Lead and cadmium are among the most common toxicity issues when it comes to pottery glazing.

Ancient pottery and ceramics often utilise earthenware, a porous type of clay that needs to be glazed before it can hold food or liquids. If you are using these ceramics for demonstration purposes, they should still be safe as long as you prevent children from touching them and putting their fingers in their mouths.

However, if you cook with them, they can be a problem. There is still a risk of lead poisoning, especially when used with acidic foods and drinks as well as hot drinks such as coffee. This can apply to lead-glazed pots used at high temperatures, as this can cause the lead to fuse with the pottery.

These high temperatures and the pottery firing process are really important. If these are done incorrectly, the glaze will not adhere properly to the clay body.  Instead it sits on top of the clay surface and there is a danger that it will break off and mix with food and drink.  This can cause you to accidentally swallow glaze.  This is particularly problematic if the glaze contains lead. 

Avoiding cadmium and lead is a great first step, and fortunately it is easy to do. Professional potters now mostly use non-lead based glazing materials. There are many such options on the market.

Some glazes contain lead, but as mentioned above, manufacturers are obliged to clearly label them.  Ultimately, it is up to each individual potter whether or not they want to use lead-based glazes.  However, if you do use them, they should only be used for decorative purposes.  These pieces are not safe for food.

It's Not Just the Bullet

Lead is not the only potentially toxic component.  If you're just starting out, it can be difficult to know which types of glazes are potentially dangerous. Therefore, you will want to do your homework and make sure that the glazes you use do not contain other toxins such as cadmium.  If you are asking the question ‘are glazes poisonous’, lead is not the only thing you need to be aware of.

In addition, other materials such as barium, cobalt, nickel and selenium can also be potentially toxic if present in large quantities in your glaze. However, as long as they are present in small quantities, there will be no problem.

On the other hand, if you are buying secrets for your own arts and crafts work, if you want to sell them or just make them for yourself or your friends, you will be dealing with a trade secret and this requires closer scrutiny.

Most glazes are labelled ‘Suitable for food containers’ or ‘Not suitable for food containers’ on the packaging; the latter means that they contain ingredients that may pose health concerns.

Safe application of glaze

No matter what type of glaze you are using, it is important that you do not swallow when working with it. Therefore, you should avoid eating or drinking while working with glaze. 

Even if there is no food or drink around you when glazing, it is important to wash your hands after glazing.  

Even if you are careful, it is easy to contaminate your food or drink with glaze while working. 

Also, whether or not a glaze is labelled as toxic, it is important not to inhale it.  For example, some glazes contain manganese.  Although manganese is not dangerous in some cases, it causes problems when inhaled in its dry form. On the other hand, it is generally safe after firing in the kiln

It is therefore advisable to wear a dust mask when working with powdered glaze.  It is also advisable to work in a well-ventilated area.  If you airbrush your glaze, it is essential to wear a respirator.

In short, try to avoid glazes containing lead.  No glaze labelled as safe for food contains lead.  If you decide to use a lead-based glaze, you can only use it for decorative purposes.

Safe Cooking of Glaze

Glaze can emit toxic fumes when fired.  It is therefore important that the kiln room is well ventilated.  Ideally, the door to the kiln room should be left open together with a window.  This allows free circulation of air.

It is worth noting that many manufacturers state that it is not possible to say that a glaze is categorically food safe.   This is because whether a glaze is safe for food depends on how well it is cooked.  There are also factors that are beyond the control of the manufacturer, such as how the glaze is used.  These factors affect whether the glaze is food safe or not.

Glazes that are non-toxic but can be irritating

There are also materials that are not necessarily toxic but can be harmful. For example, some glazes can cause severe irritation (source). These include:

Avoid using them if possible.  If they are a necessary part of the glaze, make sure to wear a mask and gloves.  It is also a good idea to cover your skin so that the ingredients are not absorbed by your skin.