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Solving the "Black Heart" Problem in Firing: Additives for Complete Organic Matter Oxidation
A systematic technical guide — from root cause diagnosis to firing curve optimisation and body reformulation — for ceramic tile and sanitaryware producers.
By Goway Technical Team | Updated: June 2026 | en.goway-china.com
Key Facts
§1 What Is Black Heart and Why Does It Form?
Cut a fired ceramic tile along its cross-section and you sometimes see it: a dark — grey to black — zone in the centre surrounded by the normal white or buff body colour. This is black heart (also called black core). It is one of the most frustrating kiln defects precisely because it is invisible until the tile is cut: hundreds of pieces can pass visual inspection and even glazing before the defect is discovered in QC cross-section sampling.
⚠️ Organic Carbon Black Heart
- Appearance: Dark zone concentrated at the tile centre; edges and surfaces normal colour
- Glaze surface: Usually unaffected — no colour shift
- Cross-section shape: Diffuse oval / "eye" shape following the tile thickness profile
- Root cause: Residual carbonaceous matter (char) not fully oxidised before pores sealed
- Worst in: Thick tiles, high-density pressed bodies, high ball clay or organic binder loading
⚠️ Reducing Atmosphere Black Heart
- Appearance: Dark zone may extend toward surfaces; often accompanied by glaze colour change
- Glaze surface: Colour deviation likely (Cu²⁺→Cu⁰ reddening; Fe³⁺→Fe²⁺ grey-green cast)
- Cross-section shape: More uniform dark zone, less "eye"-shaped
- Root cause: Kiln atmosphere O₂ too low (CO-rich); combustion not complete in kiln chamber
- Worst in: Overfired burner sections; gas kiln with insufficient air intake
Note: Both defect types can coexist. Diagnostic approach: fire sample tiles with a controlled oxidising schedule (forced air bleed). If the defect resolves, the kiln atmosphere is the primary cause. If it persists, the body organics are the primary cause.
1.1 Formation Sequence (Organic Carbon Type)
Mechanism based on: Singer & Singer, Industrial Ceramics (1963); Reed, Principles of Ceramics Processing (1995); Boch & Nièpce (Eds.), Ceramic Materials: Processes, Properties and Applications (2007). (Ref: classic ceramics process literature)
§2 Organic Matter Sources in the Ceramic Body
Before selecting an additive, it is essential to quantify and locate the organic matter that must be burned out. The sources are multiple and often overlooked in isolation.
2.1 L.O.I Contributions from Goway Raw Materials
Loss on Ignition (L.O.I) measures the total mass lost when a sample is heated to ≥950 °C. It includes structural hydroxyl water (kaolinite dehydroxylation), carbonate decomposition, and any organic carbon. For black heart analysis, the organic fraction of L.O.I is the primary concern.
L.O.I Summary — Goway Raw Materials (Source: Goway Technical Data Sheet)
Note: L.O.I values represent total mass loss, including structural water (dehydroxylation). The organic carbon fraction of mineral clays is typically 1–5% of total L.O.I and varies by deposit. Lab LOI measurement on green body is the only way to quantify total organic burden for a specific formulation.
2.2 Additional Organic Sources
Added Organic Additives
- Spray-dry binders (PVA, PEG, acrylate polymers): L.O.I 40–90%
- Plasticisers (PEG, glycerine): high organic carbon content
- CMC, HPMC (for wet-forming): burn out at 250–450 °C but can char at fast rates
- Starch-based additives: burn out at 300–500 °C
Note: Dosages and L.O.I values for organic additives other than FG-ZM01A/D are industry-typical reference values; actual values depend on specific product grade.
Natural Ball Clay Organics
- Humic acids, fulvic acids: derived from geological organic matter in ball clay deposits
- Lignite or carbonaceous clay fragments: burn out unevenly; coarser particles take longer
- Contribution is batch-variable — same product code, different mine batch can have +/−1% organic carbon
See Ceramic Body Binder FG-ZM01 for context on controlled organic binder selection to manage total organic loading.
Inorganic Gas-Generating Sources
- Carbonates (CaCO₃, MgCO₃): CO₂ release at 600–900 °C — no black heart contribution directly, but gas pressure can slow O₂ diffusion inward
- Sulfates: SO₂ release above 900 °C in some raw materials — can contribute to glaze blistering if coincident with pore closure
- Kaolinite dehydroxylation (–OH): 450–650 °C, produces H₂O vapour, not combustion
2.3 Calculating Total Organic Burden
A simple first-pass estimate of total gas volume (mL/100g dry body) can be obtained from formulation L.O.I:
§3 Oxidation Kinetics: Why the Core Is Always Last
The physics of why the core is always the last region to oxidise follows from Fick's First Law of diffusion: O₂ must travel from the surface through a progressively narrowing pore network to reach carbonaceous matter at depth. This is not an additive problem alone — it is a geometry and kinetics problem that fundamentally limits how fast any body can be fired through the oxidation window.
3.1 The Critical Temperature Zone
3.2 Factors That Amplify Black Heart Risk
| Factor | Effect on Black Heart Risk | Controllability |
|---|---|---|
| Tile thickness / format size | O₂ diffusion path ∝ thickness²; doubling thickness increases diffusion time ~4× | Product spec — fixed unless format changes |
| Green body density after pressing | Higher press pressure → denser green body → fewer open pores → slower O₂ ingress | Adjustable: reduce press pressure, add pore-former |
| Total organic burden (body L.O.I organic fraction) | More organic = more O₂ consumed per unit body volume; longer oxidation time required | Adjustable: reduce binder dosage, select lower-L.O.I ball clay |
| Heating rate through 600–900 °C | Fastest route to black heart; fast heating closes pores before oxidation completes | Adjustable: primary lever in kiln schedule |
| Kiln atmosphere O₂ level | Below ~2% O₂, surface oxidation also slows; below ~1%, both organic and Fe³⁺ reduction risk | Adjustable: burner air-to-fuel ratio, setting density, exhaust damper |
| Particle size of organic additives | Coarser organic particles (e.g., granulated binder) burn out more slowly — localised char pockets | Adjustable: select finer-particle binder grade |
| Carbonate content | CO₂ from CaCO₃ decomposition (700–900 °C) creates competing gas flow in the same critical zone | Partially adjustable: raw material selection |
§4 Additive Solutions: Types & Mechanisms
Oxidation-promoting additives work by one of three mechanisms: (a) providing supplemental oxidant within the body matrix; (b) acting as catalytic mineralizers that lower the activation energy for carbon oxidation; or (c) modifying the physical pore structure to extend the window of O₂ accessibility. No single additive type is universally superior — selection depends on firing temperature, body composition, and the primary black heart mechanism.
§5 Additive Application & Dosing Matrix
All dosage values below are industry-typical starting reference values. Effective dosage depends on body composition, pressing parameters, kiln type, and tile format. Always start at the lower bound of the range and conduct a five-point dosage curve test before full-line implementation.
| Condition | Primary Black Heart Type | Recommended First Action | Supplemental Additive Option | Starting Dosage |
|---|---|---|---|---|
| Standard tile (6–8 mm), low ball clay (<15%) | Firing curve | Reduce heating rate through 600–900 °C | Usually none needed | — |
| Thick tile or large format (≥10 mm) | O₂ diffusion geometry | Extend hold time at 700–850 °C | Pore-former (burn-out agent) | 0.2–0.5% (P3) |
| High ball clay body (>25%) with persistent black heart after firing curve fix | Natural organics + body density | Switch to FG-B88 (lower L.O.I); confirm total body L.O.I | Fe₂O₃ catalyst (if colour acceptable) | 0.2–0.5% Fe₂O₃ (P3) |
| High organic binder loading (>1.5% polymer binder) | Added organic load | Reduce binder dosage; switch to FG-ZM01A (higher active content) | Nitrate salt at onset temperature | 0.1–0.3% Ca(NO₃)₂ (P3) |
| Kiln atmosphere O₂ <1% confirmed by in-kiln probe | Reducing atmosphere | Increase combustion air; check burner air/fuel ratio | None — atmosphere fix required first | — |
| White body / premium tile (no iron addition allowed) | Any organic type | Firing curve + pore-former; avoid Fe/Mn oxides | Organic pore-former (cellulose) | 0.3–0.8% (P3) |
| Recycled body with elevated ionic/organic contamination | Variable — monitor batch | Reduce recycled fraction; review incoming material L.O.I | Transition metal oxide catalyst | Start 0.2% Fe₂O₃ (P3) |
| Intermittent / batch-variable black heart | Raw material variation | Implement incoming L.O.I QC on ball clay batches | Adjust firing curve to worst-case batch | — |
All dosages are industry-typical reference values (P3 — engineering experience). No Goway proprietary oxidation-promoter product code exists in the current database — see Data Availability Notice in §4. Starting recommendations are based on published ceramic process literature and collective industry experience.
§6 Process Optimisation Guide
Additive selection is only one dimension. Sustainable elimination of black heart requires a coordinated review of the entire production flow — from raw material receipt through firing and kiln atmosphere management. The four process levers below are listed in order of impact.
Lever 1: Firing Curve (Highest Impact)
- Map your current curve against the 600–900 °C critical window
- Identify the heating rate through 600→900 °C — this is your primary control variable
- Add a deliberate "oxidation hold" at 700–800 °C: 5–20 minutes for standard tiles; 20–40 minutes for thick/large format (industry-typical starting points — validate with cross-section sampling)
- Avoid rapid "skip-through" heating in this zone to save kiln time — the cost of black-heart scrap typically exceeds the fuel cost saved
Lever 2: Kiln Atmosphere (O₂ Level)
- Install or calibrate an O₂ probe in the hot zone (600–900 °C section of roller kiln)
- Target: ≥2% O₂ in kiln gas throughout the oxidation zone
- Increase combustion air if O₂ probe reads below 1.5%
- Review setting density and tile stacking — high setting density restricts combustion gas circulation
- Ensure exhaust damper is not over-restricted — back pressure reduces O₂ sweep of kiln interior
Lever 3: Body Formulation Adjustment
- Calculate total body L.O.I from formulation (see §2.3 model)
- If body L.O.I >12%, evaluate whether ball clay proportion can be reduced with a higher-purity kaolin substitute
- Review organic binder dosage — is the minimum dosage for acceptable green strength being used, or is there a legacy "safety margin" that can be reduced?
- If using FG-ZM01D, evaluate whether FG-ZM01A (higher active content, potentially lower dosage needed) reduces total organic burden
Lever 4: Incoming Raw Material QC
- Implement L.O.I testing on every incoming ball clay batch — not just spot-check audits
- Set an incoming L.O.I specification limit for ball clay (e.g., max 13.5%) and reject or quarantine batches above this threshold
- Track kiln performance vs. ball clay batch L.O.I over time — correlations often reveal that "random" black heart spikes are actually raw material batch events
- For recycled material bodies, implement additional organic contamination screening — see our guide on Recycled Materials in Ceramic Body
6.1 Spray Dryer Settings and Their Impact on Black Heart
The green body arriving at the press is largely determined by spray dryer conditions. Granule moisture content, particle size distribution, and binder distribution within the granule all affect how quickly the granule softens and how porous the pressed body is — which directly affects oxidation rate during firing. Slurry viscosity upstream of the spray dryer is a key upstream variable — see our Reduce Ceramic Slurry Viscosity guide for how slurry properties influence granule quality and, by extension, pressed body porosity and black heart susceptibility. Two specific spray dryer parameters are most relevant to black heart risk:
| Spray Dryer Parameter | If Too High | If Too Low | Recommended Range |
|---|---|---|---|
| Granule moisture content | Excess steam during early heating — may push pore structure to close faster | Over-dried granules → brittle, poor filling, density gradients in pressed body | 5–7% for standard dry-press (industry-typical; validate with your press spec) |
| Mean granule size (D50) | Coarse granules → poor filling in thin areas; localised density voids; uneven organic distribution | Fine granules → over-compaction; reduced open porosity in pressed body | 250–400 µm D50 for standard floor tile (industry-typical; kiln/press dependent) |
All ranges are industry-typical reference values. Optimal settings are kiln- and formulation-specific — validate with cross-section black heart evaluation at each setting change.
§7 Lab Validation Protocol
When investigating or solving a black heart problem, a structured laboratory evaluation is faster than trial-and-error on the production line. The following seven-step protocol can be completed with standard ceramic lab equipment.
Baseline Cross-Section Evaluation
Cut a representative sample of defect-affected tiles along the thickness axis. Photograph and measure the dark zone depth and width. Score the severity on a simple scale: 0 (no dark zone) → 3 (dark zone extends >50% of tile cross-section). This establishes the baseline before any changes are made.
Body L.O.I Measurement
Measure L.O.I on the dry pressed green body at 950 °C (1 hour, muffle furnace). This confirms total gas volume and is the single most informative quick measurement. Compare with your formulation-calculated L.O.I from §2.3.
Firing Rate Sensitivity Test
Fire the same body through the critical zone at three different heating rates: (a) current production rate, (b) 30% slower, (c) 50% slower. Use a lab kiln with programmable controller. Evaluate cross-sections. If the black heart is eliminated or significantly reduced at 50% slower rate, the kiln schedule is the primary cause.
Atmosphere Sensitivity Test
Fire a replicate set in a kiln with confirmed oxidising atmosphere (>3% O₂ measured). If black heart is eliminated under forced oxidising conditions but not in the production kiln, the kiln atmosphere is the primary cause — focus on burner air-fuel ratio and exhaust configuration.
Additive Dosage Curve (Five-Point Method)
For the additive type identified in the selection matrix (§5): prepare five body batches at D1 = 0%, D2 = 0.1%, D3 = 0.2%, D4 = 0.5%, D5 = 1.0% additive loading. Hold all other parameters constant. Fire all five batches at the same (current production) rate. Evaluate cross-section black heart score and fired colour at each dosage level. Identify the minimum effective dosage.
Mechanical Strength Confirmation
Verify that the selected additive dosage does not negatively affect fired Modulus of Rupture (MOR). Use three-point bending test per ASTM C648 (fired) or equivalent. Compare additive-treated tiles with control batch. Any reduction in MOR >5% relative is a concern that warrants investigation before production trial.
Production Trial & Documentation
Run a limited production trial (500–1000 tiles minimum) with the validated additive dosage and/or firing curve modification. Sample cross-sections at kiln inlet, midpoint, and outlet. Document: black heart score, fired colour L*, MOR, glaze surface quality, and firing energy consumption change. Decision gate: proceed to full-line implementation if black heart score ≤ 0.5 and no mechanical or colour regression.
§8 Troubleshooting
§9 Frequently Asked Questions
Q1: What causes the black heart (black core) defect in ceramic firing?
Black heart is caused by incomplete oxidation of carbonaceous materials — residual organic additives, natural organics in ball clay, and sometimes carbonates — during the critical oxidation window between approximately 400–900 °C. When the heating rate through this zone is too fast, the tile body is too thick or too dense, or kiln atmosphere oxygen is insufficient, residual carbon is trapped as a black core.
Q2: At what temperature does organic matter oxidise in a ceramic body?
Most organic binders and natural organics begin to burn out between 250–400 °C and are substantially decomposed by 600 °C. However, fine carbonaceous residues and char trapped in dense matrices can persist until 800–900 °C. The critical control zone is typically 600–900 °C, where complete oxidation must be ensured before the body begins to sinter and pores close off.
Q3: Can additives alone solve the black heart problem?
No. Additives (oxidation promoters, mineralizers) can lower the activation energy for organic oxidation and meaningfully assist, but they cannot compensate for a fundamentally wrong firing curve. The root fix is usually optimising the heating rate and hold time in the 600–900 °C window. Additives are most effective as a supplement — not a replacement — for correct kiln settings.
Q4: How does ball clay contribute to black heart defects?
Ball clay contains natural organic matter (humic acids, lignite fragments) in addition to its mineral structure. Goway FG-B82 ball clay has an L.O.I of 12.5%, and FG-B88 has 11.8% (Source: Goway Technical Data Sheet). A portion of this L.O.I represents organic carbon that must be fully combusted during firing. Higher ball clay proportions in the body formulation increase the total organic burden and raise the risk of black heart if the firing curve is not adjusted accordingly.
Q5: What is the difference between a 'reducing atmosphere black heart' and an 'organic carbon black heart'?
A reducing atmosphere black heart is caused by insufficient oxygen in the kiln gas — it tends to affect the entire cross-section and often accompanies a glaze colour shift. An organic carbon black heart is concentrated in the centre and is more common in thick or high-density bodies — the glaze surface is usually normal. The two types may coexist and their solutions are different: atmosphere black heart requires kiln burner and exhaust adjustment; organic carbon black heart requires firing curve and formulation work.
Q6: How do I know if the firing curve is the problem vs. the body formulation?
A reliable diagnostic: fire the same body at 30–40% slower heating rate through 600–900 °C. If the black heart disappears or reduces significantly, the firing curve is the primary cause. If it persists, investigate the body — reduce organic binder dosage, switch to lower-L.O.I ball clay, or add an oxidation promoter. See the lab validation protocol in §7 for a structured evaluation sequence.
Request a Black Heart Diagnostic & Solution Package
Goway's technical team can help you diagnose the root cause of your specific black heart defect pattern and recommend an optimisation pathway — whether through raw material selection, firing curve adjustment, or targeted additive supplementation.
Request Black Heart Diagnostic →To submit a technical inquiry, visit the Ceramic Body Binder FG-ZM01 product page (/products_detail/12.html) and use the inquiry form. Please reference "Black Heart Guide" when submitting and include the information fields below where possible.
Related Resources
- Improve Ceramic Green Strength: Binder Selection — framework for minimising organic binder dosage while maintaining green strength (visit /News_detail/158.html)
- Recycled Materials in Ceramic Body — organic contamination management for recycled-content bodies (visit /News_detail/153.html)
- Ceramic Body Binder FG-ZM01 — FG-ZM01A (Active 95–98%) and FG-ZM01D (Active 90–95%) full specifications (visit /products_detail/12.html)
- Reduce Ceramic Slurry Viscosity — upstream slurry and spray dryer management that affects granule organic distribution (visit /News_detail/150.html)
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