Tension Cracks. Tension cracks usually form at a horizontal distance of 0.5 to 0.75 times the depth of the trench, measured from the top of the vertical face of the trench. Type A Soils are cohesive soils with an unconfined compressive strength of 1.5 tons per square foot (tsf) (144 kPa) or greater. Examples of Type A cohesive soils are.
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Objects Specialty Group Conservation Wiki Contributors: Kari Dodson, Emily Hamilton, Julie Unruh Your name could be here! Please contribute. Copyright: 2012. The Objects Group Wiki pages are a publication of the Objects Specialty Group of the American Institute for Conservation of Historic and Artistic Works. The Objects Group Wiki pages are published for the members of the Objects Specialty Group. Publication does not endorse or recommend any treatments, methods, or techniques described herein.
Sao Earthenware Pot (2008.128), Worcester Art Museum, Worcester, Massachusetts, Austin S. Garver Fund
Ceramics are objects made from clay (or clay mixed with other materials) that are subjected to high heat. This application of heat causes irreversible changes in within the clay body, rendering the form permanent. Ceramics can take the form of pottery (utilitarian vessels), sculpture, casting cores for bronzes, or architectural elements such as tiles, pipes and bricks.
Generally speaking, ceramics are brittle and weak under shearing or tensile stresses but very strong under compression. In addition, they can withstand very high temperatures and are not prone to chemical erosion. They consist of both crystalline and amorphous phases, though the ratio of these phases is determined by the specific mineral composition.
Materials and technologyHistory
Ceramic technology in its simplest form has been around since the Neolithic period. There are a host of regional differences in available materials, forms, style and technology that have developed in different parts of the world, making the ceramic arts highly variable across time and space.
MaterialsClay
Kaolin, a primary clay
The primary component of the ceramic body is clay, though it is seldom free of other naturally present or intentionally added materials in smaller quantities (see Tempers and Fluxes below). Clay (the hydrous silicate of aluminum) is the decomposition product of sedimentary mineral deposits, and is characterized by microscopic, plate- like particles with a large surface area to volume ratio. The size and shape of the particles, which varies from deposit to deposit, are the keys to this material’s utility. When just the right amount of water is added, the plate-like particles easily slide past one another, making clay easy to manipulate into almost any shape. After this “free” water (as distinguished from water that is chemically bound to the clay minerals) is again lost to evaporation, the resultant form is hard enough to handle without deforming and can be finished and fired to permanently set that shape.
Tempers
Tempers are materials added (or inherent) to the clay that modify its working properties and the characteristics of the fired ware. Mineral tempers (such as sand) reduce the amount a ceramic shrinks during the firing process and will therefore prevent cracking and warping in the finished product. Grog, or previously fired and ground ceramic material, is often used to this end in high-fired ceramics. Organic tempers (for example, grasses or dung) are added to create porosity in low-fired ceramics. When the ceramic is fired, the organic material burns out and leaves small pockets through which gasses can escape, preventing explosions in the kiln.
Fluxes
Fluxes are alkaline additives (or components inherent to the clay) that serve to reduce the melting point of the silica content of clay, glaze or overglaze pigments. Fluxes include iron oxides and calcia (from naturally occurring impurities in the clay or ground shell, for example) for low temperature firings, and potash, soda, and magnesia (often contributed by feldspars) for high temperature firings.
Types of ceramic wares
Ceramics are often categorized based on the characteristics of the clay from which they were formed, the temperature at which they were fired, and the resultant structure of the ceramic body.
TechnologyPreparation of the clay
After a clay is mined, it will likely require further processing to refine its qualities. Impurities can be removed by filtration or through levigation, a process whereby particles of different sizes are separated in a series of water baths. Afterwards, excess water can be allowed to evaporate and the clay can be left to age. This process increases a clay’s plasticity by allowing the particles to settle into alignment with one another. Before creating a ceramic object, the clay mass must also be compacted and the moisture within it evenly distributed. This is accomplished by wedging or pugging, done by repeatedly slamming the clay against a surface to eliminate any trapped air.
Creation of the structure
When clay is wet and plastic, there are any number of ways it can be shaped into the desired final form. Hands are the most important tools but some techniques also employ paddles, rollers, cutting wires, forms, molds, pottery wheels, etc.
Handbuilding describes several different vessel construction techniques – basically, those that do not employ a pottery wheel. Vessel walls can be gradually raised from a ball of clay by pinching between the fingers, or they can be built up with a rolled length of clay coiling upwards from the base (coil-built). In slab construction, vessels are formed with rolled or cut slabs assembled with slip or wet clay. Any seams resulting from the latter two techniques are usually smoothed out after the basic structure is complete.
The use of molds and forms enable the rapid creation of a series of nearly identical wares. By incorporating decorative patterns or motifs into the mold or form, a potter can simultaneously shape and decorate a vessel. Flat slabs of clay can be paddled over a form or pressed into a mold. The clay can also be made fluid with the addition of water and poured into a mold.
Another, later development in potting technology involves the use of a pottery wheel – a variously powered spinning platform on which a vessel can be raised. Wheel-shaped or thrown pots can be made much more perfectly round than handbuilt vessels.
More modern techniques include extrusion, powder pressing and lathe turning.
The Johns Hopkins Archaeological Museum blog on Recreating Ancient Greek Ceramics contains extensive information on ceramic technology.
DecorationStructural
After the basic structure of a vessel has been established, the clay is allowed to dry to a leather hard state. It is at this stage that further refinements and embellishments can be made. Trimming, or fettling is done to smooth the form. The surface can be incised or impressed with designs or textures, or can be burnished to a high sheen. Additional elements such as handles, spouts or feet can be separately formed (either by hand or with the aid of molds) and attached, or luted, to the body of the vessel with clay slip. When relief decorations are molded and applied in this way it is called sprigging.
The color of clay can also be utilized for decorative effect. For example, clays with differing mineral content (and, therefore, different colors) can be layered or wedged together until they are marbled together. Beautiful patterns can be exposed by selectively cutting away layers or cross-sections. This technique has several different names: millefiore, agateware or neriagi.
Surface
Decorative slips, or fluid slurries of clay, can be applied in a multitude of ways to add color to a vessel. The slip can cover the entire pot or can be locally applied to the surface with brushes, quills, combs or other tools. For example, the Ancient Greeks employed slips painted onto the surface and fired in carefully controlled environments to create the sharp and highly contrasting designs found on red-figure and black-figure vessels. Slip can also be inlaid into recesses prepared in a clay body. In another technique called sgraffito, a contrasting colored slip is applied overall and when dried, is carved away to expose the clay of the vessel body.
Glazes are vitreous or semi-vitreous coverings on ceramics. Glazes may have multiple functions, including waterproofing a porous ceramic and creating a decorative layer that contributes color or gloss to a ceramic body.
Glaze colorants
Glaze application (ie. dipping, pouring, grounding, hand painting, transfer decoration, spraying)
Underglaze decoration: The decoration and the overall glaze are usually applied to a sintered ceramic in the same step, requiring only two firings of the ceramic. The limitation of underglaze decoration is that as firing temperatures increase, few glaze colorants retain their color. At firing temperatures for porcelain, only cobalt blue and copper red function as underglaze colorants (usually fired between 1200 and 1400°C). Overglaze decoration: Overglaze is applied over a base glaze, and may be fired in one or several separate firings depending on what is appropriate for a particular color. This allows for a full range of colors, even on high-fired wares. Bole Cold painting with oil or lacquer based colors Gilding Firing
Oxidizing and reducing atmospheres, biscuit and glost firings, temperatures, types and features of kilns
Identification
Thin-section microstructure of unetched 99.9% Al2O3 ceramic in polarized, transmitted light. A full-wavelength rectifier provided the color contrast.
While a simple visual examination, or examination under a binocular microscope, can provide a great deal of information about an object, there are several analytical techniques that can shed further light on ceramics. When used in conjunction with relevant art historical information, information gleaned from these techniques can indicate the ceramic fabrication method and process conditions, aid in authentication, and may even contribute provenance by correlating objects to specific geological deposits. Further, conservators use these techniques to determine the type and degree of deterioration and to predict or evaluate the efficacy of different treatments.
Thin-section petrography uses a polarized light microscope to identify the mineral content of a 30 μm thick, epoxy-mounted and polished ceramic thin-section based on several optical properties. These include the size, shape, color, pleochroism, refractive index, optical symmetry and cleavage characteristics of individual grains, as well as the abundance of and relationships between grains. Stains and dyed epoxies can be employed to highlight particular minerals and to more easily assess porosity in a ceramic structure (Reedy 2008). Neutron Activation Analysis (NAA) also requires a small (~50 mg) sample which is powdered and irradiated. The artificial radioisotopes that result are detected, giving both qualitative and quantitative measures of major, minor and trace elements. Using this chemical fingerprint, ceramics can be related to known geological deposits. Thermoluminescence (TL) dating involves the measurement of radiation accumulated in the crystalline structure of a ceramic since the time of its last firing (Aitkin 1985). Dates are given as ranges. The accuracy of the technique can be affected by x-ray radiography, so TL dating must be considered prior to any radiography of the ceramic. Many archaeological ceramics suffer from contamination by soluble salts (nitrates, chlorides and sulfates) (Buys and Oakley 1993). The presence of soluble salts can be detected with a conductivity meter or by the use of microchemical testing (Odegaard, Carroll and Zimmt 2000), and can be addressed by desalination if necessary. Deterioration
Factors causing ceramics deterioration can be divided into four broad categories: flaws in the materials, design or circumstances of manufacture; use; if archaeological, the burial environment; and the circumstances while in storage or on display.
Flaws in materials, design or manufacture
Use
The archaeological burial environment
The storage and display environment
The Michael C. Carlos Museum website provides useful K-12 teaching materials, including this tutorial on salts. Conservation and care
This information is intended to be used by conservators, museum professionals, and members of the public for educational purposes only. It is not designed to substitute for the consultation of a trained conservator.
Documentation
Written and photographic documentation should be undertaken for conservation treatments. Condition reports of ceramics should note any condition issues such as cracks, losses, and previous repairs. Guidelines for the photographic documentation of ceramics and other three-dimensional objects can be found on the AIC Guidelines for Practice.
A manually drawn horizontal intersection, or profile line, may be appropriate in situations such as archaeological sites. 3-D scanning and modeling may be useful in complex reconstructions. Identification of previous restorations may be aided with x-ray radiography, examination under ultraviolet and infrared radiation, and a range of other analytical techniques.
Preventive conservation
For material/object type specific issues regarding recommendations for storage and display, handling, inhibitive conservation measures, supports, mounts, labelling, transport, condition surveys, monitoring, etc. In order to reduce overlap, general preventive care issues should refer to or be discussed in the appropriate Preventive Care section of the main AIC wiki.
Interventive treatmentsCleaning
Mechanical, solvent, chemical, aqueous, poultices, pastes, or gels; reduction of surface dirt, grime, accretions, or stains; removal/reduction of non-original coatings or restorations; etc.
Stabilization
Consolidation, desalination, etc.
Structural treatments
Removal of deteriorated previous structural repairs, structural fills, joining, etc.
Aesthetic reintegration
Aesthetic reintegration involves disguising, to varying degrees, the fills and cracks in a restored vessel to allow them to blend in with the ceramic body. Considering the level of aesthetic reintegration is an important final step in any ceramic treatment. Reintegration can include creation of fills to compensate losses, inpainting of such fills, and finishing of the surface. Matching restorations to better integrate them with the surrounding pieces can involve the toning, texturizing, and painting of such fills. Treatments often aim to improve the viewer’s interpretation of an object. Reintegration should, however, be carried out to the minimum degree needed, with sufficient evidence to support the restorations (Buys and Oakley 1993). As with all conservation treatments, aesthetic reintegration should be fully documented, particularly so that original materials may be distinguished from the restorations.
Considering Object History
Finding an appropriate solution for reintegration will be dependent upon the object’s history and context, and should be decided on a “case by case” basis (Elston 1990). This is a decision that should be discussed among different parties, such as the object’s owners and perhaps art historians, curators, or others who can offer insight into the piece’s original appearance. Factors such as the original “purpose” of the object and its history will also be important considerations. The aesthetic reintegration applied to an art object to be displayed in a fine arts museum will likely be very different from that applied to an archaeological object, for example. Materials
Materials used should be as durable, but also as reversible and retreatable as possible. See structural treatments for more on fill materials used for structural repairs. Commercial vinyl and acrylic fill materials are often used to fill fine cracks and provide easily sculptable surface for aesthetic reintegration. For more on this see: Craft and Solz 1998. Synthetic resins bulked with fillers (such as fine silica or cellulose powders) are also commonly used for fills. Plaster of Paris (calcium sulfate) also has a long history of use in conservation and is still commonly used, especially for archaeological ceramics. See Koob 1998 for discussion of obsolete fill materials commonly used in restorations of the past.
Mimicking the Original Piece
Aesthetic reintegration involves mimicking the object’s original appearance on some level. Polishing with abrasives to retouch the surface or using particular fillers so that the restoration matches the texture or reflectance of the adjacent areas may be considered. Paints (lightfast, applied with a paintbrush or airbrush) may also be used to provide the appropriate color or approximate the original texture and other surface details. Regular patterns (such as a repetitive motif) may be extrapolated if deemed appropriate. While components such as sheen, color, and texture may be adjusted to closely match the original piece when deemed appropriate (see Surface Treatments, below), slight distinctions between the original and the restoration may also be used to allow the viewer to readily distinguish the two.
See Two Approaches to Vase-Painting Restoration on the Getty website.
Surface treatments
Polishing, coatings, etc.
Other treatmentsHistoric repairs and re-treatment
Most archaeological and historic ceramics collections contain historic ceramics repairs. In some cases, those repairs are valuable in themselves. In archaeological collections, pre-depositional repairs are evidence of use value, and should never be removed. Similarly, a good case can often be made for retaining repairs in historic collections that were introduced during the period of original usage. Repairs made by early conservators are valuable documentation of early conservation practices, and may be of significant value historically. The traditional Japanese technique of Kintsugi, or repairing ceramics with gold leaf powder in lacquer, is an art in itself. Kintsugi repairs, which are intended to honor the damage, are considered to enhance the value of the ceramic and would normally not be reversed.
However, modern conservation ethics stipulate that repair materials should be detectable and reversible, and should not damage the object. Historically, repair materials have not always met those criteria. Ceramics collections frequently contain inappropriate repairs that are likely to cause damage to the object. As a result, conservators regularly re-treat previously restored ceramics in order to prevent, arrest or repair damage caused by the old repairs themselves.
Historic ceramics adhesives
Adhesives for ceramics repairs should not only be non-damaging and reversible, but must be strong enough to support the weight of the ceramic, are ideally insensitive to temperature fluctuations, and may also need to be be water-resistant and heat-resistant. Until the 20th century, adhesives with those characteristics were limited. Some historic repair adhesives are listed below.
Identification: Aged animal glue ranges in color from transparent to dark brown. It fluoresces bright white under UV light. It is soluble or somewhat soluble in hot water, and may retain an animal glue smell when wet. Spot tests: Protein with copper (II) sulfate (Odegaard, Carroll and Zimmt 2000 144 – 145).
Identification: Bitumen and asphalt appear dark brown or black. They may fluoresce orange under UV light. They are soluble or partially soluble in petroleum distillates (petroleum benzine, stoddard's solvent).
Identification: Cellulose nitrate appears transparent or transparent yellow. Under UV, cellulose nitrate appears yellow, milky white, or yellow-green: it can be dull, or it can be bright. It is soluble in acetone. Spot test: test for nitrates with dipheynalamine (Odegaard, Carroll and Zimmt 2000 164 – 165).
Identification: Epoxy appears transparent or milky white. Under UV, epoxy is bright white. It is generally insoluble; it may swell in methylene chloride. Spot tests: look for nitrogen with CaO2 and pyrolysis (Odegaard, Carroll and Zimmt 2000 142 – 143. The test is published as a test for protein, but it is a test for bound nitrogen); look for a false positive with the PV(OH) test (see 'White Glues' below).
Identification: Resins and gums range in color from transparent to brown. Under UV, there is a range of fluorescence from no fluorescence to yellow-green: it can be dull, or it can be bright. Resins and gums are soluble in water or ethanol.
Identification: Plaster is opaque white. It is insoluble, but it may soften in water. Spot tests: if lime plaster, will test positive for carbonates (Odegaard, Carroll and Zimmt 2000 102 - 103); if gypsum plaster, will test positive for sulfates (Odegaard, Carroll and Zimmt 2000 124 – 125).
Identification: Aged shellac is glossy dark brown or black. It usually fluoresces orange under UV light, but may also fluoresce yellow, green, or white. If unaged, it is soluble in ethanol; aged, it may be only slightly soluble in ethanol or in an acetone/ethanol mixture.
Identification: Visually apparent. Spot tests: Test for lead using lead spot test papers (Odegaard, Carroll and Zimmt 2000 66 – 67). Test for ferrous alloys with a magnet.
Identification: Visually, white glues are transparent or milky white. They may be water soluble, acetone soluble, or insoluble. Under UV, they show a range of fluorescences: milky white, bluish, greenish, yellowish, or no fluorescence. Spot test: test for PV(OH) derivative with KI/I2 and glacial acetic acid (Odegaard, Carroll and Zimmt 2000 166 – 167). Note that this test gives a false positive for epoxies.
References
Abend, K., S. Caspi, and N. Laneri. 2010. Conserving fragments of icons: Clay votive plaques from Hirbemerdon Tepe, Turkey. Conservation and the Eastern Mediterranean: Contributions to the 2010 IIC Congress, Istanbul, International Institute for Conservation. 157-164.
Aitkin, M. J. 1985. Thermoluminescence dating. Orlando: Academic Press.
Buys, S. and V. Oakley. 1993. The conservation and restoration of ceramics. Oxford: Butterworth Heinemann.
Craft, M.L. and J.A. Solz. Commercial vinyl and acrylic fill materials. Journal of the American Institute for Conservation 37(1): 23-34.
Elston. 1990. Technical and aesthetic considerations in the conservation of ancient ceramic and terracotta objects in the J. Paul Getty Museum: Five case studies. Studies in Conservation 35(2): 69-80.
Halsberghe, L., L. T. Gibson, and D. Erhardt. 2005. A collection of ceramics damaged by acetate salts: conservation and investigation into the causes. ICOM Committee for Conservation preprints. 14th Triennial Meeting, The Hague. London: ICOM. 131-138.
Koob, S. 1984. The continued use of shellac as an adhesive: Why? Adhesives and consolidants: preprints of the contributions to the Paris congress. London: IIC: 103.
Koob, S. 1998. Obsolete fill materials found on ceramics. Journal of the American Institute for Conservation 37(1): 49-67.
Odegaard, N., S. Carroll, and W. Zimmt. 2000. Material characterization tests for objects of art and archaeology. London: Archetype Publications Ltd.
O'Grady, C. 2005. Morphological and chemical analyses of manganese dioxide accretions on mexican ceramics. In Materials issues in art and archaeology, vol. 7. Materials Research Society Symposium Proceedings 852, ed. P. Vandiver et al. Pittsburgh: Materials Research Society: 183-192.
Reedy, C. L. 2008. Thin-section petrography of stone and ceramic cultural materials. London: Archetype Publications, Ltd.
Seaward, M.R.D., C. Giacobini, M. R. Giuliani, and A. Roccardi. 1989. The role of lichens in the biodeterioration of ancient monuments with particular reference to central Italy. International Biodeterioration and Biodegradation 25(4): 49-55.
Selwitz, Charles. 1988. Cellulose nitrate in conservation (accessed 02/17/13). Los Angeles: The Getty Conservation Institute.
Further reading
Cohen, D. H., and C. Hess. 1993. Looking at European ceramics: A guide to technical terms. Malibu: The J. Paul Getty Museum.
Craft, M. L. 1994. A visual review of compensation philosophies for Islamic ceramics. Proceedings of the AIC Objects Specialty Group 22nd Annual Conference. Nashville, TN. 73-88.
Evershed, R. P., C. Heron, S. Charters and L. J. Goad. 1990. Chemical analysis of organic residues in ancient pottery: Methodological guidelines and applications. In Organic residues and archaeology: Their identification and analysis., ed. R. White and H. Page London: UKIC, Archaeology Section. 11-25.
Farnsworth, M. 1959. Types of greek glaze failure. Archaeology 12: 242-250.
Kingery, D. W., and P. B. Vandiver. 1986. Ceramic masterpieces: Art, structure, and technology. New York: The Free Press.
Koob, S. P., and W. Y. Ng. 2000. The desalination of ceramics using a semi-automated continuous washing station. Studies in Conservation 45(4): 265-273.
Ling, D. 1996. To desalinate or not to desalinate? That is the question. Le Dessalement des Materiaux Poreux: 7es Journees d’etudes de la SFIIC, Poitiers, 9 – 10 Mai 1996. SFIIC, Champs-sur-Marne. 65-74.
Lizee, J. M., T. Prindle, and T. Plunkett. 1995. Glossary of ceramic attributes. (accessed 01/03/12)
Myers, T. 2003. A preliminary investigation of compounds extracted when soaking low-fire ceramics. AIC Objects Specialty Group postprints, 10. American Institute for Conservation. Washington, D.C.: AIC. 81-91.
Pohoriljakova, I., and S. A. Moy. 2011. Adhesive testing at Kaman-Kalehöyük. Poster presented at the CCI Symposium, Adhesives and Consolidants for Conservation: Research and Applications, Ottawa, Ontario, Canada.
Rice, P. M. 1987. Pottery analysis: A sourcebook. Chicago: University of Chicago Press.
Rhodes, D. 1973. Clay and glazes for the potter. Radnor, Pennsylvania: Chilton Book Company.
SaltWiki (accessed 01/04/12)
Shepard, A. O. 1956. Ceramics for the archaeologist. Washington: Carnegie Institute of Washington.
Sigel, T., and S. P. Koob. 1997. Conservation and restoration under field conditions: Ceramics treatment at Sardis, Turkey. Proceedings of the AIC Objects Specialty Group 25th Annual Conference. San Diego, CA. 98-115.
Strahan, D. K. 1996. Preserving unstable painted surfaces on freshly excavated terracotta: dilemmas and decisions. In Archaeological Conservation and its Consequences: Preprints of the contributions to the Copenhagen Congress, 26-30 August 1996. London: International Institute for Conservation. 172-176.
Tite, M. S., V. Kilikogl, and G. Vekinis. 2001. Strength, toughness and thermal shock resistance of ancient ceramics, and their influence on technological choice. Archaeometry 43(3): 301-324.
Unruh, J. 2001. A revised endpoint for ceramics desalination at the archaeological site of Gordion, Turkey. Studies in Conservation 46:81-92.
White, C., M. Pool, and N. Carroll. 2010. A revised method to calculate desalination rates and improve data resolution. Journal of the American Institute for Conservation 49(1): 45-52.
White, J. C., and W. Henderson. 2003. Pottery anatomy: Review and selection of basic nomenclature as a step toward a searchable rim form database for the Sakon Nakhon Basin. Bulletin of the Indo-Pacific Prehistory Association 23:35-49.
ICON Ceramics & Glass Group Forum (accessed 01/17/13)
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