Unless they are hermetically sealed, caskets do not preserve bodies. They do protect corpses from water, the weight of the soil and shifting earth. Sealed caskets can aid in preservation by greatly slowing decomposition, and they are often used in conjunction with embalming. Caskets are frequently placed in burial vaults made of concrete and other materials, which can protect the casket and the body inside. Vaults can also be sealed to offer further preservation.
Bodies begin to decompose immediately upon death, and preservation techniques need to be implemented rapidly in order to slow decomposition. The rate of decomposition varies and depends on temperature, moisture, exposure to insects and many other factors. In addition to placing a body in a sealed casket, embalming is often used to delay decomposition. Embalming has been practiced for centuries, and modern embalming involves the replacement of bodily fluids with stable chemicals like formaldehyde. The combination of embalming and a sealed, oxygen-free casket offers the best preservation.
Typically and historically, the primary material in casket-making is wood. Where available, preference is given to decay-resistant hardwoods like oak. However, wood is not a good preservative and rapidly decays when buried in moist soil. By the mid-20th Century, the use of metals in casket-making became popular. Tightly sealed metal caskets made of steel or copper can greatly aid in preservation of the body inside. Other materials such as concrete and fiberglass likewise have greater preservation capabilities than wood.
Hermetically-sealed caskets became more common and more widely available by the 1970s. These caskets provide an air-tight seal and deprive the interior of oxygen and insects, which greatly reduces decomposition of the corpse. In addition to their use in the funeral industry, sealed containers are necessary for transporting bodies between countries. Disinterment of bodies in sealed caskets has demonstrated remarkable preservation, but some controversy remains about their longevity and ability to permanently maintain an air-tight seal.
Burial vaults have been widely used since the mid-20th Century. They protect the casket from the weight and shifting soils they are buried in. The casket is simply placed within the vault, and the vault itself can also be buried or extend all the way to the surface. Vaults that have their openings on the ground surface are used in swampy environments. Burial vaults are usually made of concrete and protect the casket on all four sides, plus provide drainage for water. They can also be hermetically sealed.
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