Slip Casting Wall Thickness Calculator
Enter one timed test cast, the stage where you measured it, and the shrinkage for your clay body. The calculator returns the drain time and timing window needed to hit a fired wall thickness.
Recommended drain time (min)14.83
- Earliest acceptable drain time (min)
- 13.38
- Latest acceptable drain time (min)
- 16.35
- Required drain-stage wall thickness (mm)
- 4.23
- Measured casting-rate constant (mm2/min)
- 1.21
- Time multiplier versus test cast
- 0.99
- Expected fired thickness at test time (mm)
- 3.52
Drying and firing shrinkage are applied sequentially, so the percentages compound rather than add.
Assumes the same slip, solids loading, viscosity, temperature, plaster mold, and drainage routine as the calibration cast.
How to use this calculator
- Make a timed test cast with the same slip, mold condition, and draining routine you plan to use.
- Measure a representative wall section and choose whether that measurement was taken at drain stage, bone dry, or fired.
- Enter the target fired thickness, drying shrinkage, firing shrinkage, and acceptable fired-thickness tolerance.
- Use the recommended drain time and the early-to-late window as a starting point, then verify it with another cast.
How the drain time is calculated
In drain casting, plaster pulls water from the slip and leaves a clay layer on the mold wall. Under steady material and mold conditions, a common model is the square-root casting law: wall thickness grows with the square root of casting time. This calculator does not ask you to choose a generic clay constant. It derives the constant from your own timed test.
Let tc be the test casting time, m the measured test thickness, d drying shrinkage as a decimal, f firing shrinkage as a decimal, xf the target fired thickness, and q the tolerance as a decimal.
The first step converts the test measurement back to its equivalent drain-stage thickness, xc:
xc = m if measured at drain stage
xc = m / (1 - d) if measured bone dry
xc = m / [(1 - d)(1 - f)] if measured fired
The calibrated casting law is x^2 = C t, so C = xc^2 / tc. The target fired wall must be expanded back through both shrinkage stages: xg = xf / [(1 - d)(1 - f)]. The recommended drain time is then t = xg^2 / C.
The tolerance window uses the same formula with the lower and upper fired thicknesses: xf x (1 - q) and xf x (1 + q). A zero tolerance makes the earliest, recommended, and latest times identical.
What moves the result most
Thickness has a squared effect on time. Doubling the required drain-stage wall thickness takes about four times as long when the casting law holds. Shrinkage also matters because drying and firing are applied sequentially. Six percent drying shrinkage followed by twelve percent firing shrinkage leaves 0.94 x 0.88 = 0.8272 of the drain-stage size, not eighty-two percent from simply adding the rates.
What this calculator leaves out
The estimate is for average wall thickness under conditions matching the calibration cast. It does not predict thicker seams, bottoms, corners, rims, or areas where slip pools before draining. It also does not adjust for a mold getting wetter over a casting run, plaster aging, contamination, slip temperature, deflocculant changes, settling, viscosity drift, or pressure casting. A single test point cannot prove that your slip and mold follow square-root behavior over a wider range, so production work should check several timed casts.
Worked example
Suppose a test cast stayed in the mold for 15 minutes and measured 4.0 mm at bone dry. With 6% drying shrinkage and 12% firing shrinkage, the equivalent drain-stage test thickness is 4.0 / 0.94 = 4.26 mm, and the casting-rate constant is 4.26^2 / 15 = 1.21 mm2/min.
For a 3.5 mm fired target, the required drain-stage thickness is 3.5 / (0.94 x 0.88) = 4.23 mm. The recommended drain time is 14.83 minutes. With a 5% fired-thickness tolerance, the acceptable window runs from 13.38 to 16.35 minutes, and the test cast would be expected to fire to 3.52 mm.
Common questions
Why does slip-cast wall thickness grow with the square root of time?
As the clay layer builds on the plaster wall, water has to move through a thicker deposited layer. That added resistance slows the growth rate, so equal added thickness usually takes more time later in the cast than it did at the beginning.
How do I make a test cast for calibration?
Use the same slip batch, mold, fill level, room conditions, and drain routine planned for the real piece. Time how long the slip sits in the mold before draining, then measure a clean wall section away from seams, rims, bottoms, and pooled-slip areas.
Should wall thickness be measured wet, bone dry, or fired?
Any of those stages can work if you enter the matching stage. A fired measurement often represents the finished process best, but a bone-dry measurement is faster and avoids waiting for a firing. A drain-stage measurement is direct but can deform if the cast is soft.
Why does doubling wall thickness require about four times the casting time?
The model is <code>x^2 = C t</code>. If thickness <code>x</code> doubles, <code>x^2</code> becomes four times as large, so the time has to be about four times as long when the same casting rate constant applies.
Why does the same drain time produce different walls as a mold gets wetter?
A damp mold pulls water from the slip more slowly than a dry one. The calibrated rate constant is only valid for a mold condition like the test cast, so long production runs often need periodic checks or separate calibrations.
When should I recalibrate?
Recalibrate after changing slip recipe, specific gravity, deflocculant level, temperature, mold material, mold age, firing schedule, or the amount of drying between casts. Also recalibrate if several pieces in a run begin drifting outside the fired-thickness tolerance.