What Are The 3 Basic Ingredients In Glaze?

Glazes are vitreous coatings applied to ceramic surfaces to provide decoration and protection. A glaze is made up of three main ingredients – silica, alumina, and a flux. The silica provides the glass structure, alumina adds strength and stability, and the flux lowers the melting temperature. When fired, these ingredients fuse together to create a glossy, impervious coating.

Glazes serve both decorative and functional purposes in ceramic art. Visually, they allow for an infinite range of colors, textures, and effects. Glazes can make surfaces waterproof, easier to clean, and more durable. They enhance the beauty of clay forms and are an essential part of most ceramic ware.

Silica

Silica, usually in the form of quartz, is the main source of glassy structure in glazes. Quartz is a hard, crystalline mineral composed of silicon and oxygen atoms. When heated to high temperatures, the quartz melts and forms the glassy network that gives glazes their characteristic sheen and structure. Silica typically makes up around 60-65% of a glaze’s composition.

The amount of silica has a significant effect on the glaze’s melting temperature. Higher silica levels raise the melting point, requiring higher kiln temperatures to mature the glaze. Too much silica can prevent the glaze from melting fully, resulting in a matte surface. Lower silica levels reduce the melting point, allowing the glaze to flow smooth at lower temperatures.

In addition to quartz, feldspars are another common source of silica in glazes. Feldspars melt at lower temperatures than quartz while still providing silica for glass structure. The interplay between quartz, feldspar, and other minerals is key to tuning the melting behavior of glazes.

Alumina

Alumina is the second most common ingredient in most glazes. It is added to the glaze through the clay body. Alumina provides strength and hardness to the fired glaze, making it more durable and resistant to wear. It also helps prevent dunting, which is cracking or crazing caused by thermal shock during heating and cooling of the pottery.

The alumina content in clay bodies can vary widely, from around 15% to 30% or more. Clays high in alumina like kaolin are often used in glazes to provide the needed amount of this oxide. When the glaze is fired, the alumina from the clay mixes into the glassy matrix, enhancing the physical properties of the hardened glaze surface.

Too much alumina can make the glaze overly hard and prone to crawling, where it beads up and pulls away from the clay surface. Insufficient alumina leads to a glaze that is too soft and easily scratched or worn away with use. Most stable glaze recipes aim for around 20-25% alumina content to provide an optimal combination of hardness, durability, and resistance to dunting.

Flux

Flux is another critical ingredient in glazes. Its purpose is to lower the melting temperature of the glaze and allow the glass phase to form at lower kiln temperatures. Without flux, glazes would require extremely high firing temperatures. The most common fluxes used in glaze recipes are feldspar, soda ash, and borax.

Feldspar is a naturally occurring mineral that contains significant amounts of aluminum, silica, and potassium. When feldspar melts, it forms a glassy phase that allows the glaze to fuse and become glossy at typical stoneware firing temperatures around 2200°F. Soda ash and borax also help lower melting temps, making them useful fluxes.

The percentage of flux in a glaze recipe impacts the melting behavior. Too much flux can cause glazes to become underfired and susceptible to leaching or crazing. The ideal amount of flux depends on the recipe and the firing temperature. Most stoneware glazes contain 10-20% flux to promote glass formation at cone 6 while remaining durable.

Ratio of Ingredients

The typical starting point for glaze recipes is a 1:1:4 ratio of silica to alumina to flux. This means for every 1 part silica and 1 part alumina, you would use 4 parts flux. However, this ratio can be adjusted based on the desired properties of the finished glaze.

Increasing the amount of silica makes the glaze more viscous and improves its stability on vertical surfaces. Too much silica can make the glaze too matte. Alumina improves the glaze’s hardness, glossiness and resistance to scratching. More flux lowers the melting point of the glaze so it vitrifies at a lower kiln temperature. Flux also increases glossiness and fluidity.

Most potters will start with a 1:1:4 ratio and then do test firings, adjusting the proportions as needed. A higher ratio of flux to silica and alumina will result in a more fluid, glossy glaze that melts at a lower temperature. Reducing the amount of flux will raise the melting point and make a more durable, matte glaze.

Frits

Frits are an important ingredient used in glazes as a substitute for the raw materials like silica, alumina and fluxes. Frits are made by heating a mixture of these raw materials until they partially melt together forming a glass. The resulting frit is then ground into a powder to be incorporated into glaze recipes.

Using frits has several key advantages for potters and glaze makers. Because the raw materials have already been melted together into glass, the frits are much more chemically stable and consistent compared to using raw materials directly. This helps improve the reliability and workability of the glazes. Additionally, frits are considered much safer because the hazardous materials like silica have already reacted, reducing exposure during glaze preparation. Finally, frits enable more options for glaze effects by carefully controlling the glaze chemistry through the frit composition.

In summary, frits are an indispensable component of most modern glaze recipes. Their pre-melted and consistent nature makes them easier and safer to use while providing glaze makers more control over the final glaze chemistry and aesthetic effects.

Other Additives

In addition to the three main ingredients of glaze (silica, alumina, and flux), there are several other additives that can be incorporated for functional or aesthetic purposes:

Colorants

Glazes can be colored using metal oxides, stains, or pigments. Common colorants include cobalt oxide for blue, chrome oxide for green, iron oxide for brown, copper oxide for green/blue, manganese dioxide for brown/black, nickel oxide for brown, and tin oxide for white.

Opacifiers

Opacifiers are added to glazes to create an opaque, non-transparent finish. The most common opacifiers are tin oxide, zirconium oxide, and titanium dioxide. They reflect light to hide the clay body underneath the glaze.

Suspending Agents

Suspending agents help keep glaze ingredients properly dispersed in the solution. Clay and alumina hydrate are commonly used for this purpose. Without suspending agents, ingredients may settle during storage.

Glaze Recipes

Glaze recipes can vary greatly depending on the desired properties and ingredients used. Here are some sample recipes using common ingredients:

Glossy Transparent Glaze

This recipe produces a very smooth, glass-like surface.

• 25% silica
• 25% alumina
• 50% calcium carbonate flux

Textured Matte Glaze

The high grog content gives this glaze an earthy, textured look.

• 20% silica
• 20% alumina
• 35% dolomite flux
• 25% grog (alumina and silica particles)

Crawling Glaze

This recipe intentionally creates a crawling effect by limiting the flux.

• 35% silica
• 30% alumina
• 15% calcium carbonate flux
• 20% zirconium silicate

By adjusting the ingredients and ratios, glaze artists can achieve many different visual qualities in their finished pieces.

Firing

The firing process is critical for glazes as this is when the magic happens and the glaze melts and fuses with the clay body beneath. Each glaze has an optimal melting range that the potter must achieve in the kiln. This is influenced by the glaze ingredients and their ratios. The flux, in particular, lowers the melting temperature.

It’s essential that the glaze and clay body are compatible in terms of their thermal expansion and firing range. If the clay wants to vitrify at a lower temperature than the glaze melting point, this could cause problems like glaze crawling or peeling. The glaze should melt and mature at the peak temperature for the clay body. Special low-fire glazes are required for firing at earthenware temperatures, while stoneware glazes are formulated to mature at higher stoneware firing ranges. Testing is always recommended to ensure the glaze and clay work well together.

The glaze melting range determines what type of kiln firing is required. Low-fire glazes can be fired in an electric kiln, while high-fire stoneware and porcelain glazes require a gas kiln capable of reaching cone 10 temperatures between 2200°F and 2300°F. The kiln atmosphere – whether oxidation or reduction – will also affect the finished glaze colors and surface effects. Mastering the firing process enables potters to produce stunning glaze results.

Conclusion

In summary, the three main ingredients in ceramic glaze are silica, alumina, and flux. Silica provides the glass structure, alumina controls glaze viscosity and surface tension, and flux promotes melting at lower temperatures. The ratio of these ingredients determines the physical and chemical properties of the glaze such as its appearance, melting temperature, and durability.

Glaze plays a critical role in ceramic ware by providing an impermeable and decorative coating. Properly formulated glazes enhance the beauty of pottery and strengthen it against damage from use, handling, and exposure to foods or liquids. Glazing transforms plain clay into a functional, aesthetically-pleasing vessel. With its ability to be customized in infinite ways, glaze makes each handcrafted ceramic piece truly unique.