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Frequently Asked Questions

What Cone has your piece been fired to? What Cone do I fire my glaze? 

All of our bisque has been fired to 1940 degrees or Cone 04. Most Glaze will fire to cone 06. Be sure to read the label on your glaze to know the correct firing temperature. We do not recommend firing to higher temperature than 1940 degrees unless the piece is stoneware. 

Does Ceramic Planters need to be glazed? 

No, but glazed versus unglazed and porous vs vitrified (which has to do with both the clay body and the firing temps) will provide different environments for the plants housed within. Fully vitrified and glazed pots will tend to behave a lot more like plastic pots and will also be more cold weather resistant; unglazed and highly porous pots will act more like bare terracotta (which is low-fire/porous and unglazed pottery). I make and use a lot of terracotta style stuff specifically because the more porous pot wicks water away from roots and evaporates it at the pot surface, which provides quicker drying and some measure of evaporative cooling in the summer. 

Crazing: 

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. 

What are pinholes and craters? 

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 color or clear glaze application. By making sure the application is appropriate, you can also help eliminate these issues.  

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, what causes it and what can I do to limit it in the future? 

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 colors or bisque you use. 

What are the potential causes of shivering? The following is a list of key potential causes: 

Several things can potentially cause 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 color application.

  • Not allowing the colors or clear glazes to dry enough before firing. When heated, moisture from the color 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 color 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 color 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.  

Running Glazes: 

When this occurs it will likely be a darker color like black, cobalt blue or dark green.

The most common causes are:

  • Thick application of color.

  • 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 colors do this too but they are often not noticeable to the naked eye because the pigment is not as strong.

Hazy or milky colors: 

The main causes of hazy or milky colors, 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 Glaze Surface: 

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 a non-toxic glaze and a toxic glaze together:

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 together to get different colors:

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: 

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

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:

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 plates and platters split or cracking during the firing:

  • 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 on the bisque after the clear glaze firing:

These 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.

Cause and the cure for the tiny air bubbles trapped under the surface of the clear glaze:

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.

 

What’s the difference between food-safe and dinnerware-safe glazes? Food-safe glazes are non-toxic and safe for contact with food, while dinnerware-safe glazes also meet these criteria but are durable enough to withstand daily use.

When should each type of glaze be used? Use food-safe glazes for items that will hold food but won’t face heavy use. Dinnerware-safe glazes are best for everyday items like plates and cups.

What to look for when shopping for a glaze? Look for labels that specify “food-safe” or “dinnerware-safe,” and check for durability and resistance to acidity.

Which is more durable? Dinnerware-safe glazes, as they’re designed to withstand daily wear and tear.

Are pieces with these glazes microwave and dishwasher safe? Generally, yes, but always check the manufacturer’s recommendations. Some glazes may be susceptible to slight dulling of the vibrancy after repeated dishwasher use due to having a lower resistance to acidity. Other factors such as the type of clay you use and the firing temperature may also impact the microwave and dishwasher safety of a piece.

Are pieces with dinnerware glazes oven safe? It depends on the clay body and firing temperature, in addition to the glaze used. Check for “oven-safe” labels and manufacturer recommendations.