Something has caused an uproar in the pottery community lately. There have been diverse questions regarding pottery or ceramic and its compatibility with hot water. A few ceramic or glass items are safe for stovetop cooking and boiling water obviously, although some will be usable for induction ranges. The issue is mostly the crystal structure of the ceramic or glass and its heat proliferation properties. If the ceramic’s structure does not heat smoothly and fast, some parts of the pot will expand while you place it on a stove or when you boil water with it.
Rapid temperature changes from one extreme to another is one thing that can cause a problem on your pottery. Expansion of your ceramic pottery piece is likely to occur when going from hot temperatures to cold than it is from cold to hot. If you put boiling water in ceramic in the freezer, you are likely not to have a problem, but if you pour boiling water into a ceramic that just came out of the freezer, there is a high probability that the ceramic will expand and crack (if not shatter).
The phenomenon which causes ceramic pottery to shatter when pour boiling water into it is known as thermal shock. Wikipedia has a very technical article on Thermal shock that is probably more than most of you would need to know about it. Not all ceramic cups or vessels are suitable for holding hot liquids easily. A typical pottery pan is not able to handle the heat of boiling water very well. The reason is that as the pottery heats, its density changes; it expands. As said earlier, pouring boiling water into ceramic is highly likely to shatter it, because the hot water contacts part of the ceramic first, whereas other parts of the ceramic remain cool. The disparity in the temperature over the ceramic makes it expand and break. The ceramic pottery piece does not expand as a whole, but it is being pulled in different directions as part of it expands and part remains constant due to the partial heat; this difference produces the shattering.
|TYPES OF clay||ABILITY TO HOLD BOILING WATER|
|EARTHENWARE||The clay body starts to shrink once filled with boiling water|
|MID-FIRE STONEWARE CLAY||It can hold boiling water without shrinking.|
|HIGH-FIRE STONEWARE CLAY||It can hold boiling water without shrinking.|
HOW BOILING WATER CHANGES POTTERY
1. Vitrification and Maturity
(com1)The maturity of a clay body is the balance between the vitrification of the body to bring about hardness and durability. Vitrification is a gradual process during which the pottery/clay materials that melt most easily melt. The clay body of a piece melts/shrinks and fills in the spaces between the more refractory particles. The melted materials promote further melting, as well as compacting and strengthening the clay body. during this Vitrification stage, mullite (aluminum silicate) is formed. These are long, needle-like crystals which act as binders, and also knits the clay body together. Summarily, boiling water strengthens the clay body if evenly matched on the body of the clay. This technique works with glazes also, if the glaze expansion ratio is lower than the clay body, it might cause cracks and crazing while firing.
2. Atmospheric Drying
When a pottery piece is placed into the kiln, it always bone dry (you should make sure your ceramic is completely dry before placing it in the kiln). However, there is still water trapped within the clay particles. As the clay is slowly heated in the kiln, this water evaporates out from the clay. If the clay is heated too quickly, the water will turn to steam right inside the clay body, expanding with an explosive effect on the pot.
By the time the boiling water is poured into the fired pottery (212 F and 100 C at sea level) after all of the atmospheric water should have evaporated out of the clay body, a new amount of water is infused into the clay body. This will result in the clay compacting and some minimal expansion. It has to be fired all over again to make it completely water-free again.
3. Chemical Drying off
Clay can be often being categorized as one molecule of alumina and two molecules of silica bonded with two molecules of water. After the atmospheric water is gone, the clay still contains a percentage of chemically bonded water by weight. The pot will be substantially lighter, but with no physical shrinkage.
This chemically combined water’s bond loosens when heated. Overlapping the carbon and sulfur tend to burn off, the chemically bonded water escapes from the clay between 660 F and 1470 F (350 C and 800 C). If the water boils too quickly, it will cause the explosive production of steam inside the pottery piece. It is due to all these changes (and more) that the firing schedule must allow for a slow buildup of heat.
4. Glaze removal
A few potters and experimentalists have carried out tests on glazed pottery by putting it in boiling water. They found out that the overglaze used on the pottery started peeling off immediately the ceramic bounces off the water. The change in the glazing structure will cause the pottery to expand in size by a certain percent while heating or boiling, and lose this same percent as it cools. The effect of the thermal shock is strong enough that it even can cause special types of clay-like earthenware clay or mid-stoneware clay, used by a lot of commercial potters to shatter.
Although, Porcelain pottery pieces are stovetop safe. This is because they are baked at very high temperature and the clay body components are made in such a way that it can handle high thermal shocks. You can get your porcelain clay by clicking here.
Summarily, Ceramic pottery has to be the same temperature all over the body (not hotter on the bottom, cooler t the top) so that expansion or contraction is evenly dispersed. It’s not a good idea to take a ceramic pot out of the freezer and pour in boiling water or put it on fire for the same reason. A lot of pottery pieces are not heat conductive. Hence, the heat from the boiling water would not pass well through to the clay body sufficiently.