Did you find a big hairline crack in one of your ceramic pieces? In most cases, the easiest thing to do is to simply buy or make a new one.
But if this particular cup, bowl, or plate has a lot of sentimental value because it was given to you by someone special, then you might want to repair the hairline cracks.
There are several ways to fix a hairline crack. But in general, the earlier you can identify one, the easier it will be to fix it.
Repairing hairline cracks in clay
Spotting hairline cracks in clay is normally very difficult, especially in the first two stages of greenware. But when you do find one, you want to fix them right away. So when you finish molding your clay, make sure to go over it carefully to check for any cracks.
You can normally find them where the clay is the thinnest or around structural weak points. For example, where two different pieces are attached like a handle on a mug. These types of cracks are especially common in hand-building projects and coil pots.
To fix the hairline cracks, follow the following steps:
- Open up the crack and score the area. It might sound counterintuitive to open a crack, but this is to make sure that the old clay will attach to the old clay.
- Take some soft clay and apply it around the crack on both sides of the wall.
- Compress the clay by applying pressure from both sides.
- Wrap your pieces with plastic to make sure the old and new clay dry evenly.
When you find a crack, new potters often try to fix it and smooth things over by applying water. In most cases, this will actually make things worse! Stay away from the water when you spot cracks.
If you find and repair a crack at this stage with clay, then the resulting piece will be food-safe after firing at the right temperature and if necessary applying a glaze.
Repairing hairline cracks in bone dry clay
If you missed any hairline crack in the wet clay, you might find one after the clay as dried.
Unfortunately, there is little you can do to fix a crack in bone dry clay since wet clay and water don’t stick to dried clay.
So at this stage, you can either start completely from scratch again or try to repair the crack after bisque firing with one of the methods below. However, in most cases the cracks will grow bigger during the firing process.
Repairing hairline crack in ceramics: pegging method
There are several ways to repair hairline cracks in ceramics but the pegging method is the best way to restore the structural integrity of the piece.
However, this method can only be used if the wall of your ceramic is thick enough to accept a peg.
Here is how the pegging method works:
- Grind a small channel perpendicular to the crack deep enough to embed a copper wire. And create a second channel along the crack for the epoxy.
- Cut a copper wire of the same size as the channel you made and insert it. You can temporarily open the crack by inserting a razor blade.
- Heat the epoxy and cup to ~120F and apply the epoxy across both channels. Warming the epoxy before mixing will make it more liquid and penetrate the crack completely. Make sure the epoxy went through the crack and is visible from the other side of the wall.
- Remove the razor blade (if you used one) and apply a clamp before the epoxy hardens
- Place the mug in an oven preheated at 120F until the epoxy completely hardens.
- Remove the excess epoxy with a razor blade.
- Fill any gaps with 2-part filler epoxy and grind it down with sandpaper.
Although this method is great for fixing cracks, the resulting piece is not food, liquid, or heating proof. So it shouldn’t be used for cooking or eating when repaired with the pegging method. Therefore the pegging method works best for decorated pieces of ceramics.
Repairing hairline cracks in ceramics using super glue or epoxy
Depending on how bad the crack is, simply applying some super glue or epoxy might be enough to prevent your ceramics from deteriorating further.
However, if the crack is already pretty severe this might only be buying you some time and using the pegging method would be better.
You want to follow these steps:
- Clean the crack with ethanol and a clean cloth to remove any dirt.
- Apply the glue or mixed epoxy and make sure the entire crack is filled. You can carefully wedge the crack open a little by using a razor blade.
- After a few seconds wipe away any excess glue from either side.
- Wait until the glue or epoxy has fully hardened.
Although this is a quick and easy method, it’s important to realize that most glues and epoxies are not food-safe and should not be heated again. So only use this method on your decorative pieces and not on functional ceramics.
Repairing hairline cracks with Kintsugi
Kintsugi is an ancient Japanese art of repairing broken pottery and ceramics with lacquer mixed with precious metals such as gold, silver or platinum.
You can use kintsugi to repair hairlines as well. The process is very similar to using glue or epoxy.
Traditional kintsugi using real gold and lacquer is believed to be food and liquid-safe up to a certain temperature (<200 F).
However, most readily available kintsugi kits you can buy nowadays use epoxy rather than lacquer, which means that these kits are not food-safe.
So if you want to repair the hairline crack with kintsugi, make sure you buy the right kit if the resulting piece needs to be food-safe.
Is pottery food safe after repairing hairline cracks?
If you spot a crack early when the clay is still wet, you can repair the crack with clay and the resulting piece will be food-safe after firing and glazing. However, most methods to repair hairline cracks in ceramic pieces are not food-safe because they use some kind of glue or epoxy. If you find cracks in a mug or plate it’s best to buy a new one.
What is the difference between crazing and cracking?
At first glance crazing and hairline cracks look very similar. However, crazing normally results in a web of small interconnected lines, whereas cracking is normally one big line. Moreover, crazing is surface deep and only affects the glaze, whereas cracking goes through the entire ceramic piece.
The cause for cracking and crazing is also different. For more information, you can read our article on preventing and fixing crazing.
There are many ways to fix hairline cracks in ceramic pieces, including using superglue, epoxy or kintsugi. However, most of these methods are not food-safe and should only be used on decorative pieces. Your best bet for repairing hairline cracks is to spot them in the clay before the clay dries out. If you find a crack in a ceramic mug or plate, it’s best to simply buy a new one.