giving surface protection). With these factors in mind, types of deterioration can be classified into three categories:

-Firstly, the material decay, which involves different types of metal corrosion and damage such as flaking, detachments, low-adhesion and losses in the coat and paint layer.

-Secondly, the mechanical decay that implies lack of mobility or lubrication and misalignment of the moving parts.

-Thirdly, the optical decay which refers to stains, breaks, scratches, dents, missing elements, and damaged silver emulsion layer.

Clearly, it is fundamental to understand the sextant’s operation, mechanisms, and assembly before one can diagnose this object and formulate a conservation procedure.

After evaluating and creating an experimental design based on the characteristics and condition of the sextant Tamaya 6100, I was able to obtain the authorization from UNHICUN and Escuela de Conservación y Restauración de Occidente (ECRO) to develop a dissertation project focused on researching the 20 pieces that constitute SEMAR’s sextant collection. Thanks to the support and generosity of both institutions, this study (and adventure), focused on the diagnosis and the conservation of these complex and wonderful objects, has just begun.

To tackle all these challenges, the interdisciplinary participation of many specialist has been fundamental, and it is one of the most valuable aspects of this work. The project has been developed under the supervision of the Art Conservator Alfredo Adolfo Ortega-Ordaz; the support and assessment of Naval Captain C.G. DEM Víctor Manuel Aguilar (Naval Museum’s principal), who has provided references and explanations regarding the optical-mechanical operation; the mentoring of the Lt. SAIN L. Rest. Liliann Velázquez García (head of the conservation department), who not only contributes knowledge to the project, but has also linked the military to the conservation field; and the mentoring of Lt. C.G. Carlos Alfredo Cruz Martínez and Captain Rosas, who taught me the applied function of a sextant.

I would like to close by expressing my enthusiasm for this project; my realization of this project is based on the desire to find each sextants’ origin, to explore beyond the subject and into its context, and above all, to contribute to the preservation of the Navy’s symbolism and history. To sum it up, this project opens new paths for a thousand possibilities of reflection and for teamwork with the different assets of this honorable institution.


Braña, F. (1996). Catalog of instruments for reflection for navigation of the Provincial Museum of the Sea of San Cibrán Lugo. Spain: Provincial Museum of Lugo.

Heroic Naval Military School. (2007) Astronomical navigation. Veracruz: Heroic Naval Military School.

Quinde, J. and Tomala, P. (2012). The marine sextant. Ancón: Institute of technologies.

* The General Corps, or GC, refers to the officers graduated from the engineering career in naval sciences, graduates of the Heroica Escuela Naval Militar; these are the persons in charge of commanding the ships and vessels.


Diana Citlalli Martínez Jiménez graduated in art conservation at Escuela de Conservación y Restauración de Occidente (ECRO). Currently, she is working at the Naval Museum Mexico-Veracruz, where she is also completing her bachelor’s degree dissertation project.

(See the full article in the August-September 2020 "News in Conservation" Issue 79, p. 36-39)

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It was during a building inspection season, while cataloging and diagnosing the Naval Museum Mexico-Vallarta’s collection, that I was able to study the “Astronomical Navigation” exhibit; the visitor is first introduced into a unique environment, a dark-walled space in which the constellations are made up of white LED light luminaires that create a cosmic and hypnotic appearance.
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