What Is The Definition Of A Glaze?

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What is a Glaze?

A glaze is a thin coating applied to ceramic wares like pottery, tile, and porcelain. According to Merriam-Webster, a ceramic glaze is “a glassy usually opaque coating fused on to the surface of ceramic ware by heat.” The main purposes of a glaze are decoration and protection.

Glazes serve both aesthetic and functional purposes. They can make pottery waterproof, easier to clean, and more durable. Glazes come in limitless colors, textures, and finishes like glossy, matte, or crackled. They transform plain clay into a beautiful decorative object with visual depth and richness.

There are several main types of glazes used in pottery and ceramics:

Raw glazes made from mixtures of silica, fluxes, and aluminum oxide or calcium oxide.
Fritted glazes made by pre-melting raw materials into glass that is ground into a powder.
Commercial glazes purchased pre-made by ceramicists.
Underglazes applied to bisque ware and covered by a transparent overglaze.

History and Origin

Glazes have been used to coat and decorate ceramic wares for thousands of years. Evidence shows that ancient civilizations in Mesopotamia, Egypt, and China were using glazes as early as 3000 BCE. Glazes originated as a way to waterproof earthenware vessels and prevent liquids from seeping through unglazed porous clay. Over time, glazes took on decorative purposes as well.

One of the earliest glazing techniques was to apply a slip, or liquid clay mixture, to the surface of unfired pottery pieces. The pieces were then fired, causing the slip to melt and turn glassy, creating a glaze. Other early techniques involved brushing mixtures of ground quartz pebbles or sand mixed with water and plant ashes directly onto fired vessels and refiring them. The alkali component of the plant ash helped lower the melting point so the silica could form a glassy coating.

By 1500 BCE, potters had discovered that lead could be added to glazes to create brilliant shiny finishes with vivid colors. This lead glaze technique spread through Mesopotamia, Egypt, Greece, and China over the next millennia, allowing ancient potters to create glazes in a full spectrum of colors.

Glaze Ingredients

Glazes are composed of several key ingredients that provide distinct properties when fired.^[1] The most common ingredients are:

Silica – The main glass former, usually in the form of quartz, feldspar or clay. Provides structure.
Flux – Helps lower the melting point, usually an alkali or alkaline earth oxide. Soda ash and potash are common fluxes.
Clays – Provide aluminum and fluxing oxides. Kaolin and ball clays are frequently used.
Colorants – Metal oxides that impart color when fired, like cobalt (blue), copper (green/blue), iron (brown/green), chromium (green), nickel (brown).

The ingredients are combined and milled together into a fine powder. When fired, the silica forms a glass, while fluxes help melt the mixture to a glossy coating at lower temperatures. Clays and oxides provide color and effects.

By varying the ingredients, ceramic artists can create a wide array of glazes with differing appearances like crystalline, matte, high sheen, crawling, breaking, crazing, and more.

^{[1] https://potterycrafters.com/what-is-glaze-made-of-ceramic-glaze-ingredients/}

Glaze Chemistry

The chemistry of a glaze determines how the ingredients interact and react during firing to produce the final glaze surface.^[1] Glaze chemistry examines the molecular structure and bonding of the raw materials.^[2] It looks at how ingredients like silica, fluxes, and stabilizers break down and recombine at high temperatures.

When clay and glaze ingredients are fired in a kiln, they undergo chemical reactions and form glass. The key components are silica, which forms the glass structure, fluxes like soda and boron, which lower the melting point so glaze flows and fuses at attainable temperatures, and stabilizers like alumina and zirconia, which prevent components from leaching out.^[3]

The glaze firing process takes the raw materials through multiple phases. As temperature rises,ingredients decompose, melt, flow, then cool and harden into glass. The final molecular structure determines glaze properties like surface texture, color, hardness, and durability.^[1] Understanding how ingredients interact chemically allows potters to adjust recipes and achieve desired effects.

Glaze chemistry is complex because numerous factors are at play. Ingredient choice, firing temperature, kiln atmosphere, and cooling rate all significantly impact the chemical reactions. A strong foundation in ceramic materials and glaze recipes is needed to fully utilize glaze chemistry principles when formulating new glazes.

^{[1] https://ceramicartsnetwork.org/daily/glaze-chemistry}
^{[2] https://www.eastfork.com/journal/my-chemical-romance-glazing-101-with-kyle-crowder}
^{[3] https://ceramicmaterialsworkshop.com/10-reasons-why-you-should-take-a-glaze-chemistry-course/}

Glaze Types

Glazes can be categorized into different types based on their characteristics and composition. Two main categories are raw glazes and fritted glazes. Raw glazes are made directly from raw materials like silica, fluxes, and colorants. They require extensive mixing and grinding to develop the glaze chemistry and ensure a smooth consistency. Raw glazes are inexpensive but require more testing and expertise to formulate correctly. Fritted glazes use premixed frits, which are mixtures of raw materials that have been melted into glass and granulated. Frits simplify glaze chemistry and preparation for beginners. However, fritted glazes can be more expensive.

Glazes are also described as transparent, opaque, glossy, or matte. Transparent glazes allow the clay color and texture to show through. They contain high amounts of fluxes like soda, potash, or boron to promote glass formation and transparency. Opaque glazes completely cover the clay body to hide the underlying color and texture. Opacity comes from crystals that form during firing or the addition of opacifiers like tin oxide or zirconium. Glossy glazes have a smooth, glassy shine from high melting, glass-forming ingredients. Matte glazes have a soft, muted look from materials that prevent glass formation like alumina, quartz, or calcium. The ingredients and firing process determine the visual characteristics.

Glazing Techniques

There are several common techniques used to apply glaze to ceramic pieces:

Dipping involves submerging the bisqueware into a container of glaze either partially or fully, depending on the desired effect. This allows the glaze to coat the piece evenly. For areas that should remain unglazed, wax resist can be used to prevent glaze adherence. Dipping is best for smooth, rounded shapes and can result in drips for more intricate shapes.

Pouring glaze directly onto the surface of the ware is useful for controlled coverage. The glaze can be poured uniformly or directed to specific areas as desired. Multiple pours can create layered effects. Pouring gives the artist more control compared to dipping.

Spraying glaze through an airbrush or spray gun allows thin, even layers of glaze to be applied. Spraying also avoids unwanted glaze drips. Masking or stencils can be used with spraying to create patterns and textures.

Using a single coat vs multiple coats of glaze produces different visual results. A single coat provides a thinner glaze layer that exposes more of the underlying clay color and texture. Multiple coats create a deeper, opaque glazed surface. Layering complementary glazes on top of each other is also popular for multicolored effects.

Some special glazing techniques include trailing, where lines of glaze are purposefully dripped onto the ware for decoration. Sponging glaze on with a sea sponge can create mottled textures. Sgraffito involves scratching through one glaze layer to reveal a different color underneath. Glaze is an extremely versatile and creative part of the ceramic process with endless effects possible.

Firing Glazes

Proper firing is critical for achieving the desired characteristics of a glaze. There are two main firings in the glazing process:

Bisque Firing: This initial firing is done on raw clay to convert it from its soft, wet state into a harder, porous bisque material. Bisque firing temperatures typically range from cone 04 (around 1100°F) to cone 6 (around 2200°F) depending on the type of clay body. The relatively low temperatures preserves the solubility needed for glazes to properly melt and fuse in the subsequent glaze firing.

Glaze Firing: This second firing melts and fuses the glaze onto the bisque ware. Glaze firing is done at much higher temperatures, usually between cone 6 (2200°F) and cone 10 (2350°F). The temperature depends on the makeup and chemistry of the glaze itself. Specific glazes are formulated to mature and melt at certain cone ranges. Glazes should be fired to at least their maturity temperature for proper melting, surface quality, and durability.

Glaze firing can be done in either an oxidizing or reducing atmosphere. Oxidation firing introduces more oxygen into the kiln and produces vibrant colors and glossy surfaces. Reduction firing limits oxygen, creating special effects like metallic lusters in certain glazes. The atmosphere can significantly impact the look and qualities of the finished glazed piece.

Firing to the right temperature in the proper atmosphere is essential for the glaze to bond to the clay body and produce the desired appearance and properties in the finished ceramic ware. Factors like soak times, cooling rates, and kiln uniformity also influence the outcome.

Glaze Defects

Glazes can develop defects during the firing process that negatively impact the appearance and durability of ceramic wares. Some of the most common glaze defects include:

Crazing

Crazing occurs when the glaze contracts more than the clay body during cooling, causing a network of fine cracks in the glaze surface (Digitalfire). It’s often caused by a mismatch between the glaze and clay thermal expansion, overly quartz-rich glazes, or fast cooling. To prevent crazing, match the glaze and clay expansion, increase fluxing oxides like CaO in the glaze recipe, and slow the kiln cooling cycle.

Crawling

Crawling happens when the glaze pulls away from areas of the ware surface, leaving bare clay. It’s typically caused by poor clay prep, contamination from kiln wash, or a glaze that is too fluid. Proper clay cleaning, kiln maintenance, and adjusting the glaze recipe to be more viscous can help prevent crawling (Ceramic Arts Daily).

Blistering

Blisters form when gases get trapped between the glaze layer and clay body during rapid heating in the kiln. Firing at a slower ramp rate and adding oxides like Li2O or K2O to the glaze can help reduce blistering by promoting gas release (Digitalfire).

Glaze Safety

Glazes contain many toxic ingredients that require proper handling and storage. Lead and cadmium are common ingredients in older glaze recipes that are highly toxic if ingested. According to this source, lead and cadmium glazes should always be avoided, especially on surfaces that come in contact with food. Crystalline silica is another common ingredient that can cause lung disease if inhaled.

When mixing dry glaze materials, it is important to wear a protective mask to avoid breathing in toxic dusts. Glazes should be stored in clearly labeled containers out of reach of children and pets. Work surfaces should be kept clean and free of glaze materials. Anything used for glaze mixing should not be used for food preparation. Fired glaze surfaces should be tested for leaching before using for food or drink.

Proper handling and storage of glaze materials is crucial for health and safety. Glaze makers have a responsibility to understand ingredient toxicity and follow safety guidelines. With care taken during the glaze process, the risks of hazardous exposure can be minimized.

Innovations in Glazes

The exploration of new materials and techniques has led to exciting innovations in glazes over the past few decades. Some key innovations include:

New glaze types and effects – Glaze artists have developed new glaze recipes and effects including metallic lustre glazes, crackle glazes, ombré glazes, and more. Lustre glazes create an iridescent metallic sheen, while crackle glazes deliberately form a fine crazing pattern. Ombré glazes transition smoothly between colors.

Computerized glaze formulation – Glaze chemists use special software programs to formulation glaze recipes and predict how they will turn out. Programs like HyperGlaze and Glazesim allow users to input ingredients, make adjustments, and view renderings of the fired results.(1)

These innovations allow ceramic artists to achieve stunning new effects and streamline the glaze design process.