How to Correct Five Common Ceramic Glaze Defects

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It is especially true in the ceramics world that one person’s fault is another person’s fancy – especially when it comes to glaze “defects.” Many ceramic artists deliberately create faults in their glaze surfaces to achieve a particular aesthetic.

But, of course, there are some cases in which a glaze must be perfect for reasons of safety or hygiene. So just in case glaze defects are driving you “craze-y” (sorry, I just couldn’t resist), today Robin Hopper gives some expert pointers on how to solve five of the most common pottery glaze problems (such as crawling, shown at left). – Jennifer Harnetty, editor.


Five Common Ceramic Glaze Faults and How To Fix Them

by Robin Hopper

Crazing

Crazing is the most common defect, and normally the easiest to correct. In both crazing and shivering the eradication of problems relies on matching the thermal expansion characteristics of both body and glaze. In practice, the most effective ways to correct crazing are:
1. increase the silica, in body or glaze
2. decrease the feldspar, in body or glaze
3. decrease any other material containing sodium or potassium
4. increase the boron
5. increase the alumina, i.e. the clay content
6. increase lead oxide.

Shivering

Shivering is the most problematic of glaze defects, particularly in any functional ware, since slivers of glaze from the edges can drop into food or drink. These slivers are usually small, sharp, jagged-edged pieces that detach themselves from the body. It is the most dangerous of glaze faults. Shivering is the reverse of crazing, therefore the remedies are the opposite as well:
1. decrease the silica in either the body or the glaze
2. increase the feldspar, especially sodium feldspar or nepheline syenite, or other alkaline bearing materials.

Crawling

Crawling is caused by a high index of surface tension in the melting glaze. It is triggered by adhesion problems, often caused by bad application. It occurs where a glaze is excessively powdery and does not fully adhere to the surface of the clay. This can be alleviated by the addition of a small amount of gum to the glaze batch. Crawling is more common in matt glazes than in fluid ones; sometimes the problems of crawling can be reduced by the addition of a small amount of extra flux. Crawling can also occur when one glaze is applied over another, particularly if the first is allowed to dry out completely before the second application. Some fluxes, particularly zinc and magnesium, are likely to cause crawling when used in excess. Calcining all or part of the zinc can help this problem.

Pitting and Pinholing

These are the most annoying and difficult glaze flaws to cure. They can be caused by a badly controlled firing cycle, the glaze composition, or can originate with the body, particularly highly grogged clay bodies. The following remedies should be tried to cure pinholing or pitting:
1. lengthen the firing cycle
2. apply the glaze less thickly
3. add more flux to the glaze to make it more fluid
4. decrease the content of zinc or rutile in the glaze
5. where zinc is used, try calcining half or all of the zinc content
6. increase the maturing temperature of the glaze
7. hold the kiln at the glaze maturing temperature for a soaking period of up to two hours
8. cool the kiln slowly.

Blisters

Blisters and blebs are usually the result of either an excessively thick application of glaze or incomplete clay preparation, wedging, blunging, etc. Sometimes, however, these faults can be due to overfiring or to the use of soluble fluxes in the glazes. The following fluxes might cause this problem: borax, boric acid, potassium carbonate, magnesium sulfate, and sodium carbonate. If these materials are present in a problem glaze, it would be well to replace them with other fluxing agents, or fritted materials.


This post was excerpted from by Robin Hopper’s popular book The Ceramic Spectrum, which is available in the Ceramic Arts Daily Bookstore.


**First published in July 2013
Comments
  • Great post. We’ve had all of these problems at one time or another at the high school where I teach. Crawling and pinholing are most common. Luckily, I’ve only seen shivering once or twice in 30 years in experimental mixes. It seems to be the least common defect. Another way to reduce crawling is making sure the glaze is thin enough when it is applied. Our glazes sit mostly in gallon plastic salad dressing jars for long periods between use. The suspensions seem to get thicker over time.

  • Shivering and crawling may be also caused when manipulating the biscuit with bare hands; the natural oil of the skin acts literally as a resist for the glaze.I always wear gloves to avoid this.
    Pitting and pinholing are also often produced when the glaze is still wet when fired.

  • We have a big problem with pinholes at my hIgh school. i never saw that we glazes can be a problem! IS there also a bisque temp issue that can cause this? Refiring doesn’t seem to correct the pinholes. Any advice? We use Sheffield clay and mostly Amoco low fire glazes. The clear crazes like crazy!

  • Hello and thanks for the informative article.I am having more of a glaze application problem. Once the glaze dries, it cracks and falls off in some areas leaving telltale marks, especially on white clay..
    The glaze applies smooth and there is no coverage problem.What can I add to make this glaze stick to the ware. When it stays on, it is a beautiful glaze, exceptionally nice over black clays…where it becomes metallic looking. It accepts some other glazes well with over spray and then it seems to stick a little better. I have had some beautiful results so I am not ready to give up and ditch it because of this problem.I use cone 6 stoneware clays.

  • I’ve gotten rid of some pinholes by feathering (rubbing lightly with two fingers) the glaze after it is bone dry. Do it lightly so you don’t remove too much glaze and wear a mask since the dust might be toxic to breathe. And you can do it over the glaze bucket if you want to recapture that precious glaze.

  • to reduce pinholes, make sure you bisque to maximum temp. then in the glaze fire take it up slowly through quartz inversion.

  • Thankyou – great post as I am a beginner. My first firing today had crazing. After taking it out of the kiln I could hear it continue to ‘ping’. My question is – How much do I need to ‘increase the silica’ by. (eg<1%, 2% or 5%)? Looking forward to your replies 🙂

  • And another question…. can I re-glaze over the existing pot and fire again – or will it just continue to crack underneath the new glaze? Refire – or move on???? Thanks.

  • Crazing is just another name for crackle glaze. It is not always a defect – especially if you are firing to stoneware temps. At stoneware temps the clay body starts to vitrify and that is what makes a pot waterproof, not the glaze. If you want to get rid of crazing add silica (flint) 1 percent at a time.

  • I work in low fire, Highwater white earthenware. I have found that shivering can occur if the clay gets burnished in the leatherhard state. Students oftern run their fingers repeatedly over an edge. This slicks up the surface and the glaze will shiver off after firing. Quick fix: run a damp spnge over any shiny surfaces after completing and before bisque firing.

    Also, I bisque my clay to cone 02, the highest temp recomended by the manufacturer (Highwater clay). This eliminates the crazing.

    For pitting, apply thinner coats of glaze and end your firing with a 10 to 20 minute hold at temperature.

    Lots of good info here, thanks all!

  • Lorraine Bates Woodsetton Art Pottery UK, To Gill Fry – I’m new to Ceramic Arts so let me off if I’m not doing the forum technology thing correctly!
    But I re-apply and re glaze fire up to 6 -10 times , pots may become more brittle eventually but you can really abuse them if the glaze fit is OK to start with. Bare in mind that you need to increase your final temperature by about 10 degrees or soak for 10 minutes longer each time you then glaze fire, in order to have the same heat work in the subsequent firings.
    By far best way to cure crazing is to biscuit fire higher than you glaze fire, obviously you’ll need to be using or making your glazes brush on for this.

  • Too much raw clay in the glaze can cause crawling. To correct, calcine half the clay. The thickening over time that Tim Smarzo mentions could be due to the thixotropic nature of glazes. If this is the case, stirring the glaze will return it to its usual thinner viscosity. Otherwise, the glaze may be evaporating if the jars are not tightly sealed. Measure the specific gravity when the glaze thickness is optimal and before each time it is used. Of course the glaze must be thoroughly stirred before measuring.

  • My glazes are turning out streaky and transparent. I feel that I am coating them long enough in the glaze, it doesn’t want to stick. Any help?

  • How much gum would you recommend per liter or 1 kg batch of glaze. I am developing beautiful jade glaze but am still having problems with small dots of crawling. When the glaze is applied I see little air bubbles, which i rub down. The firing is cone 6 -7 with a slow cooling down cycle. I have experimented with different glazing techniques and consistencies of glaze. If it is too thin I don’t get the result I want but a double dip of thinner glaze still leaves little air pockets. Any help/advice appreciated.

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