Understanding the fundamental differences between underglaze and glaze is crucial for any ceramic artist, whether a beginner or an experienced professional. These two distinct materials play vital roles in the decorative and functional aspects of pottery, and their correct application can dramatically impact the final piece’s aesthetic and durability.
At its core, underglaze is a type of ceramic decoration applied to unfired clay (greenware) or bisquited clay (bisque ware). It’s essentially a colored slip, a mixture of clay, water, and mineral oxides or stains, designed to provide color and design before the final glassy coating is applied.
Glaze, on the other hand, is a liquid suspension of minerals that, when fired, fuses to the clay body, creating a glassy, impermeable surface. It serves both decorative and functional purposes, offering a protective coating, adding color, and enhancing the overall visual appeal of the ceramic piece.
The Essence of Underglaze: Color and Design Before the Seal
Underglaze is primarily a decorative medium. It allows artists to paint intricate designs, create detailed illustrations, or add solid blocks of color to their ceramic creations before they are permanently sealed with a glaze. Because it’s applied to the clay body itself, underglaze becomes an integral part of the surface, offering a different textural and visual quality than surface decoration applied after glazing.
The composition of underglaze is key to its performance. It typically contains fine clay particles, colorants, and a fluxing agent, though the flux content is generally much lower than in a glaze. This lower flux content means that underglaze does not melt and fuse into a glassy layer during firing; instead, it matures into a matte, porous surface that is then covered by a subsequent glaze layer.
This characteristic porosity is what allows underglaze to adhere well to the clay body and to accept a clear or colored glaze over it without bleeding or running excessively, provided the correct firing temperatures are maintained. The colorants used in underglaze are typically metallic oxides or ceramic stains, which are stable at high firing temperatures.
Underglaze Application Techniques
The application of underglaze is as varied as the artistic visions it serves. From delicate brushwork to bold graphic designs, the techniques are numerous and offer a wide range of expressive possibilities.
One common method is brush painting. Artists can use various brush types, from fine-tipped detail brushes for intricate patterns to broader brushes for filling in areas. The consistency of the underglaze is crucial for successful brushwork; it should be fluid enough to flow from the brush but thick enough to provide opaque coverage without excessive dripping.
Another popular technique is sgraffito. This involves applying a layer of underglaze to the clay body and then scratching through the underglaze layer to reveal the clay beneath. This creates a contrast in color and texture, allowing for detailed patterns and designs to be etched into the surface. It’s a technique that demands precision and a good understanding of how the underglaze will behave during the firing process.
Underglaze can also be applied through stamping, sponging, or even airbrushing. Stamping involves pressing a carved stamp into the underglaze-coated surface, transferring a design. Sponging can create subtle textural effects, while airbrushing allows for smooth gradients and soft, diffused color application. Each technique offers a unique aesthetic outcome.
Underglaze on Greenware vs. Bisque Ware
The stage at which underglaze is applied significantly impacts the outcome. Applying underglaze to greenware, or unfired clay, offers a different set of challenges and advantages compared to applying it to bisque ware.
When applied to greenware, the underglaze becomes a part of the clay body. The clay is still very porous and can absorb the underglaze readily. However, greenware is also very fragile, making detailed work and extensive coverage more prone to damage. Any manipulation of the piece after underglaze application must be done with extreme care to avoid smudging or cracking the delicate surface.
Applying underglaze to bisque ware, which is clay that has already been fired once to a lower temperature, is generally easier for beginners. The bisque surface is hard and porous, but much less fragile than greenware. This allows for more freedom in application, including techniques like overglazing or applying multiple layers of underglaze. The underglaze adheres well to the porous bisque surface, and the piece is less likely to be damaged during the glazing process.
The choice between greenware and bisque ware for underglaze application often depends on the desired effect and the artist’s comfort level with handling fragile materials. Some artists prefer the integrated look achieved by applying underglaze to greenware, while others opt for the greater control and durability offered by bisque application.
The Role of Colorants in Underglaze
The vibrant palette of underglazes is made possible by the careful selection and use of colorants. These colorants are typically inorganic compounds that can withstand the high temperatures of ceramic firing without degrading or changing color significantly.
Metallic oxides are the most common colorants. For example, iron oxide produces a range of colors from earthy browns and reds to deep blacks, depending on the firing atmosphere and the presence of other elements. Cobalt oxide is famous for its intense blues, while copper oxide can yield greens and reds, again influenced by firing conditions. Tin oxide is used to create opaque whites and can also act as a flux or opacifier in glazes.
Ceramic stains are another important category of colorants. These are pre-mixed, manufactured compounds that offer a wider and more predictable range of colors than raw oxides. They are made by firing mixtures of metal oxides and other minerals, then grinding them into a fine powder. Stains are often preferred for their consistency and for achieving specific hues that are difficult to obtain with raw oxides.
The concentration of these colorants in the underglaze mixture determines the intensity of the color. Higher concentrations generally lead to more vibrant and opaque colors. It’s important to note that underglaze colors can appear different in their raw, unfired state compared to their fired appearance, so test tiles are essential for predicting the final result.
Common Underglaze Colorants and Their Results
Understanding the behavior of specific colorants is crucial for achieving desired effects. Each oxide or stain has unique properties that influence its color and interaction with other materials.
Cobalt oxide is a highly potent colorant, yielding brilliant blues. Even small amounts can produce intense coloration, making it economical but also requiring careful handling to avoid overly strong or muddy blues. It is generally stable across various firing temperatures and atmospheres.
Iron oxide is incredibly versatile. In oxidation firing, it typically produces shades of brown, red, and orange. In reduction firing, it can shift towards grays and blacks, offering a dramatic difference in appearance. Its color range makes it a staple for many ceramic artists.
Manganese dioxide is known for its ability to produce browns and purples. It can also contribute to speckling effects and is often used in combination with other oxides to create complex color blends. Its behavior can be somewhat unpredictable, adding an element of surprise to the firing process.
Chromium oxide typically produces greens, but its behavior is heavily influenced by other glaze components and firing conditions. In the presence of tin, it can create pinks. It’s a colorant that requires careful testing to achieve consistent results.
The Versatility of Glaze: Protection, Decoration, and Functionality
Glaze is the final, transformative coating applied to ceramic pieces. Its primary function is to render the pottery waterproof and food-safe, but it also plays a significant role in the aesthetic appeal of the finished work.
Composed of silica (glass-former), alumina (stiffener), and flux (melter), glazes melt and fuse to the clay body during firing, creating a durable, non-porous surface. This glassy layer protects the clay from moisture, stains, and chemical reactions, making the pottery suitable for everyday use.
Beyond its functional benefits, glaze offers an immense spectrum of decorative possibilities. It can be clear, colored, opaque, translucent, matte, or glossy, and can be applied in various ways to achieve a myriad of visual effects. The interaction between the glaze and the clay body, as well as the firing process, can lead to unique textures and color variations.
Glaze Composition and Its Impact
The specific ingredients in a glaze recipe dictate its properties, including its melting point, viscosity, color, texture, and how it interacts with the clay body.
Silica is the fundamental glass-forming oxide. It melts at very high temperatures, and its presence is essential for creating a durable glass. Alumina, typically introduced through feldspar or kaolin, acts as a stiffener, increasing the viscosity of the molten glaze and preventing it from running off the piece during firing. Fluxes, such as feldspar, whiting, or talc, are added to lower the melting point of silica, allowing the glaze to fuse at achievable kiln temperatures.
Colorants, similar to those used in underglazes (metallic oxides and stains), are added to glazes to achieve desired hues. However, their behavior in a molten glass matrix can be very different from their behavior in an underglaze. Opacifiers, like tin oxide or zirconium silicate, are added to create opaque glazes, preventing the underlying clay body or underglaze design from showing through.
The balance of these components is critical. A glaze that is too fluid might run off the pot, while one that is too stiff may not melt properly, resulting in a dry or crawling surface. Understanding these relationships is fundamental to successful glazing.
Types of Glaze Finishes
The visual and tactile qualities of a glaze can vary dramatically, offering a wide range of aesthetic choices for the ceramic artist.
Glossy glazes are characterized by their smooth, reflective surface. They are achieved with glazes that melt smoothly and have a lower viscosity. These glazes are often chosen for their ability to enhance colors and provide a bright, lustrous finish. They are also generally easier to clean.
Matte glazes, in contrast, have a dull, non-reflective surface. They are created by glazes that have a higher viscosity, crystallize upon cooling, or contain specific opacifiers that create a micro-textured surface. Matte glazes can offer a sophisticated, earthy feel and are excellent for showcasing subtle textures or detailed underglaze work without glare.
Satin glazes fall between glossy and matte, offering a soft sheen. They can be achieved by adjusting the glaze formula or by applying a clear glossy glaze over a matte glaze, or vice versa, in specific ways. These finishes provide a balanced aesthetic, offering some reflection without being overly shiny.
Textured glazes incorporate materials that create a tactile surface. This can include adding grog or sand for a rough feel, or using glazes that develop crystals or crackle patterns during cooling. These finishes add a unique dimension to the ceramic piece, inviting touch and adding visual interest.
Glaze Application Methods
How glaze is applied is just as important as the glaze itself in determining the final outcome. Different application methods yield distinct visual and textural results.
Dipping is a common and efficient method for glazing. The ceramic piece is submerged into a bucket of glaze, allowing the glaze to adhere to the surface. The thickness of the glaze coating can be controlled by the speed and duration of the dip. This method is excellent for achieving an even coating on functional ware.
Pouring involves pouring glaze over the interior or exterior of a ceramic piece. This technique is often used for glazing the inside of bowls and cups, or for creating layered effects by pouring different colors sequentially. It allows for more control over specific areas and can create interesting drips and runs.
Brushing is a versatile method, ideal for detailed work, applying multiple layers, or for achieving specific textural effects. Artists can use brushes to create patterns, apply underglaze colors, or build up layers of glaze. Multiple thin coats are generally recommended for even coverage when brushing glaze.
Spraying, often done with an airbrush or spray gun, allows for very thin, even coats of glaze. This technique is excellent for achieving smooth gradients, subtle color transitions, and for applying glazes over intricate underglaze designs without disturbing them. It requires a spray booth and proper ventilation for safety.
Glaze Defects and How to Avoid Them
Despite careful preparation, glaze defects can occur, often due to issues with application, firing, or glaze formulation. Understanding common problems helps in troubleshooting and achieving desired results.
Crazing is the formation of fine cracks on the glaze surface. It typically occurs when the glaze has a higher coefficient of thermal expansion than the clay body, causing it to contract more during cooling. To avoid crazing, ensure the glaze and clay body have compatible thermal expansion rates, or consider using a glaze specifically designed for the clay you are using.
Crawling is when the glaze retracts from certain areas of the clay body during firing, leaving bare spots. This can be caused by dust or grease on the clay surface before glazing, or by a glaze that is too thick or has too much flux. Thoroughly cleaning bisque ware and applying glazes at the correct thickness are key preventative measures.
Pinholing is the appearance of small holes or pits on the glaze surface. This often results from trapped air or gasses escaping from the clay body or glaze during firing. Ensuring proper firing schedules, allowing adequate ventilation in the kiln, and avoiding over-application of glaze can help mitigate pinholing.
Blistering can occur when gases are trapped within the glaze layer and expand during firing, creating bubbles that burst. This is often related to the firing temperature, firing atmosphere, or the glaze composition itself. Slowing down the firing rate, especially during the peak temperature period, can sometimes help release trapped gasses.
Underglaze vs. Glaze: Key Differences Summarized
The distinction between underglaze and glaze lies in their fundamental purpose, application, and behavior during firing.
Underglaze is applied before the final glaze layer, primarily for decoration. It is essentially colored clay slip that matures to a matte, porous surface. Its purpose is to provide color and design detail that is then sealed by a subsequent glaze.
Glaze is the final, glassy coating that fuses to the clay body. Its primary functions are to waterproof the ceramic, make it food-safe, and enhance its aesthetic appeal with color, texture, and finish. It transforms the porous clay into a durable, non-porous surface.
When to Use Underglaze
Underglaze is the artist’s choice when detailed decoration is desired before the final protective coating is applied.
Use underglaze for intricate painted designs, illustrations, or graphic patterns on pottery. It is ideal for achieving sharp lines and distinct color areas that would be difficult to replicate with glaze alone. The matte finish of fired underglaze also provides a good surface for subsequent clear glazing, allowing the colors to pop.
Consider underglaze for sgraffito techniques, where designs are scratched through the underglaze layer to reveal the clay beneath. It is also excellent for creating detailed landscapes, portraits, or geometric patterns. The ability to apply underglaze to greenware or bisque offers flexibility in the creative process.
Underglaze is also used for achieving specific color effects that might not be possible with glazes, such as vibrant, opaque colors that remain true after firing. It allows for a level of control and precision in decoration that is unparalleled by most glaze techniques.
When to Use Glaze
Glaze is essential for the functional integrity and overall finish of a ceramic piece.
Use glaze to make your pottery waterproof, food-safe, and durable. It seals the porous clay body, preventing absorption of liquids and making the piece suitable for holding food and beverages. The protective glaze layer also makes the pottery easier to clean and more resistant to staining.
Glaze is also used for its aesthetic qualities, offering a wide range of colors, textures, and finishes from glossy to matte. It can be used to create a unifying surface over underglaze decoration, or as the primary decorative element itself through techniques like slip trailing with colored glazes or creating textured surfaces.
The choice of glaze also impacts the overall look and feel of the piece. A glossy glaze can make colors appear brighter and richer, while a matte glaze can give a more subdued and sophisticated appearance. The interaction of glaze with the clay body during firing can also create unique effects.
Interplay and Synergy: Using Both Underglaze and Glaze
The most compelling ceramic art often arises from the intelligent combination of both underglaze and glaze, leveraging the strengths of each material.
Underglaze provides the canvas for detailed artistic expression, laying down the foundation of color and design. Once fired, this underglaze design is then protected and enhanced by a clear or colored glaze, which adds depth, shine, and the crucial functional properties of a ceramic finish.
This layering allows for a richness and complexity that would be difficult to achieve with either medium alone. For instance, a painterly underglaze design can be brought to life with a transparent glossy glaze, making the colors luminous and vibrant. Conversely, a matte glaze can provide a soft, subtle backdrop that allows intricate underglaze details to stand out without the distraction of glare.
The synergy between underglaze and glaze opens up a vast creative landscape for ceramic artists, enabling them to achieve a wide array of effects from the exquisitely detailed to the boldly graphic, all while ensuring the functional integrity of their work.
Practical Examples of Combined Use
Consider a ceramic artist creating a decorative plate. They might first sketch a detailed floral pattern using fine-tipped underglaze brushes on bisque-fired clay. This underglaze application would define the petals, leaves, and stems with precise colors.
After the underglaze has been fired to its mature state, the artist would then apply a clear, glossy glaze over the entire piece. This glaze would seal the underglaze, making the plate food-safe and giving the painted design a luminous, protected finish. The glossy surface would also enhance the vibrancy of the underglaze colors.
Alternatively, an artist might create a stoneware mug with a textured exterior. They could apply a slip to create raised patterns, then use underglaze to add color to these raised areas. A matte glaze could then be applied over the entire mug, providing a durable, food-safe interior and a sophisticated, tactile exterior finish that highlights the underglaze colors and the slip texture.
Another example is the use of underglaze for sgraffito. An artist might apply a dark underglaze over a lighter clay body, then scratch through the dark layer to reveal the clay underneath in a swirling pattern. A transparent glaze applied over this would seal the design, making the piece functional while preserving the etched imagery.
Choosing the Right Firing Temperature and Atmosphere
The firing process is the crucible where underglaze and glaze are transformed, and the correct temperature and atmosphere are paramount for achieving desired results.
Underglazes are typically formulated to mature at specific temperature ranges, often corresponding to cone 06 to cone 6. It’s crucial to match the underglaze to the firing range of your clay body and glaze. Firing too low might result in a weak bond, while firing too high can cause the underglaze to melt and bleed into the overlying glaze, obscuring the design.
Glazes also have specific firing ranges, indicated by cone numbers (e.g., cone 06 for low-fire, cone 5-6 for mid-fire, cone 10 for high-fire). The glaze must be compatible with the clay body and the fired underglaze. If the glaze melts too thinly, it can obscure the underglaze, and if it melts too thickly, it might crawl or run.
The firing atmosphere—oxidation or reduction—significantly impacts the color of many oxides. For instance, copper can be green in oxidation and red in reduction. Understanding these atmospheric effects is essential for predicting and controlling color outcomes, especially when combining underglaze and glaze.
The Importance of Test Tiles
To navigate the complexities of underglaze and glaze interactions, firing test tiles is an indispensable practice for every ceramic artist.
Test tiles are small samples of the clay body you are using, coated with the underglaze and glaze combinations you intend to use on your actual work. These tiles are fired under the same conditions as your finished pieces, allowing you to see exactly how the underglaze colors will appear beneath the glaze and how the glaze itself will behave.
By firing test tiles, you can verify glaze fit (preventing crazing or cracking), assess color accuracy, check for any unintended interactions between the underglaze and glaze (like bleeding or bubbling), and determine the final surface texture (glossy, matte, etc.). This proactive approach saves time, materials, and potential disappointment.
Creating a comprehensive test tile library is a valuable resource. Documenting the clay body, underglaze colors, glaze recipes, application methods, firing temperature, and atmosphere for each test tile provides a reliable reference for future projects, ensuring consistency and enabling informed creative decisions.
Conclusion: Mastering the Art of Ceramic Surface Decoration
The journey of mastering ceramic surface decoration involves a deep understanding of both underglaze and glaze. These materials, while distinct in their function and application, work in concert to elevate a humble clay form into a work of art.
Underglaze provides the initial layer of artistic expression, offering a palette for detailed decoration applied directly to the clay body. It’s the artist’s brushstroke, the painter’s detail, the foundation upon which visual narratives are built.
Glaze then acts as the final seal, transforming the piece into a functional, durable object while adding its own layer of aesthetic contribution through color, texture, and finish. It is the protective shield, the luminous veil, the finishing touch that completes the ceramic transformation.
By understanding their unique properties, mastering application techniques, and diligently testing combinations, ceramic artists can unlock a world of creative possibilities, transforming their creations with both vibrant detail and enduring beauty.