Parts of bones, especially the interior, are often rotted into dust while the harder outer surface is still preserved.

The dust must be scraped out, the interior filled with a plaster cement, and the surface pieces re-set in position.

Very often a steel rod is set into the plaster filling the interior of a bone, to secure additional strength.
After this preparation is completed, each part being soaked repeatedly with shellac until it will absorb no more, the bones can be handled and laid out for study or exhibition.

Then, if they are to be mounted for a fossil skeleton, comes the work of restoring the missing parts.
For this a plaster composition is used.
Where only parts of one side are missing the corresponding parts of the other side are used for model; where both sides are missing, other individuals or nearly related species may serve as a guide.

But it is seldom wise to attempt restoration of a skeleton unless at least two-thirds of it is present; composite skeletons made up of the remains of several or many individuals, have been attempted, but they are dangerous experiments in animals so imperfectly known as are most of the dinosaurs.