In ceramic slip casting, bentonite is often added in a very small amount but can have a disproportionate effect on production stability. It may improve particle suspension, green strength and handling of the cast body. However, an unsuitable grade—or an unsuitable addition level—can also increase water demand, disturb deflocculation, slow casting, create excessive shrinkage or cause drying defects.
For this reason, bentonite should not be selected only by appearance, whiteness or a single laboratory viscosity value. The correct choice depends on the complete ceramic system: body composition, solids loading, deflocculant package, forming process, drying conditions and firing target.
This article explains how ceramic manufacturers can evaluate bentonite more systematically for slip casting applications.
Why ceramic slip performance is not only a viscosity issue
A stable casting slip must do more than flow through a pipeline. It must remain homogeneous during storage, fill the mould consistently, release water at a predictable rate, form a uniform cast layer and produce a green body strong enough for demoulding and handling.
This requires balance between several properties:
Low enough viscosity under mixing and pumping
Controlled yield stress to reduce particle settling
Predictable thixotropic recovery after shear
Stable density and solids loading
Good casting rate and mould release
Sufficient green strength after demoulding
Controlled drying shrinkage and low cracking risk
Consistent fired colour and fired properties
Bentonite can support several of these requirements because its fine plate-like particles interact strongly with water and other mineral particles. But this same activity is why it must be controlled carefully.
A bentonite grade that provides excellent suspension may be too active for a highly deflocculated slip. A grade that improves green strength may also raise drying shrinkage if the addition is excessive. The goal is not to maximize bentonite activity. The goal is to create the most stable processing window.
What bentonite does in a ceramic casting slip
In a ceramic body, bentonite can contribute in four main ways.
1. Improving particle suspension
Fine bentonite particles can help maintain a more uniform distribution of clay, feldspar, silica and other body components. This can reduce hard settling during storage and improve consistency from one casting batch to another.
However, suspension must not be confused with simply making the slip thicker. A casting slip with high apparent viscosity may still perform poorly if the particle structure is unstable or difficult to break down during mixing.
2. Supporting green strength
After water is removed by the plaster mould and the body begins to dry, bentonite can improve bonding between mineral particles. This may increase the green strength of the cast piece and reduce breakage during demoulding, trimming and transfer.
For thin-wall sanitaryware, tableware or complex cast shapes, this can be particularly important. But green strength must be assessed together with drying behaviour. A body that becomes stronger but cracks more easily is not an improvement.
3. Influencing rheology and thixotropy
Bentonite affects how the slip behaves at rest and under shear. In practical terms, it can influence:
Flow during mixing and pumping
Recovery after agitation
Resistance to sedimentation
Stability during storage
Surface quality after casting
The desired rheology differs between pressure casting, traditional plaster mould casting, slip casting for sanitaryware and small-batch technical ceramics. Therefore, there is no universal “best” bentonite viscosity value.
4. Affecting water demand and shrinkage
Because bentonite has high surface area and strong water interaction, it may increase the amount of water required to achieve a given flow condition. This can reduce solids loading and increase drying shrinkage if not properly managed.
In ceramic processing, more water is rarely a free benefit. It can affect drying time, energy consumption, casting rate and defect risk. The bentonite grade must therefore work with the deflocculant system rather than force the formulation to use more water.
Five properties to evaluate before choosing bentonite for ceramics
1. Mineral composition and montmorillonite content
Montmorillonite is the active clay mineral responsible for much of bentonite’s swelling and rheological behaviour. Higher activity is not automatically better. Ceramic formulators need activity that is consistent and compatible with the rest of the body.
A highly active bentonite may be useful where stronger suspension or green strength is required. A more moderate grade may be easier to control in a highly deflocculated, high-solids slip.
2. Particle-size distribution
Particle size affects dispersion rate, water demand, surface finish and rheology. Very fine material may react quickly but can make the slip more sensitive to mixing conditions and electrolyte balance.
A suitable particle-size distribution should be evaluated in the actual ceramic formulation—not only by dry sieve data. The question is whether the bentonite disperses uniformly without creating local gel particles, inconsistent viscosity or surface defects.
3. Soluble salts and electrolyte sensitivity
Ceramic slips are sensitive to sodium ions, calcium ions, pH and the selected dispersant. Small changes in soluble salts can shift the deflocculation curve and change viscosity significantly.
For this reason, a new bentonite should never be evaluated only by adding it into an established slip at the previous dispersant dosage. The deflocculant demand must be re-optimized after any change in clay grade.
4. Colour-related impurities For white ceramic bodies, engobes and light-coloured glazes, whiteness alone is not enough. Iron-bearing and titanium-bearing minerals, dark accessory minerals and variation between production lots can influence fired appearance.
A visually white raw bentonite may still affect fired colour differently depending on the firing temperature and atmosphere. Ceramic producers should therefore assess fired test bars or fired body samples—not only raw powder colour.
5. Moisture and batch consistencyMoisture variation changes dosing accuracy. Batch-to-batch variation in mineralogy, particle size or soluble salts can also make a previously stable formulation difficult to control.
For industrial ceramic production, consistency is often more valuable than extreme laboratory activity.
A reliable bentonite supplier should be able to provide stable technical parameters and support evaluation in the customer’s actual body system.
A practical evaluation method for ceramic manufacturers
A structured trial is more useful than a simple “pass/fail” sample test.
Step 1: Establish the baseline bodyRecord the existing formulation and operating conditions before changing the bentonite grade:
Raw material composition
Solids loading and density
Water addition
Deflocculant type and dosage
Mixing time and mixing energy
Slip viscosity at defined conditions
Casting rate
Demoulding time
Green strength
Drying shrinkage
Fired appearance and key fired properties
Without this baseline, it is difficult to identify whether a performance change comes from the bentonite, mixing conditions or a variation in another raw material.
Step 2: Screen bentonite at small controlled increments Evaluate the bentonite at several controlled addition levels rather than selecting one dosage immediately. The objective is to observe how the grade changes suspension, deflocculant demand, flow, casting behaviour and green strength.
Do not compensate for an unsuitable bentonite by simply adding more water. First review dispersion and deflocculant optimization.
Step 3: Re-optimize the deflocculant package This is one of the most important steps. Bentonite can alter the electrochemical balance of the slip, so the original dispersant dosage may no longer be optimal.
A proper comparison should include a deflocculant curve for each bentonite grade. The ceramic team should identify the dosage range where the slip reaches the desired flow and stability without over-deflocculation or rapid viscosity drift.
Step 4: Test after ageing, not only immediately after mixing A slip may look acceptable immediately after preparation but become unstable after storage. Evaluate at defined intervals, such as after mixing, after standing and after controlled re-agitation.
Important observations include:
Viscosity change over time
Sedimentation or hard-pack formation
Ease of remixing
Foam behaviour
Casting-rate consistency
Surface quality of the cast part
Common ceramic slip problems and what to investigate
| Production symptom |
Possible bentonite-related cause |
First point to check |
| Slip becomes too thick or requires more water |
Bentonite is too active, too fine, poorly dispersed or incompatible with the dispersant |
Re-optimize dispersant dosage and mixing sequence |
| Viscosity rises during storage |
Soluble salts, pH shift or insufficient dispersion |
Check ageing behaviour, conductivity, pH and deflocculant curve |
| Poor green strength after demoulding |
Bentonite addition is too low, grade has insufficient binding contribution or body is not balanced |
Compare green strength with drying shrinkage and casting rate |
| Cracks during drying |
Excessive bentonite activity, high water demand or non-uniform drying |
Review addition level, moisture profile and drying conditions |
| Slow or inconsistent casting |
Slip rheology is not balanced, solids loading is too low or deflocculation is unstable |
Review density, viscosity curve and mould condition |
| Fired colour variation |
Iron/titanium-bearing impurities or lot-to-lot mineral variation |
Evaluate fired test pieces, not only raw powder whiteness |
The same bentonite should not automatically be used in every ceramic layer.
In a ceramic body, the focus is usually on suspension, plastic bonding, casting behaviour, green strength and drying control.
In an engobe, the focus may shift toward rheology, coating uniformity, adhesion between body and glaze, drying compatibility and colour control.
In a glaze slurry, the key concerns are suspension, application behaviour, settling resistance, thickness control and the effect on fired surface quality.
A grade that performs well in one system may not be appropriate in another. This is why application-based selection is more reliable than purchasing bentonite only under a general ceramic grade description.
Final recommendation: select for process stability, not a single number The best bentonite for ceramic slip casting is not necessarily the one with the highest swelling value, the lowest price or the whitest raw appearance. It is the grade that provides a stable balance between rheology, green strength, casting rate, drying behaviour and fired quality in the actual production system.
For ceramic manufacturers, a useful technical discussion should begin with the formulation and process conditions—not only with a request for a viscosity specification.
HERMANO NEW MATERIALS supplies application-oriented bentonite grades from controlled mineral resources in Liaoning, China. For ceramic slip casting evaluation, our technical team can recommend suitable sample grades based on your body composition, firing temperature, target colour, forming method and current processing challenges.
Contact us with your ceramic application and current slip parameters for an initial grade recommendation.
