Raman spectroscopic analysis of an English soft-paste porcelain plaque-mounted table

JOURNAL OF RAMAN SPECTROSCOPYJ. Raman Spectrosc. 2004; 35: 656–661Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jrs.1211

Raman spectroscopic analysis of an English soft-pasteporcelain plaque-mounted table

Howell G. M. Edwards,1∗ Philippe Colomban2 and Bryan Bowden1

1 Chemical and Forensic Sciences, University of Bradford, Bradford BD7 1DP, UK2 Laboratoire de Dynamique, Interactions et Reactivite, UMR 7075 CNRS, Universitaire Pierre et Marie Curie, 2 rue Henri Dunant, 94320 Thiais, France

Received 5 September 2003; Accepted 24 December 2003

Raman spectroscopic studies were made of a porcelain fragment of a unique 18th century table withinlaid porcelain plaques prior to its restoration and compared with the body and glaze of a damaged,marked porcelain plate from the Rockingham factory. The spectra are consistent with the two specimenshaving the same manufacturing source around 1840. The analysis of the body of the plate and the plaquefragment shows that both contain b-wollastonite, feldspar and calcium orthophosphate; in addition, atrace of gypsum was found on the bottom surface of the plaque fragment, indicating that plaster wasused as an adhesive and bedding agent for the plaques on their wooden table-top substrate. The porcelainbodies of the plaque and the plate are classified as English soft-paste porcelain with bone-ash additiveand differences in the spectra can be ascribed to changes in the firing temperature and kiln cycles. Theglaze compositions are also very similar with the identification of borax in each specimen. It is concludedanalytically that the porcelain plaques were derived from the Rockingham factory around 1835–40, whichcan be correlated with historical provenancing. Copyright 2004 John Wiley & Sons, Ltd.

KEYWORDS: micro-Raman spectroscopy; porcelain artefacts; Rockingham; glaze; bone ash

INTRODUCTION

The adoption of micro-Raman spectroscopy as a non-destructive technique for the characterization of ancientceramics has produced a literature extending from Egyptian18th Dynasty (14th century BC) faience,1 Vietnamese proto-type porcelains and celadons (13–16th centuries),2,3 Greekterracotta,4 Islamic (13–17th centuries) faience and pottery,5

mediaeval-type glazed roof tiles (14–17th centuries),6 Roman(2nd–3rd centuries) terracotta paint pots7 and Chineseporcelains.8,9 From such an eclectic and diverse range of syn-thetic ware it has been possible to deduce the salient featuresof the ceramic bodies, glazes and applied pigments, whereappropriate. The analysis of ceramics provides some spe-cific challenges because of their composite nature, in whichthe material microstructures10 consist of sintered grains anddomains ranging from several micrometres up to perhaps500 µm.

Crystalline and glassy phases are found together, alongwith unreacted starting materials; the composition of theceramic body and of the applied glaze, where present,is critically dependent on the processing technologies,

ŁCorrespondence to: Howell G. M. Edwards, Chemical andForensic Sciences, University of Bradford, Bradford BD7 1DP, UK.E-mail: [email protected]

firing sequence and the kiln temperatures. Because thetemperature homogeneity in ancient kilns was poor, theactual placement of the ceramic artefacts in the kiln can giverise to compositional differences at the microscopic level.In early porcelain manufacture, this could be compensatedby the use of a range of compositions before firing; forexample, the Meissen factory in the 18th century had morethan five recorded porcelain body compositions which hadbeen adapted for different kiln levels.

In the 16th and 17th centuries, the native Europeanglazed terracotta, Delft tin-glazed earthenware and Italianmajolica had to compete with imports of hard, translucentporcelains from the Far East. Small, isolated and localEuropean factory production was generally unsuccessfuluntil the rise of the great porcelain manufactories in theearly to middle 18th century, such as those at Sevres,Mennecy, Meissen, Worcester, Derby and Chelsea, enabledthe increasing demand for the new ceramics to be met. Thetwo types of porcelain in use from the mid-18th centuryare termed soft-paste (pate tendre) and hard-paste (pate dure);the latter involved the addition of kaolin to the compositionbefore firing. The development of bone china in Staffordshireshortly before 1800 and its commercial exploitation by JosiahSpode introduced a third type of porcelain body, whichcontained calcined animal bones and provided a phosphatic

Copyright 2004 John Wiley & Sons, Ltd.

Analysis of a soft-paste porcelain plaque-mounted table 657

component to the ‘paste’. Bone china provided a strong,durable, smooth, translucent material of a white appearancein contrast to the often greyish, rather gritty texture of thetrue hard-paste porcelains.

Hence, from the early 19th century, there were severalgeneric types of porcelain in circulation; each factory hadits own, jealously guarded formulation for the porcelainbody and applied glaze, and many experimental pasteswere tried involving various additives. Table 1 shows sometypical chemical analysis data for the three main types ofporcelain discussed above. Although there were variationsin composition between factories, and even during differentperiods in the production cycle, a distinctive chemicaldifferentiation centres on the high calcium phosphatecomponent in bone china (arising from the calcined boneash) and the high alumina and low calcium oxide content ofthe hard-paste porcelain compared with the soft-paste body.

It should be appreciated, however, that the wet chemicalanalyses reported in the literature would have involved thecomplete destruction of the whole or a significant part ofthe artefact;11 the classical work of Eccles and Rackham,12

for example, required a whole cup, saucer or a plate asthe specimen for an analysis. Broken pieces or fragmentedsamples from artefacts are accessible to other methodsof analysis such as scanning electron microscopy–energy-dispersive x-ray analysis (SEM–EDX) and x-ray diffraction(XRD).13 The XRD technique in particular has provideda valuable means for the identification of paste phasessuch as ˇ-wollastonite �CaSiO3� in soft-paste, tricalciumorthophosphate [Ca3�PO4�2] in bone china and mullite(formulated as 2Al2O3ÐSiO2 or as 3Al2O3Ð2SiO2, dependingon the raw materials and the firing temperature) in hard-paste porcelains.

The potential for non-destructive Raman spectroscopicanalysis of porcelains, especially using confocal microscopyto image through the lead oxide, feldspathic or tin oxideglazes on to the porcelain body, is immediately apparent,but relatively little has appeared in the literature to datewith regard to applications for the analysis of 18th and 19th

century European porcelains. Some key papers have beenpublished for 18th and 19th century French and Germanporcelains; in particular, comprehensive studies have beenundertaken for specimens of the porcelain bodies for theSevres manufactory14 and comparisons made with otherFrench factories. The applied pigments have also beencharacterized on these porcelains.15 Hitherto, there has beenno Raman spectroscopic study made of English porcelains toevaluate the possibilities of the technique; this is the topic ofthe present work.

An opportunity has arisen to examine a small fragment ofporcelain from an early 19th century damaged table top froman important artefact prior to restoration being undertaken.The ability to fashion durable shapes which did not distort inthe firing process gave rise to a creative technology in whichdiverse decorative and functional items other than teawareswere manufactured in porcelain; hence large, unusual itemssuch as garden furniture and bedposts were constructed byseveral china and porcelain factories. Not many of these havesurvived. Here, a wooden table from the George II period(ca 1730) had been modified in the early 19th century bythe inclusion of a brass-mounted top composed of porcelainplaques decorated in brilliant colours with motifs of birdsand flowers. During the dismantling process for restorationpurposes, a small piece of one of the porcelain plaques wasfound to be detached and this provided the specimen foranalysis in the present work. The questions posed for theanalysis are :

ž Can the Raman spectra of a porcelain fragment beobtained non-destructively to determine porcelain bodyand perhaps glaze composition?

ž Is it possible to determine the type of porcelain comprisingthe body?

ž Can a comparison with a documented, marked piece fromthe factory to which the manufacture of the unmarkedporcelain plaques is attributed provide analytical supportfor this attribution?

Table 1. Chemical composition of typical soft-paste, hard-paste and bone china porcelains in the early 19th century

Chemical composition/wt%

Sample Ref. Components wt% K2O Na2O CaO Al2O3 SiO2 P2O5 MgO

Soft-paste 16 Glass frit 75 4 2 16 3 75 0 0(Sevres) Chalk 151200 °Ca Marl clay 10Hard-paste 17,18 Kaolin 75 4 0 0 26 70 0 0(Limoges) Feldspar 101400 °Ca Quartz sand 15Bone china 13 Bone ash 50 2 1 26 16 35 19 1(Rockingham) Kaolin 251260 °Ca Feldspar 25

a Firing temperature.

Copyright 2004 John Wiley & Sons, Ltd. J. Raman Spectrosc. 2004; 35: 656–661

658 H. G. M. Edwards, Ph. Colomban and B. Bowden

ž Does the analysis of the pigment provide additionalsupport for the assignment of the factory and/or theperiod of decoration?

EXPERIMENTAL

SpecimensThe tripod table of high quality, made of padouk wood,of height 75 cm and with a hexagonal top ca 75 cm across,was constructed in ca 1730, probably by John Channon; ithas cabriole legs ornately carved with shells and acanthusleaves, parcel gilt and terminating in carved paw feet, andthe stem has alternating flat and concave facets inlaid withbrass stringing. The unique feature of this table, however,is the top, which comprises six radiating porcelain plaqueswith shaped reserves, decorated with flower groups andwith butterflies and insects on a rich bleu de roi ground withelaborate raised gilding (Plate 1). The plaques are securedwithin engraved brass edging and radiating stays from acentral engraved brass boss; the underside has a gilt ball ateach corner.

The provenance gives the origin of the table as theWentworth Castle Collection, which was built in 1730 forThomas Wentworth; in 1739, William Wentworth inheritedthe title of Earl of Strafford. In 1830, the WentworthWoodhouse estate passed to Earl Fitzwilliam, patron ofthe Rockingham Porcelain Manufactory,19 and it is possiblethat the plaques were an early fabrication of this factorywhich were set into the original table in ca 1839 to markthe centenary of the inheritance of the Earldom of Strafford.The artistic style of the decoration of the porcelain plaques isattributed to Thomas Brentnall, who was described by JohnHaslem,20 one of his contemporaries, as an excellent flowerpainter. This view is supported by the commissioning ofBrentnall to undertake the decoration of the most famousporcelain dessert service to leave the Rockingham factory,namely the Royal Rockingham service of King William IV,completed in 1837 after 7 years’ work and used first at theCoronation banquet of Queen Victoria. A small triangularfragment broken off from one of the plaques and measuringca 10 ð 5 ð 8 mm was discovered in 2002 when the table topwas dismantled for cleaning and restoration. The fragmentwas decorated in underglaze blue and a remnant of the oldadhesive was still attached on the lower surface (Plate 2). Thisfragment could not be sacrificed for analysis but it would beimportant to discern if the fragment could be classified asa bone china, phosphatic (English type) porcelain typical ofthe Rockingham factory.

To assist in the analytical interpretation of the spec-troscopic data, since the Rockingham factory retained astandard porcelain body composition satisfactorily through-out its period of production, a damaged porcelain dessertplate with the red griffin passant Rockingham mark for theperiod 1830–42 and with decoration attributed to ThomasBrentnall was also presented for analysis (Plate 2). The

detailed analysis of 12 porcelain items from the Rocking-ham factory by XRF and XRD techniques has been reportedby Cox and Cox13 and the bone ash content was found to beremarkably constant at about 46 wt%, which translates to aP2O5 content of about 19 wt%, as quoted in Table 1 .

Raman spectroscopyMacro-Raman spectroscopy was performed on the glazedand unglazed regions of the specimens (including the sidesof the fractured body fragments). A backscattering micro-configuration was also used to analyse specifically the micro-domains in the porcelain paste bodies and glazes. Radiationat 457.9 nm from an argon–krypton ion laser (Coherent,Santa Clara, CA, USA) and at 413.1 nm from an argon ionlaser (Spectra-Physics, San Jose, CA, USA) were used as exci-tation sources. The shorter wavelength radiation was chosenfor stronger Raman scattering and to provide a large spectralwindow for one-shot accumulations. The power was typi-cally 2–2.5 mW at the sample for the micro-configuration and10 mW for the macro-configuration. Excitation at 647.1 nm(5 mW, micro-configuration) with the argon–krypton laserwas used specifically for the low-wavenumber Raman bands�<200 cm�1� with a Spex instruments back-illuminated, liq-uid nitrogen-cooled charge-coupled device (CCD) matrixdetector (2000 ð 800 pixels). The spectral resolution of theXY system (Longjumeau, France) was ca 2 cm�1. The laserbeam diameters were about 100 and 1.5 µm, respectively,for Raman macro and micro measurements; the latter con-figuration used an Olympus ð100 long-focus objective lens,with a total magnification of 1000ð. Hence the spectrumrecorded with the macro-configuration sampled a surfacearea of ca 0.01 mm2. Many spectra are needed to gain a rep-resentative distribution of phases over the typical specimensurface. Examination of the glaze was accomplished underthe microscope and the precise area of the specimen couldthereby be selected for characterization.

RESULTS AND DISCUSSION

Good quality Raman spectra were obtained from theporcelain bodies of the table-top plaque fragment and fromthe broken segments of the Rockingham plate standard.Typical spectra obtained from several regions of the plaqueand plate fragments are shown in Figs 1 and 2, respectively;these spectra are representative of replicates which wererecorded from different regions of the specimens. The resultscan be summarised as follows.

Porcelain body crystalline phasesThe plate analyses as ˇ-wollastonite, with Raman bandsat 980, 635 and 410 cm�1, feldspar with a Raman band at510 cm�1 (representing incompletely reacted raw material,anorthite) and tricalcium orthophosphate, with a band at960 cm�1.



Figure 1. Raman spectra of porcelain plate. Selectedreplicates of the porcelain body and glaze are shown asindicated; from the top: body, body, glaze, glaze, glaze.

The table-top plaque fragment analyses as a similarcomposition to that of the plate, but containing more quartz,evidenced by the Raman band at 465 cm�1, and tricalciumorthophosphate, with a band at 960 cm�1. It could besuggested that the presence of a higher proportion of quartzin this specimen reflects an attempt to give a harder porcelainbody required for the denser potting of the thicker plaque onthe table-top. On the bottom surface of the plaque specimena spectrum of gypsum is identified (an example is shownin Fig. 2, the spectrum second from the top), which wasprobably used as a plaster adhesive to secure the porcelainplaques to the table-top.

It can therefore be concluded that both porcelain bodiesare classified as English-type soft-paste bone china, but thatthe firing temperature cycles were probably different; thislatter point is not unexpected given the significantly differentpotting thicknesses of the two specimens.

Glaze analysisLead glazes were invariably used on English soft-pasteporcelains and bone china from the late 18th into the mid-19th

Figure 2. Raman spectra of the porcelain plaque fragment.Selected replicates of the body and glaze are shown asindicated; from the top: body, lower surface of section,intermediate, intermediate, intermediate, glaze on top surface.

century. The following data from Gilhespy21 give a typicalcomposition (in wt%) for lead glazes used around 1837 :

ž Frit for the glaze comprising china stone (44%), borax(28%), soda (14%), dry flints (3%) and kaolin (11%).

ž After firing of the above frit at temperatures of between1000 and 1100 °C, the resulting glaze components werethen mixed as follows: frit as above (61%), dry flints (12%),white lead (13%) and pearl ash (12%).

The use of lead as an ingredient constituted a serioushealth hazard to factory workers and to those who residedin the vicinity of porcelain manufactories. In 1820, John Roseof the Coalport China Works was awarded a prize for hisleadless glaze, which involved the substitution of sand andfeldspar for lead white and china stone in the above recipe.However, the so-called feldspathic glaze was consideredinferior to lead glazes in actual practice and its use wasnot generally adopted.21 We would expect, therefore, that


660 H. G. M. Edwards, Ph. Colomban and B. Bowden

the glaze used on the porcelain specimens submitted foranalysis here would very likely be a lead glaze.

Figures 1 and 2 also contain representative spectra ofthe glazes from the table-top plaque fragment and fromthe specimen Rockingham plate. It can be concluded thatboth specimens contain the same type of calcium-rich glaze,with the same proportion of ˇ-wollastonite from the Ramanband at 980 cm�1, and a feature at 1450 cm�1 is indicativeof a borax content in both samples. The table-top plaquespecimen has bands at 460 (vs), 800 (w), 995 (s), 1145 (m)and 1450 (w) cm�1. The plate has bands at 440–480 (vs), 695(vw), 790 (w), 995 (s), 1050 (w), 1145 (m) and 1450 (w) cm�1.In the former spectrum, the ratio of the band intensities at500 and 1000 cm�1, A500/A1000, ascribed to Si–O bending andstretching, respectively, is indicative of the firing temperatureto which the artefact had been subjected. The ratio A500/A1000

in the former case is 1.4 š 0.1 and in the latter 1.5 š 0.2; thesimilarity of this ratio shows that the two specimens haveglazes which are not only of the same composition but alsoderived from a similar kiln manufacturing process which hasa firing temperature of an estimated 1100 °C, correspondingto an A500/A1000 ratio of 1.4.10

The rather low A500/A1000 ratios (ca 1.4) found here areindicative of a low lead content for the glazes, which is

Figure 3. Raman spectra of the enamel colours on thepolychrome porcelain plate. From the top: red, dark green,yellow, emerald green, emerald green.

significantly smaller than the corresponding values derivedfrom specimens of French soft-paste porcelain glazes.22 Theuse of borax is consistent with this smaller lead content andthe relatively low intensity of the 1450 cm�1 Raman band inthe porcelain plaque glaze indicates a lower borax contentcompared with the porcelain plate.

Figure 3 gives the Raman spectra of the enamel pigmentson the polychrome porcelain plate. Here, the red pigmentis identified as haematite, the yellow and green pigmentscontain the pyrochlore phase of Naples yellow, PbSb2O7,with bands at 140 and 510 cm�1. Other phases are present inthe green colour, particularly in the execution of the greenleaf painting, where the spectrum is very closely similarto the material found in Sevres porcelain and classified aspigment 16 in an earlier study.15

The fragment from the plaque is of a monochromeblue colour, although the plaques in their entirety are verybeautifully decorated in a polychrome palette; unfortunately,for comparison purposes, other plaque specimens were notavailable for pigment analysis. The Raman spectrum of theblue pigment on the plaque fragment (Fig. 4) is typical of anearly 19th century soft-paste blue pigment. The exact nature

Figure 4. Raman spectrum of the blue enamel from thedetached fragment of the porcelain plaque.


Analysis of a soft-paste porcelain plaque-mounted table

Plate 1. Padouk wood tripod table, ca 1730, with six inlaid triangular polychrome porcelain plaques forming a hexagonal top. Duringrestoration and cleaning, a small fragment of one of the porcelain plaques was discovered to have become detached and thisprovided the specimen for non-destructive Raman spectroscopic analysis in this work.

(a)

(c)

(b)

(d)

Plate 2. (a) Porcelain dessert plate from the Rockingham factory with (b) underglaze red griffin mark designating the period ofmanufacture 1830–42. The decoration is attributed to Thomas Brentnall. The fragment, decorated in underglaze blue, detachedfrom the porcelain plaque of the table top shown in Plate 1 is seen here (c) in profile and (d) in section.

Copyright 2004 John Wiley & Sons, Ltd. J. Raman Spectrosc. 2004; 35


of the mineral phase cannot be determined; it is not theusual cobalt blue but has similar characteristics to uvarovitegarnet, a calcium- and chromium-based silicate.

CONCLUSIONS

The non-destructive Raman spectroscopic analysis of afragment of a unique and important porcelain table-topplaque has been identified as an English soft-paste/bonechina-type body with a lead glaze typical of a low leadand borax glaze and is consistent with an attribution to theRockingham porcelain factory on the basis of comparisonwith a marked porcelain plate of the 1830s. The glaze micro-composition is also indicative of similar processing in termsof the firing temperature and composition.

REFERENCES1. Clark RJH, Gibbs PJ. J. Raman Spectrosc. 1997; 28: 99.2. Liem NQ, Sajon G, Quang VX, Tan HV, Colomban Ph. J. Raman

Spectrosc. 2000; 31: 933.3. Liem NQ, Than NT, Colomban Ph. J. Raman Spectrosc. 2002; 33:

287.4. Popelka R. Senior Thesis, Washington University in St. Louis, St.

Louis, MO, 1999.

5. Colomban Ph, Sajon G, Louhichi A, Binous H, Ayed N. Rev.Archaeom. 2001; 25.

6. Edwards HGM, Tait JKF. J. Raman Spectrosc. 1999; 30: 429.7. Edwards HGM, Hassan N, Middleton PM, to be published.8. Zuo J, Xu C, Wang C, Yushi Z. J. Raman Spectrosc. 1999; 30: 1053.9. Zuo J, Wang C, Xu C. Spectrosc. Lett. 1998; 31: 1431.

10. Colomban Ph. In Raman Spectroscopy in Archaeology and ArtHistory, Edwards HGM, Chalmers JM (eds). Royal Society ofChemistry: Cambridge, in press.

11. Jewitt Ll. The Ceramic Art of Great Britain (new edn), Minet PPB(ed). Redwood Press: London, 1971.

12. Eccles H, Rackham B. Analysed Specimens of English Porcelain.Victoria and Albert Museum: London, 1922.

13. Cox A, Cox A. Rockingham Porcelain. Antique Collectors Club:Woodbridge, 2001.

14. Colomban Ph, Treppoz F. J. Raman Spectrosc. 2001; 32: 93.15. Colomban Ph, Sajon G, Faurel X. J. Raman Spectrosc. 2001; 32:

351.16. d’Albis A. Faenza Annata LXXII 1987; 1–3: 40.17. Treppoz F. Rev. Soc. Amis Mus. Nat. Ceram. 1997; 6: 68.18. Iqbal Y, Lee WE. J. Am. Chem. Soc. 1999; 82: 3584.19. Eaglestone AA, Lockett TA. The Rockingham Pottery (rev. edn).

Newton Abbot Press: Newton Abbot, 1973.20. Haslem J. The Old Derby China Factory. EP Publishing: Wakefield,

1973.21. Gilhespy FB. Crown Derby Porcelain. Leigh-on Sea, 1951.22. Ph, Colomban. J. Non-Cryst. Solids 2003; 323: 180.


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Raman spectroscopic analysis of an English soft-paste porcelain plaque-mounted table