# Structure, Armature and the Centre of Mass

Sculpture I · Modelling, Moulds and the Workshop · https://tryals.app/learn/sculpture-i/structure-armature-and-the-centre-of-mass

## Standing Up Is a Design Problem

A sculpture must stand to exist. The physics is straightforward to design with directly.

An object is **stable while the vertical line through its centre of mass falls inside its base of support**. Tip it, and the centre rises. Tip it further, and this vertical line crosses the base edge. The object then falls. **Three** consequences follow:

- **A wider base is more stable**.
- **A lower centre of mass is more stable**.
- **A projecting element moves the centre of mass towards it**.

The practical measure is the **tipping angle**. This is the angle a piece can lean before its centre leaves the base.

### The Armature

An **armature** is an internal frame that supports soft clay. Clay slumps easily under its own weight.

For ceramics, **the armature must come out before firing**. Clay shrinks during drying, but rigid armatures do not. A fixed core will crack the piece.

| Approach | How it works |
|---|---|
| **Removable armature** | Withdrawn at leather-hard stage; cavity is closed |
| **Cut and hollow** | Modelled solid, cut apart, hollowed, and rejoined |
| **Combustible core** | Paper or foam that burns out in the kiln |
| **Build hollow throughout** | Built using coils or slabs with no internal core |

For work that will **not** be fired, the armature stays inside. You can use wire, pipe, or wood. Hang **butterflies**—small wooden crosses on wire—to support clay across long spans.

### Designing the Support

You have three main design strategies for stability:

- **Widen the base.** This is simple, though sometimes visually plain.
- **Lower the centre of mass.** Keep heavier clay low and lighter forms high.
- **Add a visible support.** Integrate a strut, drapery, or plinth into the sculpture.

An unstable-looking piece can still stand securely. You must arrange the mass precisely so the centre stays over the base.

> **Common pitfall:** designing the form and then trying to support it. Settle the mass, base, and armature together with your initial design.

## Practice questions

9 of this lesson's 13 practice questions, with answers. The full set is in the app.

### 1. Why does lowering the centre of mass make a sculpture more stable, given the same base?

A. Tipping raises the centre of mass more steeply, so more work is needed to reach the tipping point
B. It distributes the gravitational load more evenly across the entire contact footprint of the base
C. It reduces the total gravitational force acting downward on the supporting armature of the piece
D. The vertical line from the centre can no longer cross beyond the outer perimeter of the base

**Answer:** A. Tipping raises the centre of mass more steeply, so more work is needed to reach the tipping point

**Why:** Because tipping **raises** the centre of mass, and a low centre rises more steeply for the same angle of tilt. More energy is needed to reach the point where the vertical leaves the base, which is what stability means physically. Total weight and base contact area remain unchanged, and the vertical line can still cross the perimeter if tilted sufficiently.

Page: https://tryals.app/practice/sculpture-i/structure-armature-and-the-centre-of-mass/why-does-lowering-the-centre-of-mass-make-a-sculpture-more-stable

### 2. Which are true of the condition for an object to stand?

A. A heavier object is always more stable than a lighter one
B. The vertical through the centre of mass must fall inside the base of support
C. A projecting element moves the centre of mass towards it
D. A wider base increases stability

**Answer:** B. The vertical through the centre of mass must fall inside the base of support; C. A projecting element moves the centre of mass towards it; D. A wider base increases stability

**Why:** Weight alone settles nothing, a heavy object with a high centre over a narrow base topples readily, and a light one with a wide base does not. Stability is about the **geometry** of where the mass sits relative to the base, which is why it can be designed.

Page: https://tryals.app/practice/sculpture-i/structure-armature-and-the-centre-of-mass/which-are-true-of-the-condition-for-an-object-to-stand

### 3. Complete the account of standing up.

**Answer:** An object is stable while the vertical through its centre of **mass** falls inside its base of **support**. An internal support carrying soft material is an **armature**. In ceramics it must be removed before firing because clay **shrinks** and the support does not.

**Why:** The last blank ties this lesson to the second one in the unit. Shrinkage is not merely a sizing inconvenience, it is the reason an entire category of support technique had to be invented for ceramic work.

Page: https://tryals.app/practice/sculpture-i/structure-armature-and-the-centre-of-mass/complete-the-account-of-standing-up

### 4. An armature must be removed before a model is moulded and cast, exactly as it must before firing.

**Answer:** False

**Why:** **False.** The armature comes out before firing because clay **shrinks** around a rigid core and because the core would not survive the kiln. A model destined for a mould does neither, so the armature stays in and can be as rigid as the job needs, pipe, batten and wire, with butterflies across a long span.

Page: https://tryals.app/practice/sculpture-i/structure-armature-and-the-centre-of-mass/an-armature-must-be-removed-before-a-model-is-moulded-and-cast

### 5. A sculpture leans until the vertical through its centre of mass reaches the edge of its base. What happens beyond that angle?

**Answer:** 0

**Why:** It falls, enter **0**. That angle is the tipping angle, and it is the practical measure of stability: a piece with a small tipping angle is one that a passing sleeve will take out, however solid it looks standing still.

Page: https://tryals.app/practice/sculpture-i/structure-armature-and-the-centre-of-mass/a-sculpture-leans-until-the-vertical-through-its-centre-of-mass

### 6. Sort each design move by what it does to stability.

**Answer:**

- Increases stability: Widening the base, Placing dense material low in the piece, Adding a visible strut into the composition
- Decreases stability: Adding a long projecting arm, Raising the mass into a tall narrow column, Cantilevering weight beyond the base edge

**Why:** The right-hand column is not a list of mistakes. Every one of those moves is available as a deliberate effect, and the demanding case is the piece that **looks** unstable and is not, which requires knowing exactly where the centre of mass sits rather than guessing.

Page: https://tryals.app/practice/sculpture-i/structure-armature-and-the-centre-of-mass/sort-each-design-move-by-what-it-does-to-stability

### 7. Arrange these decisions in the order they should be taken.

**Answer:**

1. Decide the form and where its masses sit
2. Locate the approximate centre of mass
3. Decide the base and check the vertical falls inside it
4. Decide what the armature must carry and how it comes out
5. Build

**Why:** Taking them in this order is the point of the lesson. A form settled first and supported afterwards can turn out to have no honest support at all, and the decisions constrain each other, which is why they belong together at the start.

Page: https://tryals.app/practice/sculpture-i/structure-armature-and-the-centre-of-mass/arrange-these-decisions-in-the-order-they-should-be-taken

### 8. Arrange these approaches to a ceramic armature from the least to the most intervention required after building.

**Answer:**

1. Build hollow throughout, needing no core
2. Use a combustible core that burns out in firing
3. Use a removable armature withdrawn at leather-hard
4. Model solid, then cut apart, hollow and rejoin

**Why:** The ordering is a practical guide: the further down the list, the more can go wrong after the form is already made. Cutting a resolved piece apart to hollow it is where most beginners lose work, which is a good argument for building hollow from the start when the form allows it.

Page: https://tryals.app/practice/sculpture-i/structure-armature-and-the-centre-of-mass/arrange-these-approaches-to-a-ceramic-armature-from-the-least-to-the

### 9. An armature prevents soft clay from slumping during modelling, but clay shrinks as it dries. What follows from this tension when choosing an internal support for ceramic sculpture?

A. Combustible cores are required for any hollow clay form
B. Internal framing must be avoided in ceramic construction
C. Rigid supports demand a strategy for removal or burnout
D. Armatures must remain fixed to resist drying distortion

**Answer:** C. Rigid supports demand a strategy for removal or burnout

**Why:** Rigid frames crack shrinking clay if retained, so ceramicists must hollow, withdraw, or burn out supports. Shrinkage cannot be resisted by force, nor does it preclude armatures altogether.

Page: https://tryals.app/practice/sculpture-i/structure-armature-and-the-centre-of-mass/an-armature-prevents-soft-clay-from-slumping-during-modelling-but