# Moulds I: Typology Sculpture I · Modelling, Moulds and the Workshop · https://tryals.app/learn/sculpture-i/moulds-i-typology ## The Problem a Mould Solves A mould records a form as a negative to produce positives. The craft turns on one central question: **how does the mould come off the original without destroying either?** ### Undercuts An **undercut** is any part of a shape that widens after narrowing. It locks a rigid mould in place so it cannot pull away in a straight line. Three strategies solve this problem: 1. **Destroy the mould** to release the cast. 2. **Divide the mould** into parts that draw away cleanly. 3. **Use a flexible material** that stretches over the undercut. ### The Typology | Type | Made of | Reusable | Handles undercuts | Typical use | |---|---|---|---|---| | **Waste mould** | Rigid plaster | No, chipped away | By destruction | Single cast from clay | | **Piece mould** | Rigid parts | Yes | By division | Multiples in plaster, resin | | **Flexible mould** | Silicone, alginate | Yes (silicone) | By stretching | Complex forms, life casts | | **Mother mould** | Rigid shell | Yes | Supports flexible layer | Floppy flexible liners | ### How Mould Types Work A **waste mould** produces a single cast from a clay model. Plaster is poured over the clay, the clay is dug out, and the positive is cast inside. The plaster is then **chipped away** to free the cast. A **piece mould** offers reusability. The form is split into sections that pull away without locking. The pieces align using interlocking **keys**, though every seam leaves a mark on the cast. A **flexible mould** stretches over undercuts easily. Because it lacks structural rigidity, it requires a rigid jacket called a **mother mould** to hold its shape during pouring. ### Flexible Materials * **Alginate**: Fast-setting and skin-safe for life casting. It shrinks within hours, making it strictly single-use. * **Silicone**: Highly durable and reusable. Silicone comes in two main types: **condensation-cure** and **addition-cure**. Addition-cure offers finer detail and lower shrinkage. However, its cure is **inhibited** by sulphur-bearing clay, latex, or certain resins. > **Common pitfall:** choosing the mould type based on budget rather than form. Always evaluate the undercuts first. ## Practice questions 9 of this lesson's 14 practice questions, with answers. The full set is in the app. ### 1. Sort each mould by how it deals with undercuts. **Answer:** - By being destroyed: Waste mould, A plaster mould chipped away from the cast - By being divided: Piece mould, A rigid mould made in five parts with keys - By stretching: Silicone mould, Alginate over a hand for life casting **Why:** Three strategies exhaust the possibilities, which is why the typology is short. Everything else — mother moulds, keys, release agents — is machinery serving one of these three decisions. Page: https://tryals.app/practice/sculpture-i/moulds-i-typology/sort-each-mould-by-how-it-deals-with-undercuts ### 2. Why does a flexible silicone mould usually need a rigid mother mould over it? A. The outer jacket acts as a chemical barrier preventing casting resin from bonding to silicone B. The rigid casing seals the outer seam lines completely so that low-viscosity resin cannot leak C. Silicone has no shape of its own and would distort under the weight of the casting material D. Silicone requires external compressive pressure from a rigid casing in order to fully cure **Answer:** C. Silicone has no shape of its own and would distort under the weight of the casting material **Why:** Because the flexibility that solves the undercut problem also means the mould **will not hold its own shape**. Pour plaster or resin into an unsupported silicone skin and it deforms under the weight, so the cast comes out distorted. The mother mould is a rigid shell that holds it in position. Page: https://tryals.app/practice/sculpture-i/moulds-i-typology/why-does-a-flexible-silicone-mould-usually-need-a-rigid-mother-mould ### 3. Addition-cure silicone can be prevented from curing by contact with sulphur-containing plasticine. **Answer:** True **Why:** **True**, and it is an expensive lesson. Addition-cure silicone gives finer detail and lower shrinkage and is inhibited by sulphur-bearing clay, latex and certain resins, so a form modelled in ordinary plasticine can leave a mould that never sets. Sulphur-free plasticine exists precisely for this. Page: https://tryals.app/practice/sculpture-i/moulds-i-typology/addition-cure-silicone-can-be-prevented-from-curing-by-contact-with ### 4. Which are true of alginate as a moulding material? A. It is the standard material for life casting B. It sets in minutes and is safe against skin C. It degrades within hours, so a positive must be cast almost immediately D. It can be stored and reused for many casts **Answer:** A. It is the standard material for life casting; B. It sets in minutes and is safe against skin; C. It degrades within hours, so a positive must be cast almost immediately **Why:** The fourth belongs to silicone. Alginate is single-use in practice because it shrinks and degrades within hours of setting, which makes it ideal for the one thing it is used for, taking a mould from a living body quickly and safely, and useless for anything requiring an edition. Page: https://tryals.app/practice/sculpture-i/moulds-i-typology/which-are-true-of-alginate-as-a-moulding-material ### 5. Complete the mould vocabulary. **Answer:** A form that locks a rigid mould in place is an **undercut**. A mould chipped away to release its single cast is a **waste** mould. Registration notches that make the parts reassemble in one way are **keys**. A rigid shell supporting a flexible mould is a **mother** mould. **Why:** The first blank is the concept everything else answers. Count the undercuts and the mould type follows almost automatically, which is why it is the first thing to look for on any form you are asked to mould. Page: https://tryals.app/practice/sculpture-i/moulds-i-typology/complete-the-mould-vocabulary ### 6. What is the minimum number of rigid mould pieces needed for a form with no undercuts at all? **Answer:** 2 **Why:** **Two.** Even a form with no undercuts needs the mould divided, because a single rigid negative enclosing a volume has no direction in which to come off. Two is the floor, and the count climbs from there with the undercuts. Page: https://tryals.app/practice/sculpture-i/moulds-i-typology/what-is-the-minimum-number-of-rigid-mould-pieces-needed-for-a-form ### 7. A waste mould resolves undercuts by destruction rather than division or stretching. What does this principle imply for the positive cast and the original model? A. The technique avoids seam lines whilst preserving the original clay B. The mould can yield multiples only if the original model survives C. The positive must be flexible to pull free from the rigid plaster D. The model is lost during demoulding and only one cast is obtained **Answer:** D. The model is lost during demoulding and only one cast is obtained **Why:** Waste moulding is a sacrificial process at both stages: the original clay is hollowed out to pour the positive, and the plaster is chipped off to release it. Thinking the original survives confuses waste moulds with piece moulds, while flexibility applies to rubber liners. Page: https://tryals.app/practice/sculpture-i/moulds-i-typology/a-waste-mould-resolves-undercuts-by-destruction-rather-than-division ### 8. Arrange the steps of deciding how to mould a form, in order. **Answer:** 1. Examine the form and count the undercuts 2. Decide whether to destroy, divide or stretch 3. Choose the mould material that implements that decision 4. Plan the division lines and keys, or the mother mould 5. Apply release agent and build the mould **Why:** Counting the undercuts first is the whole discipline. Choose the material first, usually from what is in the cupboard, and you have committed to a strategy before knowing whether the form permits it, which is discovered when the plaster has already set. Page: https://tryals.app/practice/sculpture-i/moulds-i-typology/arrange-the-steps-of-deciding-how-to-mould-a-form-in-order ### 9. The division lines of a piece mould can be placed anywhere, since the seams are cleaned off the cast afterwards. **Answer:** False **Why:** **False**, and it is the most expensive assumption in mould-making. Every cast the mould ever produces carries the seam, and cleaning it is hand work on each one, so the lines are placed along an edge, in a shadow, or where the form changes direction anyway. A badly placed seam is permanent, on every positive. Page: https://tryals.app/practice/sculpture-i/moulds-i-typology/the-division-lines-of-a-piece-mould-can-be-placed-anywhere-since-the