Sanitaryware dunting on DeepSeek

Has its own word so it’s common. Good ways to get better advice on a ceramic problem is to ask for a solution to the industrial equivalent.

Dunting (thermal shock cracking during cooling) in sanitaryware is a serious kiln-related issue. Here's a breakdown of potential causes and key questions to diagnose the problem:

**Primary Causes of Dunting:**

1. **Excessive Cooling Rate:** This is the **most common cause**. Cooling too quickly, *especially* through the critical **quartz inversion zone** (around 573°C / 1063°F), generates massive internal stresses due to the sudden volume contraction of quartz crystals.
2. **Uneven Cooling:**
* **Kiln Temperature Gradients:** Significant hot or cold spots within the kiln during cooling cause different parts of a piece (or different pieces) to cool at different rates.
* **Ware Density/Loading:** Overloading, uneven stacking, or blocking cooling air channels prevents uniform airflow.
* **Piece Design:** Thick sections cool much slower than thin sections (e.g., a thick base vs. a thin rim), creating internal stress differentials.
3. **Body Composition Issues:**
* **High Quartz Content:** Bodies rich in free quartz are more susceptible to stress during the inversion.
* **Thermal Expansion Mismatch:** Incompatibility between the body and any applied glaze (or engobe) can exacerbate stresses during cooling. While often causing crazing, severe mismatches contribute to dunting.
* **Body Formulation Changes:** Recent changes in raw materials or body recipe altering thermal expansion or quartz content.
4. **Insufficient Soaking/Holding Time:** Inadequate time at peak temperature can leave the body under-fired and less able to withstand cooling stresses. More commonly, insufficient holding *before* critical cooling phases (like just above inversion) prevents temperature equalization.
5. **Moisture Issues:**
* **Insufficient Drying:** Residual moisture turning to steam explosively during early heating *can* cause cracks that propagate later, sometimes mistaken for pure dunting.
* **High Humidity During Cooling:** Introduction of humid air during low-temperature cooling (<100°C) can cause "moisture shock" cracks.

**Critical Questions to Ask the Factory:**

**About the Kiln Cycle & Operation:**
1. **Cooling Rate:** What is the *actual* cooling rate profile, **especially between 700°C down to 400°C (1292°F down to 752°F)?** Is it faster than recommended for your body type? Has it changed recently?
2. **Quartz Inversion Handling:** Is there a specific *reduction* in cooling rate programmed around 600°C ± 50°C (1112°F ± 90°F)? How long is the hold or slow-cool zone at this stage?
3. **Temperature Uniformity:** Are there data loggers or thermocouples showing significant temperature differences between different zones of the kiln (top/bottom, center/edges) *during cooling*? When was the last survey done?
4. **Kiln Loading:**
* How densely is the kiln loaded? Is there adequate space for air circulation around pieces?
* Is loading consistent? Are thick and thin items mixed indiscriminately?
* Are cooling air channels/fans blocked by ware or setters?
5. **Recent Changes:** Any recent modifications to the firing/cooling schedule? New kiln controller software? Changes in kiln pressure settings affecting airflow?
6. **Kiln Maintenance:** When were cooling fans, dampers, and burner nozzles last inspected/cleaned? Are all fans operational and balanced? Are thermocouples calibrated and functioning correctly?

**About the Ware & Body:**
7. **Crack Location & Pattern:** *Exactly* where do the dunting cracks appear? Are they consistent? (e.g., Always at thick-thin transitions? Random? Near supports? This is crucial diagnostic info).
8. **Design Changes:** Have there been any recent design modifications (especially increasing section thickness or creating sharper transitions)?
9. **Body/Glaze Changes:** Any changes in raw material suppliers, body recipe, glaze recipe, or engobe within the last few batches? Has the body's thermal expansion coefficient been checked recently?
10. **Drying:** Is the drying process (before firing) robust and consistent? Is the moisture content of the dried ware consistently low enough? Any signs of steaming or explosions early in the firing cycle?
11. **Setter/Saggar Condition:** Are setters or saggers old, warped, or contaminated? Do they provide even support and allow airflow? Are they correctly sized for the pieces?

**About Occurrence:**
12. **When Did It Start?** Was the onset sudden or gradual? Correlate exactly with any other changes (material, kiln, design, maintenance).
13. **Pattern in Kiln:** Are cracked pieces located in specific areas of the kiln load (e.g., always near the door, always on the bottom layer, always in corners)? This points to temperature gradients.
14. **Seasonality:** Has humidity or ambient temperature changed significantly recently? (Can affect drying and very low-temp cooling).

**Key Suggestions & Next Steps:**

1. **Immediate Action:**
* **Slow Cooling:** Reduce the cooling rate significantly, *especially* through the 700°C to 400°C range. Implement or extend a hold period around 600°C. **This is often the quickest fix.**
* **Check Kiln Sensors:** Verify thermocouple accuracy and placement.
* **Inspect Fans & Airflow:** Ensure all cooling fans are running correctly and dampers are set properly. Visually check for blockages.
* **Review Load Pattern:** Ensure even loading with adequate airflow channels. Avoid mixing very thick and very thin pieces in the same load if possible.
2. **Data Collection & Analysis:**
* **Data Logging:** Run kiln data loggers (if not already) to map actual temperature profiles throughout the kiln during cooling.
* **Crack Mapping:** Document *exactly* where cracks appear on different products and their location within the kiln.
* **Compare Good/Bad Batches:** Analyze any differences in body composition, moisture content, firing data, or loading between batches with high and low dunting rates.
3. **Systematic Investigation:**
* **Body Testing:** Measure the thermal expansion coefficient of the current body. Compare to previous data if available.
* **Kiln Survey:** Conduct a formal kiln temperature uniformity survey during both heating and cooling phases.
* **Review Recent Changes:** Scrutinize *any* change (no matter how small) in materials, processes, settings, or maintenance that occurred just before the dunting started.

**Start by focusing on the cooling profile (Questions 1 & 2) and kiln loading (Question 4), as these are the most frequent culprits.** The specific location of the cracks (Question 7) is also vital for narrowing down the cause. Good luck!

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