Navigating the World of Special Effect Glazes: A Guide for Potters and Customers
Ceramic glazes are more than just functional coatings — they are part chemistry, part alchemy, and part artistic expression. From crystalline blooms to metallic sheens and soft matte surfaces, special-effect glazes enrich our work with depth and personality. But with these effects come understandable questions about food safety.
This guide brings together experience from high-fire stoneware practice, current EU food-safety standards, and studio testing to help clarify how special-effect glazes behave, what “acid resistance” really means, and how to use these glazes safely and confidently.
High-Fired Stoneware and Food Contact Safety
At firing temperatures above 1250°C, both clay and glaze become fully vitrified, creating a dense, stable surface. Modern commercial glazes formulated for this temperature range are typically:
Lead- and cadmium-free
Non-porous and durable
Suitable for functional ware when applied and fired correctly
In the EU, the legal standard for food safety focuses on lead and cadmium solubility.
If a glaze contains no lead or cadmium — or does not release them — it is considered safe from a toxicological perspective.
However, this is only part of the picture. A glaze can be food-safe while still having low acid resistance, which affects appearance and surface durability, not toxicity.
Acid Resistance — The Most Misunderstood Factor
Suppliers often mark glazes with “acid resistant” or “not acid resistant.” This has two meanings:
Health perspective:
Whether the glaze risks releasing toxic metals
(for modern lead-free glazes, this is rarely the issue).Optical/material perspective:
Whether the glaze surface changes visually when exposed to acids.
The glaze manufacturer explains it clearly:
**“‘Yes’ means no optical attack under test conditions.
‘No’ means visible surface attack under test conditions.
In both cases, no raw materials containing lead or cadmium were used.”**
This is crucial:
A glaze with low acid resistance may dull, discolor, or show a spot where something acidic sat, but it is not unsafe.
This becomes especially relevant for reactive surfaces such as crystalline, crackle, matte and metallic glazes.
The Spectrum of Matte Glazes
Matte glazes are another fascinating category. They range from a dry matte to a silky satin and all the way to a soft gloss. While traditional advice often leans toward glossy glazes for functional ware, many matte glazes work wonderfully for daily use. The key is knowing where your glaze falls on that matte-to-glossy scale. A silky matte might be easier to clean and less likely to stain than a very dry matte, especially if you're using it for tea or coffee.
Functional Behavior
Satin and silky mattes tend to be very user-friendly and easy to clean.
Dry mattes are generally stable but may develop a “tea patina” after years of use.
In my own work:
Some matte glazes remain spotless after long-term use, while others develop coloration. Neither affects safety — only visual aging.
Crackle Glazes: Embracing the Fine Lines
Crackle glazes feature those beautiful, intricate networks of fine lines. While they’re visually stunning, those lines can sometimes raise concerns about bacteria or staining, as tiny fissures might theoretically trap residues. In practical use, as long as the glaze is well-vitrified and the piece is properly cleaned, it’s generally safe. But it’s good to advise customers to use these pieces for dry or decorative uses if they have any concerns.
Crackle (or crazed) glazes intentionally develop fine surface lines. When well fired, the surface usually feels smooth — the crackle is visual rather than tactile.
Considerations
Staining is the most common issue (tea, coffee).
Theoretically, micro-lines could trap residues, though proper washing mitigates this.
Crackle is more of an aesthetic consideration than a safety concern.
Crackle glazes have been used traditionally in tea cultures for centuries; patina development is often considered part of their character.
Crystalline Glazes: Delicate Beauty, Functional When Used Thoughtfully
Crystalline glazes create those beautiful starburst or floral crystals through a controlled firing cycle, giving each piece a unique, expressive surface. Although the glaze matrix is very fluid during firing and sometimes viewed as delicate, well-formulated crystalline glazes matured at high-fire temperatures can be confidently used on functional tableware — and I feature them routinely in my own collections.
Toxicology vs. Durability
Most modern crystalline glazes — including the ones I use — are:
lead- and cadmium-free
fully vitrified at stoneware temperatures
safe for food contact under EU standards
This covers the health aspect of food safety.
The remaining nuance is acid resistance, which affects appearance, not toxicity.
Crystalline Glazes in Everyday Use
Based on long-term studio testing:
They perform beautifully on plates, bowls, serving dishes, and mug exteriors.
They are well suited for everyday foods: grains, vegetables, pastries, fruits, cheese, desserts, etc.
They clean well and remain stable with normal daily use.
Where Potters Should Be Mindful
Some crystalline surfaces can be more sensitive to extended contact with acidic foods. Occasional acidity (during a meal) is not an issue, but prolonged exposure — like a lemon left overnight — may cause slight visual changes.
Why I Use Crystalline Glazes on Functional Ware
My crystalline formulas and firing schedules are developed to ensure:
full vitrification
stable, non-leaching surfaces
appropriate durability for daily use
smooth textures without open pores or defects
Key Points
Crystalline glazes are lead- and cadmium-free.
Short-term food contact is absolutely fine.
Prolonged acidic exposure may cause minor cosmetic changes, not safety concerns.
Metallic Glazes: Navigating Acid Resistance and Everyday Use
Metallic glazes — bronze, pewter, iron-rich, or other metal-saturated finishes — create striking, expressive surfaces. These glazes are typically lead- and cadmium-free, meaning they comply with EU toxicological food-safety standards. The consideration with metallic glazes is not toxicity, but acid resistance, which affects how the surface may visually change over time.
What Low or Medium Acid Resistance Means
Metallic glazes contain higher levels of oxides such as copper, manganese, or iron. These components can react with acidic foods if exposed for long periods. This does not make the glaze unsafe — but it may cause:
temporary discoloration
a faint dull spot
subtle surface changes
These effects relate to optical durability, not health risk.
Examples of Acid Exposure
Metallic glazes are more likely to react if acidic foods sit on them for many hours, such as:
lemon slices left overnight
tomato sauce or balsamic dressings resting for extended periods
acidic beverages stored in the vessel
Short-term contact — during a meal — is usually not an issue.
When Metallic Glazes Are Perfectly Suitable
Metallic glazes perform very well on serving ware used for dry or low-acid foods. This includes items such as:
bread or cheese platters
dessert dishes
serving trays
This is why I personally feel comfortable using metallic surfaces on pieces like sushi plates: the foods traditionally served on them are mild, non-acidic, and do not remain on the surface long enough to cause changes.
Metallic glazes are both beautiful and functional when used intentionally and with an understanding of their material behavior.
Does low acid resistance only mean visual change, or can leaching occur?
Short answer:
Low acid resistance can indicate a higher possibility of leaching — but only of materials already present in the glaze and only under prolonged acidic exposure.
However, this does not automatically mean harmful leaching, especially if the glaze contains no toxic heavy metals(lead, cadmium).
Let’s break it down accurately:
In ceramic testing, acid resistance assesses:
whether the glaze loses material when exposed to acidic solutions
whether the surface shows visible alteration (etching, dulling, discoloration)
These two often go hand-in-hand — if the glaze is attacked by acid, you may see it and there may be some degree of leaching.
BUT:
The key question is what is leaching out?
If the glaze contains no toxic metals, then what might leach are:
small amounts of colorants (iron, copper, manganese)
minor surface components
dissolved silica or boron
These are not toxic at the levels released from a high-fire glaze under normal use.
2. Metallic glazes (copper/manganese) and leaching
Metallic glazes often contain:
copper oxide
manganese dioxide
iron oxide
These can leach in detectable but very low, generally harmless amounts if exposed to strong acids for a long time (e.g., an overnight lemon test).
This is why discoloration sometimes appears — the acid slightly dissolves the surface and pulls colorants forward.
However:
✔ These glazes are still considered food-safe if they pass EU lead/cadmium limits.
✔ Copper and manganese are not regulated metals in EU ceramic food-contact testing.
✔ Glazes that leach visually are still often non-hazardous, just less durable.
What people fear (toxicity) and what acid resistance actually measures (durability) are two different categories.
So what is the correct professional statement?
“Low acid resistance means the glaze surface may be chemically attacked by acidic foods, which can cause both visible changes and, in some cases, mild leaching of naturally occurring colorants. This is not a toxicological concern for modern lead- and cadmium-free glazes, but it may affect the long-term appearance of the piece.”
Manganese in Glazes — Understanding the Real Risks
Manganese is a common colorant in ceramics, used to create deep browns, blacks, purples, and many metallic effects. Because manganese is known to be harmful when inhaled as dust or vapor, it often raises questions about safety in functional ware. It’s important to separate studio handling risks from finished-glaze behavior, as these are very different.
The Studio Hazard: Inhalation
Manganese is most dangerous when it becomes airborne: mixing dry glazes, breathing kiln fumes during reduction firing
These forms allow manganese to enter the lungs, where overexposure can affect the nervous system. This risk is real — but it is a workplace hazard, not a user hazard.
In finished glazes, manganese is not inhaled, and therefore not associated with this toxicity pathway.
Manganese in High-Fire Glazes: Ingestion Risk Is Extremely Low
At stoneware temperatures (1220–1300°C), manganese becomes chemically bonded within the glass matrix. Because of this strong vitrification:
leaching is minimal
any release of manganese is extremely small
levels are far below known oral toxicity thresholds
This is why manganese is not regulated in standard ceramic food-safety leach testing (EU or FDA).
Lead and cadmium are the priority because they leach far more easily and are harmful at very low doses.
Why Potters Cannot Certify Glazes for Other Studios
Even when using the same commercial glaze, results vary due to:
application thickness
clay body differences
kiln atmosphere
actual achieved temperature (thermocouples can differ from reality)
cooling rate
Because of this, no potter can ethically “certify” a glaze for another maker’s functional ware.
The only reliable method:
Have the finished tableware tested in a certified lab in your country, according to local standards.
Food safety is determined by the final fired piece, not the ingredients alone.
Practical Guidance for Potters and Clear Communication for Customers If you want the safest, most conservative approach:
Use glossy, stable, high-acid-resistance glazes on all food-contact surfaces.
If you enjoy special-effect glazes:
Use them confidently on:
exteriors of mugs
serving ware
plates for dry or mild foods
decorative pieces
And pair them with a reliable liner glaze where needed.
What to tell customers who choose to use special-effect glazes on tableware:
“All glazes in my work are lead- and cadmium-free and fired to durable stoneware temperatures.
Some special-effect glazes — such as metallic or crystalline surfaces — naturally have lower acid resistance, which may cause slight cosmetic discoloration if acidic foods remain on them for many hours. Everyday use during meals is completely safe. To keep your piece looking its best, simply avoid leaving strong acidic foods on the surface overnight — dry or low-acid foods like sushi, cheese, or pastries are ideal.”
Customers appreciate clarity without fear-driven messaging — openness and guidance builds trust.
Final Thoughts
Special-effect glazes bring depth, personality, and artistry to ceramic work. Understanding glaze behavior allows potters to design functional, beautiful pieces with confidence — and helps customers enjoy them fully in their daily rituals.
When we balance creative expression with thoughtful material choices and clear communication, ceramics remain what they’ve always been: practical art meant to be lived with.
