Pottery Glazes

Pottery Glazes coat, color, preserve, or otherwise leave the Artist's signature finish on a ceramic piece.

In general, Pottery Glazes are made up of 5 basic components:

• Silica - when heated above 3100 degrees F (by itself), it melts and forms glass. Flint is often used in glazes for the Silica component.
• Alumina - this component allows the glaze to stick to the clay and not run off when the glaze is heated.
• Flux - helps the melting process of the glass (Flint) component. Bone Ash is an example of a flux used in some pottery glazes. There are a variety of other fluxes (iron, zinc, sodium, and others) that are used depending on the temperature range firing will take place.
• Colorants - Most colorants are metallic oxides e.g. iron oxide, chromium oxide. Talc is often used as a whitening agent.
• Modifiers - are used to effect the pottery glaze's surface affects such as opacity (transparency). Some examples of modifiers are:
• Opalescence - reflecting an iridescent light (Think of how an Opal stone looks). Often titanium dioxide or bone ash are used as modifiers for this affect.
• Bentonite - helps to hold the components into suspension
• Opacifiers - because glazes end up generally transparent, opacifiers are added to give a white or opaque background. Common opacifiers used include: zirconium silicate, tin oxide, and bone ash.
• Suspenders - are used to keep the heavier components of the glaze from settling out. Bentonite is commonly used as a glaze suspender.
• Gums - are used to toughen the glaze and help to protect its finish before going into the kiln for firing. Gum arabic and sodium carboxy methyl cellulose (CMC) are commonly used for this task.

Pottery Glazes come in a vast range of colors. Common ingredients used as colorants in pottery glazes include: copper oxide, copper carbonate, chromium oxide, cobalt oxide, and iron oxide. High quality handmade pottery artists spend years and years experimenting with all the variables to find that unique combination that results in a perfect, beautiful, interesting, and eye catching piece.

The end color result on the pottery is also determined by factors such as the amount of oxygen in the kiln at the time of firing. Also affecting the end color result are the base color of the clay as well as any previously applied layers of pottery glaze (under glaze), as these multi layerings will mix and create differing colors.

If all the above looks familiar, it’s probably because it is in our pottery section. But it is important that you have an understanding of how different pottery glazes are made and used so that when you are looking at a piece of high quality pottery, you know what to look for.

Application of Pottery Glazes

There are 5 primary methods used:

• Dip Glazing In this method the pottery is immersed in the glaze for about two to four seconds. The glaze usually is the consistency of heavy cream. This is the method often used with large quantities of pottery to glaze. It’s a much faster method used. And it will give an even glaze coat. If this method is chosen, the bottoms typically get a good coating of wax resist so that the glaze won’t stick there.
• Pouring The Artist typically wants the same consistency as dipping glazes that being like heavy cream. To glaze interiors, glaze can be poured into a pot; the excess can be poured into another pot. And so on. Pottery glazes can also be poured over the outside of a pot. Usually done when a thinner coat of pottery glaze is applied as a top glaze to an underglaze. The two glazes interact, often resulting in a greater visual depth. This short video will give you an idea of how it’s done.
  
  
• Brushes and Sponges
• Brushes A lot of pottery glazes are made to work well when applied with a brush. These glazes are much thicker than dipping and pouring glazes. Some are as thick as pudding. Glazes that are made for this application are made to blend well during application and will not show brush marks. Synthetic sables work well for your brush’s bristles. They will come back after cleaning and give a nice even stroke. You will want to investigate brushes more in depth, as the selection of brushes is vast. It is important as in any trade to use the right tool for the job at hand.
• Sponges The type of the sponge and texture will translate into interesting patterns in the glaze. Sponges are used in many steps of pottery creation, including glaze application. Use a fine-textured potter’s sponge, either synthetic or natural to apply the under glaze. For applying the decorative second layer of contrasting glaze, you may want to use a more porous sponge. This will cause a different look and an artistic moment of you’re own ability could show up here! There is quite a selection of sponges, also, so you will be able to experiment.
• Air Brush I would suggest you master the basics first, and then turn your creativeness loose with an airbrush. With this method remember, safety first! A respirator, gloves, glasses, clean workspace, and a thought out system. A spray booth would be a very good idea. Don’t forget, some of the glaze bases are very caustic. Volcanic ash pottery glazes are particularly dangerous, but the look of them are worth the extra work and care. Borax, Nickel Cadmium, Chrome, Cobalt, Copper, Iron, Ferrous Sulfite, Lead, and Manganese are some of the chemicals in the glazes. Check out the safety warnings of these ingredients before using them. Some of the glazes are totally non-toxic but I still would not want it in my lungs, eyes or skin. If you are using an airbrush, don’t forget to check the size of nozzle to use as there are particles in glaze and they need to fit through the gun. Most likely you will need something with larger ports to start with.

A Little History of Pottery Glazes

China developed downdraft kilns in the Shang period (around 1751-1111 BC). These kilns trapped more heat and produced higher temperatures. In part, these kilns lead to the discovery that wood ash, when heated high enough, melts into the glaze by itself.

During this time, potters mixed ash with lime or earth to create glazes. These were a greenish yellow with spots of dark green. Mixing equal parts of wood ash, feldspar, and clay can make a beautiful glaze. (Testing is required.) We were told by one of our potter friends that this ancient method is still used today - including by her.

Our cousin the potter has been at this for about 45 years and still gets excited when experimenting with a new formula for a new glaze. He makes his own glaze from volcanic ash. Depending on the volcano he gets his ash from will determine the color of the work. In other words, you can pretty much tell what volcano the ash came from by the color of the finished work.

Glazes are described as raw or fritted. Raw glazes are combinations of natural and synthetic materials such as feldspars, clays, quartz, carbonates and oxides to produce the final glaze. Fritted glazes contain pre-melted glass or frit. This is used when natural materials won’t do the job.

A Few Types of Pottery Glazes

Celadon Pottery

Celadon glaze refers to transparent glazes; many with pronounced cracks in the glaze. Used on porcelains or stoneware bodies mostly. Celadon is used so much that the work as a whole is referred to as Celadon pottery.

Celadon glaze colors are generally white, gray, yellow-blue and blue and of course jade green. It can be from light green to dark green it depends on the thickness of the applied glaze, the type of clay, and the way in which the glaze was batched. The color is produced from the iron oxide in the glaze or clay.Celadons are mostly fired in a reducing heat. This starves the piece of oxygen and that is what makes the color what it is.

Crystalline glaze

This glaze requires a lot of time and effort; and some science to. The crystals grow on the cooling down cycle and can take 12 hours to grow. Needless to say, work done with this method will be one of a kind.

The glaze crystals are literally three dimensional, much like holograms. They develop auroras, growth rings, halos, and starbursts through changing conditions in the kiln. Chance plays a great part in crystalline-glazed works. The artists of this work don’t always know what they will have until the work is removed from the kiln. This pottery art form is a must see. Pictures do not do it justice. You will know what I mean when you see it!!!

Swatow ware

This is one of the ancient glaze techniques. Swatow ware is a little coarse, and often under fired. Most are in blue and white. There are occasionally iron red and green over glaze decorations. This work made its début at the end of the Ming dynasty in China, around 1500 AD. Swatow is beautiful in its simplicity and the scenes that were created on it.

Shino glaze

This is basically a family of glaze. In the 1500s it was generally white. For decoration the potter would allow, “crawling”. Crawling is a defect that is a crack that travels in the glaze only. In modern times Americanized Shino glaze has alumina and silica added to it. This causes dark spots and short streaks.

Salt Glaze

Salt glaze is created by adding salt (sodium chloride) into the chamber of a hot kiln. A typical salt glaze piece has a glassine finish, usually with a glossy and slightly orange-peel texture, bringing out the color beneath it.

This work is not for the novice!!! It can be outright dangerous. When you heat sodium chloride to these temperatures it has a chemical change and the gas that is created turns into hydrochloric acid and the gas that escapes from the kiln is poisonous!!! Back in the day when a potter had a head cold, they would take a whiff of the exhaust to clear their sinuses. Don’t you do that. I wonder how many of them backed away with out a face?

The unique characteristics of salt glazing were discovered in Germany around the 14th century. Cobalt oxide was used to produce the blues. Americans began producing salt-glazed stoneware circa 1720 in Philadelphia, Pennsylvania and Yorktown, Virginia and became the main houseware of nineteenth century America. Potters in both North and South Carolina are well known for salt-fired production. These independent craftsmen produce house ware in both traditional and personalized forms.

Raku

Raku is a process in which work is removed from the kiln when it is at a bright red state then it is placed in a container of combustible material, like newspaper, wood chips, or other. This "reduction" or smoking phase, causes blackening and crazing (a network of fine cracks) in the glaze surface.

Defined as enjoyment and ease, the word was taken from Jurakudai, and that was the name of a palace in Kyoto (1537–1598) Japan. Raku was originally produced for the tea ceremonies. It is too porous to hold liquids but is used to hold dry goods, flowers, teas and just to look at.

It truly is beautiful to look at. This art form is done in very many forms. One of our Featured Artists, Deborah Slahta from Bethlehem, Pennsylvania has experimented for years to find the path she has taken. Her work is artistic, mathematical, colorful and gives a sense of the emotion of a true Artist! I know Deborah and she is one of those artists that never stops thinking about the next piece. It must be perfect or it will end up in the recycle bin.

Raku has a vast number of possible glaze techniques.

Western Raku is typically bisque fired at 900 °C (1,650 °F) and a final firing or glaze fire between 800–1,000 °C (1,472–1,832 °F), which is a cone 6 firing temperature range. Raku is a little unpredictable, especially in a forced reduction. Cracking or even exploding due to thermal shock is a possibility.

Another Featured Artist, Brenda McMahon, read the books did the apprenticeships and then did it her own way. Brenda is involved with many aspects of the ceramic arts, but the one that stands alone is her burnished saggar fired porcelain works.

Burnishing is when the artist polishes the piece prior to firing. Basically, the higher the shine the longer the burnishing time. This is achieved one way, by hand and using those hands for a long time.

Saggars are containers commonly made of specialized clay in which the ceramic form is placed prior to firing in the kiln. Artists will often place combustible materials such as sawdust or other organic materials inside the saggar depending on the effect they are working to achieve. Burnished saggar fired porcelain work has an incredible look about it.

Some others materials used in Raku are leaves, horse hair, corn husks, rope, and a whole lot of other secret ingredients. Raku pottery is without a doubt one of the most beautiful clay arts available.

The top potters have spent years searching for the pottery glazes they use and more years perfecting the use of those glazes. Potters use chemistry and science in their work every day. This art is very involved and time consuming. When a buyer of potter Dennis Maza’s work complained about the price, asking how long did it take you to make this bowl?

He answered, 45 years and 20 minutes.

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