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What NOT to do?
1) Avoid using bleach or chlorine. While these substances might lighten or remove stains, they can also harm your ceramic item, as shown in the examples below.
2) Do not heat the item in an oven. If the stain is caused by an old oil-based substance, such as grease, heating may cause the oil to melt and spread further under the glaze, potentially making the stain larger.
3) Do not attempt the cleaning process on repaired items. Any existing repairs may be compromised or damaged during the cleaning process described below.
What to do? Easy way for mildly stained pieces:
Buy regular oxygen bleach typically used for laundry (e.g., OXY); mix the powder in hot water and wait for it to cool off or use the liquid version of oxygen bleach. Soak your stained item in the solution for several hours or until you see the stains disappear. **
What to do? More involved and more effective process:
From a pharmacy or a beauty supply store, purchase 8% Hydrogen peroxide bleach that is typically used to bleach hair. Soak your stained item in the solution for several hours. Then, while wet place it in an oven and set to 180 to 200 degrees Fahrenheit (82 to 93 Celsius). Let the oven rise to 180-200 degrees F * while your item is in the oven. WARNING: Do not enter your item to a preheated oven. The rapid temperature change could crack the item or chip off some of the crazed glaze. After 30-60 minutes, take it out, wait for it to cool to room temperature, and wash it with room temperature water. You will see slightly colored water seeping out from the crazing washing off. **
* Warning - Heating to above boiling temperature may result in organic matter burn stains.
** While this removes the color, it does not remove the foreign matter (dirt etc.) causing the stain; therfore, stain can reappear if introduced to moisture.
WARNING!! Using a stronger solution of peroxide is extremely dangerous. It can burn the skin off your hands and cause permanent damage to mucous membranes, and unless you know chemistry very well you could have an explosion. Leave the work with stronger hydrogen peroxide to the professionals.
What if the stain is not eliminated?
For stubborn stains, you will need to repeat the above process every 24 hours using fresh materials each time. The above mentioned products are effective only about a day once in a liquid form exposed to light.
Why we do not take on such projects?
1) The results are not guaranteed and are not 100% predictable.
2) Possible glaze flaking could occur
3) The above process may not remove the material that caused the bacteria / mold in the first place, and it's possible that same contaminant will reappear.
4) Bleaching may not remove inorganic stains such as rust or other clay minerals (Calcium, Lime, etc.) contaminant marks. In this case the stain must be removed requiring an elaborate repair and restoration effort.
Is crazed or crackled glaze on pottery food safe?
NO! The glaze surface has cracks that can possibly hold substances (food, dust, dirt) in it and produce bacteria. Therefore it is not food safe!
Appendix A
The differencesin handling soot stains:
Nature of the Contaminant: Soot might be more superficial and may not require as prolonged a soak as dark organic stains embedded in crazing lines.
For soot, pre-cleaning the surface with a mild detergent and water to remove loose particles may help before soaking.
Severity and Depth: If the soot is only on the surface, scrubbing gently with a non-abrasive pad before soaking might suffice.
Crazing soot stains, which are embedded in cracks, often demand longer soaking times.
Additional Cleaning Steps: Heavily sooted pottery may require multiple cleaning cycles or additional steps, such as using an ultrasonic cleaner.
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