Master’s Thesis 16843 words
Blue enamel on glass: an investigation into microfracturing and material
loss of 17
thcentury copper and cobalt blue enamel
Student: Roy van der Wielen 10757716
Supervisor: Mandy Slager Second assessor: Ellen Jansen
20-‐6-‐2017
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
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.
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.
Blue enamel on glass: an investigation into
microfracturing and material loss of 17
thcentury 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. During an inventory project in 1993 these four Perspex frames were encountered in the museum’s storage facilities, as well as a wooden box with the other 51 panels packed inside.6 This
wooden box contained panels that were mostly broken and mended using a pressure
1 Technical art history: chapter 1.
2 Jan Balis, introduction to Avium Vivae Icones in aes incisae editae ab Adriano Collardo, by Adriaen Collaert (Brussels culture et civilisation 1967), 1-‐4.
3 Appendix I
4 Joost M. A. Caen, The production of stained glass in the county of Flanders and the duchy of Brabant from the XVth to the XVIIIth centuries: materials and techniques (Turnhout Brepols 2009), 29, 347, 281-‐
282
5 Appendix I
6 According to RMA glass-‐conservator Margot van Schinkel it is not known whether after dismantling of the lead cames the 54 panels have immediately been set into Perspex frames. There is too little documentation to comfirm that. In 1993, the four Perspex frames were also dismantled. It is also not known when tape has been applied to the 51 panels encountered in the wooden box. This could have been done right after dismantling of the lead cames but there is no documentation to support this. At this moment, all panels are individually stored in the museum’s depot.
sensitive adhesive tape (cellophane carrier with a natural rubber adhesive7) – in many instances applied directly on decorated areas. Over the years this clear tape has shrunk, turned yellow, became brittle and has attracted a lot of dirt.8
At this moment, the panels are not suitable for exhibition and the museum wishes to remove this disfiguring tape in the near future. This problem has been submitted to the department of Conservation and Restoration of Cultural Heritage of the
University of Amsterdam to form the Master’s thesis that concludes the two years programme of Conservation and Restoration of Glass, Ceramics and Stone. The nature of this present research is purely diagnostic, therefore little reference will be made to the actual practical conservation.
A full assessment of the above introduced problem should ideally contain three stages: The first two being an investigation of the type and condition of the applied tape, and an investigation of the type and condition of the glass carrier and its
decoration layers. The concluding third step would be investigating the interrelation between tape and its substrate and its implications for future conservation.
However, because of the limitations of a Glass, Ceramics and Stone Master’s thesis, only the glass carrier and its decoration are investigated here.
Incentive to present research
There are reasons to believe that removal of the tape might result in seriously harming the decoration layers over which the tape has been applied.
Figure 0.1 shows an unfortunate example of object BK. LXX b. where – most likely – tape has been applied, removed and reapplied. The arrows indicate how, in this action, some of the fragile decoration has been torn off and has been relocated. This striking example of the apparent fragile nature of the decoration layers formed the incentive to perform an investigation into the condition of the various types of decoration present on these panels.
7 See Appendix II for FT-‐IR results.
8 Elissa O’Loughlin, Linda S. Stiber, “A closer look at pressure-‐sensitive adhesive tapes: update on conservation strategies”, in Conference papers Manchester 1992 , ed. Sheila Fairbrass (The institute of paper conservation 1992), 280 -‐ 286.
Figure 0.1: BK. LXXX b. relocated areas of decoration under a layer of clear tape
Analogue microscopy transmitted light
A first visual examination of an arbitrary selection of ten panels quickly drew
attention to a large number of microfractures and a large lacuna in a blue enamelled region of panel BK. LXXX II d.. Further inspection of blue enamel on two other panels (BK. LXX b. and BK. LXXXI i.) showed signs of a similar degradation pattern.9 It is these three objects that will form the sample group around which this research is build.
When looking at the archival photograph of the window BK. LXXX II d. it seems that, however unsharp this image may be, the large lacuna in the blue enamel was not yet present.10 If this is indeed the case, the observed degradation phenomenon might still be active – partially hidden (or even consolidated) by degraded tape. Blue enamel is well represented in this collection and already three out of ten panels showed advanced signs of degradation.
Consequently, with the impeding removal of tape remains in mind as well as the preservation the blue enamel present on these objects, an investigation into this particular kind of degradation of blue enamel is a pressing and relevant scope for this current thesis.
This thesis is set up as follows: First, a technical art historical context will be provided based on non – invasive chemical analyses of the sample group, that are integrated
9 Description of investigated degradation phenomena: chapter 2. 10 Appendix 1.
into the recipes of 17th and 18th century glass and enamel makers, as well as modern glass chemistry.
Then, based on the observed degradation phenomena and the current state of knowledge on enamel degradation, a hypothesis will be formulated and tested for applicability on the three objects.
For those points, for which the plausible hypothesis insufficiently covers the
phenomena found present on objects BK. LXXX II d., BK. LXX b. and BK. LXXXI i., the results will supplemented with additional considerations, discussion and concluding thoughts for follow up research.
Chapter one -‐ technical art historical context
Introduction of sample group
Objects BK. LXXXII d., BK. LXXX b. and BK. LXXXI i. (figures 1.1, 1.2 and 1.3) each feature a bird, delicately painted on transparent glass in grisaille shades and enamel colours. Painted decoration is applied on the interior side of the glass.11 The artist’s choice of colours does not correspond to the form12 of the portrayed birds, which complicates an exact determination of the species. Still, to go around the abstract object names BK. LXXXII d., BK. LXXX b. and BK. LXXXI i., the objects will (in this thesis) hereafter be referred to as Macaw, Woodpecker and Amazona. These are names based on the artist’s choice of colour and have no real ornithological value.
On the three panels, different types of vitreous decoration have been identified. The outlines and shadowing have been done in black grisaille (usually Fe2O3, Fe3O4 and /or CuO and/or Mn2O3). Other used colours are sanguine red (usually very fine ground Fe2O3) and blue enamel (usually CoO and/or CuO).13 In many instances, blue enamel on the interior side has been used in combination with a silver yellow stain (colloidal Ag diffused into the glass network) on the exterior side forming the colour green. Clearly visible on the Woodpecker and Amazona panel is that many of the original grisaille details have disappeared, possibly due to the use of a paint with an erroneous pigment to glass phase ratio.14 The condition of the blue enamel will be discussed in more detail chapter 2.
For all three objects, the glass carrier is very thin (1.5mm) and the size of a post card (average 90-‐70 mm). The carrier glasses all have a similar gray – green hue resulting from impurities in the used ingredients. Clearly visible are large fractures in the glass and the corresponding large areas of tape, applied to mend these pieces.
11 Caen, The production of stained glass, 274. 12 Balis, introduction, 1-‐4.
13 Caen, The production of stained glass, 251-‐60.
14 Olivier Schalm, Koen Janssens, Joost Caen, “Characterization of the main causes of deterioration of grisaille paint layers in 19th century stained-‐glass windows by J. B. Capronnier”, Spectrochimica Acta B 58 (2003), 600.
Figure 1.1: BK. LXXXII d. ‘Macaw’ Figure 1.2: BK. LXXX b. ‘Woodpecker’
Figure 1.3: BK. LXXXI i. ‘Amazona’
Technical Art Historical context
This section is a treatise on the relevant aspects of the manufacture and chemical composition of enamelled glass panels like the Macaw, Woodpecker and Amazona. The information in this section is a combination between glass chemistry and historical glass and enamel recipe books, which is corroborated by the results of surface XRF-‐analyses performed on the sample group. Aside from the manufacture and composition of blue enamel found present on the three panels, attention is given to the composition of the carrier glass. The reason is that the dynamics between carrier glass and enamel glass plays an important role in many degradation
phenomena found on stained glass. What falls outside the scope of this research is the shaping of the actual flat glass.
Glass from a chemical perspective
Glass can be defined as being an “amorphous solid completely lacking in long range, periodic atomic structure, and exhibiting a region of glass transformation
behaviour.”15
Most glasses – vessel glass, flat glass, glass enamels etc. – consist of three ingredient groups: the glass network former, the glass network modifier (or flux) and the glass network stabilizer. Predominantly, the glass network former is quartz (crystalline SiO2). Pure quartz can be made into a very durable glass – vitreous silica (amorphous SiO2, see figure 1.4). However, with a melting temperature of over 2000 degrees Celsius, vitreous silica could never be produced in a pre-‐industrial glass kiln.
16
Figure 1.4: Vitreous silica: amorphous network of covalent bound oxygen with silicon
To lower the melting temperature of a glass, modifier or flux compounds are added to create an eutecticum: a mixture of two or more components that has a lower melting point than these components individually would have had. There are many elements that would have fluxing properties when added to a glass, each with its own
15 James E. Shelby, Introduction to glass science and technology (Cambridge the royal society of chemistry 2005), 3.
characteristics. Alkali oxides of sodium, potassium or lead are reliable fluxing agents that have been used throughout Antiquity up to this day.
When added to a glass recipe, a flux modifies the strong covalent network of Si and O tetrahedra by forming occasional ionic bonds with oxygen. These ions break up the strong covalent network by converting bridging oxygen bonds (covalent bound oxygen – silicon – oxygen) into weaker non-‐bridging oxygen bonds (ionic bound oxygen – modifier) (see figure 1.5)17. Because these bonds are weaker, they lower the melting point of the glass, making it much more workable in the glass kiln.
The downside to this modifying effect is that the chemical durability is greatly lowered. In fact, a glass consisting only out of silica and a flux – also known as water glass –is soluble in water. To counter this problem the third ingredient group, the network stabilizer, is added. A much-‐used stabilizer is the alkaline earth calcium ion. As opposed to a modifier ion, the stabilizer ions are able to ionically bind two oxygen ions instead of only one. This way, the covalent network is still partially broken up by the modifier ions, but the presence of ionic oxygen – stabilizer – oxygen bonds
slightly holds the network together. This significantly enhances the chemical durability of a glass.18
19
Figure 1.5: Modified amorphous network of covalent bound oxygen (bridging) with silicon that also includes ionic bound oxygen (non-‐bridging) with modifier and stabilizer ions
Historic glass ingredients
It is important to realize that, in contrast to modern glass makers, pre modern glass makers did not have access to industrially purified network formers, modifiers and stabilizers. Many ingredients needed to make a glass do not occur in a naturally pure state but had to be acquired through natural sources such as plants and minerals. The use of these natural sources is guaranteed to lead to variations and impurities in the glass composition.
It is a fascinating view into the world of pre-‐industrial craftsmen that one gains when researching the glass makers manuscripts from the 16th throughout the 18th
17 Shelby, Introduction to glass science, 82-‐83. 18 Ibid, 89-‐90.
century.20 One of the first systematic treatises on glass making, colouring and staining is L’Arte Vetraria, written in 1612 by the Florentine alchemist and glass maker
Antonio Neri. Conveniently, one can also take recourse to the 1662 English
translation and annotation – The Art of Glass – by Christopher Merrett. In 1679, the German glassmaker Johannes von Kunckel wrote a German translation and
annotation on his Italian and English predecessors – Ars Vitraria Experimentalis. Unlocking the treasures of information that lie in these manuscripts is a discipline on its own. What is relevant for this research however is that, through reading these recipes and ingredients, an explanation can be found why certain compounds – aside from the glass former, alkaline and alkaline earth ions – are present or absent in the glass. Additionally, since degradation phenomena are hardly ever limited to the chemical composition of an object, but are often related to parts of its manufacturing process, historical recipes provide a vital context to abstract analytical results.
Different from modern day glass making, the historical sources speak of two, instead of the three ingredient groups introduced above. There is the ‘glass salt’, which is a combination of the network modifier and stabilizer, and then there is the ‘material
[…] which gives consistence and body and firmness to glass’, that is the network
former.21 These two ingredient groups are melted together and then quenched in cold water to make the so called glass fritt – a powdered base glass to which a glass artist can add additional ingredients if necessary.
The Carrier Glass
The benchmark for pre-‐industrial glass making was the Venetian Crystallo glass. Between the 16th and 17th century this glass, clear as rock crystal, was the summit of glass making and the material that glass makers throughout Europe sought to imitate. Therefore naturally, Neri in his L’Arte Vetraria and his English and German
successors introduce this Muranese glass in great detail as their first recipe. Because of this available detail, the recipe for crystallo glass is used here to s an introduction into historic glass making. Obviously, there existed different recipes for different qualities in glasses that served different purposes – plain window glass or luxury vessel glass.22, 23 However, all these recipes are variations on a similar theme: a glass salt and a glass body.
Crystallo glass was made from pure white Italian Tarfo pebbles (glass body) and the
purified ashes of the Levantine soda plant (glass salt). It was rightfully believed that impurities such as iron oxides are a cause of discolouration and that by removing them a clear glass would be created.
The purification of plant ash is done by washing the ashes so that only the salts of water soluble alkaline elements are used. Also manganese was sometimes added to the recipe for the removal of undesired discolouration. 24 Ironically enough, the insoluble alkaline earth calcium ion, an important network stabilizer, is also omitted
20 See appendix III for a schematic overview of relevant historic literature. 21 Christopher Merrett, The art of glass (London A. W. 1662), 258.
22 Caen, The production of stained glass, 129.
23 Koen Janssens, Ine Deraedt, Olivier Schalm, Johan Veeckman, “Composition of 15-‐17th century archaeological glass vessels excavated in Antwerp, Belgium” in, Mikrochimica acta (supp) 15 (1998), 253-‐257.
from the glass by this method. Therefore, many of these Crystallo glasses are chemically instable.25 Aside from glass salt made of Levantine soda plant ashes, the historical sources contain recipes that include ashes of fern, bean stalk, brambles, millet, reeds and rushes (Neri).26 Additional alkali sources mentioned in the Merrett annotation are, amongst others, kelp, pine, fir, beech and oak ashes.27
The above discussed process of purifying the different ingredients was a time consuming and expensive process not feasible for every glass producer. As stated above, different qualities of glass were produced for different purposes. From the historical sources, it seems to be that ‘quality glass’ referred mostly to its aesthetic resemblance to pure rock crystal. The historic glass makers seemed less concerned with a glass’ long term chemical durability. In his annotations on Neri’s chapter on fritts, Merrett distinguishes three kinds of fritts. The first kind – ‘Crystallo fritt’ – is made by the above introduced recipe: purified Levantine soda ash with pure white
Tarfo pebbles or sand. The second type – ‘ordinary fritt’ – is made with unpurified
Levantine soda ash and sand. The third type – ‘fritt for green glasses’ – is made with unpurified ‘common ashes’ and sand.28 Obviously, the first type is the most valuable fritt, ideal for luxurious wares, whereas the third would be considerably cheaper to produce.
The ashes of different plant species result in different ratios of network modifiers and stabilizers in a glass – improving or corrupting the glass its durability.29 To further differentiate this, not only the species but also the section of the plant that is used, as well as the region and season in which the plant or tree was cut down further
influences the composition of a glass.
It is clear that through these countless variations in ingredients, a virtually limitless amount of different glass compositions can be found. However, based on type and ratio of the glass network former, modifier and stabilizer, five general historic glass types can be discerned: Soda glass, Potash glass, Mixed alkali glass and High Lime Low Alkali glass (HLLA) and Lead glass. (see figure 1.6)
Window glass (figurative and nonfigurative) of the 17th century is in most instances rich in potassium – either Potash glass or HLLA-‐glass. This changes only after 1791, when the invention of the Leblanc process enables the use of synthetic soda in glass, making soda lime glass the leading glass type. Lead glass was, in principle, never used as window glass.30
25 Hannelore Römich, “Historic glass and its interaction with the environment”, in The conservation of glass and ceramics, ed. N.H. Tennent (London James & James 1999), 5-‐11.
26 Merrett, The art of glass, 15. 27 Ibid, 262-‐270.
28 Ibid, 272-‐73.
29 Jennifer L. Mass, Instrumental methods of analysis applied to the conservation of ancient and historic glass”, in The conservation of glass and ceramics, ed. N.H. Tennent (London James & James 1999), 16-‐17.
31 Figure 1.6: Example of classification system of different hystoric glass types
Analytical results of carrier glass
To establish the category in which the glass carrier of the Macaw, Woodpecker and Amazona panels can be classified , XRF spot analyses using a Bruker Artax
Spectrometer have been performed under Helium flow. The recorded spectra were compared to Corning glass standards and this confirmed that all three panels were of a similar HLLA composition. (see figure 1.7)
The XRF-‐analyses did not provide quantitative data, however by comparing with Corning glass standard D32, an estimation of 55 % Silicon, 15% Calcium, and 11 % Potassium was made.
31 Olivier Schalm, Koen Janssens, Hilde Wouters, Daniel Caluwé, “Composition of 12-‐18th century window glass in Belgium: non-‐figurative windows in secular buildings and stained glass windows in religious buildings”, Spectrochimica acta B 62 (2007), 666.
Figure 1.7: Combined unquantified XRF-‐spectra of the carrier glass of Macaw, Woodpecker and Amazonia Comparing these spectra with Corning glass standards comfirmed a HLLA (High Lime Low Alkali) composition. (Analysis performed by dr. L. Megens RCE)
Needless to say that the five categories of historic glass are of modern times and that, no recipe for HLLA-‐glass can be found in the historical manuscripts. Therefore it is not completely clear from which recipe HLLA-‐ glass originates. The sodium content in HLLA-‐glass is much too low for it to have originated from Levantine soda ashes. Also, in the XRF-‐spectra, some phosphor and iron was detected. Especially the presence of phosphor is a reliable indicator that unpurified ashes have been used. 33 This
composition rules out the first two fritt types (Crystallo and Ordinary fritt)
distinguished by Merrett therefore, by some, HLLA-‐glass has been thought to be a variation of what Merret calls ‘green glass’.34 Indeed, the higher potassium levels can be explained by the use of ‘unprepared common ashes’. However, this does not explain the significant level of calcium found present in these glasses.
An explanation might be found in L’Arte Vetraria’s seventh chapter, in which Neri introduces a glass salt that contains salt of lime that is used in plasterworks. This salt of lime is mixed with Crystallo salt (lime salt: crystallo salt = 2 pounds : 100
pounds).35 It could well be that the glass makers in Northern Europe added their salt of lime not to Crystallo salt, but, since they lacked the Levantine soda ashes of the Venetians, used a glass salt made of ‘common ashes’ – resulting in a potassium rich
33 Schalm, Janssens, Wouters, Caluwé, ““Composition of 12-‐18th century window glass”, 666. 34 David Dungworth et al. “Glass and pottery manufacture at Silkstone, Yorkshire”, in Post-‐Medieval archaeology 40-‐1 (2006), 171.
glass with a high level of calcium. This glass could then be processed into flat glass suitable for decoration with vitreous paints.
Enamel decoration
The application of coloured enamel decoration on glass liberated the hand of the stained glass artist. Making outlines and shades in black-‐brown grisaille has been already practiced from the 9th century on and in the 13th century this palette was extended with the introduction of yellow silver stain. However, as can be read in the early 15th century Il libro dell'arte by Cennino Cennini, for every other colour a new piece of coloured glass had to be cut to shape and set in a separate lead came.36 Enamel colours enabled a glass painter to paint detailed coloured designs on a transparent glass carrier without the constant need of disturbing (and time consuming) lead cames.
It is believed that, in the Low Countries, the use of enamel colours on glass started around the beginning of the 15th century.37 Probably the earliest mention of this technique is the manuscript ‘Die maniere van ovens te maken om colueren te backen
van alle soerten. Ende hoemense backen sal.’ that is in the collection of the Plantin and
Moretus museum in Antwerp. 38 This manuscript not only gives an account on three used ovens and the firing process of stained glass panels, it gives concise recipes for blue-‐green, blue, purple enamels and how to prepare the silver salt that is used to make a silver yellow stain. Aside from the Plantin and Moretus manuscript, enamel recipes can be found in Neri’s L’Arte Vetraria. These have been translated and
annotated by Merrett and Kunckel in quite the same way as they have done for Neri’s recipes on glasses. Still, perhaps the most complete historical source on the
preparation and application of enamel decorations is Robert Dossie’s ‘The Handmaid
to the Arts’ published in 1764.
An enamel consists of two components, or phases: The glass phase (a powdered glass fritt) and a pigment phase (finely ground pigment). A very important aspect in a successful application of enamel decoration is the use of a glass fritt that has a very low melting point so that the enamel will fuse onto the glass carrier well before the glass carrier will melt. An effective flux to use in an enamel glass phase is lead.39 In Dossie’s The handmaid to the arts, the recipe for an enamel lead fritt is divided into two parts. The first part explains how a lead glass is made by taking two pounds of Minium (Red Lead Pb3O4) and one pound of white sand or calcined (roasted) flint. Once mixed together this is vitrified and quenched in cold water creating a very pure lead glass fritt. A glass with only lead as a network modifier is chemically unstable (it blackens, according to the author) therefore the second step is to take one pound of this pure lead glass fritt and mix this with 6 ounces of pearl ash (Potassium
Carbonate K2CO3) and 2 ounces of sea salt.40 This lead fritt can be ground to the desired fineness and coloured using a pigment of choice. An important job of the person preparing the enamels is to assure that, in the end, all the prepared mixtures have a similar melting point. This is to prevent that during the firing process one area
36 Daniel Thompson, trans. The craftman’s handbook (New York Dover publications 1960), 111. 37 Caen, The production of stained glass, 251.
38 Plantin-‐Moretus museum, manuscript number 64.
39 Daniel Rhodes, Clay and glazes for the potter (London Krause publications 1973), 172-‐180. 40 Robert Dossie, The handmaid to the arts (London 1758), 274-‐75.
is already fully vitrified while other areas have yet to fuse.41 This may seem fairly straightforward but one has to keep in mind that not only the major fluxes influence the melting point, every added compound in some way influences the eutecticum of a glass.42
In the lead glass fritt no calcium source such as lime or unpurified wood ash is present to stabilize the glass network. However, the high percentage of lead in a lead glass plays a dual role, being both the network modifier and the network stabilizer.43
Analytical results of blue enamel
In the researched sample group, blue enamel has been employed in two ways. To create a shade of green, a light azure blue is applied on the interior side over an area of carrier glass that has yellow silver stain on the exterior. Transmitted light blends these two colours into a light green. On all three objects, this green is used to colour feathers and leaves. Only for the Macaw, another type of blue enamel is found
alongside the light azure blue: a deep blue that is applied over the transparent carrier glass. This blue was used for some of the blue primary feathers of the wing, and to colour the iron nail onto which the Macaw is seated.
In the historic recipes44 – Neri, Merrett, Dossie and the Plantaan and Moretus
manuscript –two general types of recipes for blue enamel can be found. An azure blue enamel containing copper oxide (collected from the scales that fall from the brass worker’s hammer) and, if a more intense blue is desired, additional cobalt is added coming from either zaffer (roasted cobalt ore, CoO+ contaminations) or smalt
(vitrified cobalt oxide, SiO2–K2O–CoO + contaminations). The other type – a deep blue enamel – is made by mixing smalt or zaffer with some enamel glass fritt. Cobalt as a sole enamel pigments tends to turn to an almost black blue and in that case some copper is added to counter this.45 Both types of pigment phases – copper and cobalt blue –are to be mixed with some enamel lead glass fritt.
XRF spot analyses on the various areas of blue enamel were performed using a Bruker Artax Spectrometer in combination with a helium flow. The results largely matched the historic recipes that were consulted beforehand. (see figures 1.8 and 1.9).
41 Dossie, The handmaid, 228-‐230. 42 Rhodes, Clay and glazes, 162-‐ 169
43 Frederic Angeli et al. “Structure and chemical durability of lead crystal glass”, in Environmental science & technology 50 (2016), 11550.
44 Schalm et al., “Enamels in stained glass windows: preparation, chemical composition, microstructure and causes of deterioration”, in Spectrochimica acta B 64 (2009) 815.
Figure 1.8: Combined unquantified XRF-‐spectra of three areas of copper blue enamel (Analysis performed by dr. L. Megens RCE)
Figure 1.9: Combined unquantified XRF-‐spectra of two areas of cobalt blue enamel found on the Macaw (Analysis performed by dr. L. Megens RCE)
Both types of blue enamel consist of a similar glass phase containing lead and potassium as its major fluxes. This composition does not require calcium as a network stabilizer and none was detected. The lead content of the glass phase matched that of 17th century enamels.46
As expected, two types of pigments have been detected: a copper based azure blue enamel that is used in combination with yellow silver stain, and a deep blue enamel based on cobalt and some copper. The copper blue enamelled areas did not need additional blue – given that they were meant to be green – no additional cobalt was added. However, in both areas of cobalt blue enamel, some copper is detected. Most likely added counter the almost black blue colour that pure cobalt can give to an enamel.
Painting and firing of enamel colours
To paint the dry enamel fritt on a glass carrier with a paint brush, a temporary organic binder was added. This can be a resinous substance like gummy water (gum Arabic) which is recommended in the Plantin and Moretus manuscript or through essential oils like oil of lavender, which is recommended in The handmaid to the Arts. This medium burns away during the firing process.
Perhaps the most technically challenging part in art of stained glass is the firing process. A lot can go wrong here and it was not uncommon for whole loads of
valuable pieces of art to go to waste. The Plantin and Moretus manuscript opens with a beautiful and quite detailed drawing of a tripartite stained glass makers oven. (figure 1.10)
47
Figure 1.10: 15th century drawing of a tripartite glass makers oven
According to this drawing, the firing process starts in the left chamber, moves to the middle chamber and is concluded in the chamber on the right. The first oven is meant
46 Geert van de Snickt et al. “Blue enamel on 16-‐17th century window glass: deterioration, microstructure, composition and preparation”, in Studies in Conservation 51-‐3 (2006), 216. 47 Plantin-‐Moretus Museum, manuscript 64.
