Slip Casting Calculator: Cast Time for a Wall Thickness, Slip Used, Casts per Day and Mold Life

How it works

Wall thickness grows with √time: L = k √t, so k = L_test ÷ √t_test and t = (L_target ÷ k)² · Wall volume = inner area × thickness · Slip solids (g/mL) = (SG − 1) × 2.6 ÷ 1.6 (Brongniart, clay SG 2.6) · Cast solids fraction from its moisture m: φ = (1/2.6) ÷ (1/2.6 + m/(1 − m)) · Slip used = dry clay in wall ÷ slip solids per mL · Water into plaster = slip used − cast weight · Fired size = cast size × (1 − shrinkage) · Casts per mold per day = min(casts before drying, working minutes ÷ (cast time + handling))

In slip casting the plaster draws water out of the slip and a layer of clay builds up on the mold face. Because the water has to pass through the layer already formed, the wall grows with the square root of the time: doubling the thickness takes four times as long. This parabolic law is the traditional rule in slip-casting practice, though strongly deflocculated slips and very porous molds can deviate from it. One timed test cast calibrates your slip and mold together. From it the calculator gives the time for any thickness and a table of times for other walls. For material use it does a mass balance. The Brongniart relation gives the dry clay in each millilitre of slip from its specific gravity, assuming a clay-body specific gravity of 2.6. The cast wall is denser than the slip: at release it holds only about a fifth of its weight as water (adjustable under More options), so the piece uses more slip than its own volume, and the difference in water goes into the plaster. That water is what eventually saturates a mold, so the calculator shows it per cast and per day as a share of the plaster weight. Fired size follows from the total shrinkage, and production is the smaller of the casts a mold takes before it needs drying and the casts that fit in the working day. Defaults for handling time, casts before drying and mold life are typical studio figures; replace them with your own.

Worked example

With the defaults: a test cast gives 4 mm in 8 minutes, so k = 4 ÷ √8 = 1.414 mm/√min and a 5 mm wall needs (5 ÷ 1.414)² = 12.5 minutes; at 12% shrinkage it fires to 4.40 mm. Pour 0.45 L × 1.15 = 0.52 L. The wall is 400 cm² × 0.5 cm = 200 cm³; at 20% moisture it contains 315 g of dry clay and 79 g of water (394 g). Slip at SG 1.78 holds (1.78 − 1) × 2.6 ÷ 1.6 = 1.268 g of dry clay per mL, so the piece uses 315 ÷ 1.268 = 249 mL of slip (443 g), about $0.40 at $1.60/L, and 0.27 L drains back. The plaster absorbs 443 − 394 = 49 g of water per cast. With 45 minutes of handling the cycle is 57.5 minutes, so 10 casts would fit in a 10-hour day, but each mold is limited to 5 casts before drying: 15 casts a day from 3 molds, and at 90 casts per mold the set makes about 270 pieces over 18 casting days.

Sources: Differentiated slip casting (Ceramics 2025, 8(1), 6): wall thickness traditionally proportional to the square root of casting time · Slip casting: typical slip SG about 1.8, cast times and worked batches (Digitalfire) · Slipcasting: slip densities 1.80–1.85 g/cm³ for vitreous china, cast times (Wikipedia)

Frequently asked questions