Casting concrete and Jesmonite in a silicone mould
Which mix to choose, how much water it really wants, how to get the bubbles out of a 6 mm wall, and why a Mouldery vessel is cast upside-down with its bottom open. Demould times, drying, sanding and sealing, and how to make the mould last.
Updated 6 November 2026 · 10 min read
Four castables, one mould
A silicone mould in its printed frame takes anything that pours cold: fine concrete, Jesmonite, plaster, even casting resin. What changes is the mix, the time before you can open the frame and the finish. The table gives the figures that matter for a vessel with a 5 to 7 mm wall; the README of each kit gives the mass to weigh for your geometry, at a density of 2.1 for concrete.
| Material | Mix | Demould | Density | Finish | Indicative price |
|---|---|---|---|---|---|
| Rapid-set CSA cement (Rapid Set Cement All and similar) | 2.8–3.5 L of water per 25 kg, thick milkshake | 1–3 h; next day for thin walls | ≈ 2.1 | smooth, hard, light grey or white; pinholes possible | low, ≈ 1–2 $/kg |
| Portland cement + fine sand | 1 : 3 by volume, 1 : 2 if thin walls crack; minimum water | 24 h; 48–72 h for thin walls and fine detail | ≈ 2.1 | smooth grey, slightly more porous | lowest, under 1 $/kg |
| Jesmonite AC100 | 2.5 : 1 base to liquid by weight; 1 750 g per litre of mould | 30–60 min; full strength 24–48 h | ≈ 1.8–2.0 | very smooth, white, takes pigment and terrazzo; ≈ 80 Shore D | high, ≈ 10–15 $/kg |
| Casting plaster | as the bag says | 30–60 min | ≈ 1.1 | matt, soft, chalky; must be sealed | low, ≈ 1–2 $/kg |
Concrete: the mix and the water
Rapid-set calcium sulfoaluminate (CSA) cements are what the fastest makers use: initial set in about 15 minutes, final set at 35, 20 MPa (3 000 psi) after one hour. A vessel can come out after an hour and the mould is free again; for a 5 mm wall, a night in the mould costs nothing and avoids a chipped rim. Water is 2.8 to 3.5 litres per 25 kg bag, about 11 to 14 % of the cement weight: a thick milkshake. If 15 minutes is not enough to fill a tall planter, use the maker's set retarder rather than more water.
Ordinary Portland cement with fine sand is cheaper, slower and just as smooth: one part cement to three parts fine sand (0–1 mm) by volume, richer at one to two if thin walls crack. Sandless grout fills engraved detail best. Whatever the cement, water is the enemy of strength: every drop beyond what the mix needs to flow leaves a pore, and too much gives a weak, chalky vessel. A polycarboxylate superplasticiser at 0.1 to 0.3 % of the cement weight makes the mix flow with less water; overdosed, the sand settles and the set is delayed. Oxide pigments go in dry with the cement, weighed so the next batch matches, within the pigment maker's limit.
Jesmonite AC100
Jesmonite AC100 is a water-based acrylic-mineral composite, a powder base and a liquid mixed 2.5 to 1 by weight. Count 1 750 g per litre of mould cavity: a 200 ml vessel needs about 350 g, 250 g of base to 100 g of liquid. Pigments, up to 2 % (20 g per kilo), go into the liquid first. Mix 30 to 45 seconds with a high-shear paddle on a drill at around 1 000 rpm, scrape, mix again, and work within the 5 to 10 minutes of open time.
Brush a 1 to 2 mm coat onto the silicone first, into the pattern and the wick socket, then fill and tap. It sets in 15 to 20 minutes at 18 °C and can be demoulded as soon as it has its initial strength, within the hour for a vessel; full strength takes 24 to 48 hours. Retarder at 2 to 8 g/kg buys time in a warm workshop; a thixotropic additive at 2 to 6 g/kg makes a brushable coat. Cured AC100 is about 80 Shore D and needs the maker's acrylic sealer against water.
Mixing, filling and getting the bubbles out
Mix in a jug with a spout. Fill the mould half way, tap the frame on the bench a dozen times, fill the rest and tap again. Then vibrate until the bubbles slow: knock the sides of the frame, hold an orbital sander without paper against the base plate, or stand the whole thing on a board driven by an eccentric motor at 60 to 120 Hz. The frame is what makes this possible: clips on, it takes the knocks and keeps the block square. Stop when water pools on top; beyond that the sand settles and the top layer, which will be the foot, gets weak.
A 5 to 7 mm wall is a narrow channel for a mix with sand in it: use fine sand or grout, a fluid consistency and a slow pour down the gap while vibrating, in two or three lifts. For engraved patterns and the wick socket, brush the mix onto the silicone with a soft brush before filling.
Upside-down, bottom open
The opening of a Mouldery mould is the foot of the vessel, not its rim: the master was pushed out through it, and it is exactly the outline of the plinth. The vessel is therefore cast upside-down. The rim, the engraved pattern and the ⌀23 mm wick socket are formed against silicone and come out crisp; the only surface you touch is the foot, which nobody sees.
Put the rigid core back into its silicone sleeve first: it stiffens it, and on planter kits it carries a peg that forms the drainage hole, so there is nothing to drill. Put the block back into the frame, close the split, clip it, opening up, and stand it on the base plate. Pour, vibrate, then screed the open bottom level with the top of the silicone. For a perfectly flat foot, press the base plate upside down onto the fresh mix instead: its 1 mm recess forms a plinth that is flat to the print, and the ⌀40 mm relief disc gives a slightly hollowed foot that stands without rocking. Wipe the plate with release first and lift it straight off once the mix has set.
Demoulding and drying
A corner of the rim left in the mould means you demoulded too early or vibrated too little; a dusty surface means too much water; a foot out of square means the frame was not on a flat surface when you screeded.
- Wait: 12 to 24 hours for concrete (an hour is possible with CSA cement, but a thin rim chips easily that early), 30 to 60 minutes for Jesmonite or plaster.
- Unclip and unscrew the bridge, pull the core out of the sleeve with a slight twist.
- Unclip the split, open the frame, slide the block out.
- Flex the block to release the vessel through the foot opening, then peel the inner sleeve out of the cavity; the wick socket releases last.
- Let the vessel dry for 48 hours before sanding and sealing. Concrete keeps hardening for a week: no direct sun or radiator on the first day.
Sanding and sealing
Flatten the foot on 120 then 240 grit wet-and-dry taped to a glass tile, with water, and break the rim edge lightly with 400 grit. Pinholes can be filled with a slurry of the same mix, rubbed in with a finger and sanded the next day.
Seal according to use: a penetrating silane or siloxane sealer for planters that live outside or hold wet soil; a water-based acrylic sealer in two or three thin coats for indoor planters, trays and anything that must not chalk, which also stops efflorescence; the maker's acrylic sealer for Jesmonite; a heat-resistant sealer inside candle vessels, covered in the candle guide. Plaster takes shellac or acrylic and still should not hold water. Felt pads on the foot finish the job.
Mould life and release agent
Concrete is abrasive. Fine detail softens after a few dozen casts in a Shore A 15–25 silicone, and the rim of the cavity, which stretches at every demould, goes first. A light coat of non-silicone release before each pour roughly doubles that; wash the mould with soap and water, dry it, store it in the frame, flat. Jesmonite is gentler: no release is needed on silicone and the mould lasts correspondingly longer.
Keep the master. The inner sleeve is the most stressed part of the mould, and a new one is a small pour of silicone into the master cavity around the core: a free sleeve held by the core and the bridge works like the original. Tin-cure silicone also shrinks 0.5 to 2 % over months; when the vessel diameter starts to drift, pour a fresh mould from the same master with the same six parts and the same quantity, or move to platinum silicone for a production run.