Glazing is a transformative step in pottery, essential for both functionality and aesthetics. It involves applying a coating of vitreous substance to ceramic surfaces, which, upon firing, fuses to the clay body, creating a smooth, durable, and often decorative finish. Glazing not only renders pottery watertight, making it suitable for containing liquids and food, but also enhances its visual appeal through color, texture, and sheen. This article serves as a comprehensive guide to glazing clay pottery, aimed at beginners and intermediate potters eager to master this crucial skill.
Understanding Glazes and Clay Compatibility
The foundation of successful glazing lies in understanding the different types of glazes and their compatibility with various clay bodies. Glazes are broadly categorized into earthenware, stoneware, and raku, each requiring specific firing temperatures and suitable clay types. Earthenware glazes are designed for low-fire clays, maturing at temperatures between 1745°F (950°C) and 2012°F (1100°C). These glazes often offer a wide range of bright and vibrant colors but may result in a softer, more porous finish. Stoneware glazes, on the other hand, are formulated for mid- to high-fire clays, typically fired between 2124°F (1162°C) and 2381°F (1305°C). Stoneware glazes produce a durable, vitrified surface, ideal for functional ware, although the color palette may be somewhat more limited compared to earthenware glazes. Raku glazes are designed for rapid firing and cooling processes, often involving post-firing reduction techniques that create unique and unpredictable effects.
Compatibility between clay and glaze is paramount to prevent common issues like crazing or shivering. Crazing, the appearance of fine cracks in the glaze surface, occurs when the glaze contracts more than the clay body during cooling. Conversely, shivering, where the glaze flakes off the clay, happens when the glaze contracts less than the clay body. To ensure compatibility, it is essential to test glazes on the specific clay body being used and to consider factors like thermal expansion rates and firing temperatures. Consulting a clay glaze compatibility chart can also provide valuable guidance.
Furthermore, the color of the clay body can significantly influence the final appearance of the glaze. For instance, a matte glaze like Cornwall stone may exhibit warm yellow-brown hues with deep speckles when applied to iron-rich clays, while appearing whiter on porcelain or white clays. Similarly, a glaze like Bringle Yellow might appear granny-smith green on darker clays and bright optic yellow on white stonewares. Understanding these interactions allows potters to harness the unique qualities of both clay and glaze to achieve desired aesthetic outcomes. It’s important to remember that each clay and glaze has its own expansion and contraction rate, and significant differences can lead to problems. To explore creating your own unique color schemes, you may be interested in learning the art of ash glazes.
Glaze Application Techniques
Various techniques exist for applying glaze to pottery, each offering distinct advantages and suitable for different projects and equipment. Dipping is a quick and efficient method for coating larger pieces, providing a uniform layer and the potential for creating gradient effects. To dip glaze, submerge the bisque-fired pottery into the glaze solution for a specific amount of time, ensuring even coverage. Brushing allows for precise application and artistic control, ideal for intricate designs and detailed work. Use soft brushes to layer glaze evenly, taking gravity into account to prevent runs and achieve smooth or textured surfaces. Pouring involves flowing glaze over the piece, covering it with a solid color or creating decorative details through dripping. This method requires practice to achieve an even layer on both the interior and exterior of the pottery. Spraying is perfect for achieving thin, consistent coats, particularly for base layers. Airbrushes or spray guns can be used, along with a banding wheel to rotate the pottery, allowing for various creative possibilities, including layering different colors. Other techniques include splattering, stippling, sponging, and glaze trailing, each offering unique textures and effects. Wax resist can also be used in conjunction with glazing to repel glaze from specific areas, creating decorative patterns.
This video from NowWhat?!PotteryWorks demonstrates a wax resist technique for glazing bottles. It’s a great visual guide to complement this text.
When combining and mixing glazes, compatibility and firing temperatures are crucial considerations. Not all glazes work well together, and incompatible combinations can lead to blistering or cracking. Additionally, underglaze and glaze should not typically be mixed, as underglazes do not flux (melt) and may distort or obscure the glaze’s color and surface. Experimenting with layering opaque and transparent glazes or combining glazes with contrasting textures can add depth and dimension to pottery. If you are interested in alternative methods, you can explore sgraffito to scratch the surface of pottery art.
Safety, Troubleshooting, and Firing
Safety is paramount when working with glazes and kilns. Many glaze materials are hazardous and can cause lung damage, skin irritation, or poisoning if inhaled, ingested, or absorbed through the skin. Always wear a NIOSH-approved respirator when handling dry materials to prevent inhalation of harmful particles. Avoid eating, drinking, or smoking in the glazing area, and wash hands thoroughly after glazing. Spray glazes only in a properly vented and filtered spray booth to prevent inhalation of airborne particles. Kilns also pose potential hazards, including the production of carbon monoxide and other toxic gases. Ensure proper kiln installation, ventilation, and adherence to safety controls to mitigate these risks. Wear welders goggles when looking into the kiln spy holes to protect your eyes from intense heat and infrared radiation.
Common glazing problems include crazing, shivering, crawling, pinholing, and blistering. Crazing can be corrected by increasing the silica content in the body or glaze and decreasing feldspar or other materials containing sodium or potassium. Shivering, the opposite of crazing, can be addressed by decreasing silica and increasing feldspar in the glaze or body. Crawling, caused by high surface tension in the melting glaze, can be alleviated by adding gum to the glaze batch or calcining zinc. Pinholing, often due to a badly controlled firing cycle or glaze composition, can be remedied by lengthening the firing cycle, applying the glaze less thickly, or adding more flux to the glaze. Blisters and blebs are typically the result of excessively thick glaze application or incomplete clay preparation and can be avoided by proper wedging and glaze application.
Firing glazed pottery in a kiln involves carefully loading the pieces to ensure even heat distribution and following a specific firing schedule. The firing schedule includes gradual temperature increases, hold times (soak), and controlled cooling to allow the glaze to mature properly and prevent defects. Understanding kiln atmospheres, such as oxidation and reduction, is crucial, as they can significantly affect the final color and appearance of the glaze. By mastering these techniques and considerations, potters can confidently transform their clay creations into durable, beautiful, and functional works of art.
