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Pottery and Ceramics Tips and TechniquesTroubleshooting Bisque, Glaze and FiringAbout the best way to make sure yourself of the quality of the bisque, |
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BISQUE How to tell if a piece of bisque has been fired properly. It is very hard for the studio to do. They can do absorption tests and test for heat expansion but these tests can be very hard to do and don't give definite answers. About the best way to make sure yourself of the quality of the bisque is to use well known bisque suppliers with a history of good quality products. 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 are actively testing all of the ceramic and pottery's leading color lines. Work with those who give you good reliable quality products and service. There are many very good suppliers out there who are willing to work with you on this matter. How to tell if the bisque has a small crack that may not be visibleDo 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. Warped bisqueIncorrectly removing ceramics from the mold can cause the piece to warp. Ceramics should not be removed from the mold until it is leather hard and releases easily.This is extremely important in porcelain casting. Porcelain has a memory and cannot be reshaped. OverfiringPlacing the ceramics too close to an element can cause one section of the ceramic to mature prematurely and warp.Keep the ceramic about one inch away from the kiln side walls and kiln sitter and thermocouple if one is installed, to insure proper ventilation. SupportImproper support on stress areas could cause warping.Large flat pieces should be supported with either stilts or posts. Porcelain should be supported with prop or silica sand. Repair warped bisqueIt is impossible to salvage warped bisque.GLAZESCrazing  Crazing is a bunch of fine hairline cracks that appear all over 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, months or years later, it can show up and it is caused by moisture getting into the pottery. Causes of CrazingUnder Fired BisqueOpening the kiln too soon before it has cooled properly.Do not open the kiln until it is below 150 degrees Fahrenheit or is cool to the touch. Incompatibility Between Bisque and GlazeMoisture can get into the piece through stilt marks and the water expansion contraction caused by heating and cooling of the pottery, repeated microwave or dishwasher use can cause this over time.Pinholes and Craters  Pinholes and craters are two of the most common glaze surface defects. After firing, there may be small holes on the surface of the pottery, which are called pinholes or there may be larger dish shaped like impressions that look like the surface of the moon, which are called craters. While there are many things that can cause pinholes and craters, the causes are both 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 glazes contains clays and other minerals mined from the earth. These contain some plant matter, carbon and more. These materials form gases during the firing process and can result in pinholes and craters. CausesFast firing or under firing of the bisque.This does not give the pottery enough time for the foreign materials to burn out. Fast firing or under firing of the glazes. This 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 figure out, without a lot of help from the shop concerning the bisque, glazes, firing schedule and cones that were used and where the pieces were in the kiln. The following two paragraphs show what can be done to reduce or eliminate pinholes and craters. There have been far greater appearances of pinholes and craters 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. They can be reduced by maintaining a proper firing schedule. There is also a much greater incidence of these glaze surface defects with a heavy color or clear glaze application. If you make sure the application is correct, it will certainly help stop pinholes and craters. Pinholes and Craters RepairHere are some suggestions if you want to try to fix a piece that has either pinholes or craters.If the cause is the glaze firing, either under fired or fast fired, you may want to re-fire to the correct time and temperature. If the problem may have happened 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 correct Cone 06 schedule. This doesn't always work, but it is the best choice that you have. If the glaze has been put on too heavy, a re-firing will probably not work. Heavy Application of GlazeSmall spots where the clear glaze seems to crawl away from the bisque and bead up, especially on the inside bottoms of mugs and vases, because it is a likely spot for the clear glaze to pool after dipping.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 the piece. The best remedy is to make sure that the bisque is properly cleaned with alcohol and a damp sponge prior to painting including the inside bottom of the pieces. Shivering or Popping Off When bisque is fired in a glaze firing, the bisque will usually expand during the firing and contract when cooling. Shivering is usually defined as happening when the bisque and glaze shrink at different rates. If the bisque shrinks more than the glaze, the glaze has nowhere to go but to shiver 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 bunch of fine cracks all over in the glaze. All pottery or ceramic workers know that shivering happens in almost all shops at one time or another, no matter whose colors or bisque is use. Most of the glaze manufactures goal is to help educate shop owners and managers to the causes so the affects of shivering can be limited. CausesUnder or Over FireUnder 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, because clay is mined from the earth and is an inaccurate science. Sometimes there can be issues with the clay that cannot be detected until you do a glaze firing. This doesn't happen a lot, but does happen because bisque suppliers are always getting their clay from many different suppliers in different parts of the world. This means there are more types of clay being introduced into the market place all the time. Make sure you are using a reputable bisque supplier that has a history of consistently good quality. Even bisque suppliers that have a good history on quality may have over or under fired bisque, as they get more different clays and can deliver bisque that shivers. Wet Glazes and Hard SpotsNot letting the colors or clear glazes dry enough before firing can cause shivering.When heated, moisture from the color can turn to steam and cause a poor fit between the glaze and the bisque. I feel this is a much bigger problem in the busy season when both customers and shops are rushed for time. It is thought that using hairdryers on a hot setting can increase the potential for shivering or 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 stick. Oils, dust and soluble salts on the bisque can cause shivering also because they do not permit the glaze to bond properly with the bisque. As you can see from the list of potential causes, it can be quite difficult to find the cause of the problem. To help find almost any ceramic problem, it is best to keep a thorough log of problems which includes such things as the type of bisque, color used, application of color, firing history and where on the piece the shivering occurred. 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. Avoid Shivering and Popping OffUse bisque suppliers with a history of good qualityMake sure that the application of color is not too heavy Don't rush the ceramic process by either firing too fast, using hairdryers on a hot setting, or firing potentially wet color or glaze. Make sure customers hands are clean and the bisque is wiped down with a dampened alcohol sponge before painting so any dust or oils from lotions or food have been cleaned from the surface. Glaze and Under GlazeWithout testing each and every manufacturer's products with each possible combination, it is impossible to guarantee results.Most colors, if not all, should be compatible with each other, but the best way to assure a good outcome is to do some test tiles. Running Glazes When this occurs it will likely be the darker colors like black, cobalt blue or dark green. 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 part 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 ColorsThe main causes of hazy or milky colors, which are tiny bubbles trapped in the clear glaze, are a heavy glaze application or a glaze firing which is not hot enough. |
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