Here we will talk about you as a user of handmade ceramics, not as a ceramist
All your cups are shiny and in bright, nice colours. It’s a glaze. And it is the only dangerous part of a ceramic piece because the glaze is touching your food. The worst part is that the glaze can leak toxic metals into your food, slowly killing you.
Let’s talk about ceramics that you use for food. All your vases and decorative elements are safe even if they contain some toxic metals. They will leak only under the influence of an acidic or hot environment. In other words, it’s a chemical reaction.
Sometimes the amount of lead in a product is tiny, but even trace amounts can contaminate whatever you're eating or drinking. Over time, exposure to lead in small doses can lead to heightened blood pressure, lowered kidney function, and reproductive issues. Lead can cause even more serious problems in kids, including slowed physical and mental development.
As the dangers of even small amounts of lead have become more widely known, the ceramics industry has gradually eliminated the additive from its products. If glaze contains lead traces it would be mentioned. But you as an end-user don’t know what glaze was used.
According to the US FDA, consumers should be attentive to antique or handmade ceramics, which might still be baked in old kilns with residue lead in them. In particular, bright orange, red and yellow homemade pottery may be of concern, because lead is sometimes used to intensify these colours. I can add mat glaze to this list.
Test your handmade ceramics
- Lead testing kit to evaluate the lead poisoning risk posed by a product (you can buy it at Amazon, but it's the most expensive variant)
- Vinegar Test: Fill a glazed container half full of vinegar and leave it for several days. If the vinegar turns yellow, this is an indication of lead release.
- Lemon Slice Test: Lemon juice is more acidic than vinegar. Keep a lemon slice against a glazed surface for several days. To make sure that it stays wet, wrap the whole thing in plastic. If needed, put a weight on top to keep the lemon in contact with the surface.
- Your beverage changed its taste. This is a good sign to stop using this tableware as well
Cracks on a glaze
One more concern is the crackles on a glaze.
We should not fear low doses of bacteria such as what might hide in a crack in a glaze.
However, if we then place into such a pot a food, and keep it there for a while, we may find that the small colony in a crack has become a large culture in the food. Then we are in trouble.
The greatest risk is in food storage containers, especially for wet foods and liquids. My feeling is that these should be entirely free of crazing and surface blemishes on the food contact side.
But for a cup where you pour your coffee, crackles are not so dangerous, your immune system will deal with it.