Mass-Produced Mullite Crucibles in Medieval Europe: Manufacture andMaterial Properties

Marcos Martinon-Torres,w,z Ian C. Freestone,y Alice Hunt,z and Thilo Rehrenz

zInstitute of Archaeology, University College London, London WC1H 0PY, UK

ySchool of History and Archaeology, Cardiff University, Cardiff CF10 3EU, UK

Crucibles from the German region of Hesse have been famoussince the Middle Ages due to their exceptional quality, regardedby many as a mystery. We analyzed 50 Hessian and non-Hes-sian archeological crucibles using SEM-EDS, FESEM, andXRD to investigate their technology and material properties. Itwas revealed that Hessian crucibles were systematically made ofkaolinitic clay with a low flux content, mixed with quartz sand,and fired to temperatures in excess of 13001C. Primary mullitedeveloped in most of the glass matrix, with secondary mullite insome regions of clay–feldspar relict mixtures. Consequently, thevessels showed superior creep and thermal shock resistance,high-temperature strength, and thermal and chemical refractor-iness. These crucibles represent the earliest industrial exploita-tion of mullite in Europe, which explains their historical success.

I. Introduction

CRUCIBLES are ceramic vessels used for dry, i.e. high temper-ature, reactions. Since the late Middle Ages, the crucibles

manufactured in the German villages of Epterode and Almerode(current GroXalmerode, in the Hesse region) have beenrenowned among assayers, alchemists, chemists, metallurgists,jewellers, and coin minters.1,2 The crucible-making recipe was aclosely guarded secret, and several potters failed at attempting toreplicate them3—which led some to speak of ‘‘the mystery ofHessian wares.’’4 Crucibles were mass produced in Hesse sincethe 12th century,2 and recent archeological and analytical workhas identified Hessian crucibles in a wide geographic region,reaching Norway, Britain, Portugal, and even Jamestown in theAmerican colony of Virginia, and ranging from the 15th to the19th century.5–7

In a previous paper we suggested that the main secret behindthe superior quality of these vessels was the development ofmullite during ceramic manufacture.8 Here we report furtherwork on the technology, microstructure, and material propertiesof these crucibles.

II. Experimental Procedure

We analyzed ca. 50 Hessian and non-Hessian crucibles from 10archeological sites, both used and unused, prepared as cross sec-tions in epoxy resin and polished to a 1 mm finish with diamondpastes after initial grinding with SiC and water. Most specimenswere first examined under a Leica DM LM reflected light optical

microscope (Wetzlar, Germany) and subsequently carbon coatedfor analyses on a scanning electron microscope with an energy-dispersive spectrometer (SEM-EDS). A few specimens were etchedby dipping for 10–25 min in 5%–12%HF solutions, then neutral-ized in acetone, and washed in industrial methylated spirit. Thesewere subsequently gold coated to optimize electron imaging.

Carbon-coated specimens were analyzed using a PhilipsXL30 SEM-EDS (FEI, Eindhoven, the Netherlands) with anINCAOxford spectrometer package (Oxford, UK), operating at20 kV and a working distance of 10 mm. Compositional ana-lyses of ceramic matrices were obtained through EDS measure-ments of areas of B70–100 by 100–150 mm, with 5–10 analysesper crucible section. These were performed with spot sizes of4.7–5.3 (INCA conventional units) and process time 5, corre-sponding to a detector dead time of 25%–40%, and an acqui-sition time of 75 s. Results are reported as normalized weightpercentages. Etched and gold-coated specimens were examinedusing a field emission SEM JEOL JSM-6301F (Akishima,Japan) operating at 6 kV and 15 mm.

For XRD analysis, a small portion of sample was crushed inan agate pestle and mortar until fragments passed a 72-meshBritish Standard sieve. The powder was put into an aluminumsample holder using the back-packing method to avoid sampleorientation. Data were obtained using a Philips 1720 diffracto-meter (FEI, Eindhoven, the Netherlands) fitted with a curvedgraphite crystal monochromator using CuKa radiation, 40 kV,30 mA, a 0.5 mm divergent slit, a 0.2 mm receiving and 0.5 mmscatter slit, ratemeter 5� 102 cps at time constant 5. Scan rangewas from 21 to 601 2y at 0.51 2y per minute. The data wereprocessed and interpreted using the Philips PC-APD softwareversion 1.6.

III. Results and Discussion

All the unused Hessian crucibles analyzed have very similarcharacteristics, which indicates that raw materials and manu-facturing techniques were kept standardized for centuries. Theceramic matrices are composed of a relatively pure kaoliniticclay, with a mean Al2O3 content of 39.6% and very low levels ofalkali and earth alkali oxides, their sum being just about 2%(Table I).

The most typical nonplastic addition consists of 20–40 vol%subrounded or spheroidal quartz sand grains, moderately wellsorted in the medium to coarse sand range (+0.25–1 mm)(Fig. 1). These grains appear internally cracked and show dis-solution interfaces with the surrounding ceramic, even in unusedvessels, which indicates a firing temperature above 12001C.9 Afew (o5 vol%) smaller inclusions were identified, namelymonazite, humboldtine, rutile, and some concentric ferruginousconcretions—all of which are known to occur in Hesse. Somerelict structures of molten potassium feldspars were also noted.Overall, the scarcity of mineral inclusions other than the quartzsand suggests that the clays were levigated, i.e. size sorted bysettling in flowing water.

W. Lee—contributing editor

Financial support for M. Martinon-Torres was provided by the Arts and HumanitiesResearch Council (UK) and Caixanova (Spain).

wAuthor to whom correspondence should be addressed. e-mail: m.martinon-torres@ucl.ac.uk

Manuscript No. 24118. Received December 18, 2007; approved February 7, 2008.

Journal

J. Am. Ceram. Soc., ]] []]] 1–4 (2008)

DOI: 10.1111/j.1551-2916.2008.02383.x

r 2008 The American Ceramic Society

1

Porosity is about 20 vol% and manifest in three forms: (a)subrounded pores around quartz grains; (b) long, subangularpores parallel to the wall surfaces; and (c) fine vitrificationporosity (Figs. 1–2). The first type is because clay plateletstend to be aligned with their plane faces tangential to the sur-face of the nonplastic particles, and the cavities are then en-larged due to the high expansion/contraction coefficient ofquartz grains during lattice inversion upon firing and cooling.10

The long pores are due to the shrinkage of the ceramic matrixduring firing, and their alignment is a reflection of the overallclay orientation during the manufacture of the crucibles on arotating potter’s wheel. The closed microporosity in the ceramicmatrix results from the development of glass during firing, withthe subsequent filling of small pores and gas diffusion intogrowing, larger ones.10–11

XRD of a powdered sample from an unused crucible showedthe presence of mullite, cristobalite,z quartz, and hematite. Thepresence of hematite is consistent with the generally reddish col-or of these fabrics, in spite of the relatively low FeO concentra-tions (B2%), and suggests that the crucibles were fired in anoxidizing atmosphere. It should be noted, however, that a recentstudy3 has identified metallic iron within the vitrified matrix of aHessian crucible.

Most important here is the presence of a mullite phase, mostlikely crystallized following the decomposition of kaolinite un-der high temperatures, and which we interpret as the main secretbehind the superior material properties of the vessels. The XRDresults are consistent with the SEM observations on etchedspecimens of unused crucibles. The vitrified groundmassappears composed of cuboid, primary mullite. In areas of high-er flux content caused by molten feldspars, a network of inter-locking needles (r120 mm) of secondary mullite is present (Figs.3–5). As noted by Iqbal and Lee12,13 and Lee and Iqbal14 in theirstudy of porcelains, primary mullite crystals formed in the pure

clay agglomerates have the lowest aspect ratio because of thehigh viscosity of the matrix. Conversely, secondary mullite crys-tals can grow unhindered in the low-viscosity clay–feldspar relictmixtures. The needles of secondary mullite can be clearly seen tooriginate from the surface of the clay and grow into the lessviscous feldspar relict (Fig. 6), in agreement with previousobservations of the possible transformation of primary into sec-ondary mullite.12–14 In addition, the substantial growth of thesecrystals is consistent with its firing in an oxidizing atmosphere.15

The presence of well-developed mullite indicates firing temper-atures in excess of 12001C, while the limited presence of bubblesand the lack of mullite dissolution suggest that a temperature of14001C was not exceeded during firing.12 Thus we estimate thatthe firing of Hessian crucibles involved sustained temperaturesin the 13001–14001C range, in agreement with a recent, inde-pendent estimate.3

IV. Material Properties and Performance

The characterization of these crucibles allows inferences abouttheir material properties and performance. The use of levigatedclays would minimize the risks of crucible failure during firing oruse due to the presence of mineral inclusions of unpredictablethermal behavior. The abundant and relatively coarse quartzgrains would be beneficial in two ways: firstly, the presence ofthis refractory filler would prevent the body from shrinking ordistorting excessively during firing; secondly, it would increasethe toughness and thermal shock resistance of the fired vessels. Ithas been suggested that the cracks and pores induced by thepresence of quartz grains in vitrified stonewares may be theirmain structural flaw.16 However, experimental studies of coarse-tempered ceramics have demonstrated that this network ofmicrocracks and hard inclusions help arrest and dissipate frac-ture lines caused by mechanical or thermal stresses, requiringmore energy for them to propagate catastrophically through theceramic body.17,18 High toughness would be an asset for han-dling and long-distance transportation of the crucibles. Thermal

Fig. 1. Backscattered electron image of the fabric of an unused Hessiancrucible, showing cracked quartz grains with dissolution interfaces in avitrified matrix. Note presence of porosity (black) around the quartzgrains and as elongated voids in the glass.

Fig. 2. Secondary electron image of an unetched, unused Hessian cru-cible, showing fine vitrification porosity in a matrix of primary mullite.

Table I. Chemical Composition (wt%) of Unused Hessian Crucible Matrices

Na2O MgO Al2O3 SiO2 P2O5 K2O CaO TiO2 FeO

Mean (n5 5) 0.1 0.5 36.9 56.8 0.2 1.3 0.3 1.9 2.0Standard deviation 0.05 0.09 0.39 0.22 0.11 0.13 0.05 0.08 0.18Maximum 0.2 0.7 37.5 57.1 0.4 1.5 0.3 2 2.2Minimum 0.1 0.5 36.5 56.5 0.1 1.2 0.2 1.8 1.7

zThe identification of cristobalite is based on a reassessment of a previously publishedXRD spectrum.8

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shock resistance would be of utmost importance, as these vesselswere repeatedly moved in and out of furnaces operating at10001C and above.

We believe that the main factor leading to superior materialproperties was the development of mullite through high-tem-perature firing. Mullite is deliberately developed in a wide rangeof modern ceramics, including building materials, refractories,optical materials, and ceramic matrix composites.19 All of theseapplications exploit relevant properties of mullite such as lowthermal expansion—and the associated excellent thermal shockresistance—high creep resistance, high-temperature strength,and an outstanding stability under aggressive chemical environ-ments.19,20

Because of the scarcity of feldspar grains in the crucible fab-rics, high aspect ratio secondary mullite is only present in certainareas, whereas the bulk of the matrix is dominated by primarymullite. A larger concentration of interlocking secondary mullitecrystals might have conferred these vessels with even higherstrength and toughness. However, more flux would have beennecessary for this, for example through the addition of groundfeldspars as in porcelain technology.14 This would have resultedin a lower bulk Al2O3 concentration in the matrix, with thesubsequent decrease in thermal refractoriness—and thus a lim-itation for the performance of the crucibles. Overall, it appearsthat the raw materials and manufacturing techniques employedwere optimum for the required material properties of the cru-cibles, which would be used under extreme mechanical, thermal,

and chemical stresses. This may explain why the recipe was keptsecret and uniform for centuries, even if crucible makers wouldnot be aware of mullite as such. The exceptionality of Hessiancrucibles is better understood with reference to earlier and con-temporary ceramics. Before the 16th century, most cruciblesmanufactured across the world were made of clays with lowerAl2O3 concentrations than the Hessian ones and fired to lowertemperatures.21 From the Renaissance, developments in alche-my and fire assay led to the systematic conduction of chemicalreactions under increasingly high temperatures, using new acidsand fluxes. Only standardized, mass-produced mullite crucibleswere capable of withstanding these conditions, hence allowingfor experimental replication. Thus, the changing demand mayhave encouraged the changes in ceramic technology that ledHessian potters to produce this earliest industrial exploitation ofmullite ceramics in Europe. Their only competitors would be thegraphite crucibles manufactured in Bavaria.5–7

Previously, mullite has been identified in postmedieval cruci-bles used for melting glass22 and manufacturing wootz steel.23

However, in both cases, mullite developed during the high-tem-perature utilization of the vessels, rather than the first firing inthe potter’s kiln. The prefiring temperature documented in theHessian crucibles is exceptional for European premodernceramics. The only other ware produced in Europe at temper-atures above 11501C was German salt-glazed stoneware,24 andthe possible link between both technologies deserves furtherresearch. Another aspect warranting investigation is the possiblerelationship between the manufacture of Hessian crucibles andthe discovery of porcelain in Europe: although the earliest Eu-

Fig. 4. Secondary electron image of another etched specimen, showingwell-developed secondary mullite in a clay-feldspar relict area, sur-rounded by a matrix of primary mullite.

Fig. 5. Detail of the highly acicular prismatic needles of secondarymullite in a flux-rich area.

Fig. 6. Secondary electron image showing the needles of acicular mul-lite growing out of a matrix of primary mullite.

Fig. 3. Secondary electron images of etched specimens, showing anarea of highly acicular secondary mullite formed in a clay–feldspar relictmixture. The surrounding matrix is composed of primary mullite.

2008 Communications of the American Ceramic Society 3

ropean porcelain was based on the use of a lime flux, rather thanfeldspar, and the developments in porcelain firing were based onexperiments with burning lenses, it is interesting to note thatJ. F. Bottger, the discoverer of porcelain, worked as an assistantto an alchemist25 who could have utilized Hessian crucibles.

V. Conclusions

Potters in the Hesse region were mass producing mullite cruci-bles several centuries before mullite was identified as a mineralphase. Mullite was synthesized by firing quartz-tempered kaoli-nite to temperatures above 13001C, with primary mullite devel-oping in most of the glass and highly acicular secondary mullitegrowing in regions fluxed by feldspars. The resulting cruciblesshowed superior material properties, including high-tempera-ture strength, creep and thermal shock resistance, and thermaland chemical refractoriness. This explains their outstanding suc-cess in the international market from at least 1500.

Acknowledgments

We are very grateful to H.-G. Stephan, who provided the reference samples ofHessian crucibles, and to many other archeologists who facilitated our access tofurther samples from across the world. We are also indebted to Kevin Reeves andPhilip Connolly for technical assistance.

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