Pottery is a time-intensive handcraft. The number of factors at play is extensive, thereby increasing the possibility of mistakes snowballing which often result in unexpected outcomes.
While beginners can avoid some materials and techniques in pottery due to the increased level of difficulty, not all problems can be avoided. Crazing, for instance, is one such issue that can be seen as a blessing or curse depending on your artistic abilities.
There is no way to reverse the fine cracked lines on your ceramic, but you can make the most of this artistic mistake by applying your creative abilities. Unfortunately, while crazing may be fixed by refiring, the resulting ceramic is not food safe.
Although the methods of fixing a crazed ceramic are limited and not guaranteed to show results, solving the problem of crazing is an easier process that can be done by making prior changes. So, here in this article, we will discuss how to prevent crazing and fix it if it has already occurred in your pottery piece.
What Is Crazing?
Crazing is a network of small cracks on a ceramic surface. There is no fixed time for when these small cracks appear. They may be noticed immediately after firing or can occur even after several years.
The lines are superficial and do not affect the structural integrity of your pottery as soon as they appear. However, they may cause your ceramic to weaken and eventually shatter based on the cracks’ pressure and the glaze surface’s tension.
What Causes Crazing?
The primary cause of crazing is the difference in the expansion of the body and the glaze after firing. So, technically, crazing is a concern caused by glazing. However, the process until the ceramic is fired and ready to be glazed is usually very straightforward.
The glaze is usually very thin and is applied to the fired pottery piece by pouring, painting, or dipping. After its application, the glazed body is fired again to help mature it and mold it together.
Once in the kiln, since the glaze is thin in consistency, it heats up sooner than the clay. Due to the difference in the temperature and heating and cooling speed in the kiln once removed, the glaze and pot body expand and contract at different rates.
Crazing occurs due to the difference in expansion and contraction rates. The glaze tends to cool faster than the clay body, resulting in rapid contraction of the glaze while the body is still hot or just beginning to contract. The excess tension on the glaze causes tiny cracks on the surface, known as crazing.
Are Crazed Ceramics Weak?
Crazed ceramics are not significantly more fragile than uncrazed pieces. It is because the cracks that usually appear like a spider web are only as deep as the glaze layer and do not affect the clay body underneath.
However, with time, your ceramic’s structural integrity may be affected due to the craze. Since glazes are usually thin, crazing is a common occurrence that can be dealt with using quick fixes, but the pottery cannot be strengthened using these tips.
Is Crazed Ceramic Food Safe?
No, a crazed piece of pottery has tiny cracks on its glazed surface. While these cracks do not run deep, they provide enough area for accumulating dirt, food particles, and microorganisms.
It is recommended to replace ceramic tableware and containers meant for handling food to ensure the maintenance of food safety standards as soon as crazing is noticed.
Can Crazed Pottery Be Used?
Crazed pottery can still be used depending on its functionality. For instance, if the ceramic body is used for decorative purposes like a vase or any centerpiece, it is safe to use for a long time before you should be concerned about its strength.
Crazing doesn’t immediately weaken the structure. Therefore, such ceramics can be used for several years based on the material and firing involved in their production.
However, the same does not apply to ceramic tableware or food containers. The cracks collect dust and food, making them a breeding ground for bacteria, ultimately making the ceramics unfit for use.
How Can You Fix Crazed Pottery?
There are not many ways to repair crazed pottery. Since it is an irreversible change, there is only so much that can be done to resolve the problem temporarily. Ideally, starting over with an altered clay, glaze, and firing temperature is suggested to get a smooth surface without cracks.
Make Preliminary Observations
Before taking corrective measures, it is essential to identify the difficulty in resolving the crazing. If you notice any of the below-stated characteristics, you may need more steps to fix the problem.
Craze Pattern: If the network of lines is closely spaced with little to no gap, the chances of eliminating or reducing crazing are low. If you observe the lines to be less than 1/8th of an inch apart, it will be challenging to resolve the issue.
Absorption Rate: If your ceramic has an absorption rate of over 4% even after glaze firing, you will have greater difficulty correcting the crazing.
Cover The Crevices
Once you notice crazing in your pottery, you can follow the step below to help reduce the appearance of the cracks. While they cannot be reversed, you can undoubtedly camouflage them. However, remember that if you notice any of the two characteristics stated in the section above, the chances of reducing the crazing become less.
This tip is only a temporary solution as once crazing appears; it cannot be eliminated. You can attempt to cover up the cracks, but the body is bound to shatter if excessive stress is put on the ceramic over extensive periods.
Refire The Crazed Pot
Place your crazed ceramic back into the kiln. Fire it for a short period at a high temperature.
Ensure that you give it plenty of time to heat and cool down and do not hasten the process. It can help temporarily get rid of the cracks.
Tips To Prevent Crazing In Pottery
Instead of dealing with crazed ceramics and covering up the spider web of cracks, it is recommended to deal with the root cause of the problem, i.e., the firing in the kiln, the clay, and the glaze. It is easier to correct the variables before the firing process than to deal with the aftereffects. Crazing can be avoided by finding the right combination of glaze, clay, and firing temperature.
Understanding the concepts of thermal expansion of clays and glazes and their correlation in different temperatures is key to preventing your ceramics from crazing.
So, here are some effective tips to prevent crazing while making your pot.
Apply Thinner Glaze Coat
Dipping, pouring, or painting excess amounts of glaze onto your clay body is one of the easiest ways to facilitate crazing. However, most glazes are thin in consistency, so it is not a product problem.
While applying it, ensure that you aren’t applying unnecessarily generous layers. The more the glaze, the higher the chances of crazing.
Take An Appropriate Combination Of Clay And Glaze
Sometimes, you may notice that even after keeping the layer of glaze as thin as possible, the ceramic shows crazing. It may simply be due to the incompatibility of the clay type and glaze.
Feldspathic, lead, tin, and salt are the most common types of glazes. The temperature at which they are fired is given in the table below.
|Glaze Types||Most Suitable Clay Type||Firing Temperature in ºF||Firing Temperature in Cones|
|Feldspathic glaze||Porcelain||1565-1652ºF||Cone 10-13|
|Lead glaze||Earthenware||1850ºF||Cone 5-6|
|Tin glaze||Earthenware||1742- 1832ºF||Cone 6-8|
|Salt glaze||Stoneware||2232-2281ºF||Cone 6-10|
It is essential to know the basic chemistry behind the compatibility of glazes and clays to understand their expansions and contractions to find the appropriate temperature to fire them at and prevent crazing.
Add More Silica
Adding extra silica to your glaze or clay can help dilute the mixture. It is an oxide that has low expansion characteristics. It will cause your clay and the glaze to expand slowly and fasten the cooling and contraction process.
However, you must keep track of the amount of silica added to the mix and ensure that it doesn’t affect the strength of your ceramic.
Alter The Glaze Composition
Altering the clay may not be a great idea since it takes time, effort, and resources that go to waste if the result is not as expected. Instead, you can make changes to the composition of the glaze. For example, add more silica, boric oxide, and alumina, which expand less.
When added to the glaze, they can help reduce contraction and prevent crazing. In the same way, reducing the quantity of feldspar, potash, or soda, which expand quickly, can help minimize contractions.
Fire At An Appropriate Firing Temperature
If you underfire your pottery pieces, it can lead to expansion and contraction of the clay every time there is a change in temperature and humidity. So, while you may not see the crazing immediately, it is bound to show up on the surface with time.
Since the problem has existed for a while, hidden under the surface, the crazing may be extreme and spread across your ceramic’s entire surface and may weaken the ceramic.
Fire At A Higher Temperature For Greater Periods
Higher temperatures and longer periods help the glaze and the clay mold better. In addition, it will help the glaze mature perfectly on top of the clay. However, this is a rather forceful option.
If the clay hasn’t fully contracted and cooled down before its removal from the kiln, this effort is meaningless and is bound to result in a crazed ceramic.
Allow Your Ceramic To Cool
Cooling down is just as crucial as the firing process. Not allowing your clay to cool during bisque firing or before glaze firing is a sure-shot way to get a crazed surface.
Allow your ceramic to cool down before taking it out of the kiln each time it is fired. It may increase the time spent in the kiln by an hour or two, but a prudent step must be taken to minimize the chances of crazing.
Open The Kiln At The Right Time
Opening the kiln immediately after firing is not a good idea, considering the chemical changes that the clay is undergoing.
A sudden temperature change will likely cause a thermal shock to your glaze and the clay and instantly cause cracks. Therefore, keep the kiln shut until it reaches a temperature of 125°F or 51°C.
Incrementally Adjust The Temperature
If you wish to increase or decrease the kiln’s temperature, do so in small increments instead of making considerable changes in a single go.
Ideally, the temperature should not increase or decrease more than 150-200°F in an hour. The same criterion is applicable while cooling the pottery.
Carefully Select Your Clay
If crazing is a frequent problem that you deal with, you may be better off using a different clay type that is easier to use. Earthenware, for instance, is a relatively difficult clay to prevent crazing as it tends to be porous even after firing.
Therefore, if you make pottery pieces intended to hold water, it is only a matter of time before it is crazed. Instead, try using stoneware or kaolin clay. The Craftsmart Natural Air-Dry Clay is an excellent choice for beginners to get used to working with glazes.
While working with clay, some difficulties cannot be avoided entirely. Crazing is one such technical challenge that can only be overcome by familiarizing oneself with the clay, glazes, and kiln. While beginners cannot be expected to experiment with the composition of clays and glazes, they can make the process easier by opting for clays and glazes that are easier to work with.
As an experienced potter, you can experiment with the compounds in your clay and glaze and adjust them to prevent crazing. The firing cycle is another essential part of the cycle that can be understood through experimentation and adjustments.
Moreover, with the growth of your expertise and knowledge, you can avoid crazing better. However, there is no permanent solution for a crazed ceramic. While it can be hidden for a short while, it cannot be reversed. Therefore, it is essential to understand the process to take preventive action while making pottery.