Geology Home Page physical geology historical geology planetary gems
Roger Weller, geology instructor
by Angela Lambiase
Whatís in a Glaze?
We see it everyday when we eat our dinner, or when we use our favorite mug. We see it when we are at the store, and when we put our flowers in a vase. Itís true, glaze is on many of the objects we use everyday, but many people donít think about itís presence. I was one of those people, until I took an Introduction to Ceramics class. There I was, learning how to make basic objects like cups and plates that I had previously taken for granted. As intriguing as the clay aspect of ceramics was to me, it was the glazing that really grabbed my imagination. There were so many options! I only learned the very basics, but it was enough to get me interested. So, letís take a look into the world of ceramic glazes.
Pottery and ceramics has played a major part in human history. Even in Neolithic
times, humans were using clay to make vessels to hold food and water, as well as
for sculptural purposes. Many of the very first pieces however, lacked the
colorful pizzazz of their more modern counterparts: glaze.
Around the 8th or 9th century B.C., in China, Egypt, Mesopotamia, and Greece, people started to develop ways to glaze their ceramics to make them more durable and more attractive and unique. These first glazes were usually just ash mixed with sand. Later processes included lead glazing (not very popular now, due to lead being harmful to people), and tin glazing. Today, we use a mixture of glass particles and oxides to glaze our ceramics. Modern technology has allowed for thousands of glaze colors and techniques.
Glazing seems almost magical, especially when your muddy- pink glazed pot comes
out of the kiln as a brilliant blue! So whatís the trick? Some basic geology
and chemistry should answer that, but first letís take a look at what makes a
Glazes are generally comprised of three basic materials: Silica, Alumina , and
Flux. Silica ( such as flint, quartz, and pure silica) acts as the glass
component in the glaze. As the glaze is heated to extreme temperatures, it melts
into glass. The Alumina ( can be kaolin, added clay, or some feldspars, or
alumina hydrate) is added in order to stiffen the glaze as it is heated,
preventing in from sliding off the piece. Finally, Flux ( feldspar, calcium
carbonate, lime) is added so that the glaze will melt at a low enough
temperature so that it can fired in the kiln.
Now about the colorants. The reason that an unfired glaze mixture appears a different color is because of the metal oxides (rust) used for the color. As they are heated, they will undergo chemical reactions that will change their color to the desired shade. The color is also effected by the amount of oxygen present, and by the type of kiln used ( electric vs. gas). For example: Copper oxide (which can be added to a glaze in the form of copper carbonate which will change to copper oxide in the firing process) will produce green and blue hues, but in lower temperatures, it can produce a coppery-red color.
Types of Glazes
Now that we have a basic understanding of how color works in glazing, letís look at different types of glazes.
temperature alkaline glazes: This type of glaze depends on alkalis, such as
sodium, potassium, or lithium as itís flux. These glazes can be fired at a low
temperature. Alkaline glazes were commonly used in the early wares of China,
Egypt, Mesopotamia, and Persia. The general characteristics of this type of
glaze are : Fluid melting and a generally glassy appearance, softness, and
producing brilliant colors when coloring oxides are added to it.
glazes: Lead glazes are fired at low to medium temperatures. They are very
versatile, being easily controlled, colorful, and durable. Discovered in 2500
B.C., this type of glazing allowed practical glazed ware. The characteristics of
lead glaze are: smooth-flowing, producing a even, glassy surface, and a smooth
surface. All these qualities make lead glaze ideal for food use; unfortunately,
there is the danger of lead poisoning.
Glazes: These glazes contain boric oxide. They can be fired at very low
temperatures. When being fired, glazes high in boric oxides have the tendency to
boil as they melt, leaving spotty patterns behind. It may also give an
opalescent-like finish to pieces. Boron glazes are also commonly used in mass
Glazes: These middle-temperature glazes depend on zinc oxide as their flux.
Indicative of itís name, it was developed in England in order to create a
replacement for the lead based glazes. As technology has advanced and found
safe ways to use lead compounds in glaze, and Bristol glaze id difficult to use,
it has mostly fallen out of use.
Porcelain and Stoneware Glazes: These glazes are fired at very high
temperatures. Porcelain glazes are typically made of feldspar and calcium, and
are smooth, clear and colorless. Overall, they have the advantages of being very
hard and durable, resistant to acid, and resistant to decay.
There are as many ways to apply glaze as you can think of, but the three most
common methods are dipping, pouring, and painting on the glaze. Dipping is
sometimes not as popular a method as painting because the areas where the piece
was held when dipped will be bare. This can, however, be touched up later.
Painting on glaze can give you much more control, but you may not achieve the
even coat that pouring and dipping will give.
No matter what method you use, make sure to coat the bottom or feet of the piece
with special ceramic wax to prevent the glaze dripping onto the kiln during
firing and cementing your piece down! As you experiment with glazes, be sure to
record your results, you may find a technique unique to you. Another important
step is to prepare test tiles every time a glaze is mixed, using the same type
of clay you are working with. Not only will glaze appear different on different
surfaces, but because of human error, the color may not come out the same every
time. Itís better to spend the extra time to avoid an unpleasant surprise when
unloading the kiln.
There are many interesting techniques you could try as well, such as Raku firing, in which the piece is removed from the kiln at itís maximum temperature, resulting in crackled glaze!
As you can see, there are nearly endless possibilities in glazing! This is only
the tip of the iceberg, as there are many ways of creating glazes. Many ceramic
artists have developed their own techniques, and there are sure to be many
others yet to be discovered. The only real limit is your imagination. If the
basic glaze formula is followed, any manner of color and technique are possible.
The only way to find out for sure is to experiment and give it a try yourself!
There is a vast amount of information on glazing, the pages and books listed on my works cited list offer information, including recipies for glazes as well as more in-depth calculations and chemistry behind glazing.
Rhodes, Daniel, and Robin Hopper. Clay and Glazes for the Potter. Iola, WI: Krause Publications, 2000. Print.
All photos taken by Angela Lambiase. All photos are of Angela Lambiaseís work.