When a fossil arrives in a laboratory, it is usually embedded in a chunk of the rock it was found in. The first job for the specially trained technicians, called preparators, is to free the fossil from the rock and clean it up. Sometimes they can remove the rock from the fossil with chemicals. The fossil is left in a bath of acid for several months while the rock around the fossil, called the matrix, dissolves. Preparators also repair bones, strengthen any weak parts with glues and resins, and may make missing bones if the skeleton is to be reconstructed. Preparation is very time-consuming work. The Tyrannosaurus skeleton in the Field Museum in Chicago, USA, for example, took 12 people a total of 25,000 hours to complete.