Does Clay Need Glaze To Be Food Safe?

Clay is the primary material used in pottery and ceramics. It is an earthy, fine-grained, natural material that can be molded when wet and hardened by firing in a kiln. Glaze is a vitreous, glass-like coating applied to pottery or ceramic ware and fused to the body during firing in a kiln. Glazes serve both decorative and functional purposes.

When making pottery and ceramics that will come in contact with food, an important consideration is whether the clay body needs to be glazed to be food safe. Glazes provide a protective barrier and make clay non-porous, which improves sanitation. However, some types of high-fired clay can be non-porous without needing an additional glaze layer. The safety of unglazed clay depends on factors like the clay body composition, firing temperature, and intended use.

Porosity of Unglazed Clay

Unglazed clay is porous in nature, meaning that it has small holes or pores in the clay body. This porosity is due to the nature of clay particles, which create small spaces between them. When unglazed clay is fired, the small particles of clay bond together, but they still leave behind interstitial spaces or pores.

The pores in unglazed clay can present a health risk. Bacteria, mold and other microorganisms can get lodged inside the pores. When the unglazed clay comes into contact with food or drink, the bacteria can contaminate the food or drink. Some types of bacteria that create toxins, such as E. coli, salmonella and listeria, could potentially survive in the pores of unglazed clay and contaminate anything the clay container holds.

Lead in Unglazed Clay

Historically, many clay bodies contained lead that could leach into food and pose health hazards. Prior to the 20th century, lead oxide was routinely added to clay to help make it vitrify (become non-porous and glass-like) at lower firing temperatures. Potters also often used lead glazes on utilitarian earthenware to allow firing at lower temperatures and reduce risk of cracking.

Over time, lead can leach out of the clay body or glaze layers, especially when used with acidic foods like tomatoes, citrus, or vinegar. Lead exposure builds up in the body over years and can lead to many adverse health effects, especially in children, including behavioral issues, learning problems, seizures, and more. Research studies have demonstrated alarmingly high lead release in certain antique ceramic vessels.

Thankfully, using lead in ceramic glazes and clay bodies has been long discontinued in commercially produced ceramics. However, ceramic artists or hobbyists firing at lower temperatures may still intentionally or unintentionally be using clay or glaze formulas containing lead. Pieces made before the late 20th century should also be assumed to have lead unless tested otherwise.

Making Unglazed Clay Non-Porous

Unglazed clay can be made non-porous through the process of vitrification. Vitrification refers to the transformation of clay into an impermeable, glass-like material when fired at extremely high temperatures. As the clay is heated, its molecules become more tightly bound together, eventually fusing into a solid mass with no pores or pathways for liquid to penetrate.

For effective vitrification to occur, the clay must reach temperatures between 2,200°F and 2,400°F. At these high heat levels, the clay undergoes intensive sintering, causing the material to become denser as particles partially melt and compress together. This leaves no room for pores or perforations where bacteria can reside. The result is an impervious clay body that blocks the transfer of liquid, rendering the clay non-porous.

Unglazed vitrified clay is often considered food-safe, as its impermeable structure prevents the migration of bacteria into the material. The high firing temperatures also kill any bacteria initially present in the raw clay. With no pores for food particles or germs to hide, properly vitrified unglazed clay has a non-absorbent surface that resists staining and is easily cleaned.

Safety of Vitrified Unglazed Clay

An important factor in the safety of unglazed clay is how vitrified or non-porous it has become during firing. Studies have shown that properly vitrified clay can be safe for food contact when used properly.

One study published in the International Journal of Food Science & Technology examined Soluble heavy metals and fluoride leaching from glazed and unglazed clayware. Researchers tested drinking water samples that had been stored in vitrified clay pots for heavy metals and fluoride leaching. They found the leaching levels to be well within safety limits, concluding vitrified clay was safe.

Another study published in Applied Clay Science looked at the Lead release from glazed and unglazed porcelain. Researchers measured lead release from unglazed porcelain dishes and found negligible amounts. The levels were thousands of times lower than permissible limits, even when exposed to acidic foods. The study provides evidence that vitrified unglazed porcelain can be safe for food.

As long as the clay body has been fired to over 1200°C and is fully vitrified, meaning non-porous, it can be considered food-safe for serving and storing purposes. Unglazed terra cotta or earthenware which is more porous would be an exception and require a glaze barrier for food contact. But properly fired stoneware and porcelain do not necessarily require a glaze to make them non-leaching and food safe.

Glazes Provide Additional Protection

While properly fired clay becomes vitrified and non-porous on its own, glazes provide an additional layer of protection against bacteria and liquids. Glazes are a type of glass coating that creates an impermeable seal over the clay body beneath. The glaze fills in any remaining microscopic pores in the clay to prevent absorption.

In addition to enhancing non-porosity, glazes allow potters to decorate clay in vivid colors and patterns. Glazes come in many vibrant hues that bring containers to life. Glazing transforms basic clay into beautiful works of art. While plain terra cotta has an appeal of its own, a colorful glaze design adds visual interest and charm to ceramic wares.

Lead in Ceramic Glazes

Historically, lead was commonly used in ceramic glazes. The lead helped the glaze melt at lower temperatures and gave it a glossy, glass-like finish. However, we now know that lead can leach from the glaze into food and drink, posing a health risk.

Thankfully, most modern glazes are lead-free. Glaze manufacturers have reformulated their recipes to eliminate lead and rely instead on other materials like zinc, barium, and boron to achieve similar results. Lead-free glazes are now widely available and just as durable and beautiful as leaded glazes.

When shopping for ceramic ware, look for items marked as lead-free or lead-safe. Reputable manufacturers will indicate compliance with lead safety laws on the product description or packaging. It’s also a good idea to avoid antique or handmade pottery from unknown origins, as these are more likely to contain lead glazes.

Choosing Safe Containers

When selecting clay containers for food use, look for ones made from vitrified clay that has been fired to high temperatures. Vitrified clay is non-porous so liquids cannot seep in. Also choose pieces marked as lead-free. Reputable manufacturers will have their products certified and labeled as lead-free. This means the items have been tested and confirmed not to leach unsafe levels of lead under normal use. Lead-free labeling gives an assurance that the clay and any glazes applied are safe.

In general, it’s best to avoid old or antique clay vessels, handmade pottery from unknown origins, and imported ceramics, as you cannot be sure of the manufacturing process or materials used. For maximum safety, choose pieces from American companies that state their products meet FDA standards for lead content. With quality lead-free labeled clay cookware and tableware, you can confidently use these items for food storage and serving.

Safety Tips for Use

When using an unglazed clay vessel for food, it’s important to take some safety precautions:

• Avoid using metal utensils, which can scratch the surface. Use wood or silicone instead.
• Don’t put an unglazed clay vessel directly on a stovetop, under a broiler, or in a microwave. The thermal shock can cause cracking.
• Handwash only using mild detergent and a soft sponge. Harsh dish soaps and scrubbing can erode the clay.
• Don’t soak the vessel or expose it to acids like tomato sauce for extended periods, as this can draw out any lead.
• Limit use to non-acidic and cooler foods and beverages. Avoid cooking tomato sauces in unglazed clay.
• Throw away any cracked or chipped vessels, as bacteria can grow in damaged areas.

With reasonable care and limiting use to low-risk foods, unglazed clay can be a safe, environmentally-friendly option.

Conclusion

In summary, unglazed clay that is properly vitrified through high temperature firing does not require an additional glaze in order to be food safe. The vitrification process makes the clay nonporous, preventing bacteria or contaminants from being absorbed. However, applying a lead-free glaze provides an extra layer of protection against acids or staining from foods and liquids.

To answer the original question, clay does not strictly need glaze to be food safe. But many potters still choose to use glazes on functional pottery like mugs and dinnerware, both for aesthetics and to enhance durability. The key is ensuring any glazes used do not contain lead or other toxic materials. When shopping for clay vessels, look for makers who can verify their pieces are lead-free. With thoughtful production methods and proper use, beautiful handmade pottery can be enjoyed safely for food and drink.