Brongniart Slip Solids Calculator
Use a weighed, measured slip sample to estimate its dry-solids content and the dry material and water in the whole batch. The calculator also works backward from a target solids percentage to show a one-direction correction.
Current dry solids71.43%
- Measured slip specific gravity
- 1.75
- Dry material in batch (kg)
- 12.5
- Water in batch (kg)
- 5
- Dry material to add (kg)
- 0.36
- Water to add (kg)
- 0
- Corrected batch weight (kg)
- 17.86
Batch masses are kilograms; the estimate treats the slip as water plus one dry material phase.
Add the indicated material gradually, mix thoroughly, remove bubbles, and measure again before correcting further.
How to use this calculator
- Weigh a clean, dry container, then fill it exactly to its calibrated sample-volume line and weigh it again.
- Enter those two weights and the sample volume to calculate the slip specific gravity.
- Enter the current batch volume and the average specific gravity of the blended dry material.
- Enter the target dry solids by wet weight to see either a dry-material addition or a water addition.
- Add material in small portions, mix and de-air the slip, then remeasure before making another correction.
How Brongniart's formula estimates slip solids
Brongniart's formula turns a measured slip density into an estimate of dry material concentration. First, subtract the empty-container weight from the filled-container weight. Divide that net mass in grams by the measured volume in milliliters. The result is the slip density in g/mL, which is numerically the same as specific gravity relative to water.
For a slip with measured specific gravity ρ and blended dry-material specific gravity D, the calculator uses:
q = D × (ρ − 1) ÷ (D − 1)
Here q is dry material in kilograms per liter. The current solids fraction by weight is x = q ÷ ρ, so the reported solids percentage is 100 × x. For a batch volume B in liters, wet batch mass is M = ρ × B, dry material is S = q × B, and water is M − S.
How the target correction works
If the requested target fraction t is higher than the current fraction, the calculator solves (S + A) ÷ (M + A) = t for dry-material addition A. That gives A = (tM − S) ÷ (1 − t). If the target is lower, it solves S ÷ (M + A) = t for water addition, giving A = S ÷ t − M. Only one addition is shown because the tool holds the existing batch composition constant and adjusts in the needed direction.
What affects the estimate most
The sample density and the dry-material specific gravity have the largest effect. A fill line that is not exact, trapped air, foam, or sediment that is not evenly mixed changes the measured density. The dry-material value is an average for the whole blend, not a value for one ingredient; use a measured or supplier value when available. Casting slips often need a high solids content, but the useful target depends on the body, deflocculant, moulds, and casting process.
What this calculation does not measure
This is a two-phase mass-balance model: water plus one blended solid phase. It does not separately model deflocculants, dissolved material, non-water liquids, evaporation, or volume change after an addition. Specific gravity and solids percentage also do not predict viscosity or thixotropy. Test flow and casting performance separately, and let a correction mix completely before deciding whether more is needed.
Worked example
Suppose a 100 mL container weighs 50 g empty and 225 g when filled with slip. The net slip mass is 175 g, so the measured specific gravity is 175 ÷ 100 = 1.75. With dry material specific gravity 2.5, Brongniart's formula gives 2.5 × (1.75 − 1) ÷ (2.5 − 1) = 1.25 kg/L of dry material.
For a 10 L batch, wet mass is 17.5 kg and dry material is 12.5 kg. Its current dry solids are 12.5 ÷ 17.5 = 71.43%, leaving 5 kg of water. To reach 72%, add (0.72 × 17.5 − 12.5) ÷ 0.28 = 0.357 kg of dry material. The corrected batch weight is 17.857 kg.
Common questions
What is Brongniart's formula?
Brongniart's formula estimates the mass of dry solids in a unit volume of ceramic slip from the slip's specific gravity and the specific gravity of its dry material. It is useful when a small weighed sample must represent a larger batch. The result is an estimate, because a real slip can contain several materials and additives.
How do I measure casting-slip specific gravity?
Weigh a dry container, fill it to a known volume with well-mixed slip, and weigh it again. Subtract the empty weight, then divide the net grams of slip by the milliliters filled. For example, 875 g of slip in a 500 mL flask has a specific gravity of 1.75.
What dry-material specific gravity should I use?
Use the measured or supplier value for the complete dry blend when you have it. A value of 2.5 is a practical working assumption for many clay-based slips, but it is not universal. An inaccurate value changes the calculated solids fraction and batch composition.
What is the difference between solids by weight and solids by volume?
This calculator reports solids by wet weight: dry-material mass divided by total slip mass. Solids by volume instead compare the volume occupied by dry particles with the total slip volume. The two percentages differ because dry ceramic material is denser than water.
Why can two slips with the same specific gravity have different viscosity?
Specific gravity measures mass per volume, not how particles move through the liquid. Particle-size distribution, particle shape, deflocculant type and dose, dissolved salts, temperature, and mixing history can all change viscosity and thixotropy. Check viscosity separately before casting.