What Is An Example Of A Glaze?
What is a Glaze?
A glaze is a vitreous coating applied to ceramics. According to Merriam-Webster, a ceramic glaze is “a preparation of minerals fused onto a ceramic body at high temperatures to produce a glassy decorative or impervious surface.”
The purpose of a glaze on ceramics is primarily decorative and functional. Glazes add color, gloss, and visual appeal to ceramic pieces. Functionally, glazes provide protection and create a non-porous barrier that makes the ceramic impervious to liquids and resistant to stains. Glazes also strengthen ceramics by filling in pores and flaws on the surface. Additionally, glazes can modify thermal properties, making ceramics more resilient to extreme temperatures during use.
Types of Glazes
There are many different types of ceramic glazes that can serve various purposes in pottery and ceramics. Glazes fall into a few main categories based on their visual appearance and surface texture.
Glossy glazes have a shiny, glass-like finish. They are smooth and reflective. Common examples include clear glazes and colored glazes like celadons. Glossy glazes often show off the natural color of the clay body underneath. Source
Satin or semi-matte glazes have a soft, silky sheen. They are less shiny than glossy glazes but have more luster than matte ones. Satin glazes are popular for a variety of ware types. Source
Matte glazes completely absorb light, leaving a non-reflective, velvety finish. Matte glazes often accentuate texture and work well on sculptural forms. Common matte glaze types include temmoku and ash glazes. Source
In addition to surface finish, glazes can be categorized as natural glazes made from raw materials or commercial glazes that are pre-mixed and formulated. Both natural and commercial glazes have their pros and cons in terms of cost, availability, and ease of use.
Glaze Ingredients
Glazes are made up of several key ingredients that each serve a specific purpose. The main ingredients in most glazes are:
Silica – Silica, usually in the form of quartz or flint, acts as the glass-former in glazes. It melts at high temperatures to form the glassy matrix that gives glazes their hardness and gloss. More silica leads to harder, more durable glazes. According to (https://www.thesprucecrafts.com/what-goes-into-glazes-2746232), silica makes up 45-65% of most glaze recipes.
Flux – Flux materials help lower the melting point of glazes so they vitrify at pottery kiln temperatures. Common fluxes include feldspars, borax, and calcium carbonate. Fluxes enhance glaze flow and surface quality. Using more flux creates smoother, shinier glazes. Excessive flux can cause glazes to become underfired and soluble (https://potterycrafters.com/what-is-glaze-made-of-ceramic-glaze-ingredients/).
Clay – Kaolin or ball clay is frequently added to glazes. Clay acts as an alumina source to add strength and prevent leaching. According to (https://ceramicartsnetwork.org/freebies/guide/ceramic-raw-materials), clay comprises 15-30% of typical glaze recipes. Too much clay can result in glazes that are matte, opaque, or prone to crawling.
In addition to these main ingredients, small amounts of colorants and opacifiers are added to create the desired glaze appearance. The ingredients and their proportions have a significant effect on the melting behavior, appearance, and durability of the fired glaze.
Applying Glazes
There are several techniques for applying glazes to ceramic pieces:
Brushing – Using a soft bristle brush to apply glaze is a common technique. Multiple thin coats are recommended over one thick coat for an even finish. Brushes with tapered tips allow for detailed glaze work. Some quality brush brands for glazes include Pottery Crafters and Royal Langnickel.
Dipping – Dipping involves fully submerging the bisqueware piece into a container of glaze. Care should be taken to avoid trapping air bubbles. Pieces should be dipped and withdrawn vertically. Multiple dips can be used to build up glaze thickness.
Pouring – Pouring glaze directly onto a piece is suitable for items with flat surfaces. The glaze can be distributed evenly with a brush. Pouring allows precise control over glaze placement.
Spraying – Glaze spray guns or airbrushes allow glaze to be misted on efficiently. Spraying is excellent for covering large, complex forms evenly. Multiple light coats are better than one heavy application for a smooth finish. Spray guns tailored for ceramics help optimize results.
Proper glaze thickness is also important for creating an attractive finish. Generally 2-4 thin, even coats are recommended. Too little glaze can create a spotty appearance while excessive thickness can lead to cracking or peeling.
Glaze Chemistry
The chemical composition of a glaze determines how it will react during firing and the final color, texture, and other properties it will exhibit. Glazes contain a mixture of fluxes, stabilizers, and glass formers that melt and fuse with the clay body during high temperature firings. The fluxes (such as sodium, potassium, and calcium) lower the melting temperature of the glaze so it vitrifies at stoneware or porcelain temperatures. Stabilizers such as alumina and silica help prevent leaching, crawling, and other defects. Glass formers like silica and boron promote glass formation in the glaze.
During firing, complex chemical reactions occur between the components of the glaze and the clay body. The fluxes interact with the silica in the clay body, promoting glass formation. As the temperature rises, the glaze components go through various phase changes from solid to liquid to glassy solid. These chemical changes determine the final appearance of the glaze in terms of color, surface texture, opacity, and gloss.
Glaze chemistry involves carefully balancing fluxes, stabilizers, and glass formers to achieve the desired color, texture, and other properties after firing. Slight variations in the chemical composition can dramatically affect the fired results. By testing and adjusting glaze recipes, ceramicists control the chemical reactions to obtain aesthetically pleasing glazes that are also functional in terms of durability, food safety, and other considerations.
For more details on glaze chemistry, see this excellent overview from Ceramic Arts Network: https://ceramicartsnetwork.org/daily/glaze-chemistry
Firing Glazes
Properly firing glazes is critical to achieving the desired visual effects and ensuring the glaze has melted and matured correctly. Glazes must be fired to a high enough temperature for the glaze ingredients to melt, bond to the clay body, and fuse together through sintering. However, firing too hot can cause glazes to become underfired, overfired, or ruin the underlying clay body.
Most glazes are designed to be fired at specific cone temperatures which refer to pyrometric cones that bend at calibrated temperatures. Popular cone firing ranges for glazes are cone 04-06 for low-fire glazes (1945-2232°F) and cone 7-10 for mid-range to high-fire glazes (2282-2381°F). It’s important to follow the cone temperature recommended by the glaze recipe or manufacturer. Proper melting depends on firing to the optimal temperature for that specific glaze chemistry.
The ramp speed, or firing rate, must also be controlled carefully. Firing too quickly can cause glazes to crawl, become greasy, bubble, or form pinholes. Slow ramps of 60-200°F per hour are typical for the key glaze melting phase. Glazes require sufficient time at peak temperature to mature fully before the kiln starts cooling.
Troubleshooting Glazes
Glazes can develop common faults like crazing, shivering, and pitting during the firing process. Troubleshooting and adjusting the glaze recipe or process is often required to achieve the desired results.
Crazing occurs when the glaze contracts more than the clay body during cooling, causing a network of fine cracks in the surface. This is often caused by too much alkali flux in the glaze or insufficient silica. Reducing the alkali content, adding silica, or choosing a clay body with a lower expansion rate can help prevent crazing.[1]
Shivering happens when the clay body contracts more than the glaze during cooling, causing the glaze to peel off the form. This is generally caused by too much silica or alumina in the glaze recipe. Adding fluxes like sodium, potassium, or calcium to the glaze can help it better match the thermal expansion rate of the clay.[2]
Pitting occurs when small cavities form in the glaze surface during firing. This is often due to gases escaping from the clay body or impurities in the glaze ingredients. Using a slower bisque firing, adding a quartz inversion during firing, or switching clay bodies can help reduce pitting issues.[3]
Decorating with Glazes
Glazes can be used decoratively in many creative ways to accentuate the form and surface of ceramic pieces. Some popular glaze decoration techniques include:
Dripping – Allowing glazes to drip down ceramic forms creates organic, fluid patterns. Glazes with different viscosities and melting temperatures can be layered to produce unique interactions as the glaze melts and blends in the kiln.
Masking – Applying masks made from wax or latex resist allows glazes to be selectively applied to the ceramic surface. Peeling off the mask after glazing reveals the bare clay body underneath for a striking contrast.
Layering colors – Multiple layers of colored glazes interact in stunning ways in the high heat of the kiln. Transparent glazes layered over opaques produce depth and dimension. Bold patterns can be built up with sharp edges by leaving spaces unglazed between colors.
Thoughtfully applying glazes to accentuate surface textures, carved lines, or embellishments on the clay form brings out the best in both the ceramic work and the glazes themselves. With so many options for creatively decorating ceramics with glazes, the possibilities are endless for producing visually striking works of art.
For more information, see: Create Stunning Ceramic Art: Layering Celadons over Velvet Underglazes
Notable Glaze Examples
Throughout history there have been many famous and significant glazes that have been developed and used on pottery and ceramics. Some of the most well-known historical glazes include:
Chinese celadon glaze – One of the most famous glazes in history originating from China around the 10th century. Celadon glazes often have a distinctive pale green “sea-green” color and crackled texture.[1]
Japanese Raku glaze – Originating in Japan in the 16th century, Raku glazes are known for their unpredictable cracking patterns and colors when removed from the kiln and cooled quickly.[2]
Some more modern innovations in glazes include:
Mid-range silky matte glazes – These glazes bridge the gap between a glossy finish and a matte look, providing a soft satin-like finish.
Crystalline glazes – These glazes contain compounds like zinc oxide that cause crystalline growths and textures during firing, creating unique visual effects.
Rare earth glazes – Glazes containing rare earth elements like copper or cobalt allow vibrant colors and effects.
Glazing Safety
Glaze ingredients can pose potentially serious health risks if not handled properly. Many common ceramic glaze materials contain heavy metals like lead, cadmium, and barium that are highly toxic if ingested or inhaled.[1] It’s crucial to take safety precautions when mixing, applying, and firing ceramic glazes.
Always check ingredient labels and safety data sheets to identify hazardous substances in glazes. Wear gloves, goggles, and a respirator when handling dry glaze materials. Work in a well-ventilated area to avoid breathing in dust. Never eat or drink in the studio where glazes are used.
After mixing liquid glazes, transfer them to clearly labeled containers. Store all glazes securely out of reach of children and pets. Clean up spills right away using wet mopping methods – never dry sweep glaze dusts. Rinse mixing equipment thoroughly before reuse to prevent cross-contamination.
During kiln firing, toxic fumes can be released, so proper ventilation of the kiln area is essential. After firing, allow adequate cooling time before removing ware. Finished pieces should be tested for leaching if they will hold food or drink. [1] Following basic studio hygiene and reading safety data will help mitigate risks when working with ceramic glazes.
