Why does china craze

These stain marks are typically form in older vintage china or antique ceramic objects. For reasons we will explain below, we do not take such tasks on but thought it will be useful to provide the information you may want to know if you choose to remove these stains yourself.

See details on this tutorial and all of our repair and restoration lessons and tips. What are these stains on pottery? They occur due to seepage of moisture through very small and sometimes invisible to the naked eye cracks in the glaze often referred to as crazing, crackle or pin holes in the glaze.

Therefore, scrubbing the surface will not help. Bacteria stains are more likely to occur on earthenware or stoneware due to the ceramic higher porosity allowing the moisture to penetrate deeper and stay wet creating perfect enjoyment for bacteria to flourish. High fire ceramic such as porcelain, which has almost no porosity, will less likely stain.

They are a network of lines or cracks in the fired glazed surface. They usually occur at the end of the kiln firing process when the outside surface cools before the clay body under the glaze. Often, for aesthetics reasons, the crazing effect is introduced by design and on purpose on Japanese, Chinese or Raku dishes or vessels — see examples above. More about crazing. Bleach or chlorine may remove the stains but also can damage your ceramic item as illustrated below.

If the stain is old oil based substance e. The repair will be damaged. Easy way for mildly stained pieces: Buy regular oxygen bleach typically used for laundry e. General stain removal: apply solution to surface. Allow to absorb for minutes. Soak your stained item in the solution for several hours or until you see the stains disappear.

Soak your stained item in the solution for several hours. Then, while wet place it in an oven and set to to degrees F. Let the oven rise to degrees while your item is in the oven. The rapid temperature change could crack the item or chip off some of the crazed glaze. After minutes, take it out, wait for it to be cooled to room temperature and wash it with room temperature water.

You will see slightly colored water seeping out from the crazing washing off. Using a stronger solution of peroxide is extremely dangerous. It can burn the skin off your hands and cause permanent damage to mucous membranes, and unless you know chemistry very well you could have an explosion. This happens generally as the wares cool after firing. It is upon the cooling of the kiln and the contraction of the wares that cracks form. Heating and then cooling too rapidly can cause the glaze to shrink too quickly and cracks appear more readily.

There are a few main ways of avoiding crazing on your pottery piece. One of these methods is either changing the glazing or changing the clay. This may sound simplistic but the expansion and contraction rates are generally the underlying cause and so experimenting with different types can lead you to a solution. The aim of changing the glaze makeup is to reduce the expansion of the glaze and therefore to stop it contracting as much on cooling.

In simple terms, this means adding materials with low levels of expansion and decreasing materials with high levels of expansion. This can be a tricky process and may take some experimentation to get the right balance.

Here are some tips for changing the makeup of the glaze to avoid crazing:. In addition to the above, thinning down the glaze may also help reduce or eliminate crazing. The thicker the glaze, the more liable it is to craze.

One of the key things you can do to the body of the clay is to add silica, which helps to dry out the body of the clay and helps it to expand and contract with the glaze. Too much silica can cause more cracks however, and too little can make the clay difficult to glaze, so there is some experimentation required as to what works for you and your chosen finish.

This advice is fine if you have the option of testing and wasting clay regularly, but many potters do not have this luxury or finance to experiment. Changing the formulation of the glaze may be an easier and less expensive option. Larger pieces will need to be adjusted accordingly. Crazing can also be reduced by increasing the firing time and slowing down the cooling. One example of how to achieve this is to extend the last section of the firing section to a few hours so that the glaze expands slowly.

This will also give the clay body the best chance to tighten and achieve a good glaze fit. You should also avoid opening the kiln too soon to ensure the wares are sufficiently cooled and do not expand by cooling too quickly. Again, experimentation may be required to ensure that you have a program that suits. This is a post to a discussion on Clayart May by Gavin Stairs stairs stairs. This discussion about the safety of crazed ware for food service is a bit of a puzzle.

We have been down a long country lane on the problem of infectious disease over the last century or so. From the time of Jenner who introduced antisepsis in medical practice until today, we have passed from the early, heroic struggles of people like Pasteur and Koch, through the miracles of sulfa, penicillin and the antibiotics, to this point.

Certainly, elementary hygiene, including washing food ware in hot, soapy water, must be considered to be one of the most consistent successes. Bacteria and other infectious agents seem to be able to adapt to antibiotics and other agents, but a good, hot wash is still more than most can handle, and I've not heard of any cases of adaptation.

There are organisms that can survive in hot baths hot tubs, public baths, hot springs, etc. In the end, we rely on our immune systems to protect us from the everyday risks of infection. Many of the disease causing agents that we fear are endemic in our environment, meaning that we come into contact with them every day.

A completely sterile environment is not good for this immunity, since it gets no reminders of what to fend off, and can essentially go to sleep. Our daily contact is a sort of inoculation against the diseases, which we therefore only contract if we come into contact with an infectious agent in large doses. If this analysis is correct, we should not fear low doses of bacteria such as what might hide in a crack in a glaze.

However, if we then place into such a pot a nice culture medium, like some food, and keep it there for a while, we may find that the small colony in a crack has become a large culture in the food.

Then we are in trouble. Greatest risk is in food storage containers, especially for wet foods and liquids. My feeling is that these should be entirely free of crazing and surface blemishes on the food contact side.

This includes the lip. And this should be the case in both pots for sale, and for individual use. Next are food preparation surfaces and pots. I think these should also be craze free, certainly on articles for sale, and for the food service industry. For personal use, I would not necessarily throw out a favorite pot that developed a glaze crack or two. Finally, individual plates, cups and bowls.

These are the least sensitive, provided they are consistently washed in hot, soapy water, and are not used for food storage. So one might easily accept a crazed glaze on these for one's personal use.