What is clay and glaze?

Hardwick Glaze Notebook

Bags of ceramic materials, clearly labeled and ready to use.

What is Clay and Glaze and how do we define them?

They are made of the same materials, but are very different in how they look, feel and act.

Together we can call these materials ceramics or the ceramic arts as they are closely related as many of the materials are the same in clay and glaze. Let’s start by studying what makes up ceramics and hope by learning what this encompasses we will better define differences and similarities between clay and glaze.

Ceramic art encompasses the study and exploration of oxides, carbonates and other salts and materials, how they mix together and when combined, how and when they interact. Some individuals are interested in the chemistry aspect of ceramics, while others purchase readymade materials and concentrate on how those clays and glazes interact. No matter how you approach ceramics, the combination of molecules and materials is as important as the form, color and texture with which the materials are subjected. Each artist has a since of how much tooth they want in their clay body, how much they want the glaze to flow and the color of both clay and glaze before and after each is fired in a kiln. These considerations can be bought already mixed and bagged or dug from the earth locally. There is no right or wrong way, each artist figures this out as they create things and their work evolves. Different work has different priorities.

But no matter how you work, how things interact should be considered. If not for your knowledge, then certainly for the clay suppliers’ who have invented and mixed the clay and glazes you use. Thousands of hours have gone into these considerations by industry all over the world and these processes have been going on for thousands of years, since clay was first squeezed between the fingers.

When we think of all the materials out there it can be very confusing and overwhelming the quantity and complexity of their attributes, but ceramics really only encompasses about a third of the elements found on earth. These elements are not found naturally, by themselves, but connected and combined with other elements such as hydrogen, oxygen, silica, alumina, carbon monoxide and carbon dioxide to name a few. These combinations form molecules and these molecules are combined together to create the ceramic materials we dig and buy.

Oxides are molecules that contain oxygen. Carbonates have carbon monoxide and carbon dioxide in them which bubbles out at different temperatures when fired, usually at higher temperatures in the firing. As clay and glaze are raised in temperature they expand, contract, combine together and break apart molecularly. These changes in firing create the surfaces, colors and strength that we want from the materials we use.

Ceramic materials, clearly labeled and ready to use.

As indicated before, knowing the different aspects of the materials that make up our clay bodies and glazes is not integral to working with clay and glaze, but the more one understands, the easier it is to correct or change clay and glaze, and solve problems and difficulties that develop while working.

Each material and the oxides they possess have a different melting temperature. A large part of understanding how oxides work in the clay and glaze one is working with, is how these materials melt, and once melted, how they cool. Some materials such as silica and alumina have very high melting temperatures by themselves. Put a ¼ teaspoon in a cone 10/11 firing and you will notice that neither of these materials melt by themselves. It is by combining materials that one lowers the melting point through a process called eutectics. Each additional material mixed together (in the correct proportion) will lower the melting point of the mix and change the look and feel of the clay body or glaze. It is through the study of not just materials and oxides, but how they combine that one finds an understanding of ceramic chemistry.

What is Clay and glaze and how are they different?

Clay and Glaze are made of the same materials- alumina, silica and a flux. Other materials can also be added to the mix, but these are the base materials. Generally speaking, the more materials that are added, whether they are individual oxides or complex materials made up of multiple oxides, the lower the materials melting point or eutectic.

Alumina and silica form a ratio that allows one to understand the basic dimensions of a clay body or glaze. Some will write this as the A/S ratio or AS ratio. A clay body with high alumina will be open, porous and refractory. A clay body with high silica content can be vitreous, translucent and/or tight. High additions of either alumina or silica can also have negative traits as well, but it is AS ratio that gives a clay body  the refractory, porosity or translucency and surface one sees and touches. This ratio also determines a glazes surface texture, finish and sometimes color. By determining the type of clay body one wants, one finds the characteristics of how a glaze bonded to that clay body will look, feel and react.

As mentioned above, alumina and silica are both refractory materials. These materials melt at very high temperatures by themselves. It is by adding additional material called a flux that these materials are able to melt at lower temperatures. Melting at lower temperatures is necessary due to costs and potentially safety concerns. In antiquity, the kilns would have melted before pure alumina or silica melted, so building eutectics was mandatory. Maybe, it is needed due to the type of glaze already established or vise versa because of the type of clay one has available. Regardless, the amount and type of flux added determines how fast and how low the temperature of the mix will changed. Different fluxes and different proportions of these materials will create a low, medium or high fire material. The reason that we use this terminology is that materials have naturally mixed this way and naturally melt at these temperatures. Low fire clay is dug and fired to the temperature it naturally matures at and the first humans that create ceramics figured this out and then made glazes that naturally melted at the same temperature. Lead, Boron and Sodium Salts were early materials used to create low fire glazes that fit or clung or permeated to the first low fire clay bodies. High fired ceramics happened in the same way, but was more difficult and therefore took longer in human history to create as the brick kilns needed to fire these refractory clays had to withstand the greater firing temperature and only in China was this discovered early on. Medium range or Mid-range ceramics is more of a creation where people have purposely lowered the high fire ceramic temperatures to save money and energy. You could say it is the study of high fired ceramics with additional flux to lower the temperature.

Pulling up the sides of an earthenware cup

Clay– is a combination of silica, alumina, fluxes and fillers refractory enough to hold a desired form.

Silica is the most common element on earth besides oxygen. It is rarely found alone, most usually combined with other elements such as oxygen in silicon dioxide. Alumina is the third most abundant element in the earth’s crust behind silica and oxygen. Alumina is a main ingredient found in most naturally occurring clays and in most feldspar. Feldspar is a flux that is used in most high fired clays. Fillers are added or are naturally occurring in most clay to help hold them together, add color or other surface specifications. One product of fillers is to create the flexibility of a clay body to bend without breaking. This is an essential characteristic for throwing, stretching and bending clay bodies.

How these materials are combined establishes how the clay body reacts to heat and cooling. High percentages of Alumina and/or Silica and little flux will make a clay body melt at high temperatures and when mature (the peak temperature to make these ingredients melt together), strong and vitreous. This same clay body fired at too low a temperature can be brittle and non-vitreous (allowing liquid and salts to pass through it). A clay body with too little clay and too much Silica and Flux can melt at an early temperature, but may be impractical to work with as it may not be flexible enough to create the desired form, much like building with sand or paste. It is combining these materials in a mix that is able to be formed and worked and then fired to the desired temperature that enabled early humans to create ceramic forms that lasted to present day. Luckily, clay can be found readily in nature that is able to work with the hands into the desired form and ready to fire to the desired temperature. That temperature may have been established by bricks being made of the same material, which would also melt if fired at too hot a temperature.

Clay is found naturally in the earth, ready to use as was seen fit, but how was it first utilized and why? We will never know for certain,

An answer is it was used to fulfill functional and decorative needs, early on without fire and heat to hold it together; It was seen to hold water in puddles and ponds and animals used it to protect against insect bites, perhaps inspiration for survival needs. Archeologists suggest it was used as body paint as well as wall paint, possibly to help with or explain fertility, ritual and origin beliefs.

The first sculptures and pots created did not have glaze on them, such as covering baskets and grass matts to hold water. It is thought that humanities first interaction with clay and oxide materials was to cover the body with them, but it is seen that cave walls were painted with them using the hands and fingers as stencils. Sculptures and figurines are the next clay items created, followed by pottery. We should say this is what archeology shows us happened in the past, but what really happened is up to ones imagination as humans may have made ceramic items for years without firing them, these creations disintegrating back to the earth. Look to adobe bricks and other clay wall treatments which are still in use today.

Some of the first fired vessels were created to utilize the porosity of the clay body as much as to hold and transport liquids. The evaporation produced when liquid moved through the containers porous walls when filled with water cooled the pottery’s contents just as today this porosity allows flower pots to drain or water saturated wine sleeves to chill a bottle of Chablis. Vessels were also buried in the ground that could be filled with water to slowly water plants buried nearby. Bricks were made by the millions- fired and unfired, in many different cultures.

There is no telling how the idea for pottery was created, but some say the earliest pots were woven baskets where clay was used to waterproof the grasses and possibly placed in a fire for cooking or heating, where it was found that the clay hardened when the temperature rose to the extremes necessary.

Traditionally, clay is used to create form. Pottery, Vessels and Sculpture all are formed with different mixtures of materials aiming to create a desired effect. Clay body types used in cultures all over the world to make things can be delineated into 3 different types, but there are combinations of materials that straddle these definitions, so there are really no restrictions today in creating a clay body. In the west, clay can be broken down into 3 different categories: earthenware, stoneware and porcelain. In addition there are refractory clays and montmorillonite clays which have properties all their own.

Earthenware plate with underglaze painting and clear glaze on top

Earthenware melts at a lower temperature than stoneware and porcelain. It is called a low fire clay for this reason. High fire and low fire terminology came about as the different clays available in the earth naturally melt at a high or low temperature. Earthenware is usually high iron when found naturally occurring in the ground. Found in every culture and every continent, earthenware comes in all colors and fires in a wide variety of colors, but mostly in reds and browns, but yellow, black and white are available in store and ground. The earliest figurines and pots are made from this clay type and most cultures historically only used earthenware clays. Because of the low temperatures, earthenware clays don’t allow color oxides to burn out (dissipate) and produce colors easily. When taken to higher temperatures, earthenware will turn brown and then dark greens, eventually becoming a glaze depending on material mix; deforming, melting and bloating in the process. It is only when earthenware is over fired and melts that it is truly vitreous, so without glaze most earthenware is not waterproof. A great example is the traditional unglazed flower pot.

Hand built stoneware bowl, fired in wood kiln to cone 10 with Chun glaze

Stoneware use more refractory materials that melt at higher temperatures than earthenware. When used at low a temperature it is often brittle, showing a need to fire a clay body at the temperature it requires to mature. The fluxes used in stoneware are different than at lower temperatures and naturally melt above cone 4 (around 2000 degrees F). Today the production of mullite molicules in a clay body defines it as stoneware. This process of creating mullite happens around 1832 degrees F.

Though, earthenware was used earliest historically in all cultures, it was in China where Stoneware and then Porcelain were discovered early on, so in certain places in China the high fired clays were used before low fire clays were used in other cultures. The fact that stoneware was used so early on in a culture is rare in the world as low fire kilns would usually melt trying to fire these high fired bodies. In most places where stoneware clay was first tried, stoneware was fired at low temperatures and when fired at too low a temperature the bodies would feel porous and punky which tended to be an inferior product. This led to most cultures abandoning the use of these clays. In China the bricks used to fire their earthenware pottery were refractory enough to fire to hotter temperatures and it is thought that when the kilns were taken to a higher temperature, whether accidentally or experimentally the bricks stayed strong and allowed the early Chinese ceramic makers to produce stoneware clay bodies.  It was possibly through these trials glaze was discovered.

Porcelain Chinese Song Dynasty footed bowl in the Freer Gallery, Washington, D.C.

Porcelain is naturally occurring high fired clay that is white and vitreous when fired.  Very similar to stoneware, it is lower in iron and usually titania than stoneware and usually higher in silica. Many industries all over the earth have defined porcelain differently (as their own) as it was once a high commodity, but most agree that porcelains must be translucent and fire to a higher temperature than both earthenware and stoneware. Porcelain was invented in China and the many types of European porcelain were created to imitate this material. French missionaries and explorers brought back secret recipes and material data that helped with the quest to unravel the mystery of porcelain. It took hundreds of years before someone in Germany figured it out and then the other types around Europe were invented. Most of the later porcelain bodies were created when kaolin was discovered in France.

Today the material boundaries of clay are being tested. Different materials are being mixed together and accepted as clay even though they may or may not melt, crumble, crack or hold together as a clay body in the past was understood to behave. It is through these experiments with clay bodies that we learn more of the material and how it behaves in heating and cooling.

Refractory clays melt very high temperature and are used to create kiln bricks and furniture to hold up, encase or contain the ceramic items in the firing. Very much like stoneware in nature, they have a great deal more alumina in them which make them melt at much higher temperatures. No culture was able to make high fired ceramics until they discovered refractory clays and how to use them to make kilns. The Chinese had refractory clays readily available and utilized them early on in their ceramics traditions.

Bentonite is a type of montmorillonite clay which are extremely plastic clays added to clay bodies and glazes when only a few percent of clay is desired in the formulation. The placticity of clay is needed to hold the desired form together. It is through plasticity that tall ceramic forms and long ceramics forms hold up to their own weight during production, drying and firing. Though bentonite is one way of adding plasticity to a clay body another way is by adding fillers, natural and synthetic. Montmorillonite clays are created by the breakdown of igneous rocks or volcanic rocks and are thixotrophic in nature. Some say they are part of the early building blocks of life on earth.

Test glazes mixed in 500gr batches to test for glaze characteristics.

Glaze– is a combination of silica, alumina, fluxes and fillers with enough flux to allow the material to melt or not melt at a desired temperature and characteristics. When looking at these materials that make up a glaze one can see that they are the same as what makes up clay. What makes the two different?  Traditionally, glaze is used to cover a clay form. Though, it is made of the same materials as the clay body it may cover, with more glass and flux or different glass and/or flux, it is made to function as a crystalline liquid coating. Thus, creating the desired surface with translucency and melting point that will bond with a clay body manufactured to fire at the same temperature.

How was glaze first formulated? How was it created and why? There are many hypotheses. One is obvious if anyone builds a hot fire on a beach. It is known that glass production on the earth came before the ceramic crafts, and if one digs through a fire pit on a beach one can often find bits of glass made from melted sand. Maybe, this was a building block of utilizing glass on a ceramic form? Sand in many places is mostly silica and silica is a plentiful ingredient in glaze. Silica adds the glass in what is called glaze. Glass containers may have been known to those ceramics makers who first started making glaze. Metal work was also created before glaze was produced in some places, so casting metal in sand may have been a building block. Most ceramic artists agree that wood firing ceramics was most likely the main protagonist in creating glaze. Wood ash itself is a flux and when wood firing a ceramic kiln over and over, a glaze is formed on the inside of the kiln. The wood ash from the burning logs thrown in as full flies around the kiln and is deposited on top of ceramic work being fired. When melted this wood ash also forms a glaze on items being fired. When the kiln is old enough; coated on the inside enough with glaze; that glaze eventually drips onto the ceramic work being fired. Together, the wood fly ash landing on unglazed ceramics and the kiln dripping on ceramics is enough to lead the ceramics maker to try and reproduce the effect on the surface of a clay piece before being placed in the kiln. It is thought that this was done at the beginning by dusting early items with wood ash before being placed in the kiln. Certain early Chinese ceramics show a round layer of wood ash inside the pot as if it had been sprinkled in from above. This does not happen naturally in a wood firing.

One of the earliest glazes created was a result of over-fluxed clay bodies that allowed soluble salts which moved to the clays surface when dried, to melt and become a glaze. Colors were added to these high sodium clay bodies and Egyptian paste was born. Soon after this creation, glaze chemistry advanced and was used to imitate other materials already in use such as glass, metal, wood and stone. It was around this same time that glaze was used to waterproof the low fired early clay bodies which never got hot enough to truly vitrify without glaze. As explained above, most kiln bricks of early societies melted at too low a temperature to get earthenware hot enough to vitrify, so it was not until glaze was discovered, that a ceramic vessel was truly able to hold water without a ring being left below it.

Over thousands of years this technology evolved in Egypt and the Middle East. China was also technically advanced and ideas between the empires were exchanged advancing each of the civilizations’ crafts. Formulas, materials and techniques were exchanged along with forms and design ideas. In some instances ideas went back and forth for multiple generations, getting more and more advanced.

It was though this that glaze evolved, that different clay bodies were tried at different temperatures and along with them different glazes were calculated. As advanced and as amazing as this is, it all comes back to combinations of alumina, silica, flux and fillers. The treatment of these materials and how they were fired and cooled was just as important to satisfy their goals and the goals of their customers. One tradition advanced another and families of ceramists passed knowledge on to the next generation.

How and why do alumina, silica, fluxes and fillers work in glazes?

When creating glaze one does not need to pay attention to flexibility, which is good as alumina is not usually as high a percentage in glaze as it is in clay (when alumina is too high in a glaze problems often accompany it), but alumina is important in glaze for another reason. The glaze needs to stick to the ceramic object and alumina in small percentages accomplishes this. If the clay percentage is too high, the glaze may ball up or peel off the clay body; a problem called crawling. Some glazes do have significant percentages of alumina, but this usually means that a very powerful flux is also present or a combination of materials is there to create a powerful eutectic, which will melt the alumina. Alumina in the glaze makes the glaze stick, but it also arrests the glaze from running or flaking off the clay body. Lots of alumina will cause the glaze to be stiff and not run. Too little alumina and the glaze will run off the pot and onto the kiln furniture. Tests are important when first creating or using an unknown glaze to see how it fits the clay body available and then the correct amount of alumina can be determined in the glaze.

Silica is the glass former in the glaze and is used for this purpose with only a few other materials found on earth. Glass is a liquid in phase, though a very stiff one, but because of this it needs to be melted at the precise temperature needed to fit the clay body upon which it is placed.  Glass and glaze are different in that glaze will crystallize making it a solid, while glass does not crystallize and so, is always a liquid. It is by firing glaze and causing different chemical and dynamic reactions that different effects are seen in mixing just a few materials together, thus causing silica to be seen or unseen in different ways. Silica by itself melts at an extremely high temperature, but by adding it in different percentages and through heating and cooling it at different rates many characteristics can be created. One of the main positive aspects of silica in a glaze is by adding it crazing (when the glaze does not fit the clay body and cracks on the glaze surface can be seen) can be lessened or stopped. Silica is the same as all materials in a glaze, that by adding more or less certain characteristics can be produced, but others will also be created or lessened. Adding or subtracting materials in a glaze is a give and take process.

Flux is added to cause the alumina and silica to melt at the desired temperature and create the required surface characteristics. I phrased this as such, because the same 3 materials can be combined in different percentages to create very different surface characteristics. This is because different percentages of the same materials can cause multiple eutectics which will melt and mature at different temperatures causing these different effects, but for simplicity, it is good to know that a flux is added to create a melt at a desired time and temperature. Just like in the case of alumina, there are many materials where flux can be found and added to a glaze. Most rocks and clays have flux already in them, but there are only a few that are continuously used in ceramics today and traditionally. Soda, potassium, lithium and calcium carbonate are the main ones found combined in multiple ways in the earths crust. Soda can be found in salts and feldspars. Potassium the same, but is also found in micas and volcanic stone. Lithium is also found in micaceous rock and feldspars and calcium carbonate or broken down sea shells are often found alone or combined in simple formations that are easily separated though heat and chemical processes. Another abundant source is wood ash used in ancient glazes.

The fillers in glazes add to eutectics and add color, finish or texture to the glazes. Glazes do not need fillers to form, but the fillers can add a desired characteristic to a glaze and are often necessary to satisfy an artistic need and less so, a utilitarian need. Fillers can be a glue that is used to help stick a low alumina glaze to a clay surface or a color oxide added to determine shade, strength or texture of a color (clear, speckled or streaked). Fillers can be added to keep a glaze suspended in the bucket, so they don’t get too hard and cause you to use a knife to mix them up each time you need them. Fillers can also be used to add a specific ingredient (alumina, silica or a flux) that cannot be found in the normal or typical materials available. Primarily, these materials are used to tweek or just slightly change the characteristics of a glaze.

Today glaze is not just a surface treatment, so ones perception of a traditional glaze treatment has been advanced to a contemporary vision. Some artists are using glaze as the body of their forms and the clay as a surface treatment. This is accomplished by altering the different amounts of alumina, silica and flux. By making a glaze that flows slowly at a desired temperature and stops when needed, glaze can be altered to create form. Another way has been through the use of molds, where the mold is removed and the finished product is ground and smoothed to show off its characteristics. One could say it is being treated in a similar way to how glass is treated to make form. Now it is up to the individual to determine what is glaze and how to utilize it in their search.

Glaze can be separated in many different classifications, but let’s look a few general areas:

Amber glaze engulfs the entire form, fired on seas shells
Star Tray 2016 Wood/Salt Fired 5x5x2″

Glossy glazes have a shiny reflective surface that is vitreous and contains water and other liquids. Used in functional ceramics as they are usually hard, utilitarian, vitreous and beautiful, they can also be used for any approach in low, medium or high fired ceramics. Because they were traditionally used for utilitarian purposes they confer certain aesthetic characteristics when used on certain forms, which can contrast or take away from a sculptural perspective, but on the same note can also be dualistic in this same pursuit. The reflectivity of a glaze can be on an opaque or translucent glaze depending on the materials used and the temperature fired at, but by altering this temperature a glaze that is glossy at cone 10 may become matte at cone 8. Also, cooling can affect the finish of a glaze as a glaze that is cooled slowly can cause surface crystallization that can cover the glossy glaze and produce a matte surface. Many different conditions of mixing, firing and cooling must be satisfied to develop the surface one desires. The can be a fine line between one surface and another.

Strontium matte glaze with iron oxide drawings

Matte glazes can be produced by high amounts of alumina or silica or a low amount of flux. Glossy glazes can be underfired or cooled slowly to produce matte glazes. Combinations of refractory materials with few eutectics may cause a matte glaze. As mentioned before, crystal growth can cause a matte glaze. A true matte glaze can remain matte and run off the pot by being fired too hot. A glaze that is not a true matte glaze will turn glossy when fired hot, meaning it is really a glossy glaze that was underfired to create the matte surface. This is not a wrong process to follow, it just shows the type of glaze that one is considering and how it changes as it is heated and cooled. Knowledge of how a glaze will move as it goes through the heating and cooling process allows one to alter it in later firings to change its characteristics or fix troubling issues. High barium and lithium can cause matte glazes. Calcium carbonate at specific temperatures and quantities can cause a glaze to be matte with high and low percentages, but somewhere in the middle after around 1400 degrees calcium carbonate glazes are glossy.

Glazes can also change color based on how they are mixed together. Different materials produce different colors and the percentages of alumina, silica and flux and filler can also alter the color of a glaze. Low iron materials will generally produce a whitish color. High iron will produce a dark color if not a black/brown. Each oxide that is added will produce a range of colors based on the atmosphere of the firing. If a kiln is fired in oxidation there will be one oxide range that will be different if the kiln is fired in reduction. The timing of when a kiln is placed in a reduction atmosphere will also determine the color of a glaze as will the cooling of a reduced glaze. Stains added to a glaze usually only determine one color and the amount of stain will account for how deep a color will be seen. The correct amounts of alumina and silica in a glaze can produce green or blue colors in an optical illusion called liquid-liquid phase separation in a reduction atmosphere. Glazes fluxed with magnesium can turn pink in either atmosphere as glazes with lithium tend to turn yellow, tan or brown in both.

There are many books and internet articles on clay glazes that can lead one to understanding how these mixtures combine in heat and cooling, in both oxidation and reduction. There are guides to how to mix glazes and create recipes and chat rooms on how to fix problems and understand all the information in the books and internet articles. Below are a few that are successful:

Glazy; https://glazy.org/

Ceramic Arts Network Community Forum; https://community.ceramicartsdaily.org/forum/23-clay-and-glaze-chemistry/

Digitalfire; https://digitalfire.com/glossary/glaze+chemistry

material cabinet
Glaze materials kept in a cabinet, clearly labeled and ready to use.

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