How to Cast Stone with Jesmonite Using Silicone Moulds

A Complete Guide to Creating Lightweight External Architectural Elements

Architectural stone features bring richness, permanence, and sophistication to a building, yet traditional stone is heavy, fragile, and time consuming to install. Whether you're replicating historical details or designing custom pieces for new builds, there's a modern solution that outperforms stone in every practical sense.

Jesmonite AC730, AC830 or AC930, used with silicone moulds, allows makers and manufacturers to recreate stone textures and profiles with precision, durability, and reduced weight. In this guide, we’ll walk you through the full process using a window surround as an example, ideal for residential exteriors, restorations, or architectural statement pieces.

Tools & Materials Checklist

Jesmonite System:

• Jesmonite AC730, AC830, AC930 (recommended for outdoor-grade, stone-effect castings)

• Jesmonite Colour Matching Service

• Jesmonite Sealers (Penetrating or Nano-Sealers)

  
  

Mould Setup:

• High-grade silicone mould, platinum cure silicone yields best results (custom made or off-the-shelf)

• Mould housing or containment box (e.g. plywood)

• Digital weighing scales

• Mixing buckets

• Paddle mixer or high-torque drill with a Jesmonite high sheer mixing blade attachment

• Soft bristle brush

  
  

Optional Additives:

• Jesmonite Flex Metal Gel Coat (for hybrid decorative finishes)

• Glass fibre strands or mesh (for extra reinforcement)

• Mould release agent if using a rigid mould or surface retarder (Jesmonite APW Wax Available for mould release in rigid moulds)

• Vibrating table (to reduce bubbles)

Step 1: Preparing the Silicone Mould

Silicone moulds are flexible, reusable, and perfect for high-detail castings, especially when replicating complex architectural profiles like cornices, pilasters or, in our case, window surrounds.

What to do:

• Inspect your mould for dust, damage, or leftover material from previous casts.

• Optional - Silicone moulds will release products without a mould release, but some prefer to apply a thin layer of mould release using a brush or cloth to improve de-moulding and extend mould life. A surface retarder can also work in creating a rougher surface for a different finish.

• Place the mould on a level surface and brace it inside a containment frame to prevent distortion under weight.

  
  

Pro tip: Use moulds with realistic surface imperfections, tool marks, pits, or stone grain for added authenticity.

Step 2: Mixing Jesmonite AC730, AC830 or AC930

Jesmonite AC730 is formulated to mimic the appearance of natural stone, with embedded fine aggregates and a mineral base that accepts pigment beautifully.

Mixing Ratio (by weight):

• 5 parts base : 1 part liquid - AC730 & AC930

• 7 parts base : 1 part liquid - AC830

Pigmentation:

• Add any further powder pigment to the base powder  before combining with liquid.

• For natural stone finishes, stick to powdered oxide pigments:

  • Yellow / buff: warm limestone

  • Reds: terracotta or claystone

  • Black + greys: Portland stone, slate, or aged concrete

Mix with a paddle mixer until smooth and lump-free.

Step 3: Pouring and Layering into the Mould

There are two effective techniques for casting Jesmonite into silicone moulds, depending on size and application.

Option A: Face Coat + Backfill (Recommended for Large or Detailed Moulds)

1. Apply a thin “face/gel/mist coat” (3-5mm) into the mould using a brush or by pouring and spreading gently. Push the mix into recesses and corners.

2. Allow it to firm slightly, you will see a noticeable reduction in surface sheen as this process occurs, usually 5-10 minutes.

3. Backfill with remaining Jesmonite mix. Add fibres or mesh as needed.

Option B: Direct Pour (For Smaller or Simpler Pieces)

• Pour steadily into one corner and allow the mix to flow naturally allowing the mould to release air bubbles.

Structural tip: For large surrounds, embed mesh or loose chopped strands during the backfill stage for added strength.

Step 4: Curing & De-Moulding

Jesmonite sets quickly, but like any casting material, it continues to gain strength over 24-48 hours.

• Wait 3+ hours before de-moulding, depending on ambient conditions.

• Carefully remove the mould, starting with corners and working slowly to avoid damage.

• Lay the cast flat on a foam surface or raised gridded board to air-cure evenly.

Avoid stacking wet pieces or placing them in direct sunlight to prevent warping and staining.

Step 5: Surface Finishing

Once your cast is fully cured, you can refine the surface with decorative or practical finishes.

Optional Finishing Techniques:

• Slurry coat: Fill pinholes with a tinted slurry of Jesmonite and water.

• Acid Etching

• Shot Blasting

• Surface abrasion: Use sandpaper, wire brushes or light acid etching to expose aggregates and soften crisp lines.

• Ageing wash: Apply diluted pigments to recesses for a timeworn look.

Sealing:

• Use matte or satin sealer with a brush, roller or sprayer.

• For exterior installs, ensure the sealant is UV-stable, water-resistant and breathable, all first-party Jesmonite products provide these benefits.

Step 6: Installation – Window Surround Example

Your finished Jesmonite cast can now be fixed to your project location, whether that’s an outdoor facade or interior feature wall.

Substrate:

• Acceptable bases include concrete block, cement/render board, ICF (EPS) or rendered wall.

• Clean, dry and primed surfaces ensure better adhesion.

Fixing Method:

• Apply adhesive, compatible construction glue, or mechanical fixings like stainless steel screws and brackets (always consult an engineer for best fixing advice).

Alignment:

• Use temporary props or shims to level the surround as you fix each section.

• Leave small gaps (2-5mm) between sections for grout or movement joints.

On-site tip: If joins are visible, use tinted filler or caulk to blend gaps seamlessly.

Why Jesmonite for Cast Stone?

Jesmonite doesn’t just look like stone, it performs better in many key ways:

Property	Jesmonite	Cast Cement	Traditional Stone
Weight	Lightweight	Approx 25% heavier	Heavy, hard to transport
Detail replication	High fidelity	Good with limitations	Limited to tool capabilities
Curing time	3+ hours	24-48hrs	24+ hours
Weather resistance	Excellent	Good	Variable
Pigment integration	Internal colouration	Can be added	External only
Eco credentials	Water-based, sustainable	Medium carbon footprint	High carbon footprint

Sustainability bonus: Jesmonite is solvent free, made with local materials, and contributes to lower carbon manufacturing footprints.

Common Pitfalls & How to Avoid Them

Issue	Solution
Air bubbles on surface	Brush face coat into details; tap or vibrate the mould
Uneven pigment tone	Mix pigments into base first, mix thoroughly
Fragile corners or edges	Add fibre reinforcement or thicken critical areas
Mould sticking	Apply consistent mould release; don’t skip this step!
Cracking during install	Allow full cure (24–48 hours); use flexible adhesive if needed

Project Inspiration & Adaptation

This method isn’t limited to window surrounds, use the same techniques for:

• Cornices and pilasters

• Window Surrounds

• Full Portico

• Fireplace mantels

• Wall plaques and medallions

• Exterior trims or columns

• Garden features and water spouts

Whether you're restoring a listed building or reimagining a modern facade, Jesmonite offers design flexibility without the structural compromises.

A Smarter Way to Build with Stone

Casting architectural elements using Jesmonite and silicone moulds allows you to bypass the limitations of quarried stone, without sacrificing realism. You get:

• Faster production

• Lighter weight performance

• Unmatched detail reproduction

• Full creative control over colour and texture

• Durability in any environment

From bespoke residential builds to large-scale commercial cladding, this method scales up beautifully. And thanks to Jesmonite’s water-based formulation and sustainability credentials, you’re creating responsibly too.