We often receive requests to remove dark crazing lines, stains, discoloration, and fire-related soot from antique pottery and porcelain. These problems are especially common in older ceramics, where moisture and contaminants may penetrate cracks in the glaze. Although we do not offer ceramic cleaning or stain-removal services, we have prepared this guide to explain what causes these marks, which cleaning methods may be appropriate, and when attempting removal could cause further damage.
Stains may develop when liquids or contaminants enter fine cracks in the glaze, known as crazing, or defects such as pinholes. Tea, coffee, oils, food residues, minerals, and dirt can leave brown, yellow, or black marks within the crack network or porous ceramic body. Microbial growth is possible in damp conditions, but it is not the cause of every dark stain. Surface cleaning alone may not reach discoloration trapped beneath the glaze.
Porous earthenware is particularly vulnerable to absorbed staining. Stoneware varies in porosity depending on its clay and degree of vitrification, while well-vitrified porcelain generally absorbs very little moisture through the ceramic body. However, crazing, chips, and damaged glaze can still collect contamination even on otherwise dense ceramics.
Stains along glaze crazed lines
Stain spots
What Is Glaze Crazing and Why Does It Happen?
Crazing is a network of fine cracks in a fired ceramic glaze. It commonly develops when the glaze and clay body have incompatible thermal expansion characteristics, leaving the glaze under tension as the piece cools. Crazing may appear soon after firing or develop later through moisture expansion, aging, or repeated temperature changes. Cleaning can sometimes reduce dark staining in these cracks, but it cannot repair the cracks themselves.
In some cases, crazing is intentionally created as a decorative effect, adding visual texture and character to the piece. When used for aesthetic purposes, traditional cleaning guidelines may not apply. For example, the green decorative surface shown below was created by fusing glass onto pottery. It is a deliberate decorative treatment, not an example of unwanted staining.
Can crazing lines be permanently removed? Cleaning may reduce discoloration, but it does not close or repair the fine cracks in the glaze. Staining can return if moisture and residues enter the same cracks.
Surface stains or stains beneath the glaze? Surface deposits may respond to gentle cleaning on stable ceramics. Dark lines following a crack network or marks beneath the glaze are harder to reach and may not be removable without risk.
When should you stop cleaning? Stop if glaze begins to lift, color transfers to a cloth, cracks worsen, or an old repair softens. Do not experiment on valuable or historically important pieces; consult a qualified conservator.
1) Avoid chlorine bleach and aggressive cleaners. They may alter decoration, affect old repairs, or cause further problems in porous and damaged ceramics. The photograph below illustrates glaze loss; it does not establish that bleach caused this particular damage.
Example of flaking and glaze loss on an aged ceramic surface.
2) Do not heat stained pottery in a household oven. Heat can spread absorbed residues, stress weakened glaze, and damage previous repairs or decoration.
3) Do not soak or chemically treat previously repaired, flaking, gilded, painted, or otherwise unstable ceramics without conservation advice. Moisture and solvents can compromise adhesives, finishes, and fragile surfaces.
Conservative cleaning considerations for mildly stained pieces: For sound, modern, undecorated glazed ceramics, gentle surface cleaning with a soft cloth and a little mild detergent may be appropriate. Avoid prolonged soaking. Oxygen-based laundry cleaners and peroxide are not universally safe, particularly for antique, porous, painted, gilded, previously repaired, or unstable objects. When in doubt, consult a ceramic conservator before using any chemical treatment.**
Why we do not recommend heated peroxide treatment at home: Older versions of this lesson described soaking pottery in concentrated hydrogen peroxide and then heating the wet item in an oven. We no longer recommend this as a home cleaning method. Heating peroxide-treated ceramics can cause rapid oxygen release, splashing, chemical exposure, and damage to fragile or previously repaired glaze. Do not heat peroxide solutions or peroxide-soaked pottery in a household oven. For stubborn stains, valuable antiques, or delicate glazed surfaces, consult a qualified ceramic conservator before attempting chemical treatment.
Important: Chemical treatments may lighten a stain without removing the material trapped beneath the glaze. Discoloration can return if moisture or contaminants enter the cracks again. Concentrated hydrogen peroxide can cause serious chemical burns and hazardous reactions; it is not suitable for home treatment of antique ceramics.
What If the Stain Cannot Be Removed? If staining remains, do not automatically repeat chemical treatments. Repeated soaking may damage porous clay, fragile glaze, decoration, or prior repairs. Seek advice from a qualified ceramic conservator before continuing.
Why We Do Not Offer Stain-Removal Services 1) Results are unpredictable, and complete stain removal cannot be guaranteed.
2) Fragile glaze, decoration, or old repairs could be damaged during treatment.
3) Lightening discoloration may not remove contaminants embedded beneath the glaze, so stains can return.
4) Mineral and rust stains may not respond to bleaching and can require specialized conservation treatment.
Is crazed or crackled glaze on pottery food safe?
Crazing can trap food residue and make thorough cleaning more difficult, especially when the ceramic body is porous or the glaze is damaged. Crazing alone does not establish that every piece is unsafe, but antique, heavily crazed, chipped, or previously repaired ceramics are best reserved for display rather than food use. For everyday food service, choose sound, food-safe tableware.
Nature of the Contaminant: Soot may remain on the surface or penetrate cracks and porous areas. Fire exposure can also weaken glazes, decorations, and old repairs, so assess the condition before cleaning.
Loose surface soot on a sound, modern glazed object may sometimes be removed with gentle, non-abrasive cleaning. Do not wet-clean fragile or valuable fire-damaged ceramics without professional guidance.
Severity and Depth: Avoid scrubbing, abrasive pads, or soaking if the glaze is cracked, flaking, or unstable; these actions can push residues into defects or cause additional damage.
Soot trapped within crazing may not be removable by ordinary surface cleaning. Extended soaking is not automatically a safer or more effective solution.
Additional Cleaning Steps: Heavily sooted, smoke-damaged, or historically important pottery should be assessed by a qualified ceramic conservator. Ultrasonic cleaning and repeated chemical treatments can damage fragile ceramics.
