Sovereignty Module: Pour the Mold
Complete Slip Casting: From Plaster Mold to Production Pottery
Slip casting allows production of identical ceramic pieces from liquid clay poured into plaster molds. This campaign covers slip preparation, mold making, casting, and finishing.
Chapter 1: Slip Casting Basics
Slip casting process: 1) Liquid clay (slip) is poured into a plaster mold. 2) The plaster absorbs water from the slip. 3) A layer of firm clay builds up against the mold wall. 4) Excess slip is poured out. 5) The clay layer (casting) dries and shrinks away from the mold. 6) The mold is opened and the casting is removed. 7) The casting is a hollow replica of the mold interior.
| Advantage | Explanation |
|---|---|
| Identical pieces | Every casting from the same mold is identical |
| Thin walls | Castings can be very thin (1/8 inch) |
| Complex shapes | Shapes impossible to throw or hand-build |
| Speed | Multiple castings per day from one mold |
| Consistency | Uniform wall thickness |
| Low skill threshold | Easier than throwing for complex forms |
Chapter 2: Casting Slip Preparation
| Component | Percentage | Purpose |
|---|---|---|
| Clay (ball clay + kaolin) | 65-70% (dry weight) | Body of the casting |
| Water | 30-35% | Liquid medium |
| Sodium silicate (deflocculant) | 0.2-0.4% | Thins slip without adding water |
| Soda ash (deflocculant) | 0.1-0.2% | Assists sodium silicate |
Deflocculant: 1) Without deflocculant, clay requires 50-60% water to become liquid. 2) With deflocculant, clay requires only 30-35% water. 3) Less water = less shrinkage = fewer cracks. 4) Less water = faster casting = more production. 5) Sodium silicate and soda ash change the electrical charge on clay particles. 6) Particles repel each other instead of clumping. 7) Result: fluid slip with minimum water.
Slip preparation: 1) Weigh dry clay (ball clay + kaolin blend). 2) Measure water (30-35% of clay weight). 3) Add deflocculant to water first (dissolve completely). 4) Add clay to water gradually (not water to clay). 5) Mix with drill mixer for 15-20 minutes. 6) Sieve through 80-mesh screen. 7) Allow to age 24 hours (improves casting). 8) Remix before use. 9) Target specific gravity: 1.75-1.80.
Chapter 3: Plaster Mold Making
| Plaster Type | Setting Time | Hardness | Absorption | Best For |
|---|---|---|---|---|
| Pottery plaster (#1) | 20-25 minutes | Soft-medium | High | Slip casting molds |
| Hydrocal | 15-20 minutes | Hard | Medium | Press molds, master molds |
| Ultracal 30 | 25-30 minutes | Very hard | Low-medium | Master molds, case molds |
Plaster mixing: 1) Always add plaster to water (never water to plaster). 2) Ratio: 70 parts water to 100 parts plaster (by weight). 3) Sift plaster into water over 2-3 minutes. 4) Allow to slake (soak) for 2 minutes. 5) Mix by hand for 2-3 minutes (avoid air bubbles). 6) Pour when plaster reaches cream consistency. 7) Tap mold to release air bubbles. 8) Allow to set completely (20-30 minutes). 9) Do not disturb during setting.
Two-part mold: 1) Create original form (model) from clay, plaster, or found object. 2) Build clay wall (cottle) around model at parting line. 3) Apply mold release (petroleum jelly or soap). 4) Pour first half of mold. 5) Allow to set, remove clay wall. 6) Apply mold release to plaster surface. 7) Pour second half of mold. 8) Allow to set. 9) Separate halves, remove model. 10) Dry mold thoroughly (1-2 weeks) before first use.
Chapter 4: Casting Process
| Step | Action | Time | Critical Factor |
|---|---|---|---|
| Assemble mold | Strap or band halves together | 1 minute | Tight seal, no gaps |
| Pour slip | Fill mold to top | 1 minute | Pour steadily, avoid air |
| Top off | Add slip as level drops | 5 minutes | Keep mold full |
| Cast | Wait for wall to build | 10-30 minutes | Desired wall thickness |
| Drain | Pour out excess slip | 2 minutes | Invert, drain completely |
| Set up | Allow casting to firm | 15-30 minutes | Must release from mold |
| Open mold | Separate halves carefully | 2 minutes | Don't distort casting |
| Remove casting | Lift out gently | 1 minute | Support thin walls |
| Trim seam | Scrape mold line with knife | 5 minutes | Smooth, invisible seam |
| Dry | Air dry slowly | 1-3 days | Slow = fewer cracks |
Chapter 5: Finishing and Troubleshooting
| Problem | Cause | Solution |
|---|---|---|
| Casting too thin | Drain too soon | Leave slip in mold longer |
| Casting too thick | Drain too late | Drain sooner |
| Bubbles in casting | Air trapped in slip | Stir slip gently, pour slowly |
| Cracking | Drying too fast | Dry slowly, cover loosely |
| Sticking to mold | Mold too wet or no release | Dry mold, apply release |
| Uneven thickness | Mold not level | Level mold before pouring |
| Soft casting | Too much water in slip | Check specific gravity, add deflocculant |
Reference Card
- Deflocculant is the key to casting slip (sodium silicate and soda ash allow clay to become liquid with minimal water; without deflocculant, slip would require too much water and crack during drying). 2. Always add plaster to water (adding water to plaster creates lumps that never dissolve; sifting plaster into water ensures a smooth, lump-free mix). 3. Casting time controls wall thickness (the longer slip stays in the mold, the thicker the wall; 10 minutes produces thin walls, 30 minutes produces thick walls; timing must be consistent for uniform results). 4. Plaster molds must be dry (a wet mold cannot absorb water from the slip; molds must be thoroughly dry before each use; rotate molds to allow drying between castings). 5. The parting line determines mold complexity (a simple cup needs a two-part mold; a teapot may need a five-part mold; plan the parting line to allow easy removal without undercuts). 6. Trim the seam while leather-hard (the mold seam line is visible on every casting; trimming with a knife at the leather-hard stage produces the cleanest result). 7. Slip casting enables production (a single mold can produce dozens of identical pieces per day; this is how pottery workshops produce matching sets of cups, bowls, and plates). 8. The mold is the master (the quality of every casting depends on the quality of the mold; invest time in making a perfect mold, and every casting will be perfect).