Upload
others
View
12
Download
0
Embed Size (px)
Citation preview
AnalyticalMethods
AMC TECHNICAL BRIEFS
Publ
ishe
d on
12
Apr
il 20
17. D
ownl
oade
d on
05/
05/2
017
11:4
1:52
.
View Article OnlineView Journal | View Issue
X-ray fluorescen
Cite this: Anal. Methods, 2017, 9, 2371
Received 21st March 2017
DOI: 10.1039/c7ay90044a
rsc.li/methods
This journal is © The Royal Society of C
ce (XRF) analysis of porcelain:Background paper
Analytical Methods Committee AMCTB No. 77
This technical brief outlines how X-ray fluorescence (XRF) can be used
for the investigation of cultural heritage objects made of porcelain. It
discusses the types of questions that can be answered with XRF and
provides an overview of how the method works.
X-ray uorescence (XRF) is a non-destructive analytical methodthat allows the identication and quantication of the chemicalelements present in the surface of a wide variety of objects. It iscurrently widely applied in the investigation and character-isation of art objects as well as geological and archaeologicalmaterials. Because XRF is non-destructive, the method isparticularly suitable for the analysis of complete objects, suchas porcelain vessels, when sampling is not an option because itwould affect the integrity of the item. In the last 20 years,portable instruments have become commercially available,allowing samples or objects to be analysed in situ (see AMCTechnical Brief No. 41). Most XRF instruments are capable ofdetecting the majority of elements in the periodic table, rangingfrom magnesium to uranium. This makes XRF an ideal tool tocharacterise the materials used in the manufacture of porcelainbodies, glazes, enamel decoration and gilding.
Types of porcelain
There are two main types of porcelain, dened by chemicalcomposition and ring temperature: hard paste and so paste.
Hard paste ‘true’ porcelain is a hard white ceramic that wasrst produced in China and later in Japan and Europe. It is
hemistry 2017
made from kaolinite clay and petuntse (a rock comprising mica,feldspar and quartz).
So paste porcelain is a less translucent, slightly porouswhite ceramic that began to be manufactured in Europe in the18th century, although there were attempts at production in thepreceding centuries. It is made from white clay mixed witha glass frit (sand, gypsum, soda, salt, alum and nitre) fusedtogether with chalk and lime.
Bone china is a porcelaneous body that was developed inBritain in the 18th century and is considered to be a type of sopaste. It is composed of kaolin/ball clay and Cornish stonemixed with a bone ash ux. Both so paste and bone chinawares were rst produced in imitation of the ‘true’ hard pasteporcelain as the hard paste formula and raw materials were notwidely known at the time.
Porcelain is typically coated with a silica-rich glaze, whichhas a chemical composition that is compatible with the paste.For example, hard paste glazes consist of silica with an alkalisuch as lime or potash and so paste glazes are based on silicawith lead oxide. A range of decorative components can beapplied, including underglaze, in-glaze, on-glaze (enamel) andgilded decoration. Porcelain colours are made from pigments,such as oxides of copper, cobalt and iron, which producea range of shades depending on the ring conditions and typeof glaze to which they are applied.
Applications of XRF analysis
XRF is used on porcelain and its decorative components inorder to help answer questions on authenticity, provenance,date and restoration. The raw materials and recipes employedby different production centres result in unique chemicalcompositions that can be determined by using XRF. Rawmaterials oen changed over time, possibly owing to techno-logical or economic considerations. Each time a new recipe wasused, the composition would be subtly different. An example ofone of these changes involved the use of green enamels at theMeissen factory in Saxony. In the 18th century, green enamels
Anal. Methods, 2017, 9, 2371–2374 | 2371
http://crossmark.crossref.org/dialog/?doi=10.1039/c7ay90044a&domain=pdf&date_stamp=2017-04-15http://dx.doi.org/10.1039/c7ay90044ahttp://pubs.rsc.org/en/journals/journal/AYhttp://pubs.rsc.org/en/journals/journal/AY?issueid=AY009016
Analytical Methods AMC Technical Briefs
Publ
ishe
d on
12
Apr
il 20
17. D
ownl
oade
d on
05/
05/2
017
11:4
1:52
. View Article Online
were coloured using copper oxide but, at the beginning of the19th century, chromium oxide green was introduced. Thus thepresence of chromium green is a good marker for 19th centurydecoration. Temporal compositional differences form the basisof scientic dating for most types of porcelain. In order for thisdating to be effective, results must be compared with databasescompiled from the analysis of established materials, or newreference objects must be analysed.
Fig. 2 Handheld XRF.
X-ray fluorescence
XRF is a method that can determine concentrations of major,minor and, in some cases, trace elements by using X-rays.The most common XRF set-up used in heritage laborato-ries is energy dispersive (ED) XRF. During an ED-XRFexperiment, a high-energy X-ray beam hits the surface ofthe object or sample under observation. This in turn emits X-rays of lower energy, a behaviour known as ‘uorescence’.Each chemical element produces uorescent X-rays withenergies that are unique to that element. The emitted X-raysare displayed as a spectrum with peaks of varying heights atdifferent energies. The energy of a peak identies thechemical element producing it and the height of a peak canusually be linked to the abundance of that element in thesample. Soware is then used to measure and quantify theabundance of detected elements. Reference materials arealways used to calibrate the procedure or to check for bias inthe results.
Equipment and methods
The three most common instruments used in heritage labora-tories are (Fig. 1 and 2):
� Benchtop analysers,� Mobile open architecture instruments for use in museum
galleries, and� Handheld analysers.
Fig. 1 Benchtop XRF.
2372 | Anal. Methods, 2017, 9, 2371–2374
Benchtop XRF instruments have a large chamber in whichthe object for analysis is placed. These instruments have anadjustable beam diameter, usually between 0.2 and 3 mm,which means that ne areas of paint and gilding can be ana-lysed. Some benchtop instruments can map elemental distri-butions spatially across the surface of the object, allowingvariations in the pigments to be viewed visually (Fig. 3).
Mobile models have comparable beam sizes (0.2 to 1.5 mm),but have the added benet of being able to work on largeimmoveable objects in situ. Benchtop and mobile devices areoperated in combination with soware on a connectedcomputer.
Handheld devices are fully portable, weighing only a fewkilograms, which means that they can be easily carried intomuseums, private collections and auction houses, and thatvaluable objects no longer need to be transported to the labo-ratory. The beam size is larger and usually non-adjustable onhandheld devices, with beam sizes ranging from 3mm to 8mm.Handheld devices are operated via soware on the back of theinstrument or by connecting them to laptop computers. Ashandheld and mobile instruments have open X-ray beams, theinstruments must be used within a controlled area and theoperator has to abide by strict health and safety regulations.
In order to conduct an analysis, the whole object is placedeither in the chamber (for benchtop instruments), or directly infront of the instrument head within 1 cm of the beam on themobile or handheld device. Sampling is not required and themethod is completely non-destructive. Handheld devicesusually have to touch the object to reduce radiation scatter.Measurement times typically range from 30 to 100 seconds.Longer measuring times are required for the detection of ‘light’elements (those of low atomic number such as sulfur,aluminium and silicon) and trace elements (those at concen-trations lower than roughly 0.1% by mass). Shorter measuringtimes are oen employed for handheld devices as it can bedifficult for the operator to hold the XRF in position for longperiods.
This journal is © The Royal Society of Chemistry 2017
http://dx.doi.org/10.1039/c7ay90044a
Fig. 3 Visual appearance (top) and XRF map (bottom) of iron distri-bution across the surface of an enamelled porcelain plate.
Fig. 4 Example of an XRF spectrum of gilding on porcelain.
Fig. 5 XRF spectra of chromium green (green spectra) and coppergreen (red spectra) enamels.
AMC Technical Briefs Analytical Methods
Publ
ishe
d on
12
Apr
il 20
17. D
ownl
oade
d on
05/
05/2
017
11:4
1:52
. View Article Online
Results and interpretation
Three types of output can be generated by XRF instruments: anX-ray spectrum, a list of spectrum peak areas, and relativeconcentrations calculated via calibration data. The raw outputis displayed as a spectrum, with each peak uniquely repre-senting a particular chemical element (Fig. 4). Each peak isidentied with the originating element by using sowareinstalled on the instrument or on a connected computer. Theheight of each peak is related to the abundance of the element.Several spectra can be overlaid on the screen allowing the userto discern visually the differences between the ranges andabundances of elements in different targets.
This journal is © The Royal Society of Chemistry 2017
Because of this, spectra provide a useful means to charac-terise a material quickly. For example, the colouring agent ina green enamel may be identied as copper or chromium(Fig. 5), or a glaze may be characterised as leaded or non-leadedby the presence or absence of high lead peaks. The character-isation of the basic materials can be useful for cataloguingpurposes, documenting the history of technology, assessmentof the condition of an object, identifying restorations andrepairs, and dating.
Quantitative analysis
Some types of XRF soware can calculate and compare the sizeof the elemental peaks on the spectrum: this is called ‘peak areaanalysis’ and allows the user to determine the relative propor-tions of elements present. The data can be downloaded andused in statistical soware packages. Peak area analysis is usedto nd simple patterns in the data that may indicate the use ofdifferent recipes or sources of rawmaterials. For example, at theMeissen factory, early 18th century porcelain pastes and glazeswere produced by using a calcium sulfate ux that was lateraugmented and replaced by a potassium feldspar ux. Theseux materials may be identied using peak area analysis byexamining the ratio of calcium to potassium (Fig. 6). As these
Anal. Methods, 2017, 9, 2371–2374 | 2373
http://dx.doi.org/10.1039/c7ay90044a
Fig. 6 Scatter plot of Meissen glazes produced in 1715–74, high-lighting the high calcium/low potassium composition of early glazes.
Analytical Methods AMC Technical Briefs
Publ
ishe
d on
12
Apr
il 20
17. D
ownl
oade
d on
05/
05/2
017
11:4
1:52
. View Article Online
different uxes were used at different dates, this type of infor-mation can be used as an aid to dating.
A more general type of analytical result comprises deter-mining concentrations of the elements (as contrasted withelemental ratios). These are calculated from peak areas but inconjunction with information from reference materials con-taining known concentrations of the elements. Some XRFinstruments have in-built theoretical ‘fundamental parameter’(FP) programs to calculate percentage data, based on both themeasured peak intensities and the fundamental physicsspecic to that instrument and the elements concerned. Resultsare normalised to 100% and are material-specic. FP programsare calibrated by using a specic set of elements, so if an objectunder test has a different suite of elements the normalisedquantitative results will be inaccurate. The accuracy of theseprograms must therefore be checked with a reference materialsimilar to the test object.
The widely-recommended way to generate accuratepercentage data is to conduct an ‘empirical calibration’wherebythe operator calibrates the instrument using a set of referencematerials with known compositions similar to the test material.A second (disjoint) set of reference materials is then analysed tocheck the accuracy of the calibration. The calibration can thenbe used to determine the composition of the porcelain underanalysis.
Care must be taken in attempting to analyse thin layers oflight material as the X-ray beam can penetrate into the substratematerial and the results may represent a mixture of one or morelayers. Analysis depth is typically in the region of 40 mm(micrometres) for a high lead glaze or 500 mm for a lime-alkaliglaze. Heterogeneity of the material will reduce the reliabilityof the results, and therefore several readings should be takenfrom nearby areas and the results averaged.
In order that useful interpretations may be made, resultsmust be compared against a database of established materialsor by conducting XRF analyses of comparable reference objects.
2374 | Anal. Methods, 2017, 9, 2371–2374
Published databases can be found in academic journals such asArchaeometry, Journal of Archaeological Science, X-Ray Spectrom-etry and Journal of the American Ceramic Society.
Useful literature
A. Bezur and F. Casadio, The analysis of porcelain usinghandheld and portable X-ray uorescence spectrometers, inHandheld XRF for Art and Archaeology, Studies in ArchaeologicalSciences 3, ed. A. N. Shugar and J. L. Mass, Leuven, LeuvenUniversity Press, 2012.
F. Casadio, A. Bezur, K. Domoney, K. Eremin, L. Lee, J. L.Mass, A. Shortland and N. Zumbulyadis, X-ray uorescenceapplied to overglaze enamel decoration on eighteenth- andnineteenth-century porcelain from central Europe, Stud. Con-serv., 2012, 57, 61–72.
K. Domoney, A. Shortland and S. Kuhn, Characterization of18th-Century Meissen Porcelain using SEM-EDS, Archaeometry,2012, 54(3), 454–474.
W. Kingery, The Development of European Porcelain, HighTechnology Ceramics: Past, Present and Future. Ceramics andCivilization III., The American Ceramic Society, Westerville,1986, pp. 153–179.
X-Ray Fluorescence Spectrometry (XRF) in Geoarchaeology,Springer, New York, ed. M. Shackley, 2011.
P. J. Potts and M. West, Portable X-ray uorescence analysis,AMC Technical Briefs No. 41, 2009.
Portable X-ray Fluorescence Spectrometry: Capabilities for InSitu Analysis, ed. P. Potts and M. West, Royal Society of Chem-istry, Cambridge, 2008.
K. Yu, Attribution of antique Chinese blue-and-white porce-lains using Energy Dispersive X-Ray Fluorescence (EDXRF), inRadiation in Art and Archeometry, ed. D. Creagh and D. Bradley,Elsevier Science, Amsterdam, 2000, pp. 317–345.
Kelly Domoney (Craneld University and Ashmolean Museum,University of Oxford)
This Technical Brief was prepared by the Heritage ScienceSubcommittee and approved by the Analytical MethodsCommittee on 29/12/16.
This journal is © The Royal Society of Chemistry 2017
http://dx.doi.org/10.1039/c7ay90044a
X-ray fluorescence (XRF) analysis of porcelain: Background paperX-ray fluorescence (XRF) analysis of porcelain: Background paperX-ray fluorescence (XRF) analysis of porcelain: Background paperX-ray fluorescence (XRF) analysis of porcelain: Background paperX-ray fluorescence (XRF) analysis of porcelain: Background paperX-ray fluorescence (XRF) analysis of porcelain: Background paperX-ray fluorescence (XRF) analysis of porcelain: Background paper
X-ray fluorescence (XRF) analysis of porcelain: Background paper