Cracking and dunting of porcelain mugs.

I was busy sorting through mugs; moving some from my showroom to my kitchen cabinets and some from my kitchen cabinets to the trash pile. If you are as sentimental as I am, any rejects first makes a round through your own kitchen and then you would look for more reasons not to dump it. Sometimes a mug will just beg you to trash it! This one time it became a pottery lesson that everyone should be aware of.
​I noticed these lines in the image below in one of my porcelain coffee mugs. I knew it was the liner glaze on the inside that was cracking.
(Technical term for potters is dunting. )

Although this mug looks perfect, it had a crack in the glaze on the inside.

When glaze is applied too thick on the interior, a mug may show stain lines on the inside which are actually coffee stains that seeped into the crack and that would eventually seep out on the shelf.

When glaze is applied too thick on the interior of  a mug, it may show clear cracking lines, where coffee in this case, seeped into and stained the  crack.
The dunt were probably in the clay too, but my bear eye could not see it yet. I would have known over time. Chances were that hot water would have revealed that, either with an explosion of the mug (if I am really unlucky!) or with liquid just seeping out onto the table or in my lap.

It was very visible in the light colored porcelain mug. (Image above) Many mugs will not show this kind of defect and as you all know, it is the perfect place for dirt and bacteria to gather. Any well vitrified clay object can be subject to too thick layers of glaze and it can be disastrous in any clay body if the problem is not addressed. 
I took the mug outside and knocked it on the cement to break it.( Yes I protected my eyes with safety glasses) and I have to tell you, it took me about 5 really hard knocks, before it broke. Did I mention that porcelain is a really strong ceramic medium?
Anyway, the way it broke shows clearly a too thick layer of liner glaze inside the mug (see image below).

A too thick glaze layer is clearly visible on the bottom of this mug shard.

How to prevent dunting in pottery.

Even walls and an even application of glaze on any functional clay object is crucial. In this case the glaze layer was too strong for the clay body. Since it is a clear glaze, it is possible that I double glazed it without realizing the mistake.
Prevent thermal shocking during the firing process. A kiln that is fired too fast or that fires unevenly may cause trouble, not only with dunting, but possibly with warping too.
Obtain knowledge of silica and its behavior in pottery clay and glazes and especially how it will behave during firing of a pottery kiln. ( Silica, often a culprit in the studio,  is an interesting topic for potters to review.) See: Kilns suitable for porcelain in the near future.
Potters must make sure their glaze and clay body is a good fit for each other. There are glazes that are simply too strong for the clay body in use and instead of just shivering, it may tear and crumble a pot up into pieces.
Glazing on just one side of a piece, may cause dunting, when the stress of expansion and shrinkage on one side is stronger than the other side.
Large plates, platters and trays often tends to dunt when it is not properly set up in the kiln Learn how to fire plates.

Identify dunting in a clay object

When a crack is long and ongoing with a sharp edge, it is most certainly a dunt. This type of cracking can appear in the clay or the glaze or in both. They may appear vertical, horizontal, spiral or with a ragged edge all over the object. 
If the crack is soft and rounded, it means that glaze moved and melted into the crack. The obvious observation is that the crack was there before the glaze melted. This type of crack is unrelated to dunting.

How to use pottery in the kitchen

It is important that pottery users realize that they are working with a glass-like product that can break and chip if it is abused in the kitchen.

Here are a few tips to be aware of when you buy pottery.
Always inspect your pottery directly after you received it from any ceramic artist; especially when it is shipped to you. 
Make sure it does not have hidden cracks, by tapping with a wooden spoon on the rim. A cracked piece will have a dull sound.
If you are about to subject your pottery to heat or cold, ALWAYS avoid sudden temperature changes. NEVER take a bowl or casserole dish straight from the refrigerator to the oven or vise versa. Not only is it a dangerous practice in which you can get hurt, but you will most certainly loose your precious bowl.
If you observe a crack, understand when it is a dunt and when it is a flaw in the making process. A crack that was formed in the making will have rounded corners, whereas a dunt is sharp.
Be aware that there is such a thing as delayed dunting. By discussing it with the potter, you may help him/her to solve a issue that they may be unaware of.

Links
TeachinArt - Online School of Art
Wheel thrown porcelain dinnerware
Handbuilding porcelain dinnerware
AIC_IAC - International Ceramics Academy of Ceramics
MAC - Mississippi Arts Commission

Antoinette appreciate any comments. Also visit her Porcelain shopping gallery if you are interested in collecting one or more of her ceramic art pieces.