Are Crystalline Glazes Food Safe?

What are crystalline glazes?

Crystalline glazes are a type of specialty ceramic glaze that form visible crystals on the surface during firing. They are created by controlling multiple factors like glaze chemistry, firing schedule, and kiln atmosphere to promote crystal growth (Digitalfire). Crystalline glazes have a glossy base glaze, typically with low alumina, on top of which multicolored crystalline structures form during the firing process. The crystals have a three-dimensional faceted appearance and can vary greatly in size, color, and configuration based on the glaze recipe and firing conditions used (The Spruce Crafts). Visually, the crystalline structures stand out in stark relief against the smooth glaze background, creating a shimmering landscape of crystals that make each finished pottery piece visually unique.

Brief history of crystalline glazes

Crystalline glazes first emerged during China’s Song Dynasty (960-1279 AD). Ancient Chinese potters developed early versions of crystalline glazes unintentionally while creating iron-rich “oil spot” glazes. These glazes contained small crystals, though growing sizable crystals was not yet possible (1).

In the late 19th century, American potters began intentionally developing crystalline glazes as an artistic technique. Potters including William Long, Hugh Robertson, Adelaide Alsop Robineau, and Taxile Doat worked to refine crystalline glazing methods. Key innovations included using specific clay bodies, glaze ingredients like zinc and fluorine, specialized firing processes, and techniques like glaze trailing to promote crystal growth (2).

By the 20th century, crystalline glazing became an established artistic practice pioneered by ceramic artists like Robert Hessler and national organizations like the Crystalline Glaze Association. Today, crystalline pottery remains popular among studio potters seeking stunning visual effects (3).

Sources:

(1) https://www.herriespottery.com/crystalline-history-process

(2) https://www.themarksproject.org/sites/default/files/biographies/_crystalline_glaze_and_artist_robert_hessler_american_art_pottery_association_journal_fall_2020_2-24_copy_1.pdf


(3) https://isakisaksson.com/the-history-and-composition-of-crystalline-glazes/

Toxicity concerns with crystalline glazes

Crystalline glazes contain high levels of toxic metals like lead and cadmium that can cause health issues if proper safety precautions aren’t taken. According to the Ceramics Safety guidelines from Western Carolina University, “Fluxes and colorants can be highly toxic by inhalation. Highly toxic glaze constituents include: Antimony, Arsenic, Barium, Beryllium, Cadmium, Chromium, Cobalt…” [1]

Both lead and cadmium are cumulative poisons that affect multiple body systems. Lead poisoning causes anemia, weakness, kidney and brain damage, while cadmium poisoning leads to kidney, bone and respiratory diseases. Cadmium is classified as a human carcinogen and lead is considered a probable human carcinogen. According to the safety rules from South Texas College, “Inhalation toxicity high. Both acute and chronic ingestion or inhalation can cause lead poisoning. Inhalation is more of a problem than ingestion. Lead affects the central nervous system, blood, kidneys and reproductive system.” [2]

It’s critical to follow all safety guidelines and regulations around crystalline glazes. The American Ceramic Society recommends guidelines such as: providing adequate studio ventilation, wearing respirators when handling toxic materials, using warning labels on glazes and clays, and avoiding contamination of food surfaces.

Factors that affect food safety

There are several key factors that determine if a crystalline glaze will be food safe or not:

Firing temperature – Crystalline glazes are often fired to high temperatures like cone 10. Firing too hot can cause excessive leaching of metals like lead and cadmium from the glaze (see url). Lower temperatures in the cone 6 range may be safer.

Glaze ingredients – Lead and cadmium are commonly found in many crystalline glaze recipes (see url). Choosing glaze ingredients without these metals is critical for food safety.

Clay body/decorating method – Crystalline glazes tend to craze extensively since they have a very high thermal expansion. This crazing creates cracks that can harbor bacteria. Using a clay body and decorating method that minimizes crazing will help (see url).

Testing for lead and cadmium

Testing crystalline glazes for the presence of lead and cadmium is critical for determining if they are food-safe. There are a few options for testing:

At-home lead testing kits are an affordable option. These kits typically test lead release by measuring how much of the glaze dissolves in a dilute acid mixture. One common household test is the vinegar test – fill a glazed container halfway with vinegar and leave for several days. If the vinegar turns yellow, it indicates the presence of lead. However, at-home testing can be imprecise. Lead in Ceramic Glazes

For more definitive results, crystalline glazes can be sent to a professional lab for testing. Labs use advanced techniques like ICP-MS to precisely measure the ppm of lead and cadmium. While lab testing has an associated cost, it provides a scientific determination of whether lead and cadmium levels are within food-safe limits.

Regularly testing crystalline glazes using either method gives potters confidence their work is safe for food and drink. Periodic re-testing is advised as glaze materials and recipes can vary across batches.

Tips for making food-safe crystalline glazes

Although crystalline glazes often contain lead and cadmium which can leach into food and make them unsafe for dinnerware, there are ways to modify recipes to improve food safety:

Substitute toxic ingredients like lead, cadmium, copper, and zinc with safer alternatives like titanium, tin, iron, and calcium. According to ceramic artist John Britt’s guide on crystalline glazes, small amounts of tin and titanium dioxide can help promote crystal growth while avoiding heavy metal toxicity issues.

Use food-safe clay without lead glazes. Porcelain and stoneware clays are better choices than earthenware as they vitrify at higher temperatures which prevents leaching. Ensure any clay or glaze ingredients were tested for lead and cadmium according to FDA guidelines.

Fire to a high enough temperature, between cone 6 and cone 10 oxidation, to fully fuse the glaze and clay body. Higher temperatures between 2200°F and 2300°F minimize leaching by sealing the glaze surface. Rapid cooling after firing also helps lock in surface crystals.

While 100% food-safe crystalline glazes are difficult to achieve, being mindful about material selection, firing, and testing can reduce risks when making pottery intended for food use.

Safety precautions when using crystalline glazes

When working with crystalline glazes, it’s important to take proper safety precautions to avoid potential health hazards. Proper studio ventilation is critical when using these glazes. Working in an enclosed studio with poor air circulation can allow toxic dust and fumes to build up to dangerous levels. Using a spray booth or vent hood that vents outside can help remove airborne contaminants. Wearing a N-95 rated respirator is also recommended when mixing, spraying, or sanding crystalline glazes, as breathing in fine particulate matter can cause lung irritation.

It’s also important to avoid applying crystalline glazes to any clay surfaces that will come in direct contact with food. According to the Ceramics Safety guidelines from West Chester University, crystalline glazes may contain high levels of heavy metals like lead and cadmium that can potentially leach into food over time (https://www.wcu.edu/discover/campus-services-and-operations/facilities-management/safety-and-risk-management/visual-arts-theatre-safety/ceramics-safety.aspx). So it’s best to reserve their use for decorative pots, vases, wall hangings and art pieces that will not hold food.

By following proper studio safety practices, potters can safely create beautiful crystalline effects in their work without endangering their own health.

Examples of food-safe crystalline glaze recipes

When searching for food-safe crystalline glaze recipes, it’s important to find ones that have been tested to ensure low levels of heavy metals like lead and cadmium. Here are a few reliable recipes to try:

Cone 6 Rutile Blue Crystalline Glaze

This cone 6 recipe produces a blue crystalline glaze that is safe for functional ware (source).

Ingredients:

• Gerstley Borate – 26.16
• EPK Kaolin – 15.95
• Silica – 24.32
• Zinc Oxide – 11.05
• Red Iron Oxide – 0.95
• Rutile – 6.32
• Tin Oxide – 4.74
• Cobalt Carbonate – 0.26
• Bentonite – 2.53

Firing:

Cone 6 oxidation

Cone 10 Lime Green Crystalline

This cone 10 lime green crystalline glaze has tested safe for functional ware (source).

Ingredients:

• Wollastonite – 27
• Silica – 20
• Dolomite – 11
• Zinc Oxide – 20
• Strontium Carbonate – 11
• EPK Kaolin – 11

Firing:

Cone 10 reduction

Tips for selling and marketing crystalline pottery

When selling crystalline pottery, it’s important to take steps to ensure customers understand that the items are decorative only and not food-safe. Here are some tips:

Label all crystalline pottery as “decorative only – not food safe.” This removes any ambiguity and makes it clear to customers that they should not use the items for food. You can add this labeling to the bottoms of pieces, hang tags, online descriptions, etc. A Reddit thread stresses the importance of clear labeling to prevent unsafe use.

Educate customers on why crystalline glazes may not be food-safe. Explain that the crazing can harbor bacteria and that potential leaching of metals can be unsafe for food surfaces. Providing this context helps customers understand that pieces are labeled decorative only for good reason.

Suggest decorative uses for crystalline pottery, like displaying flowers or trinkets. This steers customers towards safe ways to enjoy the beautiful glaze effects.

Offer care instructions advising not to put crystalline pottery in the dishwasher, microwave, or oven, as exposure to heat can increase leaching of metals over time.

With proper labeling and education, potters can feel confident marketing the dazzling beauty of crystalline glazes while ensuring customers use them safely.

The future of food-safe crystalline glazes

As crystalline glazes continue to grow in popularity, especially among functional potters, there will likely be increased focus on innovations to make them as safe as possible for food contact. Some key areas of focus for the future include:

Innovations in non-toxic ingredients – Researchers and glaze chemists are exploring substitute ingredients like zinc and titanium to replace potentially toxic metals like cadmium or lead in glaze recipes. New binders and fluxes are also being formulated to grow crystals while avoiding hazardous materials.[1]

Continued safety education – As more potters explore crystalline effects, it’s important they understand safety factors like firing schedule, glaze fit, and leaching potential. Educational efforts by experienced potters, guilds, and manufacturers will be key to promoting responsible use of crystalline glazes. Proper studio safety and handling is also essential.

Overall, the crystal craze shows no signs of slowing down. With thoughtful development focused on non-toxicity and safety education, crystalline glazes can have an exciting future in functional pottery.