Regardless if it’s on a plate, bowl, or perhaps your favorite coffee mug, you have probably seen some pottery with small cracks before.
Although these hairline cracks are quite common on old pieces, a lot of people don’t really know what causes it or what to do about it.
But finding these fine lines on your pottery might actually be a pretty big warning sign.
In this article, you’ll learn everything you need to know about crazing on your pottery.
What is crazing?
Crazing is a phenomenon often seen on pottery which results in a web of small hairline cracks on the surfaces of a glazed piece of pottery. Crazing can occur during the firing process in the kilns or after many years of use.
The cracks only run as deep as the glazing, so the body underneath normally remains intact.
The first signs of crazing are not always easy to spot, especially when colored glazes are used. However, over time the cracks have the tendency to become darker and become more obvious.
If there is a big crack throughout the ceramic body, you are no longer with crazing. In that case, you might want to read our article on repairing crack in pottery.
Is crazing in pottery bad?
Crazing is technically considered to be defects in the pottery. So crazing weakens the integrity of the piece. The more severe the crazing, the bigger the impact on the pottery.
However, since the hair cracks are normally only surface deep, it’s not as if your pot is going to crumble in to tiny pieces right away.
More important is that the small cracks are difficult to keep clean and can often be an ideal place for bacteria to grow.
So if you spot crazing on any mug, plate, or bowl you use for cooking or eating, it’s best to replace it as soon as possible.
Can you use pottery with crazing?
As mentioned before, crazing can weaken your pottery. But even after spotting the first signs of crazing, the pottery can still last you several years.
So you can still use certain pieces of pottery with crazing such as vases, flowerpots, or anything with purely a decorative function.
However, if you see crazing on pottery used for foodstuffs such as bowls or plates, it’s better to replace them quickly.
Pieces of food can get stuck in the tiny cracks and bacteria can start to grow in between them.
The hairline cracks themselves are initially slightly gray or not visible at all. But they grow darker in color over time due to the bacteria.
So if you see very dark lines in pottery use for eating, it’s best to stop using it right away.
What causes crazing on pottery?
Crazing is caused by the mismatch in thermal expansion between the body of the pottery and the glaze. Crazing often happens during firing, but can occur after years of wear and tear as well.
When a glazed piece of pottery is heated in the kiln at high temperatures, both the body and the glaze expand. And when the pieces cool down, both of them contract again.
The problem is that the glaze normally expands and contract more than the underlying clay. So during the cooling phase the glaze is stretched and might form cracks.
Even if there are no initial signs of crazing, small cracks might show up years after firing. For example, after exposing the piece to extreme temperature change, dropping or bumping the piece, or because the body absorbed a large amount of water while the glaze doesn’t.
However, all these cases are indirectly causes by the difference in thermal expansion of the body and the glaze.
How to prevent crazing?
Since crazing is caused by the mismatch in thermal expansion of the body and the glaze, the easiest way to prevent crazing is to use a different clay or a different glaze.
When it comes to the clay, you could try out several different brands. Since the ingredients of the clay and their ratios is slightly different for most brands. If the crazing is really severe, you could also try out a completely different type of clay. For example, by using porcelain clay instead of stoneware clay.
Similarly, you could try out different glazes to see which one gives the best result. Glazes with high expansion oxides such as sodium or potassium have a high thermal expansion.
Try out a glaze with less of these metal oxides, or try diluting them by adding extra silica to your mixture. Substituting alumina with quartz also helps in most situations.
Also note that crazing is more likely to occur in very thick glazes. So make sure you don’t overdo it.
Another trick you can use as a pottery is to heat and cool down your pottery very slowly. This gives the glaze more time to adjust.
When firing your pottery, don’t increase the temperature faster than 150~200 degrees per hour. And cool the pots just as slowly.
If you are using an electric pottery kiln, you can easily control the ramp rate using the display.
However, pieces prepared this way might still start to show cracks when exposed to quick temperature changes such as boiling water.
So finding the right clay and glaze is a better way to deal with crazing.
Finally, a little word of caution about porous ceramics with only a partial glaze, for example an earthenware flower pot that is only glazed on the outside.
Unlike stoneware or porcelain, earthenware is relatively porous even after firing. If only the outside is glazed, the ceramic body can still absorb water (when watering your plants) and expand. This may result in crazing as well because the glazing is not porous and can’t absorb any water.
So make sure your ceramic pieces are completely glazed or are non-porous after firing.
How do you fix crazing in pottery?
If your pottery is showing signs of crazing, there is little you can do to undo the crazing. Refiring the piece at high temperatures in a kiln with a slow heating and cooling rate might temporarily remove the hairline cracks.
However, crazing is bound to occur again as long as the ceramic body and the glaze remain the same. So to really remove the crazing, you’ll have to buy or make a completely new piece.