Running Moulds & Fibrous Plasterwork

Module 5: Running Moulds & Fibrous Plasterwork 

This module covers advanced techniques for creating ornamental plasterwork, including mould-making, casting decorative elements, and reinforcement methods for durability.

1. Making and Using Running Moulds

Running moulds are templates used to produce consistent plaster profiles for cornices, architraves, and other decorative elements.

Creating the Mould

• Materials Needed:

• Timber or MDF for the mould frame

• Profile gauge or CNC-cut template

• Zinc or plastic for the cutting edge (to ensure smooth plaster flow)

• Design Process:

• Trace or scan an existing profile to replicate it

• Cut the template to exact dimensions using a bandsaw or router

• Assemble the mould with adjustable arms for different project sizes

Using the Mould

• Surface Preparation:

• Fix grounds (timber guides) to walls/ceilings to ensure straight runs

• Apply a bonding agent to the substrate

• Plaster Application:

• Load the mould with a stiff gypsum or lime plaster mix

• Pull the mould along the grounds in smooth, steady motions

• Repeat passes to build up thickness and refine details

• Finishing:

• Remove minor imperfections with a small tool or sponge

• Blend joints and seams while plaster is semi-set

2. Casting Decorative Plaster Features (Ceiling Roses, Columns, Corbels)

Fibrous plaster casting allows for intricate, lightweight decorative elements.

Mould Preparation

• Types of Moulds:

• Waste Moulds (One-Use): Made from clay or plaster

• Flexible Moulds (Reusable): Silicone or latex for complex undercuts

• Piece Moulds (Multi-Part): For large or detailed items like columns

• Mould Release:

• Apply a separator (petroleum jelly or specialist release agent)

Casting Process

• Mixing the Plaster:

• Use fine casting plaster or dental plaster for sharp detail

• Add fibers (sisal, glass) for reinforcement if needed

• Layering Technique:

• First coat: Brush plaster into mould details

• Subsequent coats: Build up thickness (3-5mm) with hessian/jute embedded

• Demoulding & Finishing:

• Remove once set but still slightly green (prevents cracking)

• Trim flash lines with a scalpel or riffler

Installation

• Ceiling Roses:

• Fix with plaster adhesive and screws, then blend joints

• Columns & Corbels:

• May require internal armatures (metal or timber frames)

3. Reinforcing with Hessian and Jute Scrim

Traditional reinforcement methods prevent cracking and add strength.

Material Properties

• Hessian (Burlap):

• Coarse weave for bulk reinforcement in cornices

• Soaked in plaster slurry before application

• Jute Scrim:

• Finer weave for delicate features like ceiling roses

• Self-adhesive variants available

Application Methods

• Embedding in Wet Plaster:

• Lay hessian over fresh plaster and trowel to embed

• Overlap edges by 50mm for continuity

• Pre-Reinforced Castings:

• Dip scrim in plaster and layer into moulds

• Cross-hatch orientation for multi-directional strength

Modern Alternatives

• Glass Fiber Mesh: For high-stress areas

• Carbon Fiber: Ultra-thin reinforcement for restoration

Key Tools for the Workshop

• Profile Gauges – For copying existing mouldings

• Casting Knives – For trimming and detailing

• Hog’s Hair Brushes – To work plaster into mould details

• Running Mould Bench – With adjustable guides

Summary

This module teaches:
✓ Running mould fabrication for consistent ornamental work
✓ Casting techniques for complex decorative pieces
✓ Traditional and modern reinforcement methods

Pro Tip: For historic restoration, match original plaster mixes (e.g., horsehair in lime plaster) to preserve authenticity.