Slip Casting Ratio Calculator: Water and Dry Clay for Slip

How the slip casting ratio calculator works

Specific gravity compares a slip‘s weight with the weight of the same volume of water. Water has a specific gravity of 1, so a litre of slip at 1.75 weighs 1.75 kg. The calculator uses two facts about that litre: its weight is the dry material plus the water, and its volume is the volume of the dry material plus the volume of the water.

The dry material’s volume is its weight divided by its own specific gravity, usually about 2.6 for clay. Solving the two equations together gives the dry material as V times g times (s minus 1), divided by (g minus 1), where V is the slip volume, s its specific gravity and g the dry material’s. The water is the rest of the weight. The result shows every step with your numbers.

This is Brongniart’s formula, first published by Alexandre Brongniart in 1854 and still used in ceramics workshops and factories. In its usual form, the dry weight in a litre is the material’s specific gravity times (the litre’s weight in grams minus 1,000), divided by (its specific gravity minus 1).

What specific gravity should a casting slip be?

Digitalfire notes that hobbyists and potters typically aim for a casting slip around 1.75, while industry needs 1.8 or higher. The higher the specific gravity, the less water the slip carries into the mould, so moulds stay drier, casts form faster and pieces shrink and warp less as they dry.

The catch is flow. Clay and water at 1.75 would normally be a thick paste. Casting slips stay pourable because of deflocculants, such as sodium silicate or a polyacrylate dispersant, which keep the clay particles apart. That’s why specific gravity and flow are measured separately, and why getting one right doesn’t fix the other.

Decorating slips, dipping slips and terra sigillata are made at much lower specific gravities, because they’re meant to coat a surface rather than form a wall. The table above shows how much less dry material those thinner slips contain.

Measuring the specific gravity of your slip

The most accurate way to measure is to weigh a known volume. Put a measuring cylinder or jug on a scale, zero it, pour in slip to a known mark, and divide the grams by the millilitres. Digitalfire recommends a bigger cylinder for accuracy, because a small reading error matters less in 100 ml than in 10 ml.

A hydrometer is quicker but less reliable in thick, deflocculated slips, which can stop it floating freely. Whichever you use, stir the slip well first, because solids settle and the top of a bucket reads thinner than the bottom.

Check the specific gravity of every batch before casting, and record it. A slip that casts beautifully one week and badly the next has often just lost water to evaporation or gained it from a damp mould.

Thinning or thickening a slip

To thin a slip, the calculator keeps its dry material the same and works out how much bigger the volume must be at your target specific gravity. The extra volume is the water to add. To thicken a slip, it keeps the water the same and works out how much dry material that water can carry at the target. The extra is the dry material to add.

Add either in small amounts, mix thoroughly and measure again before adding more. Adding dry material can also upset the balance of deflocculant, so a thickened slip may need a little more to flow properly. Adding water usually does the opposite, and can leave a slip over-deflocculated, which casts poorly.

Reclaimed slip from trimmings and failed casts is worth measuring before you reuse it. It has usually dried out a little and may have picked up plaster from the moulds, which can cause casting problems. Sieve it, measure its specific gravity, bring it back to your target with the calculator, and mix it in with fresh slip rather than using it alone.

A worked example of a casting slip

Say you want 10 litres of slip at 1.78 to cast a set of cups, using a clay body with a specific gravity of 2.6. The calculator gives the dry material as 10 times 2.6 times 0.78, divided by 1.6, which is about 12.68 kg. The whole 10 litres weighs 17.8 kg, so the water is the rest, about 5.13 kg, which is 5.13 litres. That’s roughly 29% water by weight, a typical figure for a casting slip.

In practice, you’d start with a little less water than that, add the deflocculant, then add the dry material while mixing, because it’s easier to thin a slip than to thicken it. Measure the specific gravity once everything is mixed, then use the adjust mode to fine-tune it to exactly 1.78.

Choosing the specific gravity of your dry material

Most clays and clay bodies have a specific gravity close to 2.6, which is why it’s the default here and in most published versions of the formula. ClayCalc lists kaolin at about 2.63 and ball clay at about 2.58, and Glazy’s wiki uses about 2.6 for most glazes. Materials with heavy metals, such as tin or lead, run higher.

A small change in this figure makes only a small difference, so 2.6 is fine for most casting slips. If you’re making a slip from a single material with a known value, enter it for a more precise result.

Casting slip, shrinkage and absorption

Slip-cast pieces shrink as they dry and fire, just like thrown ones, and casting slips are often formulated to shrink less. Our clay shrinkage calculator works out how big to make a mould for a fired size. Once fired, a clay absorption test tells you whether the body has matured enough for functional ware.

When your casts are dry, they need firing at the cone the slip is made for. If you don’t have a kiln, a kiln firing service can fire them for you.