White craters in terracotta planters after a change of clay pit
A small ceramics unit in Khurja produces hand-thrown terracotta planters from a local red clay delivered wet and stored under tarpaulin. The body is mixed with 15% grog (0–3 mm) in a pug mill; no ageing or screening is done. Planters are dried under a fan for 24 h and fired in a 30 m³ downdraught kiln to about 1000°C over 14 h. In the last two batches, 8–12% of planters show small craters 1–3 mm across on the outer wall, with a white powdery deposit in the crater; some craters appear only two weeks after firing. The unit has a 5,000-piece festival order due in three weeks. The owner wants to re-fire the affected stock and continue with the same clay, because 'the clay has always worked'. The kiln operator reports that the clay delivery came from a new pit about 2 m deeper than before.
1. What is the most likely cause of the craters, and why is re-firing the affected stock not a safe remedy? Distinguish the fault from two other defects that can also leave white deposits or surface faults.
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The pattern points to lime popping, not a glaze fault. The new pit is 2 m deeper and has probably exposed a calcareous layer: small lumps of calcium carbonate in the clay. During firing, CaCO₃ converts to CaO. After firing, atmospheric moisture hydrates CaO to Ca(OH)₂, which expands and pushes a crater out of the wall; the white powder is the hydrated/carbonated lime. The delayed appearance over two weeks is characteristic. Re-firing does not remove the particle. It may drive off some water, but the lime remains and will re-hydrate later; re-firing also costs fuel and labour and can create new cracks. The stock should be quarantined, not sold as first quality.
Two distinctions matter:
- Soluble salt scum gives a white film on the surface, often after drying or firing, without a deep crater; it comes from salts in the water or clay, not from discrete lime particles.
- Black core is a dark, reduced interior from carbon that was not burned out; it may bloat or crack, but it does not produce a white powdery crater.
A simple confirmation is to crush a raw clay sample, slake it, sieve it, and look for pale lumps; a drop of dilute acid or vinegar on the raw clay or crater deposit will fizz if carbonate is present. The key judgement is that the defect is particulate and source-related, so screening and source control are needed, not more firing.
2. Set out immediate actions for the 5,000-piece order and the changes you would make to clay preparation. Include a simple incoming-clay test the unit can use without a laboratory.
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Immediate actions:
- Quarantine all planters from the last two batches and any unfired ware made from the new delivery. Do not re-fire them for sale. Sort visually, but accept that early craters may not yet be visible.
- Stop using the new pit clay until it is tested. Produce the festival order from a known clay or a blend that has passed a firing test.
- Make test coupons from the suspect clay: 20 small tiles, fired on the same schedule, kept in a damp atmosphere for two weeks and inspected. This is faster and cheaper than discovering the fault on 5,000 planters.
- Tell the customer the delivery date is at risk if the clay is not cleared; do not promise first-quality ware from untested material.
Clay preparation changes:
- Dry and crush the clay, then slake it in water and sieve the slurry through an 80-mesh screen (about 0.18 mm). Reject the oversize, which is where lime nodules concentrate.
- Age the screened slurry for at least 5–7 days before pugging; this improves plasticity but does not remove lime.
- Keep the 15% grog if it is clean and well graded; grog is not the cause here. Check that the grog itself is not from a calcareous source.
- Record the pit, delivery date and sieve residue for every batch.
Incoming test: dry a 100 g sample, break it up, slake it, and wash through an 80-mesh sieve. Examine the residue for soft pale lumps. Put a few drops of dilute acid or vinegar on the raw clay and on any white crater deposit; fizzing indicates carbonate. If residue or fizzing is significant, reject the load or blend it only after a successful fired test.