Clay modeling (or clay model making) was started by Harley earl (who was head of GM Styling studio) in 1930s. This design studio was earlier known as Art and color section, then name of this section was changed as design and styling department.
Today all cars born with clay models because clay is smooth and can mold in any
shape. Designer can make any shape to visualize to their creation.
In this process first of all we designer create scale model of car on the paper then on paper or any drafting software they calculate dimensions of the car.
First part of the clay modeling is creating a rig; rig is basically foundation of clay modeling. It can be made of wood or any strong material. If model is scale of ? then it can be a table so designer can stand and work around it.
Once rig is prepared it is divide in to the grids so that is dimension of the paper design can be referenced on the rig. Designer properly and strictly follow dimension of the design. It is also possible to designer can do some experiment on the clay model and get very new model.
After completing the clay model it is first used for visualization as a concept car. Once this concept is approved by management of company, designer start making hard model and start putting sheets metal and real glass etc