Terra sigillata is a vitrified red-brown half-glossy clay coating found on antique pottery of Roman andGreek origin.1 These 5–15 µm thick coatings consist of extremely fine clays that have been made as a slip,then applied to a ceramic body and fired. Unlike an engobe or a glaze, these coatings are so thin that theshrinkages of the coatings and the substrate body do not need to be closely matched.2

The materials that make up the coating usually consist of fine flake-structured clays, i.e., illite, kaolin ormica.3 Terra sigillata can be prepared from a fine-clay suspended fraction. The clay is deflocculated, dispersedand allowed to settle long enough so that even 1 µm particles settle. The suspended fraction then is used forcoating.

Fine-Clay Suspended Fraction

Clay material (20%), distilled water (80%) and sodium silicate (0.15%) were dispersed using a high-ener-gy mixer for 6 h.4 The suspension was transferred to a settling container and allowed to settle overnight. The

Terra Sigillata from ResidualKaolin-Based Slip

Oliver Muñiz-Serrato and Juan Serrato-Rodríguez

Terra sigillata slip prepared from a kaolin-based clay

suspension has reflectance properties similar to coatings

found on antique Roman pottery.

Terra sigillata red to orange bowl coatings fired at 1100°C.

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suspended portion was removed by siphon. Its specific gravitywas adjusted to ~1.2 before it was used to coat green and bisqueware. The coating was applied by dipping and brushing to athickness of 4–25 µm. The ware then was fired within the tem-perature range 1000–1200°C. The suspension was used to coatbowls.

Kaolin as Terra Sigillata Slip Precursor

X-ray diffractometry showed that ordered and disorderedkaolinite was present in the slip. Gibbsite, hematite and cristo-balite also were present, but in lesser amounts. Electroacousticmeasurements of particle size showed that half of the fine-claysuspendend fraction was finer than 240 nm. Scanning electronmicroscopy showed that the platy structure of fine kaolinite par-ticles formed large aggregates that were constituted of the fine kaolin fraction.

Kaolinite particles 15 nm in size were examined using high-resolution transmission electron microscopy.Some particles were purposely made amorphous by briefly condensing the electron beam on them for a fewseconds. This demonstrated them to be sensitive to the energy of the electron beam. Highly disorderedkaolinite particles showed d-spacings of 2.88 as compared with 2.55 of well-crystallized kaolinite for the(130) plane and 4.86 compared to 4.41 for the (110) plane. It seemed that such laminar disordered nanome-ter kaolinitic fraction, which is sensitive to heat, first became amorphous and then vitreous when sinteredat low temperatures to produce a rather peculiar shine.

Terra Sigillata Coating Reflectance

The spectrum of reflectance for the experimental sigillata coating has been compared with that report-ed by Sciau et al.5 for terra sigillata from archeological sites in southern France. The reflectance behavior iswhat might be expected from opaque and brilliant hematite crystals. At wavelengths >550 nm reflectanceincreases because iron absorbs less in that particular spectrum range.

Light dispersed by the coating may be partially absorbed by the iron atoms in octahedral coordination inthe hematite structure. This produces the attractive sigillata red color. The hematite pigment strongly influ-ences the reflectance behavior of the coating. Microstructural details of the present-time local terra sigilla-ta are considerably different from that of La Graufesenque archeological sites in southern France. However,the reflectance behaviors are similar.

Acknowledgment

The Coordinación de la Investigación Científica de la Universidad Michoacana de San Nicolás de Hidalgo is greatful-ly acknowledged for its financial support.

About the Authors

Juan Serrato-Rodríguez is a faculty member and Oliver Muñíz Serrato is a postgraduate student at the Departamento deCerámica, Instituto de Investigaciones, Metalúrgicas Universidad Michoacana de San Nicolás de Hidalgo, MoreliaMich, Mexico.

Terra Sigillata La Graufesenque (nrMillau, Aveyron/FR) from theAugustan period. Image taken fromAtlas of Roman Pottery.1

Schematic diagram for preparation of terra sigillata dispersed suspensions.

American Ceramic Society Bulletin, Vol. 85, No. 8 9402

Kaolin-Based Slip

American Ceramic Society Bulletin, Vol. 85, No. 89403

References

1Atlas of Roman Pottery; http://www.potsherd.uklinux.net/atlas/Ware/SGTS.2C.W. Parmelee, Ceramic Glazes, 3rd ed. CBI Publishing, 1973.3A.G. Verduch, “La Textura Superficial de las Piezas de Alfarería,” Bull. Soc. Esp. Ceram., 5 [3] 389–408.4O.M. Serrato, J.S. Rodríguez, M.O. Gutierrez and J.F. Pérez Robles, “Clay-Derived Mullite–Glass Thin Films,” Am.Ceram. Soc. Bull., 85 [1] 9101–9103 (2006).5P. Sciau, S. Relaix, C. Roucau and Y. Kihn, “Microstructural and Microchemical Characterization of Roman PeriodTerra Sigillate Slips from Archeological Sites in Southern France,” J. Am. Ceram. Soc., 89 [3] 1053–58 (2006).

2θ (deg)

XRD study that shows ordered (K) and disordered kaolinite (Ha). Gibbsite (G), hematite (H) and cristobalite (C) also arepresent.

SEM photograph illustrates fine kaolinite particles as part of large aggregates.

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HRTEM shows three nanometric kaolinite particles on a carbon grid. Particle on the leftwas made amorphous by condensing the electron beam on it for a few seconds.

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Diameter (µm)

Electroacoustic measurement of fine-clay suspended fractionparticle-size distribution. Half the amount of the clay is smallerthan 240 nm.

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Wavelength (nm)

Reflectance spectrum of the experimental Mexican clay slip compared with that of Sciau et al.5

for terra sigillata from archeological sites in southern France.