Ceramic Slip Casting
Ceramic slip casting pours liquid clay slip into a porous plaster mold that draws off water, leaving a solid layer that becomes the part.
- Part
- Forming
- Revised
- 2026-08-11
At a glance
- Family
- Glass and Ceramics
- Typical tolerances
- Roughly ±1–2% of dimension on the fired part, dominated by variation in drying and firing shrinkage rather than by the mold.
- Typical volumes
- 100–50,000 pieces per year with plaster molds; higher with pressure casting
- Lead time
- 3–8 weeks for model and mold making. In production, plaster molds typically yield one or two castings per mold per day, so output is set by mold count.
- Materials
- Ceramic
What it is
Ceramic slip casting pours a fluid clay suspension into a porous plaster mold, which draws water out of the slip by capillary action and leaves a solid layer of consolidated clay against the mold face. Wall thickness is controlled by dwell time — the cast layer builds roughly with the square root of time, so a sanitaryware mold might be filled for an hour to reach 0.3–0.5 in (8–12 mm) while thin tableware casts in ten or twenty minutes.
The slip is not simply clay and water. A deflocculant, typically sodium silicate or sodium carbonate at a fraction of a percent of the dry weight, disperses the clay particles so the slip pours freely at 25–35% water instead of the 50%+ it would otherwise need. That reduction in water directly reduces drying shrinkage and cracking.
Slip casting is the only economical route to hollow, complex, thin-walled ceramic bodies — toilets and basins, teapots and vases, technical crucibles and thermocouple sheaths. Total linear shrinkage from wet cast to fired part is roughly 10–18%, which the mold is sized to compensate.
How it works
- Prepare the slip. The clay body is blunged with water and deflocculant to a controlled specific gravity and viscosity. Both are checked every shift; a slip that thickens or thins changes cast rate and therefore wall thickness.
- Fill the mold. The plaster mold, assembled from two or more parts, is filled and topped up as the level drops — the mold absorbs water and the slip level falls as material is consumed building the wall.
- Dwell. Water migrates into the plaster and clay particles pack against the mold face. Cast thickness grows approximately with the square root of dwell time, so doubling the wall takes roughly four times as long.
- Drain (hollow casting) or fill out (solid casting). For hollow ware the mold is inverted and the surplus slip poured off, leaving a shell. For solid casting, the gap between two mold faces is narrow enough that the two advancing walls meet and the whole section becomes solid.
- Firm up and release. As drying continues the casting shrinks away from the plaster and reaches leather-hard, at which point the mold is opened. Timing matters: too early and the piece slumps, too late and it cracks as it shrinks against fixed mold features.
- Fettle and dry. Mold seams and the spare above the pouring gate are trimmed and sponged. The piece then dries slowly and evenly — uneven drying is the single largest source of scrap.
- Fire. A bisque firing around 1,650–1,830 °F (900–1,000 °C) gives a porous, glaze-absorbent body. After glazing, the glost firing runs roughly 1,830–2,100 °F (1,000–1,150 °C) for earthenware and 2,190–2,550 °F (1,200–1,400 °C) for stoneware and porcelain.
Design guidelines
Uniform wall thickness
Wall thickness follows dwell time and is therefore naturally uniform — but only if the mold's plaster is uniformly absorbent and the geometry does not trap slip. Design so the mold drains cleanly, without pockets that hold residual slip and cast a locally thick, slow-drying section that will crack.
Wall thickness range
0.12–0.20 in (3–5 mm) for tableware and decorative ware, 0.3–0.5 in (8–12 mm) for sanitaryware. Below about 0.08 in (2 mm) the casting is too weak to survive demolding.
Undercuts and mold splits
Plaster molds are rigid, so undercuts require additional mold parts, and every split leaves a seam to be fettled by hand. Each additional mold part adds labor to every single casting for the life of the tool — mold-part count is a direct unit-cost decision.
Draft and shrinkage
The casting shrinks away from the mold as it dries, so slip casting tolerates less draft than injection molding. Even so, 1–2° eases release on deep forms and reduces scrap.
Radii and section changes
Fillet internal corners generously. Abrupt section changes dry at different rates and crack, and a sharp interior corner in the mold is also where plaster erodes first.
Design for shrinkage
Total linear shrinkage from wet to fired is roughly 10–18% depending on the body — porcelain at the high end, coarse stoneware lower. The mold is made oversize by the measured figure for that body, so a body change is a tooling change.
Attachments
Handles, spouts, and feet are cast separately and luted on with slip at leather-hard. Design the joint with enough contact area, and keep the attached part's drying rate similar to the body's or the joint cracks.
| Feature | Recommended | Limit | Why |
|---|---|---|---|
| Wall thickness | 0.16 in (4 mm) tableware | 0.08 in (2 mm) | Below this, demolding breaks the piece |
| Mold parts | 2 | More adds fettling labor forever | Every seam is hand-finished |
| Internal radius | 0.12 in (3 mm) | Sharp corners crack | Differential drying |
| Draft | 1–2° | 0° possible | Casting shrinks away from plaster |
| Linear shrinkage allowance | 10–18% | Body-specific | Mold must be sized to the body |
Cost drivers
Labor and cycle time dominate. A plaster mold absorbs a finite amount of water before it must dry out, so a mold typically produces one or two castings per shift unless the plant runs a drying regime between casts. Plants therefore hold large numbers of molds, and mold storage and handling is a real cost. Plaster molds also wear: 50–150 castings is a typical working life before absorbency and surface definition degrade.
Pressure casting changes this equation. Using porous polymer molds and slip injected at pressure, cycle times drop from hours to minutes and mold life extends by orders of magnitude, which is why high-volume sanitaryware moved to it. The tooling costs far more.
Firing is the other major cost: two firings, each consuming kiln time and energy, with kiln capacity setting plant throughput. Scrap discovered after the glost firing has absorbed all of that cost.
Volume breakpoints: plaster slip casting suits roughly 100–50,000 pieces a year. Above that, pressure casting or press molding is more economical, and for small technical parts at high volume, ceramic injection molding takes over.
- Minimize mold parts — every seam is fettled on every piece, forever.
- Design so the mold drains completely; trapped slip is scrap.
- Keep sections uniform to reduce drying and firing losses.
- Consider whether the design can be cast in one piece rather than assembled from luted components.
- Design the ware to stack efficiently in the kiln; firing capacity is the plant bottleneck.
Questions
6 questionsHow is wall thickness controlled in slip casting?
By dwell time. The plaster mold draws water out of the slip and clay consolidates against the mold face, with thickness building roughly in proportion to the square root of time. Doubling the wall thickness therefore takes about four times as long, and slip specific gravity and viscosity must be held constant for the relationship to repeat.
Why does slip need a deflocculant?
A deflocculant such as sodium silicate or sodium carbonate, added at a fraction of a percent of dry weight, disperses the clay particles so the slip pours freely at 25–35% water instead of the much higher water content it would otherwise need. Less water means less drying shrinkage and far less cracking.
How much does slip cast ceramic shrink?
Roughly 10–18% linearly from wet cast to fired part, combining drying and firing shrinkage. Porcelain sits at the high end and coarse stoneware bodies lower. Molds are made oversize by the measured figure for the specific body, so changing body means new tooling.
How many castings does a plaster mold produce?
Typically 50–150 before absorbency and surface definition degrade enough to force replacement. A mold also needs to dry out between casts, so plants generally get one or two castings per mold per day and hold large mold inventories to make throughput.
Can slip casting produce undercuts?
Yes, but each undercut needs an additional mold part, and every mold split leaves a seam that is fettled by hand on every casting. Mold-part count is therefore a permanent unit-cost decision rather than a one-time tooling decision.
What is the difference between drain casting and solid casting?
In drain (hollow) casting, surplus slip is poured out after the wall reaches thickness, leaving a shell — this is how vases, teapots, and sanitaryware are made. In solid casting, the gap between two mold faces is narrow enough that the walls advancing from both sides meet, producing a solid section such as a handle or a knob.