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Glazing & Firing Troubleshooting

How to Resolve Common Glazing & Firing Issues

Looking for full kiln technical guidance and videos? Visit our Kiln Technical Support page .

Please find following troubleshooting tips for bisque, glazes and firing.

BISQUE

How can I tell if my bisque is firing properly?

This is very difficult for the studio to do. You can do absorption tests and test for coefficient of thermal expansion, but these tests can be complicated and inconclusive. The best way to assure your self of quality bisque is to use reputable bisque suppliers with a history of good quality. If you are just starting out or having problems with certain bisque, you may want to meet with the supplier to make sure they have a history of good quality and active testing program on all the industry's leading colour lines. Work with those who give you consistent quality and service. There are many quality suppliers out there who are willing to work with you on this issue.

How can I tell if my bisque has a small crack? (Ping test)

Do the "ping" test. Hold the piece of bisque loosely in your hand and tap the edge with a pencil. It should ring like a bell. If it sounds like a clunk, the bisque may have a hairline crack. You can also view the video on the following link to **learn how to do the ping test.**

GLAZES

Crazing — what it is and how to prevent it

Crazing is a network of fine hairline cracks that appear on the glaze surface. If the glaze, during the cooling process, shrinks more than the clay body you get crazing. There are two types of crazing, immediate and delayed. If you get delayed crazing, weeks or months later, it is probably caused by moisture getting into the ware.

Some causes of crazing are:

  • Under fired bisque.
  • Opening the kiln too soon before it has cooled properly. Do not open the kiln until it is below 150°F or is cool to the touch.
  • Incompatibility between bisque and glaze.

Moisture can get into the piece through stilt marks and the water expansion contraction caused by heating and cooling of ware, i.e.: repeated microwave or dishwasher use can cause this over time.

Pinholes & craters — causes and ways to reduce them

Pinholes and craters are two of the most common glaze surface defects. After firing, you may see small "pin sized" holes on the surface of the ware (pinholes) or you may see larger "crater like" impressions like the surface of the moon (craters).

While there are many things that can cause pinholes and craters, the causes are related to gases coming from either the clay body or the glazes that have not smoothed over after firing.

The clay body, and even the glaze, contains clays and other minerals mined from the earth. These contain some organic matter (i.e. plants, etc.), carbon and more. These materials form gases during the firing process and can result in pinholes and craters.

Several things can cause such defects:

  • Heavy glaze application
  • Fast firing and/or underfiring of the bisque. This does not give enough time for the organics, etc. to burn out.
  • Fast firing or underfiring of the glazes. Does not give enough time for the glazes to smooth over the surface of the bisque.
  • Not enough oxygen in kiln for carbon to properly burn out.

There are many possible causes of this problem and it can be tricky to properly diagnose without a lot of help from the studio concerning the bisque used, glazes used, firing schedule, cones used, where piece(s) was/were in the kiln, etc.

We believe the following two ideas are important to reducing and/or eliminating the occurrence of pinholes and craters.

  1. We have seen far greater incidence of these issues with fast firings or firings that go too fast toward the end of the firing because the glazes do not have time to smooth over any potential pinholes or craters. So, by maintaining a proper firing schedule, you should be able to greatly reduce or eliminate these surface defects.
  2. We find there is also a much greater incidence of these glaze surface defects with a heavy colour or clear glaze application. By making sure the application is appropriate, you can also help eliminate these issues.
Can I fix the piece for my customer? (pinholes, craters & crawling)

Here are some suggestions if you want to try to fix a piece that has either pinholes or craters. If the issue is the glaze firing (i.e. it might be under fired or fast fired), you may want to re-fire to the proper schedule. If the problem may have occurred in the bisque firing, try grinding down any of the thick craters with a dremel tool, brush on a thin coat of clear glaze, and re-fire to the proper cone 06 schedule. This may not always work, but it is the best option. If the glaze has been applied too heavy, a re-firing will probably not work.

Crawling

Small spots where the clear glaze seems to "crawl" away from the bisque and "bead up”, especially on the inside bottoms of mugs, vases, etc:

Several things can cause such defects:

  • Over application of clear glaze. This often happens on mugs, vases, etc. because it is a likely spot for the clear glaze to pool after dipping. The clear glaze tends to collect at the bottom. The remedy is to make sure the excess clear glaze is swirled around the inside of the piece and properly poured out.
  • More than likely though, dust or debris has collected at the bottom of the piece (or oils or lotions from hands on the surface of other pieces). The best remedy is to make sure that the bisque is properly cleaned with a damp sponge prior to painting including the inside bottom of pieces.
What is shivering and how can I prevent it?

When you fire bisque in the glaze firing, the bisque will basically expand during the firing and contract during the cooling phase. Shivering is typically defined as occurring when the bisque and glaze shrink at different rates. If the bisque shrinks more than the glaze, the glaze has nowhere to go but "shiver" off or "flake" off the bisque. "Shivering" is normally defined as the opposite problem to "crazing" since crazing is often a result of the glaze shrinking more than the bisque. The result is a network of fine cracks in the glaze.

Unfortunately, shivering happens with almost all studios over time no matter whose colours or bisque you use. Our goal is to help educate studio owners and managers to the potential causes so the affects of shivering can be limited.

What are the potential causes of shivering?

  • Under-fired bisque (and on some occasions over fired bisque).
  • Clay or slip used in the making of the bisque does not fit the glazes. There can be multiple causes for this problem. Clay is mined from the earth and this is an inexact science. Sometimes there can be issues with the clay that cannot be detected until you do a glaze firing. This happens infrequently. Also, bisque suppliers are continuously getting their bisque from multiple suppliers from different parts of the world. This means there are more types of bisque being introduced into the market place all the time. Make sure you are using reputable bisque suppliers and they have a history of consistent quality. We should emphasize that even reputable bisque suppliers may, as they develop additional sources, deliver bisque that shivers.
  • Heavy colour application.
  • Not allowing the colours or clear glazes to dry enough before firing. When heated, moisture from the colour can turn to steam and cause a poor fit between color and bisque. We feel this is a much bigger problem in the busy season when both customers and studios are crunched for time.
  • There is a theory that using hairdryers on a hot setting can increase the potential for shivering.
  • Cast bisque being cleaned too much around the edges.
  • Pressed bisque being compacted too much on certain areas of the pieces like the edges.
  • Sometimes, bisque, when shipped, can rub against the box and create a polished surface that does not allow the colour to adhere.
  • Oils, dust and soluble salts on the bisque can cause shivering because they do not permit the glaze to bond properly with the bisque.
  • Any combination of the previously mentioned points.

As you can see from the large list of potential causes, it can be quite difficult to diagnose the cause of the problem. To help diagnose almost any ceramic issue, we recommend keeping a thorough log of problems which includes such things as the type of bisque and colour used, application of color, firing history, where on the piece the shivering occurred, etc. This log will help you organize facts, to present to your supplier who will then be in a better position to help determine what may have caused the problem. Without a good log, finding a solution will be almost impossible.

How to limit shivering (practical steps)

  • Use bisque suppliers with a history of good quality.
  • Make sure that the application of colour is not too heavy.
  • Try to limit "rushing" the ceramic process by either firing too fast, using hairdryers on a hot setting, or firing potentially wet colour or glaze.
  • Make sure customers hands are clean and the bisque is wiped down with a damp sponge before painting so any dust or oils from lotions or food have been cleaned from the surface.
Glaze/underglaze compatibility — will different brands mix?

Without testing each and every manufacturer's products with each possible combination, it is impossible to guarantee. But most colours, if not all, should be compatible with each other. The best way to assure you is to do some test tiles.

RUNNING GLAZES

Running glazes — the most common causes

  • Thick application of colour.
  • Thick application of glaze.
  • Not allowing the piece to dry thoroughly before glazing.
  • Glaze firing too hot.
  • Combination of the above.

The interesting aspect of this is that lighter colours do this too but they are often not noticeable to the naked eye because the pigment is not as strong.

Hazy or milky colours — why it happens

The main causes of hazy or milky colours, which are normally tiny bubbles trapped in the clear glaze, are a heavy glaze application or a glaze firing which is too cool. If it is the latter, this may be fixed by refiring to the proper cone value. Use shelf cones throughout your kiln to verify your kiln is firing to the proper cone value. By re-firing you are allowing more time for the bubbles to work their way up through the glaze.

Grainy or rough surface — common causes

If you are getting a grainy, rough or matte surface on your clear glaze, it is often caused by either a thin application of clear glaze or an underfiring of the glaze.

Mixing non-toxic & toxic glazes — is it safe?

Toxic and non-toxic formulas can be very different so we do not suggest you try this. However, with so many different brands and formulas, contact your manufacturer for the definitive answer. Remember that in mixing a non-toxic glaze with a leaded one you will lose the non-toxic properties, and you will probably make the piece no longer dinnerware or food safe.

Mixing underglazes — can I combine colours?

The answer with almost all like brands is yes. For example, if you want to get a lighter blue you can normally mix a little white into dark blue. This can be very helpful for your customers in decorating. You may want to do some test tiles with your colors for your customers and contact your manufacturer for their information.

We do recommend however, staying within one manufacturer's product when you mix. There is no way to insure compatibility between manufacturers.

Double dipping pieces — tips to avoid visible lines

There are two techniques to remedy the situation.

  1. Use tongs and single dip and simply touch up with a brush (wear gloves).
  2. Another way is to double dip, but dip it in a way such that the line is in an inconspicuous spot. For example, if you are dipping a plate, dip at least 3/4 of the plate such that you are double dipping a small section of the plate. This will not be as noticeable as dipping a little more than half the plate so the line is in the middle.
Changing dipping glazes — switching brands or leaded/unleaded

When changing dipping glazes, from leaded to non-leaded or from one manufacturer to another, we do not recommend the practice of mixing unlike glazes. Different suppliers' products may not be compatible with one another. You could get inconsistent finishes, blistering, or other problems. If you are mixing leaded glazes that have been individually tested to be dinnerware or food safe, you cannot be assured that they remain food safe after they are mixed. It is even more risky if you mix leaded and non-leaded glazes together. You would be mixing a moving and non-moving glaze together and they may not work at all, and you probably will be destroying their food safe properties.

FIRING

Small debris on ceramic ware after firing — likely causes

There are several possibilities:

  • Bits of firebrick and flakes from heating elements that have loosened or fallen. Due to the extreme heat during the firing of the kiln, a kind of convection effect is created and debris can blow throughout the kiln and deposit onto your ware. The solution is to vacuum kiln before each firing or as necessary.
  • Dipping glaze may have become contaminated from any of a number of sources. We recommend using a 100 mesh screen sieve whenever the glaze looks contaminated. Keep the glaze container tightly covered when not in use.
  • Ceramic ware stored after dipping may have debris fall on it from heating ducts, abraded aluminum trays or shelves or other contaminants.
Large pieces splitting/cracking during firing — what to check

This could be in the making of the bisque or it could be in the glaze firing. Make sure that the plate or platter is raised properly off the shelf with stilts and there is proper kiln ventilation (a venting system is highly recommended). This will allow the ware to heat and cool at an even temperature. Placing the large platters as close to the middle of the kiln when loading will also help. Do not place on bottom or top of kiln or near peep holes where cool air can cause thermal shock.

The problem could have been caused before you even received the plate. For example, a stress crack may have developed in the making of the bisque or the bisque may not have been properly fired. Some bisque manufacturers stack their plates and/or do not give proper ventilation to the larger items and the entire piece does not get the same "heatwork". This could lead to cracking during the glaze firing.

Firing and/or cooling too fast.

Yellowish spots after glaze firing — “hard spots” explained

Yellowish spots on the bisque after the clear glaze firing are most likely "hard spots" which occur in the pouring of the greenware. If the slip hits the same spot during the greenware pouring process, it can sometimes concentrate and pack the clay in an area thus creating a "hard spot." This may appear as a yellowish spot on the bisque. However, it is not always present on the bisque and sometimes appears only after the glaze firing.

Tip: The hard spot on the bisque is sometimes difficult to paint but you can hide the yellow by covering it with white under-glaze.

Air bubbles trapped under the glaze surface — causes & tips

The tiny air bubbles trapped under the surface of the glaze are gases. They can occur for many reasons, but most likely, an over application of clear glaze or an under firing of the glaze. If you do not allow the organic materials either remaining in the bisque or in the glazes to escape, it can result in tiny bubbles trapped in the glaze.

If you see more bubbles in the bottom of a mug for example, chances are this is an over application, because that is where clear glaze tends to "puddle." The remedy is to make sure your clear glaze is the proper viscosity and your dipping technique is good. For an immediate solution, you can try to fire it again but you will get varying degrees of success depending on the cause. Make sure the glaze is properly mixed.

Speak to our team

For small technical problems, many issues can be resolved quickly with a phone call or email. Please call our Technical Team on 01782 344253 (Mon–Fri) or email Technical Support .