The Christian Island 'Astrolabe'

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CCI Newsletter, No. 34, December 2004

The Christian Island 'Astrolabe'

by Kate Helwig, Senior Conservation Scientist, Analytical Research Laboratory;
Robert L. Barclay, Senior Conservator, Treatment and Development Division - Objects; and
Jeremy Powell, Senior Scientific Documentation Technologist, Analytical Research Laboratory

CCI recently examined an important piece of Canada's scientific heritage — an early scientific instrument known as the Christian Island 'astrolabe'.

The device, shown in the adjacent photo, was found in the ruins of Fort Ste. Marie II, the site of a 17th-century Jesuit mission on Christian Island, near Penetang, Ontario. Elijah Monague, an Ojibway who lived in the area, made the discovery some time in the 1920s and later sold the instrument to a Dr. Starr. In 1950, Dr. Starr's widow donated the astrolabe to Victoria University Library (Toronto), which remains its custodian.

The instrument has been examined by a number of experts and a description of it has been published by Peter Broughton.¹ According to Broughton, all agree that while it is not a true astrolabe, it is engraved with many of the scales found on such instruments. He also notes that although it bears the date 1595, this might not be the actual date of manufacture because the maker may have been "adapting, albeit clumsily, some standard design." He goes on to say that the discovery of the astrolabe at Fort Ste. Marie II links it with the Jesuit missionaries who spent the 1649–1650 winter there.

Astrolabes are generally composed of a circular plate engraved with scales and degree markings (known as the mater), suspended by a ring at the top, and a pointer (or alidade) that pivots at the centre. These devices are used to find latitude or to determine time and calendar dates by the sun and the stars. Only two other astrolabes of similar age have been discovered in Canada. Both are mariners' astrolabes: one, dated 1603, is thought to have belonged to Samuel de Champlain, and was found near Cobden, Ontario, in 1867; the other, dated 1628, was found off the coast of Newfoundland. The Christian Island astrolabe is in poor condition compared to these other two instruments. It has no alidade, a portion from the centre of the mater where the pivot would have been is missing, and the metal is fragile and cracked.

We examined the Christian Island astrolabe to determine information about its condition and chemical composition and to provide recommendations for its storage and display.² The astrolabe was examined visually and under low-power microscopy. It was then photographed in normal light, raking light and ultraviolet fluorescence. An X-radiograph was also produced. Subsequently, areas of the astrolabe were analysed non-destructively by X-ray fluorescence to determine the principal elements comprising the alloy. Small samples of corrosion and the bronze-coloured adhesive were also studied using a variety of analytical techniques.

The metal was found to be a copper alloy that has a relatively low concentration of zinc and small amounts of tin, lead, iron, antimony and silver. The composition is consistent with historic copper alloys known collectively as 'brasses'. The multitude of cracks on the surface indicate that the metal is very brittle. An X-radiograph of the astrolabe confirmed the extent of the cracking. As well as showing the details of the fine cracking, the X-radiograph showed a thin line of radio-opaque material along one of the cracks. This corresponds to a high atomic number material, possibly lead or tin solder.

A section on one side of the astrolabe has been attached very clumsily with a bronze-coloured adhesive. The adhesive, which fluoresced bright orange in ultraviolet radiation, was found to be composed of shellac mixed with talc and brass flakes. We considered the possibility of removing this repair, but decided that the surrounding metal is so fragile that this treatment might cause more damage. This is a very conservative approach, but we believe the rarity and fragility of the object warrant it.

Microscopic examination showed that the corrosion products, identified as malachite and cuprite, are well-attached and generally stable. Malachite and cuprite are commonly found on bronzes buried in soil. They are expected to be stable at normal levels of relative humidity (RH), but we have recommended that an RH level above 80% should be avoided.

The present mounting system, between sheets of glass in a wooden frame, seems to have protected the astrolabe very well since this mounting was applied some time in the 20th century. The mount sandwiches the fragile metal between the two pieces of glass and keeps it flat and stable. The mounting system also has, by now, a certain historical quality, which also argues for retaining it. The twisted wire, which originally held the object in the mount, was removed. A metal-on-metal contact is never a good idea for long-term use because it can promote electrolytic action in some circumstances. Instead, a small, acid-free cardboard insert has been used to support the astrolabe within the glass container.

Because of glare on the glass, the astrolabe in its current mounting system is not as visible as might be desired by researchers. However, this problem will now be offset by the availability of the photographic and descriptive material that we produced during our examination. This documentation sheds new light on the fine details of the Christian Island astrolabe, as befits such an important and rare example from
our scientific past.

References

1. Broughton, P. "The Christian Island 'Astrolabe'." Journal of the Royal Astronomical Society of Canada 80, 3 (1986), pp. 142–151.
   
   
2. Helwig, K., B. Barclay, and J. Powell. "Examination and Analysis of the Christian Island Astrolabe." Unpublished report. Ottawa: Canadian Conservation Institute, 2002.