NPS Archeology Program: Managing Archeological Collections

Field conservation and preservation

(photo) Stabilizing a ceramic during lifting in the field.

Stabilizing a ceramic during lifting in the field. Photograph courtesy of Jessica Johnson.

In situ or on-the-spot conservation of archeological materials can be an essential aspect of a field project and can significantly affect the long-term preservation of objects in the resulting collection. The conservation and preservation care that an object receives in the field may be the only care it ever receives. Similarly, all the records produced in the field, including field notes, maps, photographs, standardized forms such as site forms, artifact catalogs, audio tapes, and a variety of digital data, must be created, managed, and handled with care in the field and lab.

Archeologists should consider and practice some basic principles and methods of conservation at all times. On a large site or one where there is likely to be a wide range of complex conservation needs, it is preferable to have a professional conservator on site.

Material Remains

An archeological object in the ground is in a state of equilibrium with, or is adapting to its surroundings when found (Sease 1994). When it is taken out of the ground and exposed to air and different temperature and humidity levels, that equilibrium is disturbed. The object immediately begins to react to the changes in ways that are both visible and invisible to the archeologist. Most often these changes are in the direction of object deterioration. Planning for conservation in the field is therefore essential for the long-term preservation of archeological objects through condition assessment and appropriate actions. When planning for field conservation needs, it is important to consider:

The kinds of material remains anticipated;
What types of conservation treatments may be needed in the field;
Volume and kinds of archival quality storage materials that will be required to transport the collection from the field to the lab or repository;
How the material remains can best be collected to facilitate their long-term preservation.

The type of soil in which objects lie can be used to anticipate the condition and conservation needs of the recovered objects. Below is a chart that outlines the general preservation of objects in some basic types of soil conditions. Alkaline soils have a pH above 7.0 and are most common in arid climates where evaporation exceeds precipitation. Acidic soils, with a pH below 7.0, usually occur in areas of high rainfall and low evaporation. They are also found in areas where there has been an incomplete breakdown of organic materials in an anaerobic environment (e.g., peatbog). Saline soils contain a preponderance of salt. They are usually found in areas that have been inundated by seawater, but can also be created by human objects and actions (e.g., deterioration of metal objects; concentration of waste matter or wood ash). Crystallization of the salt in the soil can occur in areas where evaporation exceeds precipitation.

Soil Type (adapted from Sease 1994)

Acidic Alkaline Saline Water-
logged Acidic Water-
logged Alkaline Desert Arctic

Ceramics R-calcar-
eous fillers dissolve P-basic structure affected P R P G-wind erosion possible G

Lithics G G P-soluble salts P P-insoluble salt encrus-
tation G-wind erosion possible G

Glass & Glazes R-alkali leaching P-basic structure affected P R P G-wind erosion possible G

Wall Plaster P G P P P G G

Shell P G P-soluble salts P P G G

Metals

Iron P-
corrosion G P-
corrosion G G G G

Copper Alloys P-
corrosion G P-
corrosion G G G G

Lead P P R G G G G

Silver P G G-slight saline
P-high saline G G G G

Organics

Bone, Ivory, Antler P G P-soluble salts P P G G

Wool, Leather, Hair deter-
ioration of protein P R-dehyd-
ration G G G G

Wood, Cotton, Linen P P R-dehyd-
ration G G G G

G=good preservation
R=reasonable preservation
P=poor preservation

While the conservation needs of different classes of material remains differ, there are a few key principles that should be followed in the field to help ensure proper conservation of all materials:

Choice of excavation tools affect the materials and condition of an object. Determine the best tools that will inflict the least harm prior to field work.
A number of factors, such as water, temperature, humidity, and sunlight, affect the stability of an object in the ground during and directly after excavation.
Always immediately cover up an object or a group of objects that seem unstable. Contact a professional conservator for advice on in situ treatment and methods to remove the object(s).
Objects should be kept in bags or containers with like materials. Improper mixing can cause damage to some objects (e.g., placing bones or soft ceramics in containers with large and heavy lithics).

Field conservation measures can range from simple, common sense activities to complex procedures. The following are some general guidelines for conservation in the field. Remember that any procedures applied to an object should be reversible -- they can be later be removed without any damage to an object and its constituents. A professional conservator should always be consulted for complex treatments or if there are any questions about correct conservation procedures. In particular, be sure appropriate adhesives and consolidants are selected.

Handling

Always assume an object is fragile. The true condition of an object may not be immediately apparent.
Handle objects as little as possible. Do not pick up objects by handles, rims, or other attachments.
Avoid bending flexible objects.

Lifting

Physical support created for lifting delicate bone in the field. Photograph courtesy of Jessica Johnson.

The method chosen to lift an object out of the ground depends on its strength, size, weight, composition, and condition, as well as the condition of the soil matrix.
Assess the object condition, then record information, sketch and/or photograph the object before lifting it out of the ground.
Remove as much dirt surrounding an object as possible before removal. Do not flick or pry an object out of the ground.
Support the object at all times. A pedestal of dirt may be left underneath the object for support while continuing to excavate around it.
Lifting an object out of the ground with its surrounding dirt (block lifting) is useful for extremely fragile objects. The appropriate method of block lifting depends on the size and weight of the object and on soil condition.

Bandaging and Consolidation

A bandage can be used to support fragile objects once they have been excavated. A bandage consists of gauze or cloth strips wrapped around an object in layers. Adding plaster or resin can strengthen the bandage, but do not glue or plaster a bandage directly to an object. It is critical to apply a separate layer between the bandage and object.
Backing an object is useful for fragile, flat objects. Backing usually involves the application of a rigid bandage to the object. Some PVA emulsion, Acryloid B72, or plaster can be used for rigidity. Do not use Elmer's Glue-All®.
Consolidants should only be used when absolutely necessary and in consultation with a professional conservator. The choice of consolidant will depend on the type and condition of the materials involved. Consolidation should not be attempted on waterlogged materials.
Consolidants can be applied to fragile objects to join pieces and allow for lifting and handling. Consolidants should have: 1) good adhesive and cohesive properties; 2) achieve good penetration; 3) be durable, stable, and reversible; and 4) not alter the appearance of the material consolidated.
Do not consolidate any material that will be used for dating or scientific analysis.
Clean an object thoroughly before applying a consolidant. The most common consolidants are PVA emulsions or resins and Acryloid B72. Allow the consolidant to dry completely before lifting the object out of the ground.

Records

(photo) Mapping a dog burial in the field.
Mapping a dog burial in the field. From the photograph collection of the Bureau of Land Management, Anasazi Heritage Center, Dolores, Colorado.

The records created in the field, as well as in the lab, are vulnerable to insects, vermin, mold, humidity, light, temperature changes, and mishandling. They are also vulnerable to a variety of environmental threats, such as roof leaks, flooding, fire, and asbestos problems, and to theft or other malicious action.

There are a number of general recommendations to follow in the field and lab in order to promote the long-term preservation and viability of the great variety of records created:

use appropriate long-lived media for all record types;
use permanent and archival stock in paper, ink, lead pencil, folders, and boxes;
inspect and redo damaged or inadequate records;
label everything, or their containers;
use appropriate storage for all media in the field in order to protect them from poor environmental conditions and threat of fire or theft;
carefully consider existing guidelines and equipment for digital and audiovisual media, make sure backup copies and hard copy printouts exist, and migrate data to updated software on a regular schedule; and,
ensure that project information and data is captured by appropriately knowledgeable staff.

There are also a number of conservation principles to consider for each of the primary types of media used for associated records.

Paper records

Use high alpha cellulose, lignin free, acid-free paper, especially for field notebooks and standardized forms.
Record information using archival (permanent carbon) inks or #4 (HH) pencils.
Protect paper from water and humidity, and minimize its exposure to light.
Try not to fold or roll paper.
Store papers in archival folders in polyethylene boxes.

Photographs

Protect all photographic materials (e.g., film, prints, slides, negatives, and transparencies) from heat, rain, and wind. Store them in archival folders in polyethylene boxes.
Maintain a log of all photographic images.
Only handle photos along their edges. Do not touch the image with bare fingers.
Do not use paper or plastic clips, rubber bands, pressure sensitive tape, adhesive or pressure sensitive labels, or Post-it® notes directly on photographs.
Do not put photographic materials, except unused film, in cold storage without reformatting them for access and duplication.

Magnetic Records

Protect all magnetic materials (e.g., audio tapes, video tapes) from heat, dust, and dirt.
Consider the equipment required to play the audiovisual material and the longevity of that equipment.
Label all records in a permanent, carbon-based ink.
Store the records in their cases in polyethylene boxes.

Cartographic and Oversized Records

Oversized records should be stored flat in folders, preferably in map cases. Do not roll or fold.
Protect paper from water and minimize its exposure to light.
During storage and use, protect oversized records from tears and rips. Do not use tape to repair tears.
Label the oversized folders in permanent, carbon-based ink.

Digital Records and Data

Produce your master records in uncompressed TIFF format, if possible. Avoid using proprietary file formats or lossy compression.
Protect all digital records from heat, dust, dirt, and ultraviolet radiation.
Choose a storage medium that is considered a standard. Research its longevity.
Keep digital records away from magnetic or electric fields that are created by old telephones, static, and field and lab equipment such as magnetometers and 12 volt transformers. Computer diskettes can be partially or completely erased by such exposure.
Label the records in permanent, carbon-based ink.

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