Shell is the hard exterior covering of mollusks such as clams and oysters. Shell is built up on layers and is composed primarily of calcium carbonate and a protein similar to collagen called conchiolin. Whole shells are often found in midden deposits or shell is made into decorative objects, jewelry, inlay, buttons and beads. Shell beads which have been coated with a lacquer or resin in the past which did not have good light-fast properties and has now turned brown.
Shell beads which have been coated with a lacquer or resin in the past which did not have good light-fast properties and has now turned brown. Photo by M. Myers. Used by permission of the Virginia Department of Historic Resources.
Shell survives fairly well in alkaline environments. In acidic environments, it becomes friable and powdery as the calcium carbonate dissolves.
Once excavated, if the shell appears to be in good condition and has no evidence of surface decoration, it can be washed in water with a soft brush. Metal and any other hard tools, even hard-bristle brushes, will easily scratch the surface of the shell. To remove dirt on soft, friable shell, Use a soft brush; if the soil is compact, first slightly soften it with a swab of water and then remove it with a wooden pick-do not try to rub, pry or chip the dirt off, as this will cause the surface of the artifact to come off with the dirt. Soft, friable shell artifacts may need consolidation, and can be consolidated using a 10% solution of Acryloid B-72 in acetone by brushing it on the surface of the completely dry artifact.
Post excavation, shell is prone to a form of deterioration known as Byne’s disease. It manifests itself as a crystalline deposit (calcium acetate) on the surface of the shell and is associated with storage in environments rich in organic acid, particularly plywood, chipboard, cardboard, newspaper, other wood products and formaldehyde resins. Therefore, shell artifacts should be stored in acid free boxes or in polycarbonate plastic boxes or polyethylene bags. Acid free tissue or inert polyethylene foam, such as EthafoamÂ® or Volara, should be used to cushion individual pieces.
Sourche from; http://www.sha.org/research_resources/conservation_faqs/treatment.cfm