JAIC 1987, Volume 26, Number 2, Article 4 (pp. 105 to 120)
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Journal of the American Institute for Conservation
JAIC 1987, Volume 26, Number 2, Article 4 (pp. 105 to 120)


Mark Gilberg


Rathgen's first experience was gained primarily from trials with archaeological stone from the Egyptian Collection of the Royal Museums of Berlin. His attention initially focused on the treatment of over one hundred and twenty-five limestone blocks excavated from the Meten Grave Chamber in Egypt. The surface of many of these blocks had cracked and exfoliated as a consequence of their high salt content. Though the destructive influence of soluble salts upon porous stone (and clay) was well established in the scientific literature, their effect upon archaeological material was not fully appreciated by many of the early archaeologists. Prior to this time it was common practice to impregnate archaeological stone and other porous antiquities with a water-insoluble resin in an attempt to render them impermeable to moisture. These coatings, however, only served to delay the onset of decay which then often proceeded at an even greater rate.

As an alternative, Rathgen recommended prolonged steeping in repeated changes of fresh water to remove soluble salts prior to the application of any protective coating. The treatment of these blocks, some of which weighed over 400kg, continued for some two to three years. The progress of the steeping process was followed by monitoring the chloride and sulphate content of the water bath by titration with standard solutions of silver nitrate and barium hydroxide, respectively, until no further salt could be detected in the wash. While not the first to attempt steeping in water baths, Rathgen was indeed the first to adopt a very rigorous, systematic approach to monitoring the progress of the steeping process. Detailed records of these results were kept, some of which extended over a period of several years. This procedure was first described in Rathgen's Handbook in 1898 and is one of the earliest accounts of what is today generally considered standard procedure (Rathgen, 1898).

A similar treatment method was adopted for the preservation of other porous materials, such as clay and earthenware, possessing a high salt content. In fact, during the treatment of the above-mentioned limestone blocks not less than four hundred chests filled with glazed tiles excavated from the Processional Wall and Ishtar Gate of the Palace of Nebuchadnezzar in ancient Babylon arrived for treatment (Rathgen, 1906). The tiles required extensive leaching due to the presence of considerable quantities of sodium chloride. The fragments, however, were so numerous that given the existing facilities Rathgen estimated that even if thirty vats were employed the leaching process would take a minimum of ten years. A makeshift shed was therefore constructed adjacent to the museum which could accommodate an even greater number of vats. Over two hundred vats, each one square meter in area, were constructed from old wine vats, cutting the treatment time to a year and a half (Fig. 3).

Fig. 3. Vat room of the laboratory of the Royal Museums of Berlin

The shed was later dismantled to make room for the construction of the new museum in 1927 when another four hundred crates filled with glazed tiles were transported from Baghdad to Berlin for treatment in Rathgen's laboratory (Andrae, 1928). Only a relatively small number of the tiles which had been excavated before World War I had in fact been brought back to Berlin. The tile fragments were subsequently pieced together and the Processional Wall fully reconstructed by 1930 (Fig. 4).

Fig. 4. Piecing together of tile fragments

Many of the bronze antiquities in the Egyptian Collection also required Rathgen's immediate attention. Much of the collection was in extremely poor state of preservation and exhibited signs of active corrosion. Frequently the surface of the object would break out in light green, powdery efflorescences typical of bronze disease. Rathgen attributed this phenomenon to the corrosive influence of chloride salts present in the patina of the bronze antiquities as a result of prolonged burial in the earth. Due to the insoluble nature of the copper chloride corrosion products, however, it was not useful to subject bronze antiquities to the steeping process. To remove the chloride salts, and to restore the strongly corroded object to its original shape and form, Rathgen employed an electrolytic reduction method originally prescribed by Adolf Finkener, a chemist at the Bergakademie in Berlin.

Just prior to Rathgen's appointment as Director of the Chemical Laboratory of the Royal Museums of Berlin, Erman had sought the advice of Finkener regarding the deterioration of many of the antiquities in the Egyptian Collection. For the preservation of bronze antiquities Finkener recommended the removal of the entire patina using electrolytic reduction. The object was made the cathode and wrapped round with platinum wire which was connected to the positive pole of the first of three Daniell cells placed in series. The object was then immersed in a dilute solution of potassium cyanide which served as the electrolyte. A platinum metal plate was used as the anode. This method was first described by Rathgen in 1889 (Rathgen, 1889). Over the years he was to employ this method with great success for the treatment of both bronze and iron antiquities and was the principal moving force in its acceptance as a standard treatment method for the conservation of metal artifacts, though today its use is primarily confined to the treatment of iron antiquities.

Although today it is generally acknowledged that bronze disease is a manifestation of the chemical transformations which copper chloride corrosion products undergo in the presence of moisture, for many years it was attributed to some sort of bacterial infection (Frazer, 1898; Petrie, 1972). This is hardly surprising given that the appearance of these efflorescences on the surface of bronze antiquities was not found to coincide with a change in the physical environment of the object; in fact, blooming often occurred many years after excavation. Moreover, it was observed that some bronze antiquities exhibiting this phenomenon, if placed in the vicinity of otherwise stable objects, would initiate the onset of blooming in the stable objects. As a consequence such patinas were said to be diseased, sick or malignant.

One of the earliest references to this phenomenon attributing the onset of decay to bacterial attack may be found in the papers of the Royal Academy of Lincel. In 1893 two chemists, Mond and Cuboni, presented the results of their findings before the Academy in which they identified the presence of the fungus Cladosporium aeris in the pits of actively corroding bronze antiquities (Mond and Cuboni, 1893). Subsequent experimental trials appeared to indicate that those objects infested with this bacterium could indeed infect other objects in close proximity which were apparently sound. Furthermore, sterilization with heat (120° C) was found to arrest the decay of infected bronzes, giving further credence to the bacterial infection theory.

Rathgen learned of these findings purely by chance while on a visit to the Waldarf-Richartz Museum in Mainz in 1896, where he found that many of the bronzes were undergoing sterilization with heat (Rathgen, 1896). Rathgen was very skeptical of the findings of his colleagues, and upon his return to Berlin, he attempted to duplicate their experimental results. Rathgen, however, failed to observe blooming on sterilized bronzes following inoculation with the fungus Cladosporium aeris. He maintained that a more plausible explanation could be sought in the examination of the chemical transformations which copper chloride corrosion products undergo in the presence of moisture as suggested by the French chemist Marcellin Berthelot.

At the turn of the century Berthelot presented a number of papers before the French Academy of Sciences attributing the poor state of preservation of many of the bronze (and silver) antiquities excavated by French archaeologists in Egypt and subsequently deposited in the Louvre to the corrosive influence of chloride salts (Berthelot, 1894, 1895, 1898, 1901). Berthelot described a cyclic process whereby chloride salts accelerated the rate of corrosion of copper metal in the presence of moisture and carbon dioxide with the formation of atacamite, the principal chloride containing copper corrosion product which had been identified in the patina of poorly preserved bronze antiquities.

This explanation, according to Rathgen, could well account for the findings of Mond and Cuboni. The varying amount of moisture in the atmosphere would influence the spread of the malignant patina; thus changes in the patina might not be apparent until many years after excavation when the object was placed in the museum. Sterilization with heat, on the other hand, would arrest the decay of infected bronzes by driving off all moisture, thus inhibiting subsequent chemical transformations.

In addition to his efforts on behalf of the Egyptian Collection of the Royal Museums of Berlin, Rathgen also conducted numerous experimental trials on antiquities from their Numismatic Collection. The treatment of large numbers of coins presented Rathgen with considerable difficulties. While Finkener's electrolytic reduction method yielded excellent results for large objects, it was entirely unsuitable for the treatment of large numbers of small artifacts, such as coins. As an alternative Rathgen employed a most ingenious electrochemical reduction method whereby he sand wiched the coins between sheets of perforated zinc laid one on top of the other which he then immersed in a bath of dilute caustic soda (Rathgen, 1896). In this manner, Rathgen successfully treated between forty and fifty thousand Roman bronze coins in a relatively short period of time. The reduction of eight thousand Danish copper coins yielded similarly good results.

Rathgen was not the first to employ electrochemical reduction for the conservation of metal antiquities. The use of electrochemical reduction for cleaning iron antiquities was described in the literature as early as 1892 by the Danish chemist Axel Krefting (Krefting, 1892; Applegren, 1897). Rathgen, however, conducted much original research evaluating the potential application of electrochemical reduction for the treatment of metal antiquities and was responsible for establishing guidelines for its proper application.

In 1898 Rathgen published the first edition of his handbook Die Konservierung von Altertumsfunden [The Conservation of Antiquities] (Rathgen, 1898). This was the first comprehensive treatment of the subject; it drew upon his ten years of experience and practical work in the field. The several works devoted to the conservation of archaeological artifacts that had been published prior to this date were more limited in scope. In 1888 Flinders Petrie had published a short article in the Archaeological Journal entitled “The Treatment of Small Antiques,” in which he described a number of different treatment methods he had employed in the field for preserving freshly excavated material (Petrie, 1888). That same year Albert Voss, Director of the Königlichen Museum für Völkerkunde in Berlin, had published the first of three editions of his archaeological manual Merkbuch, Altertümer aufzugraben und aufzubewahren [Notebook for the Excavation and Conservation of Antiquities], in which he devoted an entire chapter to the preservation of freshly excavated finds (Voss, 1888, 1894, 1914). Rathgen, himself, had previously reviewed his work in a lecture before the Polytechnische Gesellschaft [Polytechnical Society] in Berlin in 1892 (Rathgen, 1891; 1892). Several years later he published the first in a series of general review articles on the conservation of antiquities in O. Lueger's Lexikon der gesamten Technik und ihrer Hilfswissenschaften [Encyclopedia of the Complete Technical and Auxiliary Sciences] (Rathgen, 1894).

Rathgen's handbook was divided into two parts, a format which was adhered to in all subsequent editions. Part I was devoted entirely to the transformations which antiquities undergo before and after excavation, paying particular attention to the destructive influence of soluble salts upon stone and metal antiquities. Special attention was also given to the study of bronze patinas, particularly the so-called “wilde” patina exhibited by rapidly corroding bronze antiquities.

In Part II Rathgen discussed the actual treatment of antiquities, using as examples methods used in various European museums as well as at the Royal Museums of Berlin. Many of these methods had been published in the literature in various obscure journals and magazines, while others were simply personal communications from private and public museums that he had visited or with which he had corresponded. Many of the methods employed today for the treatment of archaeological materials may be found here in one form or another although Rathgen is rarely cited as a source of reference.

Whereas in Part I of his handbook Rathgen assumed a certain amount of chemical knowledge on the part of the reader, his description of the various methods of preservation presented in Part II was at times grossly over-simplified. This approach was to characterize much of Rathgen's later work, for it was his intent to present treatment methods from an elementary standpoint in order that they might be more readily carried out by those individuals lacking a scientific background, as was generally the case in most museum laboratories.

In the following years Rathgen presented numerous lectures and demonstrations on the subject of archaeological conservation before various professional organizations including:

  • Internationale Konferenz für angewandte Chemie [International Conference for Applied Chemistry] (Rathgen, 1930);
  • Hauptversammlung des deutsche Vereins für Ton-, Zement-, und Kalkindustrie [General Meeting of the German Society for Clay, Cement and Lime Industry] (Rathgen, 1906);
  • Berliner Gesellschaft für Anthropologie [Berlin Anthropological Society] (Rathgen 1906);
  • Versammlung der Deutschen Anthropologischen Gesellschaft [Assembly of the German Anthropological Society] (Rathgen, 1910);
  • Hauptversammlung des deutschen Gipsvereins [General Meeting of the German Plaster Association] (Rathgen, 1911);
  • Verein zur Beförderung des Gewerbefleisses [Association for the Promotion of Industrial Activities] (Rathgen, 1912);
  • Tag für Denkmalpflege [Meeting of the Society for the Preservation of Monuments and Historic Sites] (Rathgen, 1903, 1904, 1910, 1912);
  • Annual Congress of the Museums Association (Rathgen, 1913).

These presentations were not simply attempts to “popularize” conservation. On the contrary, by promoting conservation within the scientific community Rathgen sought to bring a sense of credibility and professionalism to conservation which would stimulate the interests of a much wider circle of individuals and provide, at least initially, an outlet for much of the published work.

In the years immediately following the publication of his handbook Rathgen devoted his attention primarily to the treatment of clay and metal antiquities. The Assyrian Collection of the Royal Museums of Berlin possessed a large number of Babylonian clay tablets only recently excavated from archaeological sites in the Near East. Many of these tablets were poorly fired or unbaked and could not withstand prolonged steeping in water to remove soluble salts. For their preservation Rathgen adopted a very simple, though innovative approach whereby he hardened the clay prior to steeping by first baking the tablets in a muffle furnace heated by bunsen burners. The temperature was regulated through the use of Seger Cones (Green, 1979) placed inside the furnace where they could be seen through the observation aperture. Firing the tablets in this manner not only removed much of the surface incrustations of lime and gypsum but also allowed the tablets to be safely immersed in water. Rathgen's findings were first published in 1901 (Rathgen, 1901), though he was to continue his experimental trials for many years in an effort to improve upon his treatment procedure (Rathgen, 1903, 1906, 1907, 1908). Much the same approach was later adopted by the British Museum and has found application even to this day, though Rathgen is rarely cited as the original source of reference (Bateman, 1966; Delougaz, 1933; Organ, 1959; Vaughan and Crawford, 1966).

In addition to his work on clay antiquities, Rathgen continued to devote considerable attention to the preservation of metal artifacts. In 1903 he published a brief account of his treatment of a number of lead medallions in the Royal Numismatic Collection for which he employed electrochemical reduction to remove disfiguring surface layers of lead carbonate (Rathgen, 1903). Later that same year Rathgen published the results of his experimental trials with molten potassium cyanide for the removal of thin corrosion layers from small silver and iron objects (Rathgen, 1903). The objects were momentarily immersed in baths of molten potassium cyanide then quenched in cold water. All traces of cyanide were then removed by repeated boiling in distilled water followed by impregnation in molten paraffin wax. Though ideal for the treatment of large numbers of small objects, Rathgen cautioned against using this method because of the toxicity of potassium cyanide, and he later employed a fused mixture of potassium and sodium carbonate with some success (Rathgen, 1905).

In addition to the above, Rathgen published several general review articles on the conservation of metal antiquities with particular reference to the use of electrolytic and electrochemical reduction methods (Rathgen, 1902; 1903; 1904). Since their original publication, a number of variations of Krefting's electrochemical and Finkener's electrolytic reduction methods had appeared in the literature employing different reducing agents and electrolytes (Krause, 1902; Rhousopoulos, 1903; Setlick, 1903). Unfortunately, Finkener and Krefting were rarely cited as the original source of reference, an omission with which Rathgen took great issue.

It was not uncommon for many authors to claim priority for the use of a particular treatment method without adequately referencing the literature. Rathgen found this highly unethical and unprofessional. In 1903 he published a short communication in the German scientific periodical Chemiker-Zeitung, in which he sought to establish priority for a number of commonly employed treatment methods in conservation, and in particular for the first use of electrochemical and electrolytic reduction methods (Rathgen, 1903). Priority, according to Rathgen, could only be established through publication in the relevant literature and not through subsequent claims. Rathgen was particularly critical of Eduard Krause, a conservator at the Museum für Völkerkunde in Berlin, who claimed that he was the first to have recognized the role played by soluble chloride salts in the decay of iron and bronze antiquities even though this had been previously demonstrated in the literature years earlier.

The turn of the century was marked by the introduction of the first synthetic resins, cellulose nitrate and later cellulose acetate (Worden, 1911). Rathgen conducted much original research on the application of Zapon, a commercial cellulose nitrate lacquer, for the preservation of antiquities. Zapon was one of the first synthetic resins to be prepared on an industrial scale and its introduction was to revolutionize conservation. In general, it was considered far superior to the natural resins because of its transparency and inertness to chemical attack and its reported impermeability to moisture and atmospheric pollutants. Though Zapon had previously found widespread application for the preservation of archival materials (Marwick, 1964), Rathgen was one of the first individuals to recognize its potential for the preservation of antiquities and was a pioneer in its application in objects conservation. His findings were first published in 1904 in the landmark paper “Zapon und seine Verwundung zur Konservirung von Sammlungsgegenstände” [Zapon and its Application for the Conservation of Museum Objects], in which he recommended the application of Zapon as a protective coating and consolidant for a wide variety of archaeological materials (Rathgen, 1903, 1904).

Rathgen later experimented with the use of cellulose acetate preparations when it was recognized that Zapon and other cellulose nitrate preparations were far too flammable to have general application in conservation. In 1913 he published a paper in the German museums journal Museumskunde, in which he recommended the use of Zellon as a substitute for Zapon in museum conservation (Rathgen, 1913, 1914). Zellon was the proprietary name for a non-combustible safety glass prepared from cellulose acetate. This was apparently available in sheets of varying thickness and, when necessary, could be dissolved in acetone for use as a lacquer. Rathgen was quick to recognize the potential of Zellon as a substitute for glass as a backing or interleaving material or support for museum objects and included a small sample of Zellon in his original publication, attached to the spine of the journal.

Both Zapon and Zellon were widely employed by Rathgen as protective coatings for plaster casts. Throughout his career Rathgen devoted considerable attention to the care and preservation of plaster casts and conducted much original research towards the development of new techniques for cleaning the surface of soiled plaster (Rathgen and Borrmann, 1904; Rathgen, 1911, 1911, 1915, 1917). Many of the natural resins traditionally employed as protective coatings for plaster casts altered the appearance of the cast. Moreover, it was often impossible to clean the cast following their application. As an alternative, Zapon retained much of their original shape and outline and could be safely washed with warm, soapy water.

The use of Zapon was later discontinued in favour of Zellon, though even this proved unsuitable with time. Various methods were therefore adopted for cleaning plaster casts that had not been so impregnated. One such method, attributed to Rathgen, involved dipping the plaster casts in “Gipswasser” (a super-saturated solution of gypsum) and going over the surface of the cast with a soft, bristle brush while the plaster was still wet (Rathgen, 1911, 1915).

In 1905 a fifteen-page supplement to the first edition of Rathgen's handbook was published (Rathgen, 1905). This essentially updated his handbook by including the results of his various experimental trials since its publication in 1898, particularly with regard to the treatment of metal and clay antiquities as well as the use of Zapon.

Later that same year an English translation of Rathgen's handbook entitled The Preservation of Antiquities: A Handbook for Curators was published; this incorporated the German supplement into the body of the text (Rathgen, 1905). With the exception of a single paper read before the Annual Congress of the Museum Associations in Hull in 1913 and entitled “The Preservation of Antiquities,” this was Rathgen's only major work to be translated into the English language (Rathgen, 1913).

Between the years 1908 and 1915 Rathgen published a series of fifteen communications from the laboratory of the Royal Museums of Berlin in the German journal Museumskunde. These dealt with various aspects of museum conservation:

  1. Über Konservierung bablyonischer Tontafeln [The Conservation of Babylonian Clay Tablets] (Rathgen, 1908);
  2. Konservierung eines Tonsarges [The Conservation of Clay Vases] (Rathgen, 1908);
  3. Entfernung trüber Glasurschichten [The Removal of Old Glazes] (Rathgen, 1908);
  4. Die Verwendung von tetrachlorkohlenstoff [The Use of Carbon Tetrachloride in Conservation] (Rathgen, 1908);
  5. Luftdichte Museumsschänke [Airtight Museum Cases] (Rathgen, 1909);
  6. Über Mittel gegen Holzwurmfrass [The Treatment of Woodworm] (Rathgen, 1910);
  7. Über Reinigung von Gipsabgüssen [The Cleaning of Plaster Casts] (Rathgen, 1911);
  8. Über die Verwendung von Kohlenstofftetrachlorid zur Abötng von tierischen Scädlingen [The Use of Carbon Tetrachloride for the Eradication of Woodworm] (Rathgen, 1911);
  9. Über Klebemittel [Adhesives] (Rathgen, 1911);
  10. Zellon [Cellon] (Rathgen, 1913);
  11. Über mikrochemische Untersuchungen [Microscopical Investigations] (Rathgen, 1913);
  12. Die “Nitrogenkassette” [Nitrogen Cases] (Rathgen, 1913);
  13. Entfernung trüber Glasurschichten [The Cleaning of Old Plaster Casts] (Rathgen, 1915);
  14. Paraffinformen für Gipsabgüsse [Paraffin Impregnation of Plaster Casts] (Rathgen, 1915);
  15. Tonabdrücke orientalischer Siegelzylinder [Clay Impressions for Oriental Cylinder Seals] (Rathgen, 1915).

Though we shall not consider each and every one of these individually, it is important to note the significance of their inclusion in what is otherwise a purely museological publication. Until relatively recently there were no journals devoted exclusively to conservation; furthermore, the relationship between conservation and the museum community was not well established. The publication of this series of communications in Museumskunde, however, reflects a marked change in attitude towards conservation and its acceptance as an integral part of the museum profession, much to the credit of Rathgen.

In the following years Rathgen undertook a number of rather difficult conservation projects which attest to his tremendous organizational skills. One such project, begun in 1916, involved the cleaning of the Kaulbach wall paintings in the staircase of the Neue Museum in Berlin (Rathgen, 1916, 1917). The frescoes, painted by Wilhelm von Kaulbach (1805–1874) in 1847, were badly damaged by sodium sulfate efflorescences which had been introduced from previous attempts to fix the colours with “waterglass” (sodium silicate). After extensive preliminary tests the paintings were laboriously cleaned with sponges moistened with distilled water.

At about this time another large-scale project was carried out in connection with the treatment of a large number of plaster casts belonging to the Berlin Institute of Archaeology (Rathgen, 1926). The casts were badly soiled as a result of poor storage conditions and repeated handling during their transfer several years earlier from the Antiquities Department of the Royal Museums of Berlin. Due to their poor state of preservation no attempt was made to clean the casts prior to the application of a protective coating. Instead the casts were freed of all loose surface dirt using a strong hair blower, tinted with ochre, and sprayed with Zapon. Due to the large number of casts requiring treatment (twenty-five hundred, of which one thousand were life-size or larger), new facilities were constructed and a special spray apparatus was designed for applying the Zapon (Fig. 5). The plaster casts were placed on exhibition in 1921. Unfortunately, the sculptures themselves were completely destroyed during World War II, though the moulds have been preserved.

Fig. 5. Workroom for spraying soiled plaster casts

It is not surprising that with time Rathgen began to assume responsibilities for the treatment of museum objects more generally and not simply archaeological materials. This change is most evident in later editions of his handbook, which were greatly revised and much enlarged in scope. Entitled Die Konservierung von Altertumsfunden mit Berücksichtigung ethnographischer und kunstgewerblicher Sammlungsgegenstände [The Conservation of Antiquities with Special Reference to Ethnographic and Folk Art Museum Objects], Rathgen's handbook was divided into three parts. Part I, published in 1915 (Rathgen, 1915) and later revised in 1926 (Rathgen, 1926), was devoted exclusively to the treatment of “stone-like” materials. The conservation of metals and organic materials was considered in Part II and Part III, respectively, and published in 1924 (Rathgen, 1924).

These later editions incorporated the results of a questionnaire which Rathgen had sent to a number of different museums and institutions throughout the world inquiring into the methods they employed for the preservation of their collections. Thus, Rathgen was in contact with Alexander Scott (Plenderleith, 1947) at the British Museum, Alfred Lucas (Gibson, 1948) at the Cairo Museum, G. Rosenberg (Thomsen, 1941) at the National Museum in Copenhagen, O.A. Rhousopoulos (Anon, 1958–62) at the National Museum in Athens and many others active in the field of conservation. These questionnaires reflect the importance which Rathgen placed upon communication as a means of ensuring the development of new and improved treatment methods at a time when there was no formal mechanism by which conservators could communicate with their colleagues in the field. In many respects these questionnaires were the forerunners of our present-day newsletters and journals.

In his last major work devoted to the conservation of antiquities published in 1926 in Ebert's Reallexikon für Vorgeschichte, Rathgen reviewed the history and development of the Chemical Laboratory of the Royal Museums of Berlin and discussed many of his most important accomplishments in the field of conservation (Rathgen, 1926).

Copyright © 1987 American Institute for Conservation of Historic and Artistic Works