Facial Approximation from the Skull
3D facial approximations are sculpted in clay over a replica of the skull. Applying clay to the actual skull should never be done because of the damage that might occur during the sculpting process. Not only is the skull an irreplaceable piece of evidence, it is a person.
The following shows the general process of the "Combination Method" of facial approximation. Here, the artist applies tissue depth markers to the replica, but adjusts the thickness of each area depending on the skull's individual shape and structure.
No matter which method of facial approximation is used, the forensic artist should always work in collaboration with a forensic anthropologist, use the most contemporary tissue depth data, and follow the most current scientific guidelines.
In the "Russian" and "Manchester"method of facial approximation, the artist sculpts the underlying musculature before adding fat and flesh to create the face.
This method is still fairly subjective, because there's no way for the artist to know how much fat and flesh should be applied on top of the sculpted muscles. This ends up being a time-consuming process that is, realistically, no more accurate than the combination method.
If an artist is unable to make replica of the skull, or simply prefers drawing to sculpting, then the 2D method can be done instead. Here, the artist takes photographs of the skull, and draws the features instead of sculpting them. Many artists, myself included, prefer sketching these digitally with a software like Photoshop. To me, it's a lot more efficient than using pencil and tracing paper, and messing around with a light box.