All ceramics are made of fired clay. Kaolinite (Al2O3.2SiO2.2H2O) is the most predominant clay mineral, but other minerals commonly found in clay are quartz (SiO2), feldspars, calcite (CaCO3), iron compounds such as iron oxide (Fe2O3), as well as other materials such as shell (CaCO3) and straw. Fillers and fluxes are added to clay to help with shape retention and control the hardening of the body. Different types of ceramics are distinguished from one another by the initial mineral content of the clay, the fillers and fluxes used, and the conditions and duration of firing.
Photos by M. Myers. Used by permission of the Virginia Department of Historic Resources
Before firing, a ceramic must air-dry so that most of the water has evaporated from the body. Once sufficiently dry, it is ready for firing, which may involve one single firing or a series of firings to achieve different effects. The hardness of the ceramic body is determined by the temperature at which it is fired-the higher the temperature, the harder, or less porous, the body. Glazes are often applied to earthenware ceramics in order to make them impermeable to liquids. A glaze is essentially a thin layer of glass that is fired onto the body of the ceramic. In addition to their functional use, glazes are often highly decorative and a number of colors can be achieved by the addition of metal oxides to the glaze. Ceramics may also be decorated prior to firing by painting, by gilding, by scoring and by applying a slip of diluted fluid clay to the surface of the body.
Generally, the most common problems seen in archaeological ceramics are cracking and flaking of the surface (either the paint or glaze), crumbling of the ceramic body, salt damage, and staining from metals or other associated materials. Often these problems are seen in ceramics with more porous bodies rather than harder ceramics. For example, surfaces that are especially prone to flaking are those found on low-fired, porous ceramics, such as delftware, due to the fact that the surface glaze is much harder than the body. Cracking and spalling of the harder surface will occur as temperature and relative humidity fluctuate, both within the burial environment and later in storage. Under-fired or low-fired ceramics are also prone to crumbling in moist soil as the ceramic body begins to re-hydrate back to clay.
Soluble salts perhaps have the greatest potential for damaging ceramics. Porous earthenwares are most likely to be affected by soluble salts, as opposed to harder ceramics, such as porcelain. When salt-contaminated ceramics are excavated from a moist burial environment, as the ceramic dries, the salts crystallize in the artifact. Damage occurs as the crystals grow and expand within the pores of an artifact, exerting great pressure. The repeated dissolution and recrystallization of soluble salts during periods of fluctuating relative humidity in artifact storage is particularly damaging (See Question 4c for further information about soluble salts).
Again, porous ceramics are most likely to be affected by staining from associated materials, such as iron. This staining will not cause any further damage to the ceramic itself, but may obscure any decoration and make the artifacts less pleasing aesthetically.
Ceramic vessels and shards are generally robust and are not at risk of being damaged during or after excavation. There are, however, several instances where fragile ceramics must be lifted using additional supports, and specially packaged in order to prevent them from incurring further damage until they can be cleaned and stabilized.
Important points to remember:
Ceramics with flaking glaze or paint must be lifted carefully and the attached soil should remain on the surface until the ceramics can be cleaned in a lab.
Low-fired ceramics, especially those from wet contexts, may be fragile and must be lifted using supports. There are various effective techniques that can be used for a lift, which should be carried out by a conservator or after consultation with a conservator, to ensure that the proper materials and techniques are being used (see Question 3c and Lifting below).
Porous ceramics from wet contexts must not be allowed to dry out until they are assessed for soluble salt content. If the ceramics are known to contain soluble salts, contact a conservator to determine a proper treatment plan (see question 4ci-v).
If a vessel is complete, it must be lifted with the contents intact. Once lifted, the contents can be excavated separately.
Package fragile and damp/wet ceramics carefully for transfer to the lab. Use acid-free tissue, Ethafoam and polyethylene bags to package fragile, dry ceramics. If ceramics are only slightly damp, store them in perforated bags so that water does not condense inside the bag.
Never use acid-free tissue to package damp/wet ceramics, because as the tissue dries out it will stick to the ceramic, causing further damage later. Use damp Ethafoam to keep damp/wet ceramics wet temporarily during transfer to the lab where they can be properly cleaned and dried.
Important points to remember for a lift (defined as any process for removing the artifact from the excavation context with special support):
A lift should be conducted either by a conservator or by an archaeologist trained in lifting techniques.
Always remember to use a barrier to protect the ceramic from the supporting material (polyethylene cling film or aluminum foil will work).
Before executing a lift, make sure that there is a plan and that all necessary materials are readily available. These include the materials needed for the lift as well as the packaging materials needed for transporting the ceramic from the field to the lab.
Most ceramics can be cleaned gently with soft-bristled brushes and deionized water. However, DO NOT clean ceramics with brushes and water if they have very soft, crumbling bodies or if they have flaking paint, gilding or glaze. Consolidation may be recommended by a conservator. Never attempt to remove stains on ceramics. Stains cannot be removed without chemical cleaning, and often this will result in further damage.
Do not attempt to mend ceramics that are particularly fragile or are crumbling and weak. For information on mending and reconstructing ceramics, see question 6ci-ii.
Once clean and dry, ceramics do not require very strict storage conditions. Damage occurs when ceramics are stored before they are completely dry or without being desalinated. Like all other materials, ceramics should be kept in an environment free from fluctuations in RH and temperature, and free from vibrations. Handling also poses a great risk to ceramics, and all sherds and artifacts should be handled with clean hands over a padded surface to avoid staining and damage.
Buys, S. and V. Oakley. (1993) Conservation and Restoration of Ceramics. Oxford: Butterworth-Heinemann.
What adhesive do you use to mend ceramics?
Mended ceramic sherds being supported during the adhesive cure in a box of wax beads. Photo by M. Myers used by permission of the Virginia Department of Historic Resources.
The most important factor in choosing any adhesive to use on archaeological artifacts is the long-term stability of the adhesive. Adhesives that discolor, become brittle, fall apart, or cannot easily be reversed (re-dissolved in order to undo the join) are not appropriate for archaeological materials. Many adhesives readily available from hardware stores, craft stores and other suppliers may be appropriate for fixing a broken coffee mug or other household item where preservation is not a factor but they are not appropriate for use in museums or other cultural institutions. Many such adhesives have been used on archaeological ceramics in the past and have caused more harm than good.
Adhesives for all archaeological artifacts should be conservation-grade and should be purchased from conservation suppliers for the following reasons:
Mended ceramic sherds being supported during the adhesive cure in a box of wax beads.
The adhesives sold by conservation suppliers tend to be purer mixtures than those sold elsewhere. They contain only the resin and solvent whereas many commercial adhesives include plasticizers and fillers that may not be archivally stable over the long term.
Most commercial manufacturers use proprietary formulas to make up adhesives. These formulas may change from batch to batch with no warning so that an adhesive that may once have been a suitable material for preservation use is no longer suitable. Unless you have the resources to test each batch for suitability prior to use, it is better to avoid commercially manufactured products altogether.
Another factor in choosing an adhesive for ceramics is that the adhesive should not be stronger than the ceramic fabric. If there is going to be a mechanical failure, or physical shock to the object, you want the adhesive to fail at the old join before the ceramic cracks in a new place.
Cellulose nitrate adhesives (Duco Cement, HMG Blue Tube) tend to be stronger than most earthenwares. They also do not age well – they darken over time, become brittle, and become less soluble. Cyanoacrylate adhesives (such as Super Glue) are also very strong and are not stable in the long term (many were developed for short term purposes such as adhering skin together after operations). Cyanoacrylate adhesives are not recommended for use on artifacts.
Epoxy adhesives create very strong bonds, yellow as they age, and are difficult to reverse. Conservators tend to avoid using them, except in special circumstances such as mending glass and porcelain, where there are no pores to help the ceramic bond. In such cases conservation-grade varieties, such as HXTAL or Epotek 301, are used. Epoxy adhesives should never be used on earthenwares or low fired ceramics.
Acryloid B-72 (also called Paraloid B-72) is the preferred conservation adhesive for most conservators, since it does not change color, and remains easily reversible with an appropriate solvent. Solidified B-72 does begin to “flow” at 40Â° C (104Â° F) so in very hot climates alternatives may be used to avoid the collapse of mended areas.
Acryloid B-72 is sold by conservation suppliers in the form of solid beads which can be custom-mixed with solvent or as premixed adhesive in tubes. Although it is possible to make up one’s own adhesive by dissolving solid resin pellets in a suitable solvent, unless one is using very large supplies of adhesive it is often easier and better to simply purchase pre-mixed tubes. The company Herbert Marcel Guest (HMG) makes a number of conservation adhesives. Acryloid B-72 is sold by HMG, available through Conservation Resources International in a purple tube.
See also: Things to Think About When Choosing an Adhesive
What do you use to fill gaps in ceramics?
Gap-filling is a technique that was once widely used by archaeologists to prepare objects for storage or exhibit. It appears to be used less often now, particularly in historical archaeology, because of the importance many archaeologists place on being able to study the body/ fabric of a ceramic vessel or item. The advantages of gap-filling are that it allows an artifact to be considered as a whole, which is useful for exhibits and for study collections, and it allows support to be provided to otherwise unsupported shards or those that have very small points of contact with each other. The disadvantages of gap-filling are that if it is done poorly it can cause significant damage to a piece, and that reconstructed ceramics require more space to store than shards.
A number of materials are used for filling ceramics depending on the size of the piece, the span of the fill, any texturing on the surface, and the body/ fabric of the ceramic. Plaster of Paris is one of the preferred materials for gap-filling ceramics, especially for earthenware and stoneware; dental plaster is often recommended as it is uniformly fine-grained and does not contain lumps or other impurities. Proprietary calcium carbonate and calcium sulphated- based fillers such as Pollyfilla and Hydrocal are also used. These materials remain workable for longer periods than plaster of Paris, allowing the material to be wet-modeled to a greater extent than with plaster. Additionally they are generally a little softer when dry allowing pieces to be sanded, shaped and cut back more easily. Resin putties or epoxy putties, such as Finebond, are sometimes used for porcelain and harder paste ceramics as they allow for finer molding and modeling and tend to match the translucency and luster of the ceramic better. The color of the fill may be matched to the color of the ceramic body or glaze either by mixing dry pigments into the fill medium or by in-painting the fill once it is dry. It is important to ensure that the fill materials and in-painting never cover the surface of the ceramic and that any techniques used to shape the fills do not cause abrasion to the ceramic. One useful technique for gap-filling, that allows any shaping work to be done away from the ceramic itself, involves creating a separate shard fill that precisely fills a gap and is then mended into place as if it were another potsherd. When in-painting, conservators often use an approach known as the “Rule of Six feet /Six inches,” meaning that when an object is seen from six feet the colors blend well and the object is read as a whole, but at six inches it is easy to discern the fill.
Making a successful gap-fill is time-consuming. It calls for patience and practice in mixing, molding, and shaping the final product. It is also necessary to exercise care in choosing materials and to ensure that the materials are sympathetic to the piece.
Buys, S & Oakley, V. (1993) Conservation and Restoration of Ceramics. Butterworth-Heinemann: London.
Koob, S & Sigel, T (1997) “Conservation and Restoration Under Field Conditions: Ceramics Treatment at Sardis, Turkey” Objects Specialty Group Postprints 5: 98-115
Koob, S (1987) “Detachable plaster restorations for archaeological ceramics.” In Black, J. (compiler) Recent Advances in Conservation and Analysis of Artifacts, Institute of Archaeology Jubilee Conservation Conference Papers. Summer Schools Press: London. Pp. 63-67.
Sourche from; http://www.sha.org/research_resources/conservation_faqs/treatment.cfm