University of Amsterdam Conservation and Restoration of Cultural Heritage – Glass, Ceramics And Stone Master’s Thesis 16843 words Blue enamel on glass: an investigation into microfracturing and material loss of 17th century copper and cobalt blue enamel Student: Roy van der Wielen 10757716 Supervisor: Mandy Slager Second assessor: Ellen Jansen 20-6-2017 All images made by Roy van der Wielen, unless specified otherwise Blue enamel on glass: an investigation into microfracturing and material loss of 17th century copper- and cobalt blue enamel Table of contents Abstract 3 Samenvatting 4 Introduction 5 Chapter one - technical art historical context 9 Chapter two – description of investigated degradation phenomena 22 Chapter three - current state of knowledge 34 Chapter four – corroboration of hypothesis 39 Chapter five – gas inclusions, surface pits and microfractures 49 Chapter six – conclusions 65 Acknowledgements 67 Bibliography 68 Appendix I Archival images 71 Appendix II FT-IR analysis applied tape 75 Appendix III historical sources and their inter-relation 79 Appendix IV unquantified xrf-spectrum Corning glass standard “D” and quantification table 80 Van der Wielen UvA 2 Blue enamel on glass: an investigation into microfracturing and material loss of 17th century copper- and cobalt blue enamel Blue enamel on glass: an investigation into microfracturing and material loss of 17th century copper and cobalt blue enamel Abstract The Rijksmuseum Amsterdam owns a collection of fractured 17th century enamelled glass panels that have been mended with clear tape consisting of a cellophane carrier and rubber adhesive. In many cases, this tape has been applied over the decorated side and has yellowed and shrunk. Future removal of this tape instigated the investigation of the condition of these objects. In this thesis, the focus was laid on microfractures and the subsequent loss of blue enamel on three panels: BK. LXXX II d., BK. LXX b. and BK. LXXXI i. XRF-analyses identified copper based and cobalt based enamel with a similar lead and potassium flux. The used carrier glass was High Lime Low Alkali (HLLA) glass. The identified degradation phenomenon was a network of vertical surface microfractures crossing both the enamel as well as a proportion of the carrier glass. These microfractures are interconnected in the horizontal plane, undermining areas of the glass carrier and blue enamel. In this research, the term ‘cross-phase microfracturing and subsequent flaking’ was used to describe this phenomenon. On the three objects, the condition of cobalt blue enamel seemed significantly better than that of copper blue enamel. In contemporary conservation literature, cross-phase microfractures and subsequent flaking is believed to be the result of tension, caused by a discrepancy in the thermal expansion coefficients of the enamel glass and the carrier glass . By visually investigating basic fractographic characteristics, this hypothesis was found plausible. However, insufficient explanation for significant difference in condition between copper and cobalt blue enamel on the Macaw panel was offered. Numerous surface pits and gas inclusions have been found in copper blue enamel layers, that were almost absent in cobalt blue enamel. The presence of these seems to be related to the formation of cross-phase microfractures, although the nature of this relation is unclear. Literature agrees that the presence of gas inclusions implicates that the enamel has not melted properly. The melting point of a glass is determined by added fluxing agents. The flux ratio in both enamel types did not differ in such a way that would result in two different melting points. However, cobalt blue enamel contained arsenic contaminations. Arsenic is claimed to be a powerful flux and has the capacity to remove gas inclusions from a molten glass. It was concluded that influence of arsenic on the durability of blue enamels deserves further investigation. The identification of thermally induced microfractures is relevant for conservation. When removing tape, care should be taken not to lose material. Another possible scenario would be shrinking cellophane backing tearing off material. The diagnosed degradation phenomenon will progress unless temperature fluctuations are kept to a minimum. Van der Wielen UvA 3 Blue enamel on glass: an investigation into microfracturing and material loss of 17th century copper- and cobalt blue enamel Blauw emaille op glas, een onderzoek naar microfracturen en materiaalverlies van 17de eeuws koper- en kobaltblauw emaille Samenvatting Het Rijksmuseum Amsterdam bezit een collectie 17de eeuws gebrandschilderd glas. Veel van deze zijn gebroken. In het verleden zijn ze gerepareerd met plakband, bestaande uit cellofaan met rubber. Dit is in veel gevallen over de beschilderde kant aangebracht, is vergeeld en gekrompen. Eventuele verwijdering hiervan initieerde dit onderzoek naar de objectcondities. In dit onderzoek ligt de focus op microfracturen en daaropvolgend verlies van blauw emaille op drie panelen: BK. LXXX II d., BK. LXX b. en BK. LXXXI i. XRF-analyses identificeerde zowel koper-, als kobaltblauw emaille met een vergelijkbare kalium/lood flux. Het glas van de dragers is HLLA-glas (hoog calcium, laag alkali). Het geïdentificeerde degradatiefenomeen is een netwerk van verticale microfracturen, die zowel emaille- als dragerglas doorkruisen. In het horizontale vlak blijken deze fracturen onderling verbonden en ondermijnen ze gedeeltes dragerglas en emaille. In dit onderzoek wordt dit ‘fasedoorkruisende microfracturen met daaropvolgend materiaalverlies’ genoemd. Op de drie objecten lijkt de conditie van kobaltblauw emaille beter te zijn dan dat van koperblauw emaille. In de literatuur wordt aangenomen dat ‘fasedoorkruisende microfracturen met daaropvolgend materiaalverlies’ resulteren uit een discrepantie tussen de thermische expansie coëfficiënten van het emaille- en het dragerglas. Deze hypothese wordt aannemelijk geacht, gebaseerd op de fractografische karakteristieken van de diverse breukvlakken. Deze biedt echter onvoldoende uitleg voor het geobserveerde verschil in conditie tussen kobalt en koperblauw emaille. Diverse oppervlakteputten en gasinsluitsels zijn aangetroffen in het koperblauw emaille. Deze zijn vrijwel afwezig op het kobaltblauw emaille en lijken gerelateerd te zijn aan de vorming van fasedoorkruisende microfracturen. Hoe exact, is onbekend. Vanuit de literatuur wordt aangenomen dat gasinsluitsels en oppervlakteputten tekenen zijn van onvoldoende gesmolten emaille. Het smeltpunt van emaille wordt in grote mate door de fluxen bepaald. Maar de fluxverhoudingen in de onderzoekte emailles verschilden niet in een dergelijke mate dat dit het smeltpunt kon beïnvloeden. Daarentegen bevatte het kobaltblauw emaille arsenicum, een materiaal dat bekend is om zijn fluxcapaciteiten en vermogen gasinsluitsels te verwijderen. Er is geconcludeerd dat de invloed van arsenicum op de duurzaamheid van emaille meer onderzoek verdient. De identificatie van temperatuursafhankelijke microfracturen is relevant. Wanneer plakbandverwijdering overwogen wordt, dient zorg gedragen te worden géén emaille te verliezen. Zelfs wanneer er geen actie wordt ondernomen kan krimpend cellofaan emaille lostrekken. Het gediagnosticeerde degradatiefenomeen zal doorzetten, tenzij temperatuursfluctuaties tot een minimum beperkt worden. Van der Wielen UvA 4 Blue enamel on glass: an investigation into microfracturing and material loss of 17th century copper- and cobalt blue enamel Blue enamel on glass: an investigation into microfracturing and material loss of 17th century copper and cobalt blue enamel Introduction The Rijksmuseum Amsterdam houses an important collection of stained glass objects from the 15th, 16th and 17th century. Among these is a group of 105 Netherlandish thin, postcard sized panels from the 17th century, decorated with vitreous paint. The use of this paint, consisting of a pigment phase and a low melting glass phase, allows the decoration to be permanently fused onto the glass plates in a furnace.1 On each panel individual floral, faunal, pastoral or genre scenes are depicted. Some have been executed in bright colours (different enamel colours), and some only with black shading and outline (grisaille) Many of the depicted scenes originate from well- known print makers.2 The museum’s documentation on these panels is scarce but they are already included in the 1890 Inventaris van Gebrandschilderd Glas (inventory of stained glass)3. The inventory describes the panels as being set into larger wooden frames – most likely by means of lead cames. Art historically seen, it is unlikely that one single stained glass window featured so many of these small individual scenes since these were usually placed as corner pieces around larger heraldic symbols and Biblical scenes.4 Therefore the windows included in the 1890 inventory should be seen as a compiled collection and its individual panels do not necessarily share provenance and history. In the available documentation, no mention is made of any degradation phenomena or conservation treatments being undertaken. However, on undated archival images5 numerous large fractures in the panels can be observed – some mended with additional lead cames, some left unattended. At an unknown point in time, the windows have been dismantled and the lead cames have been removed from the panels. Subsequently, 54 of the 105 panels have been mounted in four Perspex frames and have been on display between the 1960ties and 70ties.