Vitreous china slip maintained at a bulk density above 1.80 g/mL represents a suspension in which the volumetric solids fraction exceeds 0.49 when the particle true density is 2.60 g/mL. The corresponding weight solids is approximately 72.5 wt% at 1.82 g/mL and rises to roughly 74 wt% at 1.85 g/mL. In this density regime, the distance between edge and face charges on kaolinite, illite, and montmorillonite particles is short enough that the diffuse electrical double layer is easily compressed by excess sodium ions introduced as sodium silicate or sodium carbonate. Production-scale blungers with drive capacities between 37 kW and 75 kW, typically operating with propeller tip speeds below 8 m/s, provide sufficient shear to mask the early signs of over-deflocculation; the slip may appear thin under the blunger but thickens or sediments in the transfer line after 20 min of static ageing. The central control problem is therefore not viscosity reduction alone, but the management of deflocculant demand so that the slip remains inside a narrow electrokinetic window where casting rate, green strength, and plaster mould release are simultaneously acceptable. A density shift of 0.02 g/mL within an eight-hour production shift is often sufficient to alter casting rate and release behavior, particularly when return scrap, plaster mould moisture, and process-water hardness are not corrected.

The deflocculation curve for a vitreous china body at 1.80 g/mL is typically U-shaped. Initial additions of a mixed electrolyte system—sodium silicate with a SiO₂:Na₂O ratio between 2.0 and 3.3 and sodium carbonate—displace adsorbed calcium and magnesium from clay edges and raise the negative zeta potential from approximately −15 mV to −35 mV. Viscosity falls as particle-particle repulsion increases. Beyond the optimum addition, which may lie between 0.30 wt% and 0.45 wt% sodium silicate equivalent on dry solids depending on soluble sulfate background and clay cation exchange capacity, the further increase in sodium ion concentration compresses the electrical double layer. The suspension may re-flocculate or become strongly shear-sensitive, and the viscosity can rise again while the casting rate collapses. Published data for this specific configuration is limited, but plant trials on parallel 10,000 L blungers have shown that the optimum at 1.83 g/mL can shift by 0.08 wt% between morning and afternoon shifts when unwashed return scrap and plaster mould moisture are not corrected. Consequently, the density increase from 1.70 g/mL to 1.84 g/mL is not merely a concentration effect; it narrows the allowable deflocculant range because there is less free water to dilute the soluble electrolyte background.

What Deflocculant Demand Shifts When Return Slip Carries Soluble Calcium Above 120 mg/L?

Soluble calcium and sulfate concentrations in the water phase are among the most frequently overlooked variables when a vitreous china slip is held above 1.80 g/mL. Calcium ions originate from hard process water, plaster mould dissolution, and clay minerals that release exchangeable cations during blunging. When the soluble calcium concentration in the supernatant exceeds 120 mg/L, the conventional sodium silicate/sodium carbonate deflocculant pair is partially consumed by cation exchange and by the precipitation of calcium carbonate and calcium silicate. The result is a false deflocculation demand: the operator records rising viscosity and adds more sodium silicate, but the effective soluble deflocculant in the water phase remains low until the exchange capacity is saturated. When the calcium concentration then drops, the excess sodium silicate becomes available rapidly, and the slip can move into over-deflocculation within one cycle. Ion chromatography according to ISO 10304-1:2007 for sulfate and ICP-OES according to ISO 11885:2007 for calcium are appropriate for tracking this transition. The threshold of 120 mg/L is not arbitrary; at 1.84 g/mL, the reduced water volume amplifies the ionic strength effect, and a calcium increase of 20 mg/L can shift the deflocculant optimum by 0.05 wt% or more on dry solids.

The use of barium carbonate as a sulfate scavenger is effective at precipitating sulfate as barium sulfate below 0.1 µm when the addition is introduced as a dispersed slurry before the clay is blunged. However, its addition must be controlled within 0.05–0.10 wt% of dry solids because unreacted barium carbonate is classified under REACH and can create effluent toxicity issues if released. Over-deflocculation induced by sulfate precipitation is often delayed; the slip may appear stable for 12–24 h and then exhibit a rapid pH rise from 8.2 to 9.3 as carbonate alkalinity increases. For this reason, slip-ageing trials of not less than 48 h are required before a new batch is released to casting. The ageing period should be accompanied by sealed-tank storage to prevent carbon dioxide absorption, which can alter the carbonate equilibrium and produce a false low pH reading in a slip that is already over-deflocculated. Conductivity testing on a filtered liquor sample is more useful than pH alone, because it reflects total dissolved ionic species rather than only hydrogen ion activity.

Rheometric Signatures That Distinguish Over-Deflocculation From Under-Deflocculation at 1.84 g/mL

A rotational viscometer with a small sample adapter and a spindle geometry calibrated according to ASTM D2196-20 is used to record the 20 rpm and 100 rpm viscosities at 25 ± 0.5 °C. A slip at 1.84 g/mL that is correctly dispersed typically displays a 20 rpm viscosity between 420 mPa·s and 650 mPa·s, and a 20/100 rpm ratio between 1.4 and 1.9. Over-deflocculated slip can present either a low viscosity with a ratio below 1.3 and visible syneresis, or a re-flocculated condition with a ratio above 2.2 and a yield stress that exceeds 8 Pa. The diagnostic distinction is made by measuring the hysteresis loop area between ascending and descending shear-rate sweeps from 2 s⁻¹ to 200 s⁻¹ over 180 s. A loop area greater than 1,200 Pa·s⁻¹ in a slip that still pours readily indicates that the system is kinetically unstable and likely to form hard sediment in transfer lines. Conversely, a loop area below 200 Pa·s⁻¹ with a 20/100 rpm ratio below 1.2 suggests excessive dispersion and possible particle classification in pressure casting pipes.

The following table is compiled from repeated production observations on a sanitaryware casting line using a 3.3 m³ pressure casting unit; published data for this exact configuration is limited, and the values should be treated as representative operational benchmarks rather than universal specifications.

Corrective action on a production slip above 1.80 g/mL must be based on a combination of density, conductivity, and rheological trend data rather than a single viscosity reading. If density is high and viscosity is low, the first step is not to add water indiscriminately but to verify deflocculant dosage records and sulfate concentration; water addition may drop the density below 1.80 g/mL while leaving the electrolyte concentration sufficiently high to perpetuate over-deflocculation. If density is high and viscosity is high, sodium silicate should be added in pre-diluted form at 10–15% solids and at a rate below 2 L/min into the blunger vortex to avoid localised electrolyte shock. The slip should then be recirculated through a 150 µm vibrating sieve for at least 30 min before re-testing. If conductivity exceeds 4.5 mS/cm, the batch should be diverted to a correction tank and blended with freshly blunged slip at a ratio not exceeding 30% of the total volume; this reduces ionic strength without adding water and without stripping the necessary fines from the particle size distribution.

Control of over-deflocculation above 1.80 g/mL also requires a closed-loop record of particle size distribution. Excess deflocculant can preferentially stabilize the submicrometer clay fraction while allowing quartz and feldspar particles larger than 10 µm to sediment. Laser diffraction according to ISO 13320:2020 should be used to monitor the d50 and d90 values after each blunging cycle. For a typical vitreous china sanitaryware slip, the d50 is usually between 4 µm and 8 µm, while the d90 is below 25 µm. A shift in d50 by more than 1 µm within 24 h can indicate that over-deflocculation is selectively suspending fines and changing the packing behavior of the cast layer. The particle size data should be correlated with the casting rate constant k, measured as green thickness divided by the square root of time in minutes; a drop from 0.74 mm min^−0.5 to 0.41 mm min^−0.5 is a leading indicator of excess electrolyte before visible syneresis appears.

When Sulfate Ion Drift Exceeds 250 mg/L and Sodium Silicate Demand Reverses

At sulfate ion levels above 250 mg/L in the slip liquor, the deflocculant demand can reverse because sulfate ions compress the double layer and compete with polyvalent anions for edge sites on clay minerals. This condition is particularly common when process water from a closed-loop recycling system is returned to the blunger without sulfate removal. The slip at 1.82 g/mL may then require a higher initial sodium silicate dose, followed by a sudden viscosity collapse when the sulfate is bound by calcium released from the clay. The control limit for sulfate should therefore be set below 150 mg/L for high-density vitreous china slip, and the chloride concentration should be held below 100 mg/L to avoid corrosion of stainless-steel mould carriers and excessive ionic strength. Conductivity of the slip liquor, measured on a filtered 10 mL sample according to ISO 7888:1985, typically falls between 1.8 mS/cm and 3.5 mS/cm for a well-dispersed 1.80–1.84 g/mL slip; values above 4.5 mS/cm are associated with over-deflocculation and casting-rate collapse.

Sulfate drift can also originate from the oxidisation of pyrite or sulfide minerals in the clay feed, especially when blends are changed between mine locations or storage piles. A feed clay containing 0.1 wt% fine pyrite can release enough sulfate during acid-dispersed blunging to exceed 250 mg/L within 72 h. This delayed release explains why a slip may pass initial quality control and then fail on the casting line after weekend ageing. For this reason, the sulfate concentration should be monitored at 24 h, 48 h, and 72 h after blunging, not only immediately after preparation. If sulfate is trending upward, the batch may be stabilised by adding a small amount of dispersant based on polyacrylate or lignosulfonate, but the interaction with sodium silicate must be evaluated by jar testing before plant addition because some polymer dispersants increase sensitivity to over-deflocculation at high solids.

Pressure Casting Dewatering Limits and Plaster Mould Saturation Gradients

In pressure casting at 1.80 g/mL and above, the dewatering rate is governed by the permeability of the cast layer and the saturation gradient in the plaster or porous resin mould. Over-deflocculated slip creates a cast layer with a high packing density and a low permeability coefficient, often below 1.0 × 10⁻¹⁴ m², compared with 1.5 × 10⁻¹⁴ m² to 3.0 × 10⁻¹⁴ m² for normally dispersed slip. The result is that the cast layer thickens to 2.5 mm in 45 min instead of 25 min, and the residual moisture in the cast layer remains above 22% when release is attempted. Plaster mould dewatering is further suppressed when the slip contains excess sodium silicate because the silicate deposits at the mould interface and reduces surface porosity. The cast body may release with a soft, greasy surface, and warping during drying is increased. A standard plaster mould with a water absorption capacity of 32% and a compressive strength of 9–12 MPa can tolerate only a limited number of over-deflocculated cycles before the surface becomes sealed and the casting rate drops permanently.

The operational boundary for a high-density vitreous china slip above 1.80 g/mL is defined by a density band of ±0.02 g/mL around the target, a 20 rpm viscosity band of ±100 mPa·s, and a deflocculant dosage change of not more than 0.05 wt% per 4 h period. Sodium silicate additions should be pre-diluted to 10–15% solids and injected into the blunger vortex at a rate below 2 L/min to avoid localised over-deflocculation. The slip should be aged for a minimum of 24 h and recirculated through a 150 µm sieve before casting; slips that fail the hysteresis test or show conductivity above 4.5 mS/cm should be diverted to a correction tank rather than pumped to the casting line. Published data for this specific configuration is limited, but the operational limits described are derived from routine production control and conform to the test methodology of ISO 9001 controlled process documentation.