Is It Safe To Eat On?
Glazed ceramic pottery can leach metals, harbor bacteria, and even flake off into food and drink, making it unsafe. According to the FDA, some ceramics, both imported and domestic, have been determined unsafe for food. For glazed ceramic pottery and food safety, it’s best to error on the side of caution. Glazed ceramic pottery can be a hazard to food safety.
Why is Glazed Ceramic Pottery Potentially Not Food Safe?
The majority of ceramic glazing contains lead and other elements which can be harmful to your health if it comes into touch with food. When pottery is improperly formed and fired (unfired clay fired at too low a temperature), there is a substantial risk of lead leaching into your food and drink. While lead may not have an immediate detrimental effect, it accumulates in the body over time and can lead to major health problems.
According to John Hesselberth and Ron Roy different temperature range requires compatible glazes1.
Lead poisoning cases have been recorded all around the world as a result of long-term usage of glazed ceramic pottery kitchenware and plates. This is such a big concern that the FDA scrutinizes ceramic ware for lead leaching. Abdominal pain, constipation, lethargy, migraines, renal impairment, hypertension, and microcytic anemia are among the signs of lead poisoning. If left untreated, these symptoms might lead to major health problems, even death in extreme circumstances.
According to Morgan A. Baldinelli, at Georgia Southern University, A food-safe glaze is a shiny, thin coating that does not leach chemicals or has an excess of colorants; common food-safe glazes are white liners and clear glazes as they have no additives2.
How To Test For Lead In Glazed Ceramic Pottery
3M makes a product called LeadCheck Swabs that detects lead on most surfaces in 30 seconds. It’s simple to use. All you do is crush, shake and swab. If you see red, then you know you have lead. The swabs are non-toxic and disposable after use. If you see pink, my advice is to error on the side of caution and assume you have lead. There are other test kits available I am sure are good or just as good, but this is the one I am familiar with. These are simple tests and do not show the level of lead found. They do not tell you how dangerous the lead is.
According to Michael Welton, Alfonso Rodriguez-Lainz, and Miguel Fraga, Lead exposure from lead-glazed ceramics (LGCs) and traditional folk remedies have been identified as significant sources of elevated blood lead levels in Mexico and the United States. This study took place from 2005 to 2012 in a rural community in Baja California, Mexico3.
The Science Behind Glaze Leaching
Glaze leaching is a phenomenon where certain components of a ceramic glaze dissolve into food or liquids that come into contact with the glazed surface. This is a critical concern, especially when hazardous materials like lead or cadmium are part of the glaze composition.
According to Ralph W. Sheets, when samples of pre-1950s U.S.-made ceramic dinnerware, purchased in antique shops and flea markets, were filled with 4% acetic acid or 0.5% citric acid and heated in a microwave oven for 2–5 min, lead was leached in amounts of up to 5 mg per dish4.
Factors Influencing Glaze Leaching
- Firing Conditions: The temperature and duration of the firing process can significantly impact how stable a glaze becomes. Under-firing can result in a glaze that is more susceptible to leaching.
- Glaze Composition: Glazes containing hazardous materials are more likely to leach. Always opt for food-safe glazes that are lead-free and cadmium-free.
- pH Levels: Acidic foods like citrus fruits can accelerate the leaching process, making it essential to consider the types of food that will come into contact with the pottery.
Lab Testing for Safety
To ensure the safety of a glazed ceramic piece, lab testing is often recommended. These tests generally involve exposing the glaze to various substances that mimic food and then analyzing the results for any signs of leaching.
Preventive Measures
- Slip Casting: This technique involves creating a mold of the pottery piece and then pouring liquid clay into it. The mold absorbs water from the clay, leaving a solid clay object that is less porous and therefore less susceptible to leaching.
- Quality Control: Always source your glazes from a reliable manufacturer to ensure they meet safety standards.
- Regular Checks: Periodically test your pottery using simple home tests like the lemon test or the scratch test to monitor for potential leaching.
How to Test for Glaze Safety: The Scratch Test and The Lemon Test
When it comes to determining the safety of glazed ceramic pottery, especially for food use, two simple tests can provide valuable insights: the Scratch Test and the Lemon Test. These tests are easy to perform at home and can give you a preliminary idea of whether your glaze is food-safe.
The Scratch Test
What It Is: The Scratch Test involves using a sharp object, like a knife or a coin, to scratch the surface of the glazed pottery.
How to Perform It:
- Take a sharp object and gently scratch the surface of the glazed area.
- Examine the scratched area for any visible marks or removal of the glaze.
What It Indicates:
- If the glaze is easily scratched off, it may not be stable and could be prone to leaching.
- A glaze that remains intact is generally more stable and less likely to leach.
The Lemon Test
What It Is: The Lemon Test involves exposing the glazed surface to an acidic substance, like lemon juice, to check for any changes in color or texture.
How to Perform It:
- Squeeze fresh lemon juice onto the glazed surface.
- Let it sit for a few hours, then rinse off the lemon juice.
- Examine the glazed area for any changes in color or texture.
What It Indicates:
- If the glaze shows signs of discoloration or a change in texture, it may not be food-safe.
- A glaze that remains unchanged is generally more stable and less likely to react with acidic foods.
How To Inspect For Food Safe Ceramic Pottery And Care
The following are some suggestions for safely using glazed ceramic pots in your home.
1. Avoid The Dishwasher
Dishwashers should not be used to wash painted or glazed ceramics. The lead will be more exposed when the glaze wears away, allowing it to leach more readily.
2. Avoid Leftovers
Leftovers should not be stored in glazed ceramic pottery bowls and platters. The more food comes into touch with glazing, the greater the risk of lead exposure
3. Ceramic And Safe Food Areas
Outside of the food area, pottery with beautiful and colorful glossy elements is normally safe. The safest way for food surfaces is to apply color as a slip to the ware and then apply a transparent glaze over top. In white areas, there is frequently no lead glazing.
4. Avoid Acidic Foods
Avoid serving acidic meals on a platter or bowl with a lead glaze. Acid speeds up leaching, allowing the lead to enter food more quickly.
5. Avoid The Microwave
When heating food on lead-glazed colorful adorned kitchenware in the microwave, the rate of leaching is increased. My simple advice is to avoid the microwave for anything you are about. As a general rule, I use paper plates mostly in the microwave. I never put good porcelain in the microwave.
Leaching In Glazes
Glazes are glass and we tend to think of them as timeless, indestructible. However, all glass leaches to some extent when it comes into contact with even water. With acids, especially if the contact occurs over a period of time or the acid is hot, the effect is obviously greater.
According to the FDA, “Lead compounds, such as lead oxide, have historically been used in glaze formulations to regulate the melting properties of other glaze components and to enable the use of a broad firing temperature range in the production process. However, when fired at inadequate or uncontrolled temperatures, the lead is not fully incorporated into the glaze structure and is available to leach into food in quantities that may pose a health hazard to consumers.5 “
It’s important to note glaze leaching is only a practical and aesthetic concern if they form a glaze from innocuous ingredients like silica, dolomite, feldspar, whiting, kaolin, ball clay, and so on. However, if the glaze contains metallic colorants (other than iron) or other minerals containing lithium, barium, lead, chrome, then safety becomes a concern.
Glaze Leaching Is Complex
A leachable glaze is one that has not been properly melted. Are you aware that It’s not merely a question of whether the glaze’s chemicals are hazardous? The problem is complicated since it involves the preparation and firing of the materials as well as the formulations used. Toxic elements can be used safely, and imbalanced ratios can ruin glazing that is otherwise safe.
It is impossible to expect a glaze to be resistant to leaching if it has not been fully melted. While a simple visual assessment of a glossy glaze is generally enough to determine the degree of melt, determining the degree of melt in a matte glaze is particularly difficult.
How Do I Know If My Glazed Pottery Is Food Safe?
There are some simple tests you can perform at home. When glass comes into constant contact with acids or bases, it leaches to some extent, especially if the contact is prolonged or the acid is hot. The shine and texture of the glass surface vary with time, indicating this. The glaze’s ability to pass two easy tests is a good indication that it will provide trouble-free service.
These tests aren’t sophisticated enough to guarantee that ware is completely safe, but they will definitely reveal glazes that are clearly dangerous.
1. Vinegar Test For Safety
Pour half a cup of vinegar into a glazed jar and leave it for several days. Compare the color and surface quality of the item above and below the liquid line (e.g. sheen, gloss, texture). With an overnight vinegar leach, unstable glazes can readily turn white.
2. Lemon Slice Test For safety
Lemon juice is a powerful acid. You can also simply place a lemon slice on top of the glaze. Any disparities between the acid-exposed and non-acid-exposed surfaces suggest that the glaze is prone to acid leaching.
3. Dishwasher Test For Safety
Take two similar products and place one in your cabinet and the other in the dishwasher for two months. Take a close look at the surfaces of the two. If the washed one differs in color, gloss, or texture from the original, you have a glaze that is prone to base attack.
4. Drano Test For Safety
Drano is the most powerful base that most people have, so you could use it for a test. Pour half a cup into a glazed jar and leave it for several days. Compare the color and surface quality of the item above and below the liquid line (e.g. sheen, gloss, texture).
5. Microwave Test
Fill the glazed pottery product with water and microwave it for 1-2 minutes. The glazed pottery ware is not food-safe if the clay body absorbs water and becomes heated. Water absorption shows the presence of weak clay and glaze connections that can break at any time.
Ceramic Glaze Flaking Making It Unsafe For Food Or Drink
Inspect the rims of cups and look for flaking. I have had several pottery cups where the glaze has flaked off. I ended up having to stop using them for drinking cups. Glaze flakes are not something you want falling into your coffee. I was lucky and spotted the problem before there was any danger. But for smaller flakes, this can be challenging to spot ahead of time. The smaller flakes can go unnoticed.
My guess is that there must have been some kind of shrinkage from either the firing or a thermal imbalance over time that caused the ceramic glaze to flake off. In any case, all I can say is make visual inspections often.
Ceramic Cracking Making It Unsafe for Food Or Drink
If your glazed pottery is cracked, it invites bacterial infections and creates a breeding ground for bacteria, lowering the safety of food stored in that ware. Cracking can also cause problems, such as filling the glazed pottery product with moisture, which breaks the bindings of the glazing chemicals and clay elements, increasing the possibility of combining harsh acids with your meal.
If you expose the glazed pottery to high heat, it will begin to crack and contaminate meals, rendering it unsafe to eat on. I see this a lot with microwave use.
What Are Ceramic Pottery Glazes?
A durable coating put to the exterior surface of pottery is known as a ceramic glaze. Through kiln firing, the glaze is fused to the pottery. Glazing is used for a variety of purposes, including coloring, ornamentation, waterproofing, and creating a robust, long-lasting surface. Although most ceramic glazes are used today this is not the case for all of them. There are various lead-free ceramic glazes on the market.
In all my artwork, I only use food-safe glazes, no lead and all my work is ornamental and not made to be used to serve food. When it comes to ceramic pottery, many glazes are commonly employed.
1. Ash Glaze
Ash glaze is a form of glaze that typically consists of plant ash and contains potash and lime. In most cases, this is regarded as “food safe.”
2. Tin Glaze
Tin glaze is a lead-based glaze that has been made translucent by the use of tin.
3. Lead Glaze
Lead glaze is a dangerous form of ceramic glaze that contains lead. It must be fired at 1,470 degrees Fahrenheit in a kiln for adequate hardness, and if done correctly, it will almost certainly pass leach tests. However, unless they are properly certified, most lead-glazed goods are not automatically deemed food safe.
4. Salt Glaze
Salt glaze is a form of glaze that is manufactured in an innovative way. Salt is thrown into the kiln during the firing process. Because the sodium reacts with the silica in the pottery, a glassy, translucent outer layer forms on the piece. This glaze is considered non-toxic. In most cases, the glaze is colorless or takes on various shades of brown as a result of the iron oxide. The glaze for cobalt oxide can be blue, and the glaze for manganese oxide can be purpose.
How Does The Liner Glaze Ensure That The Glazed Pottery Is Food-safe?
Using a liner glaze is a simple and eye-catching approach to ensure that your glazed pottery has a leach-resistant surface. It also ensures that the glazed pottery is long-lasting and has a high physical strength to hold the glaze and clay connections together. A magnificent glossy, transparent, or translucent glaze layer, known as a liner glaze, is present in every food-safe glaze. It is done to eliminate the risk of metallic colorants leaching into food or drink while using that glazed pottery.
Use a liner glaze on glazed pottery is to assure food safety. One popular brand is Mayco Designer Liner. It’s certified as non-toxic and is clay-based.
What Should You Look for When Purchasing Food-Safe Lead-Free Glazed Pottery?
When purchasing glazed pottery, you should always pay special attention to whether or not the item is food-safe. Look for these unanticipated aspects because they will safeguard you and your health.
1. Glazed Pottery Color
If the glazed pottery ware is heavily adorned with bright colors such as red, orange, yellow, and so on, do not purchase it since bright colors include high-intensity pigments that are harmful to your health.
2. Glaze Inspection
Examine the texture of the glazed pottery carefully because unsafe glazed pottery might be damaged or excessively worn out.
3. Choose High-Quality Glazed Pottery
Always choose high-quality, safe glazed pottery from a reputable manufacturer who provides a warranty of the product as well as additional specifics. Purchasing food-safe glazed pottery from flea markets or vendors should be done carefully or not at all.
4. Pottery Appearance
If the glazed pottery has a rough appearance or is not a regular shape, it indicates that it was fired in an unconventional manner.
FDA Ceramic and Pottery Food Safety Testing
Did you know that the US FDA (Food and Drug Administration) scrutinizes ceramic ware for metal leaching, with often alarming results?
It has been discovered that some ceramic food wares leach substantial amounts of lead from possible food contact surfaces. Foods can extract the metal, which can induce several negative health effects, including the classic effects of chronic lead poisoning if consumed regularly.
The FDA is giving this advice to the industry in order to address our concern regarding the safety, labeling, and usage of the phrase “lead-free” for the marking of pottery in imports of traditional pottery, including imported Mexican terra cotta. This guideline also reminds the industry of the FDA’s labeling requirements for ceramics purely for ornamental or decorative applications to guarantee that they are not being used for food processing reasons likely to cause the contamination of ceramic foodstuffs.
Compounds such as lead oxide were employed to manage the melting property of glaze components in glazing formulations to allow for a wide range of firing temperatures in the production process. However, the lead is not completely absorbed into the glaze when fired at insufficient or unregulated temperatures and as a result be released in foods in proportions that may constitute health concerns to consumers.
Look for the CCIB Sticker Or Logo
The China Commodity Inspection Bureaus (CCIBs or CIQs) establish a certification system for ceramic tableware production facilities in CHINA in order to provide FDA with reasonable assurance that ceramic tableware is produced in these facilities and exported to the US will meet FDA action levels for leachable lead and cadmium.
Consignments of ceramic dinnerware originating from CNCA-certified facilities are more likely to meet US safety regulations than those originating from other, non-certified companies in China, according to the FDA.
Stay Updated with the Latest Trends in Pottery
The world of pottery is ever-changing, influenced by market demand, artistic innovation, and even cultural shifts. Staying abreast of these trends is crucial for both collectors and artists alike. Whether you’re looking to create, buy, sell, or simply appreciate pottery, understanding the current landscape can offer invaluable insights.
For the latest trends in pottery, be sure to check out our up-to-date guide.
What distinguishes Terracotta from Clay? – Terracotta is clay that has been shaped and burned, whereas clay is the basic ingredient. Earthenware clay has the shade of brown associated with the name, but terracotta objects can be made out of any type of organic clay. The porous and permeable surface of terracotta objects is created by firing at low temperatures.
What are the types of ceramics? – What are the types of ceramics? – Ceramic or pottery may be divided into three categories: Earthenware, stoneware, and porcelain. However, you should read the entire essay since there are a few very interesting differences between the ceramic categories.
When exposed to extreme heat, such as in a microwave, glazed ceramics can break and contaminate food. Cracking produces a breeding habitat for microorganisms, reducing the safety of food stored in that container. It can also cause issues such as filling the goods with moisture, which breaks the glazing chemicals’ bonds.
IDo not buy glazed pottery ware that is lavishly ornamented with vivid colors such as red, orange, yellow, and so on. High-intensity pigments in bright hues are detrimental to your health.
About the Author: Ed Shears
Hello, I’m Ed Shears, the creative force behind Artabys.com, a platform dedicated to promoting visual arts education and awareness. With a robust foundation in traditional pottery techniques, my true passion lies in creating unique ceramic wall art. Each piece I create is handbuilt with meticulous attention to detail, embodying originality and craftsmanship.
Credentials and Experience
- Education: A dedicated student of ceramic arts, my skills have been honed through rigorous training and hands-on experience.
- Artistic Philosophy: I believe in the unparalleled beauty of handcrafted original artwork, which not only adds a unique touch to living spaces but also resonates with the soulful touch of its creator.
- Achievements: I am a renowned author and contributor at Academia.edu and have showcased my artwork at Fine Art America since 2015.
For more about me and my work, feel free to visit my full artist page.
References
Hayes, T. R. (2008). Iron based earthenware in a forced reduction atmosphere. https://soar.wichita.edu/handle/10057/1372
Bond, J. J. (1976). Teaching the Historical and Technical Development of Pottery.
https://www.maycocolors.com/product/sg-401-designer-liner-black/
https://www.fda.gov/media/71764/download
https://unsplash.com/photos/7HuTGlUfQSo
Footnotes
- Hesselberth, John., Roy, Ron. Mastering Cone 6 Glazes: Improving Durability, Fit and Aesthetics. United States: Echo Point Books and Media, 2020. ↩︎
- Baldinelli, Morgan A. “Drink Up: A Study of the Food-Safe Quality of Ceramics Glazes with the Addition of Rutile.” (2021). ↩︎
- Welton, Michael, Alfonso Rodriguez-Lainz, Oralia Loza, Stephanie Brodine, and Miguel Fraga. “Use of lead-glazed ceramic ware and lead-based folk remedies in a rural community of Baja California, Mexico.” Global health promotion 25, no. 1 (2018): 6-14. ↩︎
- Sheets, Ralph W., Sandra L. Turpen, and Patrick Hill. “Effect of microwave heating on leaching of lead from old ceramic dinnerware.” Science of the total environment 182, no. 1-3 (1996): 187-191. ↩︎
- https://www.fda.gov/regulatory-information/search-fda-guidance-documents/guidance-industry-safety-imported-traditional-pottery-intended-use-food-and-use-term-lead-free ↩︎
