Hydrocal / Pottery Plaster (USG Plaster) for Hemisphere / Dome Mold Calculator
Pre-loads specific gravity (1.65) and 1.42:1 mix ratio to calculate exact Part A & Part B weights with cup scraping buffer.
A standard Hemisphere / Dome Mold (Ø10 cm minus 150 cm³ master) requires 199g (7.03 oz / 0.44 lbs) of Hydrocal / Pottery Plaster (USG Plaster). Mix 116.9g Part A with 82.3g Part B (8% buffer included).
Casting Material & Volume Mode
Pre-loads specific gravity and 1.42:1 ratio by weight.
Geometry determines the container-volume formula. Use water displacement when the cavity is irregular.
Volume displaced by your master object inside the box.
Compensates for resin clinging to mixing paddles and cup walls (8% recommended).
Includes 8% waste buffer at SG 1.65
Ratio part: 1.42
Ratio part: 1
Net volume before buffer
Order Hydrocal / Pottery Plaster (USG Plaster) Kit
High-tear strength Hydrocal / Pottery Plaster (USG Plaster) with verified 1.42:1 ratio. Ideal for bubble-free casting.
Hydrocal / Pottery Plaster (USG Plaster) vs Alternative Casting Compounds
Compare specific gravities, mix ratios, working times, and cure speeds across casting compounds.
| Material / Specification | Specific Gravity | Mix Ratio | Pot Life | Demold Time |
|---|---|---|---|---|
| Hydrocal / Pottery Plaster (USG Plaster)ACTIVE | 1.65 | 1.42:1 (weight) | 8 min | 0.75 hrs |
| RTV-2 Platinum Silicone (Smooth-On Mold Star 15/30) | 1.07 | 1:1 (weight) | 30 min | 6 hrs |
| RTV-2 Tin Cure Silicone (Condensation Cure) | 1.12 | 10:1 (weight) | 45 min | 12 hrs |
| Jesmonite AC100 (Eco-Resin) | 1.75 | 2.5:1 (weight) | 12 min | 2 hrs |
| Jesmonite AC730 (Stone Casting) | 1.85 | 5:1 (weight) | 15 min | 4 hrs |
| Deep Pour River Epoxy (2:1 Ratio) | 1.11 | 2:1 (weight) | 90 min | 48 hrs |
Step-by-Step Mix Ratio Derivation: Hydrocal / Pottery Plaster (USG Plaster)
How displacement volume, specific gravity, and ratio split combine into final scale weights.
Calculates the empty fluid cavity by taking the gross 261.8 cm³ hemisphere containment volume and deducting the 150 cm³ displaced by the master model.
Multiplies fluid volume by the specific gravity of Hydrocal / Pottery Plaster (USG Plaster) (1.65 g/cm³) plus an 8% buffer for cup scraping and pouring loss.
Distributes batch mass according to the exact 1.42:1 ratio by weight.
Engineering Specifications & Troubleshooting Guide: Hydrocal / Pottery Plaster (USG Plaster)
Factory-tested parameters, common failure modes, and workbench best practices.
- Standard pottery gypsum cement: 100 parts plaster powder (Part A) to 70 parts water (Part B) by weight (1.42:1).
- Sift dry plaster onto the water surface and let it slake (soak) for 2 minutes before agitating.
- Produces exceptionally dense, chip-resistant castings with fine surface detail.
Stirring vigorously with an egg whisk, which whips millions of tiny micro-bubbles into the gypsum slurry.
Frequently Asked Questions: Hydrocal / Pottery Plaster (USG Plaster)
Mix ratios, degas guidelines, cure times, and mold making tips.
Q1What is the mix ratio of Hydrocal / Pottery Plaster (USG Plaster)?
Q2How much Hydrocal / Pottery Plaster (USG Plaster) do I need for a Hemisphere / Dome Mold?
Q3How long before I can demold Hydrocal / Pottery Plaster (USG Plaster)?
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