No, it is not safe to use objects with glaze cracks for food and drink. Small cracks in the glazed surface can allow food to seep into the deeper layers of the dishes. Bacteria and cracks can grow. Cracks also make the dishes weaker and increase the risk of breakage in their use.

What cracks in the plates look like?

Sometimes it is very easy to spot cracks in the crockery. This is because over time these cracks absorb pigments from coloured drinks such as tea and coffee and become more noticeable. This is especially noticeable on long-used or antique pieces.

However, cracks are not always immediately noticeable. It may be more difficult to see cracks on a plate that has only recently been produced. Cracks take time to change colour. For this reason, you should first hold the plate up to the light and examine it carefully.

One way to tell if a plate is cracked is to draw a mark on its surface with a black felt-tip pen. Then wipe the line with a solvent such as thinner or acetone. If the surface is cracked, the black ink will remain in the cracks.

When testing with this method, keep in mind that the dark ink will be permanent on the ceramic. It is therefore a good idea not to use it on your favourite pieces.

The main cause of cracking is the incompatibility of glaze and clay.

In ceramic making, the paste is fired at high temperatures. Ceramics used for food are usually fired at temperatures above 1200°C.

During firing, the glaze melts and forms a smooth liquid coating on the clay. As it cools, this glaze layer becomes hard and shiny.

For the glaze to form a strong protective layer, the clay and glaze must fit well together. During firing, the clay expands as it heats up. At this point the glaze is melted.

In the cooling phase, the clay shrinks a little. It is necessary for the glaze to shrink at the same rate in order for the clay and glaze to ‘harmonise’ with each other.

If the glaze shrinks more than the clay, it stretches and is under great tension. This tension causes the glaze to crack.

When does glaze cracking occur?

Sometimes the glaze may crack immediately after the product is manufactured. However, sometimes the tension can last for months or even years. Over time, however, abrasion weakens the glaze and eventually the tension succumbs and cracks appear.

Sometimes it is said to be caused by using the plates too hard or storing them in places exposed to temperature changes. For example, a cupboard that is too cold in winter and too hot in summer.

While such wear and tear can trigger cracking, the main reason is that the glaze is under extreme stress. As a result, it is not possible to completely prevent cracking. If the glaze is under tension, it will crack at some point, it is just a matter of time.

1) Bacteria harbouring: Cracked surfaces can serve as a hotbed for bacteria. For this reason, it is generally recommended not to use cracked containers with food.

Manufacturers generally do not sell products containing cracked glaze that will come into contact with food. In 2013, the Food Administration (FDA) published the Food Code guidelines. These guidelines state that food contact surfaces used in restaurants must be free of cracks and other irregularities.

So, what should we do with the cracked plates and mugs we have? Can they be used? Depending on who you talk to, you will probably get different answers to this question.

However, there is some consensus in the pottery community about the levels of risk. For example, if you occasionally drink coffee from a mug with a little cracking, it probably won't do any harm. In contrast, storing or cooking food in a cracked vessel is considered more risky. Storing or heating food in a cracked container increases the chances of any bacteria multiplying and causing problems.

One of the problems that can arise when using cracked containers for cooking or storage is the problem of contamination.

2) Contamination Risk:

Glaze cracking is not only a superficial problem. When a glaze cracks, the crack extends from the glaze surface to the underlying clay body. Therefore, when food and drink enter the crack, it can also access the underlying substrate body.

As a result, food and drink not only settle in the cracks, but can actually be absorbed by the substructure body itself. This is a bigger problem for earthenware. As already mentioned, earthenware is also porous and absorbent. Therefore, if water from your casserole gets into the cracks of the glaze, it can actually be absorbed into the dish itself.

When the dish is reheated, the juices from your previous meal can seep under the glaze and enter your new dish. As this is not only unhygienic, it also raises allergy issues. For example, if you cook roast walnuts in your pot, traces of walnuts may remain in the next dish you cook.

Products such as earthenware and porcelain are non-porous, so the problem of cross-contamination is less of a concern.

However, earthenware and porcelain are only non-porous if they have been fired correctly. So, you trust that the production method is adequate. And if your vessel has cracks, it makes no sense to assume that it was manufactured correctly.
 

3) Cracks weaken pottery:

Leaving aside the problems of bacteria and contamination, there is another problem caused by cracks. Cracks actually weaken the vessel itself. Cracks are not superficial, they extend into the substrate beneath the glaze.

This means that cracks actually create defects in the underlying infrastructure. As a result, cracks can create weak spots. Plates with cracks are more likely to break when bumped and hit in daily use. And handles are more likely to come off items such as mugs, jugs and casserole dishes.

The fact that cracks compromise the stability of your dishes means that they are not safe to use in your kitchen or dining room. You don't want your hot casserole dish to break while being lifted out of the oven with the overflowing hot meal.