Gemmological Aspects of the Vila Pouca da Beira Monstrance in the Museu Nacional de Machado de Castro - Part 1

The gemmological content of this devotional implement from the early 1800’s is very interesting in various aspects. From its gem materials to their fashioning and settings, the monstrance also shows evidences of the use of profane jewellery adapted to this religious context and it is a good example of the use of gem materials in the period. The article, in two parts, also shows how does a gemmological approach  may be made in such historical artefacts using simple gem testing techniques.

This article serves as a call for attention to the relevance of the gemmological information in  inventory and cataloging decorative art projects and studies, not only from the descriptive point of view but also from an interpretative perspective. The fundamentals of the gem testing techniques used are not presented, but it also shows briefly how the observations can be made and presents the results of the gemmological study of this gem set goldsmith historical artefact. The study was undertook to complete the information sheet on the artefact, serving not only the descriptive aspects of the labeling in a museum’s environment, but also as a contribution to the interpretative aspects from an historic and artistic point of view, as well as a guide for conservation professionals when performing cleaning, maintenance or restoration. Another use of the gemmological information is to assist the collection holders to issuing a proper insurance policy and to undertake the necessary steps to implement security measures according to the real and perceived value of the item.

Museu Nacional Machado de Castro, Coimbra (Portugal)

The Museu Nacional de Machado de Castro in Coimbra, Portugal, houses a very significant number of medieval goldsmith and jewellery artefacts as well as an interesting selection of devotional artefacts from the second half of the 18th century and early 19th century, a period of recognised high activity of the Portuguese manufacturing workshops. The richness of the Portuguese manufacture over these decades owes much of its success to the volume of gold and precious stones that arrived from Brazil throughout the 1700’s, namely diamonds, topazes, amethysts, rock-crystal, citrine, beryl (aquamarine and goshenite), chrysoberyl and garnet. Although colourless and coloured pastes (glass) and composite products (doublets) were common in Europe during that period, the Portuguese workshops did not used these extensively as they had easier and readier access to those Brazilian gem materials, most of them of mid to low cost.

Vila Pouca da Beira Monstrance, silver gilt and gemstones, ca. 1804-1810. Photo José Pessoa © Museu Nacional Machado de Castro / DGPC

The monstrance entertained herein, from the collection of the Museum (ref. MNMC 6904; O517), known as the Vila Pouca da Beira Monstrance, is a reasonable representative of the use of gem materials in the early 19th century, as it is set with most of the previously mentioned Brazilian gemstones, also showing evidences of the use of profane jewellery whole pieces or parts set directly into the monstrance (fig. XXXX). These are significant aspects that contribute to the gemmological and historic-artistic importance of the artefact.

Materials and Methods
Most gem testing techniques use non-destructive methods and this non-invasive, non-destructive  approach gets more relevance when studying artefacts of historic and artistic value, specially when kept in museums. The irresponsible hardness test was sadly used in the past in historical jewels and not only did it contributed very little to the identification of the materials, but also, and most regretfully, often left a damaging scratch mark on the stone’s polished surface. For the gemmological study of this monstrance, a series of non-destructive techniques and instruments were used, namely magnification under the loupe and microscope, refractometer, spectroscope, dichroscope, colour filters, thermal tester, long wave and short wave ultraviolet light and various incident light sources (optic fibres, diffused CIE D65 light, incandescent bulb and LEDs). Some advanced gem testing techniques are available, namely Raman spectroscopy, PL spectroscopy and FTIR, but not only are they expensive but also sometimes challenging to use in a large gem set implement such as this monstrance.

Typical scratch marks on the table of a polished gemstone. 

The observation protocol to reach a diagnostic follow a pre-defined scheme that, step by step, indicates the adequate technique to use. The first observation is a naked eye inspection under different light sources, and many useful data can be collected in such a way. Colour, luster, sheen, transparency and fire as well as cutting style, shape and cutting quality (e.g brilliance, windowing, extinction) are really important data that can be collected under a simple eye observation. A dedicated 10x loupe observation follows, and lighting conditions are chosen accordingly, weather a direct, diffused or collimated illumination is appropriate to collect relevant information. With a 10x magnification, it is possible to inspect external characteristics (e.g. cleavage, fracture, polishing marks, abrasion) and internal characteristics (e.g inclusions). Relevant information concerning the conservation conditions of the stones and their settings can be also checked under the 10x lens. Whenever necessary and possible, a gemmological microscope can be used to observe external and internal features to a higher magnification, up to 200x. Proper lighting is key to this technique and many limitations are presented to the gemmologist, namely those related to the volume of the artefact and positioning of the microscope.
From the visual observation phase, an observation under ultraviolet light and under appropriate lighting with various color filters is made and, then, a series of confirmation tests and instruments are used accordingly to identify the materials, with a special emphasis to the spectroscope and the dichroscope.
A precision gemstone gauge is used to measure as accurately as possible the gem materiais in their settings as this, notwithstanding the limitations, enables the calculation of the approximate carat weight of the stone. This information is relevant for the description of the artefact for various reasons.

The dirt on the artefact 


The gemmological study of gem set historical artefacts comes with significant limitations, namely the fewer possible viewing angles, the near impossibility to the use of certain instruments (e.g. refractometer) and, finally, the conservation aspects of the artefact as a whole that may signify considerable dirt over the usable surfaces of the stones or underneath the setting, just to name two limiting situations that reduce the quality of the observations. Sometimes, this conservation aspects are an impediment to a reliable diagnostic. When expressly authorised by the artefact owner or curator and whenever safe, the gemmologist may use plastic or soft wood pointers covered with a flint free tissue soaked in a water and alcohol solution to remove the excess of dirt up to a point when an inspection is possible. It must be emphasised that certain gem materials and certain settings can not be in contact with such liquids and that a case by case decision must be carefully taken during the study. The inability to properly gauge the impact of such simple cleaning procedure in the artefact’s integrity must refrain the observer to perform such an action and seek advise among the conservation community.

(Part 2 features the gem testing results and conclusion)

adapted and translated from Galopim de Carvalho, R. et al. (2011) “A Custódia de Vila Pouca de Aguiar” in Normas de Inventário - Ourivesaria (pp. 162-171), Instituto dos Museus e da Conservação, Lisboa