Before I can begin sculpting a figure in clay, it is necessary to build an armature. The basic point of an armature is simply to hold up the clay. When doing a life-size sculpture it is important to build an armature strong enough to hold a lot of weight in clay, usually around 150lbs or more, depending on the physic of your figure.
Different Types of Amatures
Most of the time when sculpting a small figure (12"-36"tall) you build a wire armature from aluminum wire, and attach it to a pipe which is fastened to a board. This allows you to fashion the gesture of the armature in any way you like with the only fixed point being in the torso where the pipe meets the wire.
This is what we call an External Armature. Although there is an aluminum wire armature holding up the clay, it is given this name because it requires a support (the attaching pipe) that is visible externally to the clay model. In other words, an external armature requires a visible, external support which is not (normally) intended to be part of the piece. This external support is the main downfall of external armatures because they provide a distraction and don't allow you to view the piece entirely in the round without having to mentally "delete" the metal pipes. It also causes added problems when making a mold.
So, what if we have an armature inside the sculpture that doesn't require an external support? You can work on the sculpture from any angle without having to compromise with a pipe, and it makes it easier to visualize what the final product will look like. An armature like this would be made out of steel, welded together for strength, with its fixed points where the feet of the figure would meet the ground. This is what we call an Internal Armature.
Now, unlike an external armature, an internal armature is solid and immovable. The great thing about an external armature is that you can change the gesture at any moment. It is not necessary to commit to any certain position because the only fixed point is at the torso. The position of the legs and where the weight sits in the figure can be adjusted easily. As soon as we take the external pipe away, we take away the strength and support. In an
internal armature, the strength needs to come from within. However, like any armature, it needs to be grounded to a base, and the only point to do this with an internal armature is at the feet.
In order to know exactly where the feet are going to be, you also need to know exactly how the weight is going to sit in the pelvis, along with how the pelvis is going to relate to the ribcage and the head. Because you have to know exactly where these points are, you also have to be committed to a pose, because you cannot change it later. (Note: For this reason, internal armatures are not the best choices for small sketches because you cannot alter the pose. External armatures allow experimentation, where as internal armatures are for more finalized concepts).
To weld the armature in the correct position, you will need to plot the placement of the feet, along with important bone points on the base plate you intend for the armature to stand on. In order to find these points you will need a plum line and live model. When the model is in the pose you wish to sculpt, you drop a plum line from several key bones points. Crucial points include both trochanters (hip bones), knees, pubis synthesis, sternal notch, acromian processes, and C7. Where the plum line hits the plate, mark and label it. Also, don't forget to outline the feet! To the right is a picture of William Rinehart's Clytie, which I have used to illustrate a plum line being dropped from bone points. After these points are marked you have a guide for the angles at which your steel pieces need to be welded. For the arms you can attach adjustable aluminum wire. Because the arms are not weight-baring, their position does not need to be fixed like the rest of the pose.
This internal armature is what I have chosen to use for my sculpture of Liberty. A sturdy steel armature is perfect for holding the heavy clay I will be applying, as well as give me a 360 degree view without visual obstruction. Also, because I am certain of the pose I have no need for the ability to change positions.