🗿Sculpture Techniques Unit 7 – Mold Making and Casting in Sculpture

Key Concepts and Terminology

• Mold a hollow form or cavity used to shape a liquid or pliable material into a desired shape
• Casting the process of pouring a liquid or pliable material into a mold to create a solid object
• Positive the original object or sculpture from which a mold is made
• Negative the hollow space within a mold that represents the shape of the final cast object
• Release agent a substance applied to the mold surface to prevent the casting material from sticking (silicone spray, petroleum jelly)
• Undercuts areas on the positive object that are narrower than the widest part of the object, which can make mold removal difficult
  • Undercuts often require multi-part molds or flexible mold materials
• Parting line the seam where the parts of a multi-part mold come together

Materials and Tools

• Mold making materials include silicone rubber, latex rubber, alginate, plaster, and polyurethane
  • Silicone rubber is flexible, durable, and captures fine details, making it suitable for complex shapes and multiple castings
  • Alginate is a fast-setting, single-use material often used for body casting (life casting)
• Casting materials include plaster, concrete, resin, wax, and various metals (bronze, aluminum)
  • Resin is a versatile material that can be tinted, pigmented, or filled with other materials for unique effects
• Tools for mold making include measuring cups, mixing containers, stirring sticks, brushes, and spatulas
• Safety equipment such as gloves, goggles, and respirators is essential when working with certain materials (silicone, resin)
• Mold keys or registration marks ensure proper alignment of multi-part molds

Mold Types and Their Uses

• One-piece molds are used for objects with no undercuts and are the simplest to create and use
• Two-piece molds are used for objects with one main parting line and minor undercuts
  • Two-piece molds are often used for figurative sculptures or objects with a front and back
• Multi-part molds are used for complex objects with multiple undercuts or intricate details
• Waste molds are single-use molds that are destroyed during the casting process (often used with plaster)
• Flexible molds (silicone, latex) are ideal for objects with undercuts and can be reused multiple times
• Rigid molds (plaster, fiberglass) provide stability and are suitable for large-scale or heavy castings
• Slip casting molds are used for creating hollow ceramic objects by pouring liquid clay (slip) into a plaster mold

Preparation and Safety Measures

• Clean and prepare the positive object by filling in any holes, smoothing the surface, and sealing porous materials
• Create a mold box or containment area around the positive object to hold the mold making material in place
  • Mold boxes can be made from wood, cardboard, or foam board
• Apply a release agent to the positive object and mold box to prevent sticking and ease mold removal
• Wear appropriate safety gear (gloves, goggles, respirator) when working with mold making and casting materials
  • Some materials (silicone, resin) release fumes that can be harmful if inhaled
• Work in a well-ventilated area and follow the manufacturer's instructions for mixing and handling materials
• Properly dispose of any unused or excess materials according to local regulations

Mold Making Techniques

• Brush-on technique involves applying layers of mold making material directly onto the positive object with a brush
  • Brush-on technique is suitable for large or complex objects and allows for greater control over mold thickness
• Poured mold technique involves pouring the mold making material over the positive object in a mold box
  • Poured molds are faster to create and ensure a uniform mold thickness
• Blanket mold technique uses a flexible mold making material (silicone) that is draped over the positive object
  • Blanket molds are ideal for capturing texture and detail on the surface of an object
• Skin mold technique involves applying a thin layer of mold making material directly onto the positive object
  • Skin molds are often used in conjunction with a rigid outer mold (mother mold) for support
• Alginate molds are created by mixing alginate powder with water and applying it to the positive object (often used for life casting)
• Plaster piece molds are made by building up layers of plaster and burlap or jute over the positive object

Casting Materials and Methods

• Solid casting involves pouring the casting material directly into the mold and allowing it to fully set
  • Solid casting is suitable for small objects or when the final piece needs to be heavy and durable
• Hollow casting involves creating a hollow shell by rotating the mold as the casting material sets
  • Hollow casting is often used for large objects to reduce weight and material usage
• Slush casting involves pouring a small amount of casting material into the mold and rotating it to create a thin, hollow shell
• Pressure casting uses a pressure pot or vacuum chamber to force the casting material into the mold, reducing air bubbles and ensuring complete filling
• Cold casting involves mixing metal powders (bronze, aluminum) with resin to create a metal-like finish
• Lost wax casting is a multi-step process used for creating metal sculptures (bronze, silver)
  • A wax model is created, then a mold is made around it. The wax is melted out, and molten metal is poured into the mold.

Finishing and Post-Production

• Demold the cast object by carefully removing it from the mold once it has fully set
  • Flexible molds can be peeled away, while rigid molds may need to be chipped or broken away
• Clean up any flashing or seam lines on the cast object using tools like scissors, knives, or sandpaper
• Fill in any air bubbles or imperfections with a compatible filler material (epoxy, putty)
• Sand or polish the surface of the cast object to achieve the desired smoothness and finish
• Apply patinas, paints, or other surface treatments to enhance the appearance of the final piece
  • Patinas can be applied to metal castings to create a range of colors and textures (verdigris, rust)
• Seal the surface of the finished piece with a clear coat or wax to protect it from moisture and wear
• Attach any additional components (bases, mounting hardware) to complete the final sculpture

Troubleshooting Common Issues

• Air bubbles in the mold or casting can be minimized by tapping or vibrating the mold to release trapped air
  • Pressure casting techniques can also help reduce air bubbles
• Incomplete filling of the mold can be caused by low viscosity casting material or insufficient venting
  • Ensure the casting material is properly mixed and consider adding additional vents or pour holes
• Mold degradation can occur with repeated use, especially with flexible molds
  • Store molds properly and use a release agent to extend their lifespan
• Casting material not curing or setting properly may be due to incorrect mixing ratios or environmental factors (temperature, humidity)
  • Follow the manufacturer's instructions for mixing and curing, and control the workspace environment when possible
• Warping or shrinkage of the cast object can be caused by uneven cooling or insufficient support within the mold
  • Use a mold material with minimal shrinkage and ensure the mold provides adequate support during the curing process
• Difficulty removing the cast object from the mold may be due to undercuts or insufficient release agent
  • Carefully design molds to account for undercuts and apply release agents thoroughly to prevent sticking