Ceramic Slurry Defoamer: Selection & Dosing SOP for Body Slip and Glaze (with Starting Doses and Zero-Defect Validation)

TL;DR (30-second summary)
• Where to add:
Body slip → late-mill + continuous micro-dosing in the holding tank;
Glaze → after screen/magnet and right before application (low shear).
• What to choose:
Body slip → polyether / polyether-ester as baseline; consider mineral-oil blend for very high solids;
Glaze → silicone-free “de-gassing” types; use organically modified silicone only for body-slip emergencies, never on the glaze line.
• Starting doses:
Body slip 200–1000 ppm (mineral-oil blend 100–800 ppm); Glaze 50–400 ppm.
• Release gates:
Density variation ≤ ±0.005 g/cm³; glaze 0 pinholes / 0 fisheyes / 0 craters; spray-dry hollow rate ↓ or flat; no takt-time loss.

Body slip workflow: raw materials → ball mill → disperse/pH → screen/magnet → holding tank → feed to spray dryer.
Foam originates from high-shear milling, entrained air via dispersants/electrolytes, and pump aeration. Results: density swings, higher hollow-granule rate, and pressed density/strength variability that narrows the firing window and inflates rework.

Glaze workflow: glaze powder + water → disperse/fine-grind → screen/magnet → rheology adjust → application → drying.
The glaze face is extremely sensitive to low-surface-tension residues. Uncontrolled silicone or over-dosage of surfactants can trigger craters, fisheyes, and pinholes—a straight path to rejects.

Takeaway: foam drivers and tolerance differ by stage; one bottle for everything rarely works. You need line-specific defoamer plans and add-point discipline.

• Late-mill — add the first portion 5–10 minutes before mill discharge.

• Holding tank — keep density stable via a metering pump (continuous micro-dosing).

• Emergency — buffer tank or just before filter/screen to stabilize density and curb hollows.

• Polyether / polyether-ester: 200–1500 ppm (typical 200–1000 ppm). Robust vs. alkali/salts; low residue.
• Mineral-oil blend (very high solids/refractory mixes): 100–800 ppm. Strong hold-back for chronic entrainment.
• Organically modified silicone microemulsion: 10–200 ppm, emergency only, and keep it off the glaze line (strict asset segregation).

• Split dose: 50% at late-mill + 50% in the holding tank → steadier than a single slug.
• Continuous micro-dosing: tame density swings to ≤ ±0.005 g/cm³ (densitometer/pycnometer).
• Pre-dilute: 1:5–1:10 in process water to avoid oil eyes or localized overdosing.
• Compatibility: verify 24–48 h static compatibility with PAA, Na₂SiO₃/Na₂CO₃, SHMP and your dispersant system (watch for re-coagulation or viscosity spikes).

• Don’t add defoamer before dispersants have fully activated.
• Keep conventional silicone away from glaze assets (pumps, tanks, lines).
• Don’t “top-up by feel” — meter additions or you’ll re-foam and chase density.
• Run an alkali/salt stability check; otherwise expect phase split or slicks on tank walls.

Best add-point

• After screen/magnet and right before application, blended at low shear.
• Small make-up additions are acceptable in the recirculation pan/tray as needed.

Recommended chemistries & starting doses

• Silicone-free “de-gassing” defoamers (acetylene glycol/multi-polyol; low-residue polyether-esters): 50–400 ppm.
  Rationale: rapid surface-tension drop to liberate micro-bubbles with minimal residue risk.

Validation & release criteria

• Draw-down or spray → dry → lab kiln.
• 0 craters / 0 fisheyes / 0 pinholes is a single-vote veto for release.
• Watch for flow marks/orange peel; tune dose and rheology (thixotropy) if necessary.

Prohibitions & risk controls

• Avoid conventional silicone emulsions on the glaze line.
• Acetylene glycol chemistries demand tight dosing; over-use may leave flow artifacts.
• Introduce at low shear to prevent new bubble entrainment.

• Near the glaze line? → Choose silicone-free de-gassing (polyether-ester / acetylene glycol). Ban conventional silicone.
• pH ≥ 10 or electrolyte-rich? → Favor polyether or mineral-oil blend; confirm alkali stability for acetylene-glycol types.
• Primary KPI? → Body slip: density stability / lower hollow rate. Glaze: zero visible defects.
• Need a short-burst fix? → Body slip only: organically modified silicone at low ppm, with hard segregation from glaze.

Core chemistry families & why they fit

• Polyether / polyether-ester (baseline for body & some glazes)
  Tunable HLB, inherently alkali/salt tolerant, low residue when dosed properly, and less prone to re-foaming than very high-HLB non-silicones.
• Acetylene glycol / multi-polyol (glaze de-gassing)
  Immediate surface-tension cut to free micro-bubbles before defects form; ideal where silicone residue is a non-starter. Dose discipline is essential.
• Mineral-oil blend (body slip at very high solids / refractory)
  Provides hold-back (anti-foam) to complement knock-down; best when pumps/valves continually entrain air. Keep away from the glaze line.
• Organically modified silicone (body emergency only)
  High instantaneous defoaming power; use sparingly, never on glaze assets, and maintain strict equipment/material segregation.

Proven compatibility checklist

• 24–48 h static test with PAA, sodium silicate/carbonate, SHMP, and your dispersant system.
• Check rotational viscosity / flow cup — avoid unexplained thickening.
• Pre-dilute 1:5–1:10 in process water; inject under low turbulence.
• Build a habit: split dose + micro-dosing beats one-shot additions.

• Over-dosing → oil slicks and surface flaws, including short-shots on the glaze face.
• Very high-HLB non-silicone → re-foaming or viscosity creep.
• Silicone crossing the glaze boundary → fisheyes/craters (treat as cross-contamination; fix procedures, not only chemistry).
• Fighting your dispersant → re-coagulation or viscosity spikes (do the 24–48 h static check).
• Unmetered make-up → density drift and rising hollow rates.
• Skipping alkali/salt stability checks → phase separation on the loop.

Q1: Why must the glaze defoamer be silicone-free?
Because the glaze face is highly sensitive to low-surface-tension residues; conventional silicone leaves a trail that causes craters/fisheyes. Choose silicone-free de-gassing (polyether-ester / acetylene glycol) and prove zero defects in the lab kiln.

Q2: Body slip foams badly, but the glaze line hates silicone—what now?
Use polyether / mineral-oil blend in body slip and segregate those assets. Run a dedicated silicone-free package on the glaze line.

Q3: Why do very high-solids systems seem to “re-foam”?
Entrained micro-bubbles and shear-induced dissolved gas keep recycling. Micro-dosing stabilizes density (≤ ±0.005 g/cm³), and a mineral-oil blend adds hold-back for chronic entrainment.

Q4: What are safe starting ladders?
Body slip 200–400 ppm; glaze 50–100 ppm. Use foam half-life plus lab-kiln visuals as the two-key release decision.

Q5: How do we define “pass”?
Four gates — stable density, 0 glaze defects, hollow-rate flat or down, and no takt loss.