1Museums play a vital role as guardians of cultural heritage, carefully preserving both original artifacts and high-quality replicas within their collections. These replicas serve multiple purposes, such as replacing lost items or safeguarding delicate pieces, making them essential for the accurate representation and study of historical objects.
2In the collection known as the 'Hallepoort', housed at the Royal Military Museum in Brussels, Belgium, and property of the Museums of Art and History, a particular focus lies on a unique type of replicas or reproductions: imprints or casts of medieval seals. The original seals, often crafted from fragile materials like beeswax, are highly susceptible to deterioration over time. As a result, producing reproductions is crucial in ensuring that the historical and cultural significance of these seals is not lost.
- 1 Villela-Petit Inès. 1994. pp. 513.
3The precise reason for producing these replicas isn't always clear. Were they made to facilitate forgery, or were they mainly designed to enhance collections and promote educational efforts? These questions remain a matter of debate. Some suggest that these techniques might have been used to create entirely new stamps based on existing impressions, while others argue that the goal was to faithfully replicate the original seals without any intent to deceive. 1
- 2 Villela-Petit Inès. 1994. pp. 514-516.
4While reproductions were made for collectors as early as the 17th century, the importance of creating replicas became more evident in the 19th century, when museums and institutions began to display certain collections to the public. At this time, the need to protect original, vulnerable seals grew more urgent—not only to preserve the seals themselves but also to ensure they remained securely attached to the documents they authenticated. Yet, reproducing seals posed a challenge: how could it be done without causing damage to the originals? 2
- 3 Villela-Petit Inès. 1994. pp. 516.
5Over time, various materials and techniques were experimented with to create accurate seal impressions or counter-seal impressions. Early attempts involved materials like clay, gutta-percha, and bronze. Eventually, the ‘casting technique’ emerged as the most suitable and least damaging method. Around 1910, traditional methods of pressing seals into soft substances like clay to create an imprint were largely abandoned in favour of a new process involving casting with plaster. In this improved method, the original seal was carefully cleaned and coated with a protective layer of grease. A paper band was then wrapped around the seal, and liquid plaster was poured over it. Once the plaster set and hardened, the imprint was made using molten sulphur, which was then doubled with a layer of plaster and painted to highlight or accentuate details. This technique remained popular and in widespread use until around 1960. 3
6The origins of these seal impressions or counter-seal impressions within the museum’s collection are somewhat obscure. The old inventory records do not specify where these items came from. Some may have been acquired from private collectors who wished to expand their personal collections before eventually donating them to institutions such as the Royal Museum of Armour, Antiquities, and Ethnography, or later to the Royal Museums of Decorative and Industrial Arts, of which the Hallepoort was a branch. It is also possible that the museum itself commissioned the creation of these impressions for educational purposes, potentially produced in the casting workshop of the Royal Museums of Decorative and Industrial Arts, which had been active since the 19th century. The precise date of these reproductions remains uncertain, but based on the techniques used, they can be roughly dated to the late 19th and early 20th centuries, approximately between 1880 and 1920.
7The yellowish tint of the reproductions and the use of (molten) sulphur for making these copies, given the likely production period (late 19th and early 20th centuries), led us to suspect that sulphur was present in the impressions. Not only is sulphur highly flammable, with an ignition temperature as low as 260°C (by comparison, an electrical spark reaches temperatures of around 1.300°C), but it can also cause chemical reactions with other objects, making it important to determine the chemical composition of the impressions with certainty.
8Air typically contains a mixture of gases that can have significant impacts on materials. These gases include sulphur, ozone, nitrogen oxides, ammonia, and various acids. The introduction of additional sulphuric sources into the environment can have severe adverse effects on the preservation and management of various objects, particularly those made of metal, paper, textiles, leather, and photographic materials.
- 4 Eve Van Dael, Anne-Cathérine Olbrechts, Birgit van Laar and Marijke Hoflack. 2006. pp. 12-13.
9One notable issue is the corrosion of metals caused by sulphur-containing gases in the air, such as hydrogen sulphide (H₂S) and carbonyl sulphide (OCS). This type of corrosion, known as atmospheric corrosion, occurs when metals are exposed to both air and moisture. It can be challenging to prevent this type of damage without implementing substantial protective measures. The rate, however, at which the corrosion occurs is influenced by several environmental factors, including temperature and humidity. 4Higher temperatures and elevated humidity levels can accelerate the corrosion process.
- 5 Agnes Brokerhof, Bart Ankersmit and Frank Ligterink. 2016. pp. 145, 148.
10Nevertheless, the most significant threat comes from the formation of sulphur dioxide (SO₂). Sulphur dioxide is produced through the reaction of sulphur with oxygen in the air. Once formed, sulphur dioxide can interact with water vapour in the air to produce sulphuric acid (H₂SO₄). Sulphuric acid is highly corrosive and can lead to accelerated deterioration of metals, as well as the bleaching of dyes and degradation of materials such as paper, textiles, leather, and photographic films. 5
11In the context of a museum or other similar environments, the concentrations of these sulphur-containing gases are typically too low to pose a direct health risk to humans. However, the potential damage to valuable collections and artifacts from atmospheric corrosion and acid deposition can be significant.
12The Royal Institute for Cultural Heritage was consulted to definitively determine the nature of the material used for the casts of medieval seals. Understanding the precise composition of these objects was critical for guiding present and future conservation strategies. The primary objective of this research was not only to identify the materials but also to take any additional preventive measures needed to ensure the safe storage or display of these items, while also protecting other objects in our collection from potential degradation caused by interaction with these casts.
13To achieve this, the composition of the casts was analysed using X-ray fluorescence spectroscopy (XRF), a non-invasive technique particularly suited for heritage conservation studies. XRF is valuable for such analyses because it allows the examination of elemental composition without causing damage to the artifacts. A portable XRF spectrometer (S1 TITAN 800, Bruker) equipped with a 5 mm diameter beam was utilized for the analysis. This device is capable of detecting elements starting from magnesium (Mg, atomic number 12) and higher. The non-destructive nature of the technique allowed for repeated measurements without risking any harm to the casts.
14The XRF measurements were conducted on three carefully selected representative seal impressions or counter-seal impressions from the collection (inventory numbers 13338, 13286, and 13340).
Fig. Seal impression
Ferrand of Portugal (inv. no. 13286).
Credits: © WHI
Fig. Seal impression
Jan, Lord of Wezemaal and Fallais (inv. no. 13338).
Credits: © WHI
Fig. 3 Counter-seal impression
René I of Anjou (inv. no. 13340).
Credits: © WHI
15For each impression or counter-seal impression, at least two measurements were taken: one on the front side (with the yellowish tint) and one on the back side (with the whitish tint). An additional measurement was made on the counter-seal impression with inv. no. 13340, where a significant fracture on the front side was present that could help minimize the influence of the finishing layers.
Fig. 4 Damage to counter-seal impression
René I of Anjou (inv. no. 13340).
Credits: © WHI
16Over the years, some of the casts have sustained damage. Although this is undoubtedly harmful to the overall preservation and integrity of the pieces, it also presents an unexpected opportunity. The damage reveals cross-sections of the casts, allowing us to examine the internal structure and the different layers in greater detail than would otherwise be possible.
- 6 Laurent Fontaine and Sebastiaan Godts. 2024. pp. 4-10.
17In all three examples analysed, the spectral data revealed that sulphur (S) is the predominant element found on the front surfaces, while both sulphur (S) and calcium (Ca) are the primary elements detected on the back surfaces. The presence of calcium on the back surfaces, combined with the identification of sulphur, strongly indicates that these surfaces are likely composed of plaster or gypsum (CaSO₄·2H₂O). 6This analysis provides thus the necessary evidence that the front sides of the casts are indeed characterized by (molten) sulphur, whereas the back sides are indeed comprised of plaster.
Fig. 5 Detail of the XRF spectrum
Detail of the XRF spectrum front side of seal impression inv. no. 13338 (in red) between 0 and 5 keV. Sulphur (S, in green) is clearly dominant. Calcium (Ca, in blue) is detected in very small amounts.
Credits: © KIK/IRPA
- 7 Villela-Petit Inès. 1994. pp. 516.
- 8 Martina Stillhamerova, Jan Verbeke and Jacques Vereecke. 2009. pp. 7-8.
18Furthermore, it was pointed out in the previous chapter that organic components such as waxes, oils, or resins for the possible top finishing layer were used.7 Initially, it was suggested by previous conservators that this could be a discoloration of the sulphur due to exposure to UV rays. The colour difference allowed this layer to be clearly distinguished from the underlying layer of molten sulphur. From moulding techniques for statues around 1875 (the techniques for seals were generally the same, especially when it comes to plaster examples), we know that gelatine and fats were used (so they may contain polymerized fat acids and proteins, prone to discolouration over time). 8As previously noted, original seals were greased, leaving a residue on the cast, and were finished by being ‘painted’ (also 'colourless') to accentuate details. During the analysis, the elements of this finishing layer could not be detected by the portable XRF spectrometer, suggesting that the layer primarily consist of very light elements, such as possible organic components. Taking into account the era of their creation, the techniques and materials used, and the results of the XRF analysis, we could tentatively conclude that the top layer of the casts is likely composed of organic elements. However, to accurately determine the precise components of the top layer, additional analyses using other techniques must be carried out, including but not limited to Fourier transform infrared (FTIR) spectroscopy, Pyrolysis gas chromatography/mass spectrometry (Py GC-MS), and Raman spectroscopy.
19In addition to determining the composition, further measurements provided more clarity about the thickness of the different layers visible on the casts. Since a yellowish top layer could be clearly distinguished from a whitish bottom layer, the question arose as to the extent to which there was an even distribution of the layers between different casts. After further analysis of some of these damaged casts (they must be damaged in such a way that the different layers can be distinguished and measured in cross-section), we can observe an average depth of the (molten) sulphur layer of about 0.6 to 0.7 cm, which lies directly on the plaster base layer.
20To make the sulphur layer as unified as possible, the sides of the casts were also finished with molten sulphur. The depth of this (molten) sulphur layer averages about 0.3 cm. Since not all seals are the same size, the thickness of the layers can vary slightly; however, a general trend can be observed that these are approximately the same thickness on the tested casts.
21Given that the Royal Institute for Cultural Heritage's research has conclusively demonstrated that the casts contain sulphur, it is imperative to implement additional measures to ensure the safety of these casts, as well as other objects within the collection.
- 9 Laurent Fontaine and Sebastiaan Godts. 2024. pp. 11.
22Following their analysis, the Royal Institute for Cultural Heritage made several recommendations to mitigate potential risks. 9These recommendations have since been adopted or are in the process of being implemented by our institution.
23The primary concern is the high flammability of sulphur. To address this risk, it is essential to adopt stringent fire safety measures. This includes the provision of fire blankets and CO₂ extinguishers in close proximity to the casts. Furthermore, it is crucial to store these objects in separate display cases to contain any potential fire and prevent it from spreading to other collection items. By isolating these objects, we not only reduce the risk of fire damage but also safeguard other valuable pieces. The examples stored in our reserve storage room are organized separately within a dedicated drawer cabinet, distinct from other items. It was decided to combine all sulphur-containing casts into a single location. This approach ensures that the different copies are not scattered or stored separately, which would place excessive strain on our available space and storage capacity. By grouping all sulphur-containing casts together, we can also optimize the use of our storage facilities and maintain better organization. This method not only helps in conserving space but also simplifies the retrieval process, making it more efficient to access and manage these specific items when needed or in case of an emergency (like fire).
- 10 Eve Van Dael, Anne-Cathérine Olbrechts, Birgit van Laar and Marijke Hoflack. 2006. pp. 17-18.
24Maintaining stable temperature and humidity levels is also critical to prevent further degradation of both sulphur and plaster, and to limit the emission of harmful fumes. The gallery where these casts are exhibited, as well as the reserve storage area, are now maintained at an ideal relative humidity level of 40 to 50% and a temperature range of 15 to 20°C. 10These controlled conditions also help to control other elements such as water particles who could interact with the sulphur and create even more harmful fumes, thus minimizing the risk of chemical deterioration of the casts themselves and other artifacts.
25Moreover, effective ventilation is necessary to avoid the accumulation of potentially harmful fumes. Display cases can be provided with ventilation systems that have high air renewal rates. In museum settings, however, budget-friendly solutions are often a key factor in organizing conservation and management practices. For existing display cases, modifications can be made to install an efficient ventilation system with a high air renewal rate. An appropriate ventilation system allows for the display of casts alongside other items in the same case, provided that proper monitoring is in place. Alternatively, separate enclosures specifically for casts can be used to minimize the risk of harmful fumes or fire affecting other items. These enclosures can either have their own ventilation systems or be left without one, depending on cost considerations. Additionally, an airtight and vacuum-sealed enclosure might be sufficient to prevent or eliminate certain harmful chemical reactions.
- 11 Camille Moos. 2023, pp. 35-36, 42, 47.
26Regular inspections are also essential for monitoring the condition of casts and their surrounding environment. Recent research by Camille Moos has demonstrated that harmful sulphuric fumes (H₂S and SO₂) can be captured by various sorbents. Although these sorbents were not deemed reliable for long-term storage, alumina granules and activated charcoal proved effective for periods of several months. To ensure ongoing effectiveness, these absorbers also need to be regularly checked. This can be done by including metal plates, such as silver, copper, and lead, in the display cases. These metals are highly susceptible to deterioration from H₂S and SO₂, making their reaction to harmful elements more noticeable during inspections. Tarnishing of these metals indicates increased harmful concentrations in the display cases, allowing for early detection of potential degradation and timely intervention to preserve the objects. 11
27Maintaining these casts consequently requires a careful balance between conservation, education, and safety. Above mentioned measures are essential for maintaining the integrity of these informative objects and ensuring their preservation for generations to come.