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Manual: Making a silicone mold of an ornament or decorative concrete
Guide: Making a silicone brush-on mold from an ornament or decorative concrete
In this guide, you will learn step-by-step how to copy an existing ornament or decorative concrete using thickened silicone (brush-on mold) and an epoxy support shell. The mold created can then be used to cast exact copies in concrete, plaster, polyurethane, or other casting materials. Suitable for beginners and professionals alike.
What will you make?
You will create a flexible brush-on mold from an existing ornament or decorative concrete. The mold consists of two parts: a layer of thickened silicone that captures every detail of the original, and a rigid epoxy support shell that holds the mold in shape during casting. Once the mold is ready, you can produce accurate copies, including all textures, reliefs, and details of the original.
💡 Suitable for: concrete, plaster, polyurethane, wax, and more. The silicone mold is resistant to temperatures from -40 °C to +280 °C and is reusable for dozens to hundreds of castings.
Required materials
🛒 From the webshop
- Silicone Condensation 20 Universal Set – 1 kg A + 50 g B
- Thixo C – Thickener for brush-on molds (50 gr)
- Epoxy Support Shell Fiberglass Set – for rigid support shell
- Digital Scale 0–5 kg
- Showa 7545 disposable glove (Nitrile)
- 2 mixing cups (preferably flexible)
- Stirring stick or spatula (Mixing and 3 mm strips for rolling out Epoxy)
- Brush or paintbrush (for contact layer)
🧰 Arrange yourself
- Ornament or decorative concrete to be copied (the model)
- Household or pallet wrap
- Rolling pin or round rod
- Flat, level surface
Preparation
Before applying silicone to the ornament, it is essential that the model is clean and dust-free. The silicone will pick up every detail - including cracks, scratches, dirt, and fingerprints. Repair any damage beforehand: fill cracks, fill holes, and restore missing parts. Bring the model to its desired final state before you begin.
Check if the model needs a release agent. Porous materials such as untreated concrete, stone, or plaster can stick to the silicone. If in doubt, always do a small test on an inconspicuous part of the model first.
⚠️ Please note: Porous materials (untreated concrete, stone, plaster, wood) can stick to the silicone. In this case, use a release spray or sealer before you begin. Always test on a small area.
Step-by-step guide
Prepare model and apply release agent
Thoroughly clean the ornament. Remove dust, dirt, and loose particles. Repair any damage you do not want to replicate in the mold. If necessary, apply a release agent or sealer to porous surfaces and let it dry completely.
💡 Tip: Always do a small test on an inconspicuous part of the model to check if a release agent is necessary and if the silicone releases well.
Fig. 1 – The model: an ornament or decorative concrete ready for processingWeigh silicone and add Thixo C
Accurately weigh the A-component into a mixing cup. Add the B-component in a ratio of 100 : 5 by weight (variable between 4.5% and 5.5%). Mix thoroughly for 2–3 minutes.
Then add 0.6% Thixo C to the mixture and stir well again. Let the mixture then rest for 2 minutes so that the thickener can do its job – the mixture will become noticeably thicker and will cling to vertical surfaces.
💡 Pro-tip: Add a small amount of dye to the B-component before mixing. This allows you to visually check if the mixture is completely homogeneous – no more white streaks means well mixed.
⏱️ Mind the pot life: Silicone Condensation 20 has a working time of ± 45 minutes. Make sure all materials are ready before you start mixing.
Fig. 2 – Detail photo of damaged ornamentApply contact layer with a brush
Apply a first thin layer of silicone to the model with a brush or paintbrush. Work carefully and use a dabbing (tapping motion) in areas with a lot of detail or in deeper corners. This prevents air bubbles directly on the model surface.
Ensure the entire surface is covered. Let this contact layer set for 10–15 minutes before applying the next layer.
💡 Advanced: For maximum detail quality, you can apply the first contact layer without Thixo (liquid), and only add Thixo for thickness with the second and third layers. This minimizes air bubbles in the contact layer.
Fig. 3 – Applying contact layer with a brush, dabbing in detailsApply build-up layers until ± 0.5 cm thick
Apply multiple layers of thickened silicone until the total thickness is ± 0.5 cm. Let each layer set lightly (10–15 minutes) before applying the next. Work systematically from bottom to top so that the silicone does not run off.
Cover the model after the last layer with transparent household or pallet wrap. Lightly press the film and smooth the silicone evenly to create an even surface - this will later become the inside of the support shell.
Fig. 4 – Model fully covered with silicone (± 0.5 cm), covered with filmAllow silicone to cure
Allow the entire assembly to cure for ± 24 hours at room temperature. Cover with a cloth or cardboard to keep dust away, but ensure sufficient ventilation. Do not move the model during curing.
💡 Tip: After 24 hours, check if the silicone is fully cured by pressing on an inconspicuous edge. Cured silicone springs back without being sticky.
Mix and roll out epoxy support shell
Weigh the Epoxy Support Shell Fiberglass A-Component (paste) on a piece of film. Add the B-component (liquid) in the correct ratio and knead the mixture thoroughly until a smooth, homogeneous grey-blue mass without lumps or streaks is formed.
Place the mass between two layers of film. Place a 3 mm thick strip on both sides and roll over the strips with a rolling pin. This will give the epoxy a uniform thickness of 3 mm everywhere.
⚠️ Wear gloves: Always work with epoxy using gloves. Cured epoxy is very difficult to remove from skin and work surfaces.
Fig. 6 – Rolling out epoxy to 3 mm thickness using strips and a rolling pinApply epoxy support shell to the silicone mold
Remove the top film from the epoxy pancake. Lift the pancake with the bottom film and drape it over the model with the film side out. Press the epoxy down firmly everywhere so that no air remains between the silicone and the epoxy. Then remove the outer film as well.
Cover the entire model in this way. Allow each pancake to overlap by approximately 1 cm with the previous one and massage the seams together thoroughly to create a seamless whole.
💡 Tip: Do not mix more epoxy than you can process in ± 60 minutes. Calculate in advance how much you need based on the surface area of the model.
Fig. 7 – Epoxy support shell applied and seams massaged inCure epoxy and demold
Allow the epoxy support shell to cure for approximately 16 hours at room temperature. Then carefully remove the epoxy support shell from the silicone mold. Next, remove the silicone mold from the original. The mold is now ready for use.
First use: Rinse the silicone mold with lukewarm water and soap to remove any contaminants from the original. Dry thoroughly before use.
💡 Tip: Any contamination from the original (moss, dirt, patina) will be transferred to the silicone, but generally does not affect the quality of your castings. Remove coarse contamination with a sponge and soapy water.
Fig. 8 – Silicone mold with epoxy support shell after demolding, ready for use
End result: an exact copy of the ornament or decorative concrete, cast in the homemade silicone brush-on mold
Special instructions
- The percentage of B-component determines the curing speed. Vary between 1.5% and 4% — more than 4% can negatively affect the mold's strength.
- Always pre-treat porous materials (untreated concrete, stone, plaster, wood) with a release agent or sealer.
- Do not expose the mold to temperatures below −40 °C or above +280 °C for extended periods.
- Larger molds can be fitted with epoxy feet to facilitate casting — knead fresh epoxy into the desired shape and attach it to the support shell.
- The epoxy support shell can be post-processed with sandpaper after curing for a smoother surface.
Storage
Uncured silicone (A and B components)
Store the closed packaging between 10 °C and 25 °C, away from direct sunlight. When stored correctly, the shelf life is at least one year. Close the packaging immediately after use to prevent moisture absorption.
Cured silicone mold
Store the mold in a dark, dry place between 10 °C and 25 °C with low humidity. Store the epoxy support shell with the mold so that the mold always retains its correct shape. Do not stack molds under heavy objects to prevent deformation.
Frequently asked questions
Do I always need a release agent?
Not always. For smooth, non-porous materials (ceramics, glass, metal), a release agent is often not necessary. For porous materials such as untreated concrete, stone, plaster, or wood, a release agent or sealer is highly recommended. Always perform a test first.
Why do I use Thixo C and not liquid silicone?
Liquid silicones run off vertical and inclined surfaces before they can cure. Thixo C makes the silicone thick enough to adhere to any surface, even on inverted parts. This is essential for brush-on molds of ornaments and reliefs.
How much silicone do I need?
Estimate the surface area of the model in square centimeters and multiply by 0.5 cm (the desired mold thickness). This gives the required volume in cm³ (= ml). The density of Silicone Condensation 20 is ± 1.0 g/ml, so volume in ml ≈ weight in grams. Add an extra 10–15% for loss during mixing and application.
Can I also use the mold for concrete?
Yes. Silicone Condensation 20 is excellent for casting concrete, plaster, polyurethane, wax, and resin. When using concrete, always use a release agent to prevent sticking and extend the mold's lifespan.
How long does the mold last?
A well-maintained silicone brush-on mold will last tens to hundreds of casts, depending on the casting material and careful use. Clean the mold with lukewarm water after each use and store it in the epoxy support shell.