Glazes are used to create different effects, such as a shiny or matte finish or a multi-colored design. In addition, glaze increases durability and can make your pottery waterproof. But what is glaze made of?
Most Ceramic Glaze Ingredients contain silica or glass formant, a flux or melting agent, a refractory or stiffening agent, and a colorant. The different ingredients in each glaze go through a chemical reaction during firing, resulting in a unique finished product. Clear glazes are the exception, as they do not contain a colorant. The type of glaze used will depend on the desired outcome for the piece.
Whether you plan on making your own glazes or buying commercial brushable or dipping glazes, it’s helpful to have a basic understanding of each ingredient. In this article, we’ll take a closer look at the four main ingredients in pottery glaze recipes and how they work together to create a unique product.
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The Four Main Ingredients of Ceramic Glaze
The main ingredients of glaze are important because they work together to create a unique end product. Each ingredient has a specific property that contributes to the overall reaction. Glaze chemistry can seem complicated. Understanding the basic components can make the process easier. You can predict the outcome of different glazes and mix in your own additives to create the desired effect for brushing, dipping, or spray glazing.
Ceramic Glaze Glass Formers
Silica, also known as Silicon dioxide, is the ingredient that forms the structure for the glass in glazes and is important in sealing the clay.
Different sources of silica can be found naturally in quartz, sandstone, sand, and flint. It can also be chemically manufactured as silica oxide.
Glazes are applied to the surface of ceramics to give them a smooth, glass-like finish. Without silica, glazes would be dull and prone to chipping. However, just as each ingredient serves a different purpose in the glaze, the amount and source of silica can also affect the finished product.
For example, a quartz-based glaze would not be a good glaze to use for cookware, as rapid and frequent temperature changes in such glazes cause cracking.
Though silica can form glass on its own when heated enough, the temperature required to do so is too high for a Ceramic Kiln to reach. 3095 degrees Fahrenheit (1700 degrees Celsius). This is why adding fluxes with silica is necessary to create a lower melting point.
Ceramic Glaze Fluxes (Melting Agents)
The melting agent or flux is the key to lowering the melting point of the silica and transforming it into glass. A wide variety of ingredients can be used as fluxes in a glaze. Some fluxes are highly active, allowing for lower firing temperatures like those used on Earthenware Clay. The less active fluxes are only useful at midrange to high-range temperatures.
Fluxes Used in Ceramic Glaze
Different pottery glaze fluxes serve different purposes for Different Clay Bodies. They are meant to lower the melting point of the glaze, while others also help to prevent defects. Here are a few popular pottery glaze fluxes.
These fluxes include minerals such as nepheline syenite and sodium carbonate, which are used in mid to high fire glazes. Sodium carbonate, known as soda ash, is used in salt firing.
Nepheline syenite is a type of feldspar that contains high levels of soda and some potassium. It has a lower melting temperature than soda feldspar, making it useful for mid-range temperature applications.
Boron significantly lowers the melting temperature of a glaze. Thus it is particularly useful for lower temperature firings. It is mainly used as a low-fire flux.
Borax is often used to smooth out higher firing glazes and in Raku glazes. There are also Borons with frits such as 3134 or 3124.
These are important components in both high and low fire glazes. Magnesium carbonate is a flux for high-fire glazes, increasing glaze adhesion and viscosity.
Talc is used as a flux in both low and high fire glazes. Dolomite is a calcium-magnesium carbonate flux used in the high fire range.
Barium carbonate is a white powder that is used as a flux in ceramic glazing. When added to the glaze, it helps to prevent defects and produce a smooth, even finish.
Barium carbonate can be used to create matte, satin-matte, or stony matte finishes.
Potassium fluxes are more durable than soda-fluxed glazes. Therefore, potassium is preferred for high-fire glazes. Potassium fluxes include feldspars, such as Custer. Cornwall stone, or Cornish stone, contains mainly potassium but also has sodium and calcium.
Volcanic ash is the richest in potassium, but the chemical Volcanic ash composition will vary widely.
They are used to encourage crystal growth in crystalline glazes. Lithium carbonate is the preferred source of lithium for crystal growth. Lithium feldspars, such as spodumene and petalite, are also used as fluxes.
Calcium carbonate, calcium oxide, and lime can be used in high-fire glazes, while dolomite is a calcium-magnesium carbonate that can also be used in high-fire glazes.
Bone ash or calcium phosphate can also be used as a flux in glazes. Wollastonite is a calcium silicate that can be used in both clay bodies and glazes, and it helps promote strength and reduce shrinkage.
Different raw materials will also affect the final product. Different fluxes can also be combined to create custom effects. By experimenting with different combinations, potters can create unique glazes that suit their needs.
Ceramic Glaze Stiffening Agent
The stiffening agent or refractory allows the glaze to adhere to the Clay Bodie’s surface as it is fired and strengthens the glaze as it is heated, preventing damage from heat, pressure, and chemicals.
Alumina (Aluminum Oxide)
The most common stiffening agent used in ceramic glazes is alumina, formally called Aluminum oxide. Alumina can be added as a clay in forms such as kaolin or ball clay.
Alumina has a very high melting point and withstands both hot and cold temperatures. This helps prevent degradation during firing and prevents long-term damage afterward. However, as a result, the more alumina added to a glaze, the higher the firing temperature is required. Alumina also helps to disperse gas bubbles that can form during the firing process, preventing the glaze from weakening.
EPK (Edgar Plastic Kaolin )
EPK is one of the most popular kaolins used in glazes for its suspending capability.
Kaolins are in glaze recipes to keep glaze particles from settling out. The chemistry of the glaze particles and their interaction with the water is responsible for this behavior. Kaolins are hydrophilic, meaning they attract and hold water molecules on their surfaces. This forces the particles apart and prevents them from clumping together or settling out of suspension.
EPK can be helpful in preventing this from happening. EPK is also less likely to produce haze or bloom on the surface of the glaze. Try EPK in your next glaze recipe and see how it can improve the quality of your finished product.
Ceramic Glaze Colorants
As silica is clear and colorless once melted, additional mix-ins are required to create color and other effects. The final color and appearance of a glaze are influenced by everything from the glaze composition, clay type, and firing temperature and can be hard to predict. A bit of math and chemistry are required to correctly calculate firing temperatures. That being said, let’s look at some of the ingredients used to beautify the glaze.
Metal Oxides and Carbonates
Metal oxides and carbonates can be used to add colors to a glaze, but the fired product bears little to no resemblance to the raw color. As you notice, when using commercial glazes. For example, some raw glazes that are red and, once fired, turn into blue or purple.
Understanding the colors that oxides and carbonates create is key to producing the desired color. Some metal oxides can produce a variety of colors depending on the metal-to-oxygen ratio.
When making your own glazes, you will find there are many coloring oxides and carbonates to choose from. These are a few popular ones:
These oxides fire as red, yellow, brown, maroon, green, or black, though the unfired glazes containing it may appear very similar. In addition, some metals that do not produce colors by themselves can react with other ingredients to produce unique colors and effects.
Red Iron Oxide is also known as hematite. It is a natural red pigment that has been used for centuries to color a variety of materials. When used as a glaze colorant, it produces earthy reds and browns.
It is found in nature as a reddish-brown powder and can also be manufactured synthetically. Red iron oxide is non-toxic and safe to use in ceramics, making it a popular choice for both professional and amateur potters.
This oxide is commonly used as a glaze colorant to produce greens. However, in the presence of zinc, chrome can instead produce browns.
Glazes that contain tin oxide will often blush to pink if chrome is present in the glaze along with the tin. This often results in intense pinks. Therefore, chrome oxide is rather versatile and can be used to achieve various colors depending on the other elements it combines.
Cobalt oxide is a popular choice for glaze colorant because it yields a wide range of blue colors, from pastels to rich midnight blues. Cobalt is also used in black glazes and washes as a decorative medium.
In the presence of most fluxes, cobalt oxide yields blue colors in both oxidation and reduction atmospheres. However, cobalt can become purple, pink, or reddish blue depending on whether it was fired in oxidation (yields purple) or reduction.
This is another form of Cobalt that is used as a glaze colorant. Cobalt carbonate yields similar colors to Cobalt oxide, but Cobalt carbonate is not as stable at high temperatures. Therefore, Cobalt carbonate is usually used in lower temperature firing ranges.
Copper oxide is an active metal that produces a very wide range of colors in glazes. Copper oxide can be used to produce delicate turquoise, red, green, black, yellow, pink, blue, and copper luster. Copper oxide is also a versatile colorant and can be used in many different ways.
Copper oxide can be sprinkled on the surface of the glaze before firing, or it can be added to the glaze batch. Copper oxide can also be applied as a slip or an underglaze. Copper oxide is a powerful colorant and should be used sparingly.
It is a popular choice for glaze colorants because it can produce a wide range of colors, from greens and turquoise to copper reds. In addition, copper-containing glazes fired in reduction atmospheres will often yield ox blood or copper red glazes.
When choosing a copper carbonate for your glaze, it is important to consider the desired color, as well as the firing temperature and atmosphere.
Stains for Ceramic Glaze Colorants
When it comes to adding color to pottery glazes, mason stains are also used. Stains can be added to glaze to create a desired effect, but it’s important to keep in mind that the type of glaze can affect the final color.
To ensure that your stain turns out the way you want it to, it’s always best to make test tiles and test fire the glaze before using the stain. By doing this, you can avoid any unwanted surprises and create beautiful, colorful pieces of pottery.
How the Ingredients in Glazes Work Together to Create a Unique Product
The different ingredients in glazes go through a chemical reaction during firing, resulting in a unique finished product. By understanding how the ingredients work together when making your own or buying Commercial Glazes, potters can create beautiful and unique pieces of pottery.
What Are Ceramic Crystal (Crystalline) Glazes Made Of?
Crystal Glazes can contain components of zinc, titanium, lithium, boron, magnesium, calcium, sodium, and potassium.
These elements encourage the formation of crystals in the final product. Chemical reactions with other ingredients can also serve to further color crystals by combining various colorants with the glaze, resulting in a beautiful and unique finish. The most used colorants are copper, iron, cobalt, rutile, and nickel.
Silver, gold, uranium, and some rare earth metals are less common. These can be used alone or in combination with each other to create a variety of special effects.
What Is Ceramic Shino Glaze Made Of?
Shino glaze is a type of ceramic glaze that is known for its satin-like white color. The glaze is made primarily of feldspar, which is why it has a white color. The characteristic of Shino Glazes is that they can have bright, red-orange, or black spotting. This is a result of carbon trapping during the firing process. Some potters formulate the glaze and time the firings for different colors.
Crawling is another characteristic of the Shino glaze, and it happens when the piece is cooled too fast or when the glaze does not fit properly to the clay body. When crawling happens, it creates small bare spots on the surface of the piece, making the bare clay body visible beneath. High feldspar glazes have a cream to orange color flashing or blushing. Potters today seek to emulate the Shino appearance using a wide range of recipes.
What Is Clear (Transparent) Glaze Made Of?
Clear Glazes are made from a combination of silica, flux, and alumina. Silica is a glass formant, which means it helps to give the glaze a smooth, glossy finish. Flux is a melting agent which helps to make the glaze more malleable and easier to work with.
Alumina is a refractory or stiffening agent, which helps to give the glaze more body and structure. Clear glazes are typically used on Dark-colored Clay bodies, as they allow the natural color of the clay to show through. They are also used on Underglazed Pottery to become glossy, transparent, and waterproof.
What Are Celadon (Translucent) Glazes Made Of?
In order to produce the celadon color, a glaze containing a small amount of iron oxide is fired at a mid to high temperature kiln. In addition, the materials used in the glaze must be refined, as other chemicals can alter the desired color.
Too little iron oxide will result in a blue color, while too much will produce a dark green or black. The presence of other chemicals may also have an effect; for example, titanium dioxide will cause a yellowish tinge. Celadon glazes are mostly translucent but can be transparent or even opaque depending on the addition of colorants.
What Are Solid (Opaque) Glazes Made Of?
In order to make a solid (opaque) glaze, you will need to use an opacifier. Opacifiers are materials that make the glaze more opaque so that less light is able to pass through it. The most common opacifiers used in glazes are titanium dioxide, zirconium silicates, and tin oxide. Each of these materials has different properties that make it more or less suitable for different types of glazes.
For example, titanium dioxide is the best choice for oxidation firings, while zirconium silicates are more stable in reduction and less reactive with some colorants. Tin oxide is also an effective opacifier but can produce a white, opaque, and glossy glaze. Talc is another material that can be used to opacify a low-fire glaze. Talc is a refractory powder that can promote opacity when added to the glaze.
Ceramic Glaze Additives
What to Add to Ceramic Glazes to Make It Brushable?
Adding a gum solution will make your ceramic glaze brushable, giving you a more even application. Gum solutions slow down the drying process, making it easier to brush on the glaze and reducing shrinkage. Gum Arabic and sodium carboxymethylcellulose (CMC Gum) are some of the most commonly-used gums. Adding a Gum Solution to your glaze can improve the quality and appearance of your finished pieces.
Additive, if the Glaze Is Too Thin
If you find that your glaze is too thin, you can add a flocculate like Epsom salts to thicken it up. Simply dissolve the desired amount of flocculant in warm water and then mix it into the glaze. This will help increase the glaze’s viscosity and make it easier to apply to your project. Remember that you may need to adjust the recipe slightly if you use a different type of flocculant.
Additive if the Glaze Is Too Thick
If your glaze is too thick to work with, there are a few things you can do to thin it out. The most obvious solution is to add water, but be careful not to add too much, or you’ll change the composition of your glaze.
Another option is to add a deflocculant, which is a substance that helps reduce the viscosity of liquids. Just add a small amount at a time until you get the desired consistency. Glaze thinners like Darvan 7 are also available and can be added to achieve the desired thickness. Whatever method you choose, make sure to add the thinning agent slowly and in small increments to avoid ruining your glaze.
Additive if the Glaze Is Too Thick
Water in Dipping Glazes can cause the glaze particles to settle and become difficult to use. Amaco SuspendAid is a specially formulated mixture that helps prevent the settling of glazes, especially ones to which additional water has been added.
This Additive increases the time that glazes suspensions remain in usable form. Simply add the recommended amount of SuspendAid to suspend glaze particles, giving you more time to apply your glaze. The Additive is also AP, Non-Toxic, and safe to use. With SuspendAid, you can be sure that your glazes will stay suspended and is easy to use.
Glazes can be either clear, translucent, or opaque, depending on the ingredients you use. Clear glazes are typically used on underglazes and dark-colored clay bodies to allow the natural color of the clay to show through. Celadon glazes are mostly translucent but can be transparent or even opaque depending on the addition of colorants. Solid (opaque) glazes are made by adding an opacifier to the glaze mixture.
Glazes are an important part of the pottery-making process, By understanding the different ingredients used in glazes and how they interact with each other, will help you when you want to create your own glaze recipes, and there are many different recipes you can use to create a wide variety of effects and produce beautiful and unique pieces of pottery.
What Glaze Ingredients Are Not Food Safe?
The two most common toxic materials used in glazes are cadmium and lead. Cadmium is used primarily to create bright red and orange colors. Lead oxides are used to make glazes flow at low temperatures. There are other materials that may be toxic.
Many of these materials are toxic in high doses – nickel, barium, selenium, and cobalt. To be absolutely sure a Glaze is Food Safe, look for the words “food safe.” Also, make sure your pottery is “Dishwasher Safe.” If you’re still not sure, ask the glaze manufacturer. When in doubt, it’s always best to err on the side of caution and choose a different product.
What Are Underglazes Made Of?
Underglazes are made from a mix of clay, colorant (mason stains), and frit (Frit is silica and flux). This combination gives Underglazes the unique properties that make them so useful for a variety of applications. The clay helps to give the underglaze a smooth consistency and allows it to adhere to Greenware and bisque ware.
Underglaze works as a kiln-safe paint on the surface of the clay, and then you can cover it with a clear glaze to make your piece glossy and waterproof.
What Does Zinc Do in Glaze?
Zinc is a common element used in glazes. It can act as a flux, meaning it helps to lower the firing temperature, and it can also help to reduce the thermal expansion of the glaze. In larger amounts, zinc generally promotes crystalline effects. However, the glaze surface can dry if too much is added. Overall, zinc is a helpful element that can improve the quality of your glaze.