# Moulds II: Materials and Procedure

Sculpture I · Modelling, Moulds and the Workshop · https://tryals.app/learn/sculpture-i/moulds-ii-materials-and-procedure

## Plaster

Studio moulds are still largely **plaster**. This material rehydrates and sets into a rigid, porous solid. The reaction is **exothermic**, warming noticeably as it goes off.

Two core rules govern its use:

- **The ratio matters, and more water is not more forgiving.** Measure by weight, typically **70 parts water to 100 parts plaster** (Plaster · 100 · 70). Extra water leaves the set plaster weak and overly porous.
- **The sequence is not optional.** Sift plaster into water. Let it **slake** for two minutes, mix gently, and pour promptly.

Plaster moulds must be **thoroughly dry** before reuse.

### Release agents

Release agents prevent bonding between surfaces:

| Situation | Typical release |
|---|---|
| Plaster against plaster | Soft soap, or shellac then wax |
| Plaster against clay | Often none needed |
| Silicone against most surfaces | Usually none; seal porous surfaces |
| Resin against silicone | Usually none |

### Key Operations

- **Keys**: registration notches cut into a piece so parts lock into exact alignment.
- **Walls or shims**: temporary barriers dividing the form during multi-part pours.
- **Undercut removal**: filling minor recesses with clay to simplify the mould.
- **Venting and pouring**: providing entry for material and an escape path for trapped air.
- **Warning layer**: tinted plaster placed against the positive in waste moulds to signal where to stop carving.

### Workshop Safety

Plaster dust is a respiratory irritant. Plaster must **never** go down a sink drain, as it sets underwater and blocks pipes.

> **Common pitfall:** treating small preparation steps as optional fussiness. Keys, vents, and warning layers prevent catastrophic cast failures. They are easy to add during setup, but impossible to fix afterwards.

## Practice questions

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

### 1. Why is plaster sifted into water and left to stand before mixing? Select every true statement about the procedure.

A. Plaster is sifted into water rather than water poured onto plaster
B. Adding extra water produces a weaker, more porous set
C. It is left to slake for a couple of minutes before mixing
D. Extra water extends the working time without any cost

**Answer:** A. Plaster is sifted into water rather than water poured onto plaster; B. Adding extra water produces a weaker, more porous set; C. It is left to slake for a couple of minutes before mixing

**Why:** The fourth is the tempting mistake. Extra water does not buy working time for free, it produces plaster that is weaker and more porous, which for a mould means a face that degrades faster and a shell that breaks in handling.

Page: https://tryals.app/practice/sculpture-i/moulds-ii-materials-and-procedure/why-is-plaster-sifted-into-water-and-left-to-stand-before-mixing

### 2. The setting of plaster is exothermic, so a setting block becomes noticeably warm.

**Answer:** True

**Why:** **True**, and it is the most reliable sign that plaster has gone off. The hemihydrate rehydrates to the dihydrate and the reaction releases heat, so a block that is warming is setting, which is more informative than poking it.

Page: https://tryals.app/practice/sculpture-i/moulds-ii-materials-and-procedure/the-setting-of-plaster-is-exothermic-so-a-setting-block-becomes

### 3. Complete the account of plaster and moulding.

**Answer:** Plaster is sifted into water and left to **slake** before mixing. Registration notches that make mould pieces reassemble in one position are **keys**. For a waste mould, plaster tinted with pigment next to the positive is a **warning** layer. Plaster must never be put down a **drain**, because it sets underwater.

**Why:** The last blank is worth stating as chemistry rather than as a rule. Plaster sets by rehydration, so being underwater does not stop it, it sets **in** the pipe, which is why every plaster studio has a settling trap and a rule about it.

Page: https://tryals.app/practice/sculpture-i/moulds-ii-materials-and-procedure/complete-the-account-of-plaster-and-moulding

### 4. Plaster moulding requires careful mechanical preparation rather than relying on chemical separation alone. Why must a warning layer be tinted and placed immediately against the positive in a waste mould?

A. It alerts the caster to stop chipping before hitting the cast
B. It prevents the final casting resin from adhering to plaster
C. It ensures air bubbles are drawn away from delicate surfaces
D. It indicates when the exothermic setting reaction is finished

**Answer:** A. It alerts the caster to stop chipping before hitting the cast

**Why:** Waste moulds are destroyed to retrieve the single cast inside; without a coloured barrier, tools easily gouge the positive. Chemical release and venting resolve adhesion and trapped air, but only visual contrast prevents mechanical damage during breakout.

Page: https://tryals.app/practice/sculpture-i/moulds-ii-materials-and-procedure/plaster-moulding-requires-careful-mechanical-preparation-rather-than

### 5. Sort each situation by whether a release agent is usually NEEDED or NOT.

**Answer:**

- Release agent needed: Plaster cast against a plaster mould, Plaster against plaster with no barrier, Plaster over a porous unsealed original
- Usually not needed: Plaster mould over damp clay, Silicone against a sealed non-porous surface, Resin cast in a silicone mould

**Why:** Plaster against plaster is the failure everyone commits once: two chemically identical pieces set against each other bond permanently, and the mould and the cast become a single lump. Soft soap or shellac and wax exist entirely to prevent it.

Page: https://tryals.app/practice/sculpture-i/moulds-ii-materials-and-procedure/sort-each-situation-by-whether-a-release-agent-is-usually-needed-or

### 6. Roughly how many parts of water per 100 parts of plaster does a typical pottery plaster use, by weight?

**Answer:** 70 (within ±8)

**Why:** Around **70 parts to 100**. The exact ratio is a property of the specific plaster and is printed on the bag, which is where it should be read from, the number here is a working expectation rather than a specification, and different plasters vary substantially.

Page: https://tryals.app/practice/sculpture-i/moulds-ii-materials-and-procedure/roughly-how-many-parts-of-water-per-100-parts-of-plaster-does-a

### 7. Arrange the steps of mixing and pouring plaster in order.

**Answer:**

1. Weigh the water and the plaster to the stated ratio
2. Sift the plaster into the water
3. Let the mix slake for a couple of minutes
4. Mix gently, avoiding folding in air
5. Pour before the mix begins to thicken

**Why:** Slaking before mixing is the step people skip and the one that most affects the result: it lets the plaster absorb water evenly before agitation starts, which produces a denser, stronger set with fewer air pockets in the mould face.

Page: https://tryals.app/practice/sculpture-i/moulds-ii-materials-and-procedure/arrange-the-steps-of-mixing-and-pouring-plaster-in-order

### 8. Roughly how many minutes of working time does a typical plaster mix allow between pouring the mix and it becoming unusable?

**Answer:** 8 (within ±5)

**Why:** On the order of **eight minutes**, and highly dependent on the plaster, the water temperature and how much it has been agitated. The number matters less than the habit: everything — mould walls, release agent, containers, the original — must be ready before the plaster touches the water.

Page: https://tryals.app/practice/sculpture-i/moulds-ii-materials-and-procedure/roughly-how-many-minutes-of-working-time-does-a-typical-plaster-mix