Composite objects are defined as anything that is made up of two or more different material classes. Common examples of composite artifacts are: wooden timbers with iron fasteners, tools with wooden handles and metal components, an iron knife with a bone handle, a textile with metal fasteners, a glass bottle with a cork stopper, or a metal cufflink with a paste glass inset.
The same factors in the burial environment that act on individual materials will also affect composite materials. However, the deterioration of composite materials can also be affected by the proximity of different material types to each other. These effects may be detrimental or quite positive. For example, the rapid corrosion of certain types of metal can create casts of organic materials that are in contact with them, preserving morphological information that might otherwise have been lost when the organic rotted. Expanding corrosion products on the iron tang are causing the bone knife-handle to crack. Photo by M. Myers. Used by permission of the Virginia Department of Historic Resources.
Since there are so many variations both in the materials that composite artifacts are made from and the condition of those materials on excavation, there are no blanket treatments for composite artifacts. Instead each treatment must be designed uniquely for that artifact. Composite objects can be found either in wet or dry environments, but in either case they can pose some of the worst possible problems in conservation. The worst problems arise when one of the component materials actively contributes to the degradation of the other components – i.e. decaying wood can release acids that will attack metals and accelerate their corrosion, while metal corrosion products tend to be of greater volume than the original metal, so the expanding metal will put mechanical stress on the organic material.
Additionally, corrosion products will stain organic materials, and the metal salts will cause cellular disruption, accelerating the decay of wood and bone.
Another serious problem is that the standard conservation treatments used for one of the materials may often damage other component materials. For example, waterlogged wood is usually treated with polyethylene glycol (PEG) before being freeze-dried, but PEG is corrosive to iron. Similarly, iron artifacts from marine sites are often desalinated by electrolysis or by soaking in caustic solutions, but these solutions or electrolytes tend to have a high pH, which can accelerate the degradation of wood. Sometimes the object can be disassembled, so that the different components can be treated separately, then reassembled.
If disassembly is not possible, then it is time for an earnest talk about conservation priorities:
Does the object need active intervention to prevent its loss? Or can it simply be stabilized in its current condition? This may be an option for some artifacts from dry, terrestrial sites.
If it is a wet composite object, you may have to consider whether one component is less critical to the interpretation of the object than another: i.e. is the timber or the nails more important? The axe head or the fragment of handle?
If there are no easy answers, there are options for treating all components, or at least minimizing damage to one part while treating the other part. This can be time consuming and, possibly, expensive. It may require additional corrosion inhibitors, neutral pH electrolytes, and constant monitoring of treatment processes.
It is very important to take this type of object to a professional conservator and to discuss all the options with them.
However the object is treated, in the end, it will still be very sensitive to environmental conditions. Since the different components will prefer different conditions, it will need careful monitoring in storage. Consult with your conservator to determine how best to balance the different needs, and set up a way to monitor the object.
How do you assess architectural materials, brick and stone and decide upon a conservation treatment?
Stone falls into three geological groups: igneous, sedimentary and metamorphic. Igneous rock (i.e. granite, obsidian, and basalt) is formed when molten materials cool. They are generally hard, and have a compact and non-porous structure. Sedimentary rocks are formed through the sedimentary deposition of minerals, other rock fragments, and/or skeletal material from marine animals. Over time these sediments become cemented together creating fairly soft and porous stones, such as limestone, sandstone and shale. Metamorphic rocks are sedimentary or igneous rocks that have been changed or recrystallized through the effects of pressure and high temperature. They have a denser structure than sedimentary rocks and include stones like marble and slate.
Plaster is made out of gypsum, hydrated calcium sulphate that has been heated so that all the water is driven off. When water is added again, the gypsum reforms and sets into a hardened mass. Plaster can be cast in a mold or modeled, carved and attached to something else. Plaster can be painted when dry or mixed with pigments when still wet.
Bricks are molded rectangular blocks of clay baked by the sun or in a kiln until hard and used as a building and paving material. They have properties that are similar to those of ceramics.
Architectural materials are often thought of as being chemically and physically resistant to damage because they are robust and are part of our natural outdoor environment. Nevertheless, these materials are not immune to damage from burial, cleaning, handling and storage. Stones with a laminated structure, like redstone or shale, and those with high amounts of calcium carbonate, such as limestone and marble, are particularly vulnerable to damage. They can become softened or pitted and soluble salts may enter the matrix of the stone. Once the salts re-crystallize, they can cause spalling and destruction of the overlying layers destroying any detail or decoration and weakening the stone. Limestone and marble are particularly vulnerable to the effects of weak acids, which can dissolve the calcium carbonate components of the stone. Plaster is vulnerable to the effects of water and can easily be dissolved. It survives poorly in wet burial environments.
Architectural materials are often heavy and can be damaged physically through improper handling and packing. The weight and composition of these objects must also be considered when undertaking treatments on them.
Some of these materials are composites of many different types of clay, minerals, sand, rocks and modern polymers (in the case of more modern building materials). It is important to identify the type of “stone” or material you have before cleaning or treating the object. Artifacts can be composed of different types of rocks, and their porosity and homogeneity can vary dramatically. Some stone is very difficult to clean, as it delaminates easily. Any cleaners containing acid can attack materials made of limestone or marble. Some plasters and mortars will dissolve if they are cleaned with water. By analyzing, or identifying, the type of stone, mortar, brick or plaster you have first, the cleaning method chosen will be more successful in the long term.
It is best to test any cleaning method prior to cleaning an entire object first. Dry brushing the materials with a soft brush post-excavation is acceptable. After analyzing the materials, some wet cleaning may also be performed. Other cleaning methods used in conservation include using organic solvents, erasers, poultices, and laser cleaning. All of these techniques should be tested prior to cleaning an entire object and should be performed by a professional conservator.
Staining on the objects should be considered part of the archaeological history and evidence of the object, and should only be removed after consultation with a professional conservator.
In outdoor environments, biological growth may occur on architectural remains. Sometimes, this growth can be disfiguring to the object, such as is sometimes seen on tombstones. Most of the fungal and microbiological growth can be removed by cleaning the object with water; however, a biocide may be necessary in order to kill the micro-organisms and to keep re-growth to a minimum. The use of biocides is not without its own problems, and should be considered carefully before use. Household bleach, Lysol and other proprietary biocides should not be used on objects as they are often too strong and can leave detrimental residues in the object.
Many architectural materials are subject to damage by salts. Desalination may be necessary in order to remove salts from the interior pores of many types of brick, mortar, and stone. If the objects are not desalinated, salt efflorescence may occur causing further physical damage to the materials once they are dry. Please refer to section 4 Processing Archaeological Artifacts of this document to locate more information on checking for soluble salts, and desalinating. Because of the susceptibility of plaster to dissolution in water, consolidation may be necessary prior to desalination; it is best to consult a conservator prior to attempting to desalinate plaster. After these objects have been surface cleaned and processed, further conservation treatment may be necessary. If the object is broken it may need to be mended. Stone and brick tend to break due to their excess weight and brittleness. It is not always necessary to mend the materials, but if it is desired a suitable adhesive should be used. For small breaks or fractured pieces, Acryloid B-72 may be used. For larger fragments, or heavier materials, a stronger adhesive system, such as an epoxy or a silane, may be required. If structural support is necessary, a more intrusive method may need to be undertaken, such as inserting dowels to attach to very heavy pieces back together or using metal frames or channels to support the piece. All of these treatments have their drawbacks, and should be considered carefully.
Architectural materials should be stored in a controlled environment where there are no rapid or sharp fluctuations in RH. If there are pigments or paints remaining on the surface light levels may need to be controlled to avoid fading; otherwise most architectural materials can tolerate a high degree of light and UV radiation. Depending on the size and shape of the artifact, supports for storage may be required.
How should I care for archaeobotanical remains?
Archaeobotanical remains can be anything from a tiny fragment of wood, to seeds, soils, nuts and plant specimens. These materials are generally collected in flotation samples and should be analyzed by a professional archaeobotanist. Generally, archaeobotanical remains do not require conservation and in many instances conservation can impair future analysis. If long-term storage of such materials is necessary it is best to consult an archaeobotanist.
Rose, C. & Torres, A. (1992) Storage of Natural History Collections: Ideas and Practical Solutions. New Haven: Society for the Preservation of Natural History Collections
Sourche from; http://www.sha.org/research_resources/conservation_faqs/treatment.cfm