Often preparators make exact replicas of the specimen, known as casts. Before doing this, they must first make a mold of the fossil. Molds are made by painting on thin layers of a special liquid that solidifies into rubber. Once the rubber is completely dried or cured, the specimen is removed from the mold. If the specimen in the mold is large, the mold is encased in a removable plaster jacket before the specimen is removed. Then, the mold is ready for making casts. The casts are made by pouring or painting into the mold a high quality plaster, which when dry can be painted the color of the specimen, or of colored epoxy, a kind of plastic. The casts resemble the original so closely that they are used for study when:

• Several scientists want to study the specimen at the same time.
• Several museums want to display a particular fossil, but there aren't enough fossil bones to go around.
• The original fossil must be returned to the country or state where it was found.
• The original fossil is too fragile to be displayed or studied.
• Museum visitors want to be able to handle the fossil.