ClayCalc
Ceramic Calculator Platform
Plaster Mixing Calculator
Slip Casting
claycalc.com
Slip Casting
Plaster Mixing Calculator
Calculate exact plaster and water weights needed to fill a mold volume using the WCR method.
Updated
L
ratio
error
Results
calculate
Enter your measurements above and click Calculate.
The Water:Plaster Ratio (WCR) Method
The WCR (Water:Plaster Ratio) expresses the weight of water relative to plaster weight. For Pottery Plaster #1, the standard WCR is 0.70 — 70 parts water per 100 parts plaster by weight. Lower WCR produces harder, denser plaster (Hydrostone uses 0.32).
Formula
Given a target volume, this calculator solves for the plaster weight needed using the combined density formula:
V_total = P_weight / plaster_density + P_weight × WCR / water_density
→ P_weight = V_total / (1/plaster_density + WCR)
Pottery Plaster #1 has a set density of approximately 1.52 g/cm³. Water density is 1.0 g/cm³.
Grade Comparison
| Grade | WCR | Hardness | Best For |
|---|---|---|---|
| Pottery Plaster #1 | 0.70 | Medium | Jiggering, bat molds |
| Casting Plaster | 0.72 | Softer | Slip casting molds |
| Hydrocal | 0.45 | Hard | Master models |
| Hydrostone | 0.32 | Very Hard | High-durability molds |
Frequently Asked Questions
What is water:plaster ratio (WCR)? expand_more
WCR is the ratio of water weight to plaster weight. Pottery Plaster #1 has a WCR of 0.70, meaning 70g of water per 100g of plaster. A lower WCR (like Hydrostone at 0.32) produces harder, denser plaster.
Why do you add plaster TO water, not water to plaster? expand_more
Adding water to dry plaster causes lumping and inconsistent hydration. Sifting plaster slowly into water allows even wetting and produces a smoother, more consistent mix. Always add plaster to water.
How do I know when plaster is ready to pour? expand_more
Slake undisturbed for 2 minutes after all plaster is added. Mix gently for 2–3 minutes. The plaster is ready when it leaves a slight trail on the surface (called "trailing"). Pour before heat becomes noticeable.
Can I use builders plaster (plaster of Paris) for pottery molds? expand_more
Craft or builders plaster is softer and less durable than USG Pottery Plaster #1 or equivalent. Molds made from it wear out faster. For production slip casting, always use a dedicated pottery-grade plaster.