Crazing & Crackling & Raku - Food Safety and Handmade Pottery
This is a short note about the fine cracks you may see on a pottery piece and their causes, terminology, and safety.
Before I begin, I'd like to point out that a crack at a joint, for example at the place where a handle meets the body of the pot (often a mug), is a sign of a weak point. It may have cracked at any point in the making process, and it may never break. However, if one of your beloved ceramic pieces has such a crack, treat it gently. Joints in general are the most likely place to break in routine use, so you should treat your handmade pottery gently anyway.
Here is my favorite coffee mug. I made it years ago, use it almost every day, and it hasn't broken. The crack happened during the bisque (first) firing, and I just glazed it and hoped.
Crazing is the word for when a glaze doesn't perfectly match the clay during the vitrification process. It causes fine cracks to appear. You may see this all over, but since many people use more than one glaze on a single pot, you may only see it on part of the pot.
This is the same mug. The crazing is easy to see on the inside because of the coffee stains.
Crackling looks pretty much the same as crazing, but the effect is intentional. Glazes are made that are intended to crackle when fired. These can be left un-pigmented or an ink can be rubbed into the cracks to highlight them.
This is an example of a crackle glaze with ink rubbed into it. Looks kind of similar, right? I used a non-crackle glaze on the inside of this vase.
Raku is an ancient Japanese firing technique used all over the world. One of its most well-known effects is crackling, similar to above but there is no need to pigment the cracks, as the secondary fire will have already highlighted them in black.
The fine cracks are not very visible on this raku vase, but you can see a section of larger cracks and pinprick effect - super pretty!
Vitrification is the fusing of the clay particles (and if used the glaze particles). If the clay or glaze is fully vitrified, it will no longer be porous enough to absorb water or elements of the food you may put into the pot. This is why potters may refer to the type of clay they used and the temperature it was fired to in its final firing. While not all clays can safely be fired to high temperatures, the higher the temperature, the more vitrified the clay. Earthenware, for example, can NOT be fired to as high of temperatures as stoneware or porcelain, and then we use a glaze with a lower vitrification temperature as well, to properly seal it. Glazing the inside of a pot (food contact points) usually makes it the most non-porous and thus food-safe.
So how do those little cracks fit in?
The short answer is that yes, they do give access to the clay body and therefore the pots may be more porous. Some people therefore consider no pot with any form of cracking, intentional or not, to be food safe.
However, the raku technique has been used for hundreds if not thousands of years on food ware, so this safety issue is highly debatable.
Personally, I don't think that the limited time your dinner spends in its pot will impact your food safety much if at all. I am not alone in this, but as I mentioned, this topic gets argued about thoroughly in pottery circles. There are non-glazed fermentation pots on the market, and that's even more access to the minerals in the clay.
Of course, there's also the concern that bacteria can form in the cracks. I recommend washing with hot water and soap, same as anything you eat from (except your cast iron, of course - and there you have a desirable leaching of minerals into food, so... yeah, you can make your own judgement here).
One interesting note on your pottery and the dishwasher: yes, unglazed surfaces like the foot of your bowls or the outsides of some pots are more permeable. That explains why you might get rings of condensation or even mold in your cupboards when you use your pots in the dishwasher. Drying them thoroughly (and I mean THOROUGHLY) will prevent this. I've heard recommendations to put them in the hot sun (if you live in such an environment) or in the oven on a low temperature (always a breaking risk if they are not meant for oven use). The easiest way to deal with this, however, is to store them upside down in the cupboard, so that the unglazed parts have access to air circulation and may fully dry or at least will not condense on your shelves.
A related issue, although not strictly about crackling, is lead content. Manufactured glazes (no commercially available clays that I know of) still occasionally have lead in them, however they are always strictly controlled and not to be used for items intended for food. As I write this, I realise that while I have always carefully made sure the glazes I buy state clearly that they are lead-free, I can't remember the last time I saw one bearing the label that they do contain lead. I remember that label, but it's possible that this has been yet more strictly controlled in recent years.
Do you have questions about this? Please leave a comment below!