Object Deterioration Chart by H. Wellman. Photo used by permission of the Maryland Archaeological Conservation Laboratory
Environmental factors in the ground contribute to the deterioration of materials. These factors may include, but are not limited to, water, temperature, oxygen, salts, micro-organisms and pH. When an artifact is buried, the environment around it is different from the one in which it was created and used. As the artifact reaches chemical equilibrium with the new environment, it may undergo significant deterioration or may even be totally lost.
Upon excavation, the environment is once again disrupted and the object will again undergo changes as it reaches equilibrium with the new conditions. Some of these changes can be particularly damaging. The agents of decay found in the new environment may include but are not limited to, visible light, ultraviolet (UV) radiation, temperature, relative humidity, pollutants, insects, and handling for processing, conservation, study and exhibit.
The first step in stabilizing freshly excavated artifacts is to provide storage that minimizes the degradative effects of the artifact's environment. Providing proper handling, storage and packaging after excavation is the responsibility of every member of an archaeological team. For many objects these actions may be sufficient to stabilize the artifact and allow it to reach equilibrium gently with its new environment. If proper storage alone does not stabilize an artifact, active conservation treatment will also be necessary. As damage can occur if treatments are carried out improperly, conservation treatment is typically the responsibility of the conservator or, depending on the degree of difficulty of the treatment, of a member of the excavation team who has been trained in materials science and conservation
Freshly excavated materials: General points to remember:
Material-Specific points Metals
Photo by M. Myers. Permission for use by the Department of Historic Resources
Of course, the most important information is the site name and context, object type, number of objects included in the container, material type, and date of excavation. To supplement this information, a conservator may ask for all the other information that is recorded in the field such as the description of the context, its location on thsite grid, characteristics of the soil, inclusions, asamples taken, and interpretations. In addition, it is important for archaeologists to record all information about any lifting procedures or consolidation procedures that are used in the field, and any changes in appearance the artifact has undergone (as this may indicate ongoing deterioration). This is particularly crucial if any chemicals are used in these processes. Before sending any artifacts to a conservator, it is also important to record information about the post-excavation environment, such as the storage conditions (temperature and RH), storage materials used, and storage environment (such as whether the artifact was stored wet or dry, treated with a fungicide, stored in a refrigerator, when the silica gel was last regenerated, and so forth).
Photo by H. Wellman. Used by permission of the Maryland Archaeological Conservation Laboratory
Some artifacts are so fragile that they cannot support their own weight sufficiently to be lifted safely from their burial matrix. Several options are available for strengthening and supporting a fragile object enough to lift it:
Consolidation: This involves applying a synthetic liquid resin to the object in order to allow the resin to penetrate the object, solidify, and strengthen it. The use of this technique on bones, for example, is described in several places in the archaeological and conservation literature. A conservation-grade resin must be used for this process, since it will be applied directly to the object surface, will penetrate the surface, and may not be able to be completely removed afterwards. Resins frequently used are Acryloid B72 (an acrylic copolymer) dissolved in acetone or other organic solvent, Acrysol WS-24 (a water-based acrylic colloidal dispersion) and Rhoplex AC33 (a water-based acrylic emulsion). The choice of consolidant is determined partly by the type of environment and by the artifact material itself. Organic-solvent based consolidants are used in dry environments, and solidify through evaporation of the solvent, which is usually rapid. These cannot be used in damp environments, where they are incompatible with water and will form a white gel or skin. Water-based emulsions are used in damp (not wet) environments, and solidify through evaporation of the water. They take considerably longer to dry, and will never dry if the burial context is constantly wet.
To consolidate an object, remove as much soil as possible from its surfaces without undermining it or causing surface loss. Soil left on the object and consolidated to the surface will need to be removed later, as will thick layers of excess consolidant. Apply the consolidant a little at a time by soaking a brush in the liquid and then touching the brush to the object surface and allowing the liquid to be pulled off the brush by capillary action. When the object cannot absorb any more consolidant, the liquid will begin to pool. This will need to be done at many places across the artifact surface to ensure even, complete consolidation. After a solvent-based consolidant dries, it is possible to apply more consolidant if necessary, since the solvent will re-dissolve the polymer that has already been applied.
Water-based consolidants have a larger particle size and higher surface tension, so the artifact may not readily absorb them; they may need to be gently brushed across the artifact surface. Once a water-based consolidant is dry, more consolidant applied to the surface will not be absorbed, since the dried film will not be water-soluble.
It may be necessary to consolidate an exposed area, allow it to dry, remove more soil from around the artifact, and then consolidate the freshly exposed areas, until the object is strong enough to be removed. The object can then be lifted and placed in a well-padded container, see: How do I pack artifacts for transport to the conservation / curation facility? It is likely that excess consolidant and soil will need to be removed from the artifact later. Whatever type of consolidant is used, it is crucial that it is allowed to thoroughly dry before the object is lifted. An object that is heavy with wet consolidant is weaker than it was before, and can collapse under its own weight.
Backing or facing: A conservation-grade polymer, such as those noted above, is applied in a thicker, more concentrated form so that a layer of strengthening material can be adhered to the surface of the object. Depending on the nature and condition of the object, the object may need to be consolidated with a more diluted polymer before the concentrated polymer and backing can be applied, and two different polymers may need to be used. The backing material may be a nylon bandage, fibrous tissue, fabric (e.g., muslin, linen), or other material. This technique is commonly used for flat objects such as fallen wall plaster, mosaics, metal and ivory plaques, and crushed bone.
Wrapping: An object such as a weak or broken ceramic vessel filled with soil can be snugly wrapped with bandages, plaster-impregnated bandages, or fabric strips before lifting. A separation layer such as tissue or clingy polyethylene film must be used to protect the artifact surface.
Block lift: An object or assemblage of objects can be lifted intact by lifting along with a block of the surrounding soil, or with a block of some other surrounding support material. Block-lifting can be a difficult, time-consuming and disruptive process, and may damage surrounding archaeological contexts, so should be considered carefully. The procedure has many variations. For a large object it may involve exposing an object (or retaining the soil around it, if the soil is cohesive), building a wooden box or other walled unit around it, putting protective layers over the object surface (such as thin clingy polyethylene film or aluminum foil), filling the surrounding space in the box with a fully form-fitting rigid material such as expanding polyurethane foam or plaster of Paris, securing a lid, sliding a board or flat metal plate beneath the whole assemblage to release it from the ground, inverting the block, filling the space with more plaster, foam or soil, and closing it with a lid. Dry ice or paraffin wax are also sometimes used in this process. For a smaller object, the procedure may be less complex, and may involve, for example, wrapping the object and its surrounding soil in plaster bandages or similar materials that will set rigidly and provide support.
Obviously there are as many techniques for lifting as there are artifacts and you will need to adapt your method to the piece you are planning to lift after a careful examination of the artifact. There are however several key principles to bear in mind when planning a lift:
When an artifact is freshly excavated, its appearance, weight, color, shape and even size may all give clues about the materials it is composed of, as well as the type of artifact it is. Conservators who study materials and the degradation of those materials during burial may be able to help identify some artifacts and materials without undertaking cleaning or treatment of the objects. Different materials are cleaned, processed and stored differently after excavation, so it is important to determine the material type before beginning these activities.
Below are some indicators that may help to identify excavated materials. It is important to remember that every artifact is unique. As artifacts decay in the ground their appearance may be altered, and this change is different for every type of site or burial environment. Artifacts found on terrestrial sites will differ in appearance from those found in waterlogged or underwater sites. Some general characteristics help to identify an artifact's material, but for more detailed identification, analysis should be undertaken. Analytical testing such as spot testing, FTIR, X-ray diffraction, and X-radiography can determine material types and artifact identification prior to cleaning. More information on analytical testing of archaeological artifacts can be found in Section 7: Research and Analysis of Artifacts.
The colors of metal corrosion products are usually the most helpful feature in identifying different metals. On aerated sites:
Iron--turns orange, red and brown and is often lumpy and misshapen. Copper alloys--will be shades of green with brown, red and/or purple. Lead and other white metals--will turn white/gray, brown and/or pinkish, may be fissured. Silver--is often gray or blackened, alloys may be green and may be mistaken for copper. On poorly oxygenated or anaerobic sites: Iron--often red but may quickly oxidize to orange, may have a fissured, wood-grain appearance. Copper alloys-black, purple-black, some green products, or yellow metal exposed. Silver--blackened.
Lead and other white metal--may be black.
Other features also provide clues. Lead or alloys containing lead are typically heavier than their appearance warrants, due to the higher density of lead compared to other metals. Iron is magnetic, so a simple magnet test may indicate the presence of iron or ferrous alloys.
On aerated sites glass often appears iridescent and may contain layers of adhered insoluble salts mixed with soil and burial debris. It may be difficult to determine the original color of the glass, but the form, size and thickness of the glass may determine whether it is bottle, table or window glass. On anaerobic sites, glass may often appear unaltered, but may be worn slightly and covered in a thin layer of oxidation. Upon drying, damp glass may lose its saturated appearance and become more iridescent and opaque.
It is usually easy to identify an artifact as a ceramic of some kind, but it may not be easy to identify the specific type without some cleaning or analysis. Ceramic features can sometimes be masked by soil, soluble and insoluble salt encrustations and staining from minerals in the soil or from nearby metals. It may be necessary to wait until cleaning has been undertaken before trying to identify ceramic types.
Bone, Ivory, Antler and other Bony Materials:
Bony materials tend to be creamy white materials , however they can be intentionally dyed a range of colors, from green to red to black, and in the burial environment may become stained by proximity to metals. Bone usually retains a harder surface than other organic materials. It can often be identified by the caniculi on the surface; these usually manifest themselves as small block dots where soil has filled small blood vessels or caniculi. If bone has been burnt a mottled effect may be apparent, with colors ranging from white to black evident. Antler has a characteristic rough grooved pattern and a core of spongy tissue. Artifacts made from antler often incorporate areas of the patterning. Ivory is difficult to distinguish from bone in field situations.
Organics (other than bony materials):
Organic objects (such as wood, leather and textiles) survive primarily in waterlogged anaerobic conditions or in association with metals. They often appear blackened, soft and very spongy. Most of these materials will be recognizable, but details about the objects may be masked. However, information will be lost if cleaning is started before careful examination, preferably with a microscope. Wood can usually be identified by the grain visible on tangential or radial sections, and the concentric growth rings on cross-sections. Cracks will tend to form along the long axis, and radially out from the middle of round-wood. Highly degraded and rotted wood will crack across the grain, sometimes forming right angle intersections that break up into small rectangular or square chunks. Leather has a fibrous appearance and may show signs of delamination at the edges. Highly degraded leather in proximity to iron may look like powdery iron corrosion.
Textiles can often be identified by the weave pattern on the surface.
If cleaning of the object is necessary in order to identify it, proceed carefully. Consult sections:
The answer to this question is based on the real question, which is: What will do it the most harm: staying wet, or drying out? This can usually be decided based on the material class of the object, and the burial environment from which it came.
|Material Class||Dry||Damp (subsurface soils)||Waterlogged: Fresh Water||Waterlogged: Marine|
|In Gerneral||If it's dry, leave it that way.||Here you have to be worried about the effects of drying, versus the effects of moisture-driven corrosion or decay.||Here you have to be worried about the effects of drying, versus the effects of moisture-driven corrosion or decay.||Here you have to be worried about the effects of dissolved salts that can physically damage porous materials, or drive corrosion processes. Never allow marine concretions to dry out, they are much easier to remove when wet.|
|Ceramic (porous)||If it's dry, it's probably ok. But beware of wet-cleaning porous ceramics from salty soils, as the water may dissolve, then reactivate crystallization that may degrade the ceramic.||Low-fired ceramics are weaker when damp than when dry. Ceramics can be slow-dried, but watch carefully for salt crystallization which will damage ceramic body and surface treatments.||Similar to damp conditions||Keep wet. Do not dry out until soluble salts have been removed by washing in purified water. Washing endpoint should be determined by using some dissolved solid measuring system. Slow-dry under controlled conditions.|
|Ceramic (vitreous)||They can stay dry.||Most vitreous ceramic bodies (porcelain, stoneware) are not subject to crystalization pressures, but be cautious with glazed or overpainted surfaces. Slow-dry under controlled conditions.||Similar to damp conditions||As with damp materials, be cautious of overpaints or glazes that might be affected by salt crystallization. Desalination is always a safe step. Slow-dry under controlled conditions.|
|Glass||As for vitreous ceramics||As for vitreous ceramics||As for vitreous ceramics||As for vitreous ceramics|
|Stone||See the appropriate ceramics class, either porous or vitreous. Note that some stones are water-soluble.||See the appropriate ceramics class, either porous or vitreous. Note that some stones are water-soluble.||See the appropriate ceramics class, either porous or vitreous. Note that some stones are water-soluble.|
|Bone/ tooth/ horn/ Ivory||Leave it dry, and move to a controlled humidity environment (ca. 55%RH). Very fragile materials may be consolidated in situ to aid in excavation.||Determine the degree of decay and cellulary loss. It may be possible to consolidate fragile materials in situ with water based resins to aid in excavation. It may be possible to slow-dry very carefully, and move to a controlled humidity environment (ca. 55% RH)||Keep it wet. Waterlogged "hard tissues" must be treated very carefully to prevent cracking and loss during drying. If possible, keep in a dark cool place to retard decay.||Keep it wet. Waterlogged "hard tissues" from marine sites must be desalinated and treated very carefully to prevent shrinkage and loss during drying. If possible, keep in a dark cool place to retard decay.|
|Composite (metal/organic)||Compromise between the needs of the most sensitive material - robust organic materials can tolerate low RH (to about 30% RH) needed to reduce metal corrosion rates, but the rate of RH decrease should not be abrupt.||Compromise between the needs of the most sensitive material - robust organic materials can tolerate low RH (to about 30% RH) needed to reduce metal corrosion rates, but the rate of RH decrease should not be abrupt.||Keep it wet. Drying out a composite object can be very tricky, as the treatments for organics and metals involve materials that may cause damage to both components.||Keep it wet. Drying out a composite object can be very tricky, as the treatments for organics and metals involve materials that may cause damage to both components.|
Immediate post-excavation storage can usually be decided based on the material class of the object, and the burial environment from which it came. Important factors to consider are the relative humidity, padding system, and archival grade of the materials used. Wet storage of waterlogged materials should be viewed only as a temporary solution preparatory to the artifact receiving long-term stabilization or being transported elsewhere for treatment. It should never be seen as a long-term storage solution, as the potential for significant deterioration during wet storage is always present.
|Material Class||Dry/Damp Conditions||Wet/Waterlogged Conditions|
|Ceramic (porous and vitreous)||Ceramic can survive a wide range of conditions, but avoid widely swinging humidity, which can activate salt crystallization. Crystal growth can damage ceramic bodies and surface treatments.||Store in tap water or deionized water. Avoid prolonged wet storage, which may encourage biological growth (molds and slimes) on the ceramic, causing stains. Active water changes during desalination discourages biological growth. Cold storage also discourages biological activity. Consult a conservator.|
|Glass||As for ceramic. Note that glass "corrosion" is moisture-activated. If there is active weathering or spalling of surfaces, consider controlled humidity storage, between 20% and 40% RH.||As for ceramic.|
|Stone||As for ceramic.||As for ceramic.|
|Metal (ferrous)||Store in desiccated (less than 20% RH) environment.||Storage solutions should be kept at pH greater than 8 to reduce corrosion rates. This is usually accomplished with 2-5%sodium carbonate or sodium sesquicarbonate. Metal ions in solution often will act as a biocide. Consult a conservator as soon as possible.|
|Metal (non-ferrous)||As for ferrous metals.||As for ferrous metals, but be aware that carbonate solutions may activate new forms of corrosion. Store lead and other white metals in tap water only.|
|Wood||Keep in controlled environment, ca. 55% RH. Changes in RH will cause dimensional stresses, leading to cracking and shape change.||Store in tap water, or in mixed tap/salt water if from a saline environment. Keep in dark cool place to reduce biological activity. Consult a conservator as soon as possible.|
|Leather||As for wood.||As for wood.|
|Bone/ tooth/ horn/ Ivory||As for wood.||As for wood.|
|Composite (metal/organic)||Compromise between the needs of the most sensitive material - robust organic materials can tolerate low RH (to about 30% RH) needed to reduce metal corrosion rates, but the rate of RH decrease should not be abrupt.||Wet organic materials can tolerate pH between 8 and 9, but no higher than that. Keep in a cool, dark place to reduce biological activity (but metal ions may act as a fortuitous biocide). Consult a conservator as soon as possible.|
The care required in collecting and handling samples will vary depending on the analytical technique, and the sample itself. In general, the more specific and precise the questions you are likely to ask, the more stringent your collection and storage of samples should be. For some techniques, such as DNA analysis, the analytical specialist may insist on taking the samples themselves.
Most materials that are going to be studied have to undergo significant preparation prior to study. This may include pulverizing, polishing, sectioning, scraping - all considered "destructive" testing. In general, it is best to keep all samples away from contamination with modern or foreign materials by storing them in archivally stable containers (e.g., glass vials). Metal or geological samples should be kept in dry or desiccated storage to prevent continuing corrosion and degradation. If dry, organic sample should be kept dried and at controlled temperatures. If the organic material is wet, its condition and its storage conditions should be discussed with the analyst. Desiccation or freezing prior to analysis may cause physical changes that might compromise the analysis (such as collapse of internal wood structure making it difficult to identify), while desiccation and freezing during analysis may result in physical damage (such as shrinkage and warping) that will affect the long-term preservation of the object.
Packing for transport is a very important process, since the artifacts will be subjected to a variety of stresses that can cause permanent damage. These include not only physical shocks from handling and vibration during shipping, but also changes in temperature, humidity, and light.
Always ensure that you have enough padding around the objects to protect them from shocks - the usual assumption is that most damage occurs when a package is dropped, or slips off the back of a vehicle. Over-packing a container is a common mistake, and can either make a package too heavy to lift safely, or can cause the weight of the objects to crush those on the bottom of the pile. Choose packing materials carefully - ensure that they are strong enough to bear the load you will be packing in them, and that they will not degrade during transport and storage. Do not use common cardboard boxes, which are made with acidic materials and will quickly lose their strength if they get wet, if they are left in the sun too long, or if they are over-loaded. Wherever possible, use quality padding materials that will not release gases or chemical vapors that may injure the object. If an object is particularly sensitive to changes in temperature or humidity (for instance, waterlogged organic materials, or metal from marine sites), refer to Questions:
for the appropriate storage environment. Consider how long the objects will be in their packing materials, and plan appropriately. For example, if you are packing waterlogged objects and need to keep them wet, you can wrap them in wet open-cell foam, which will keep humidity levels high, but doesn't involve the weight of packing them submerged in water. If you are packing terrestrial iron, include desiccant in the sealed container.
As a last resort, if you are packing something and want to preserve its burial environment as much as possible (especially something that needs a moderate humidity, like bone), pack it with the soil from which it was excavated. Do not do this if the soil contains biohazards or toxic material, or if the object will not be removed from the soil and placed in stable packing materials immediately on arrival at the conservation lab.
Always include packing lists with every container, along with images of the packing or instructions on how to handle fragile or sensitive materials.
Some additional points to remember are:
For help in purchasing archival quality packing materials see the PowerPoint presentation: Storage Environments: Packing and Labeling Requirements
Copyright © 2006 Colleen Brady, Molly Gleeson, Melba Myers, Claire Peachey, Betty Seifert, Howard Wellman, Emily Williams, Lisa Young. All rights reserved. Commercial use or publication of text and graphic images is prohibited. Authors reserve the right to update this information as appropriate.