What's In a Glaze?
Types of Glaze
The term 'glaze' can be used as an umbrella word to encompass all fired finishes, however it more accurately refers to a product that produces a finished result on its own, without needing additional products (like a clear glaze). Commercial glazes are typically divided into two categories based on their firing range: cone 04/06 (low-fire) and cone 5/6 (mid-fire). They can also come in a variety of finishes (matte, gloss, textured, etc), and with an endless range of colors. Glazes are applied to bisqueware (clay that has been through the preliminary firing), and can be used on their own or layered with others to create interesting effects. There are many different sub-categories of glazes such as crystalline glazes, shinos, raku glazes, and celadons.
Underglazes are clay-heavy formulas that are very opaque and stable. One way to think about them is that they are the 'color' without the 'shiny' built in. Because they are so stable, they are often used for detailed paintings and delicate line work. They can be applied to greenware (clay pieces that have not yet been fired) or bisqueware, over or under other glazes.
Also called 'overglazes', these are a special type of fired finish that requires an additional firing after the glaze firing. Gold luster is probably the most common type of overglaze, though there are other options such as Mother of Pearl. These specialty finishes are fired to much cooler temperatures than most glazes (usually in the 019-016 range).
Choosing the Right Glaze
The most important consideration when picking out your glazes is the firing range. It is critical to match your glazes to your clay body. If you are working with low-fire clay, you will want to pick low-fire glazes. For mid-fire clay, choose mid-fire glazes. If you are taking classes at a community studio that uses high-fire clay, they will usually provide you with several glaze options that are formulated for their firing temperatures. Ask your instructor before bringing in outside glazes to use in their kilns.
Some low-fire glazes do well at mid-fire or even high-fire temperatures, but the pendulum does not swing the other direction. If you fire mid-fire glazes to low-fire temps, they will not mature. Instead of looking shiny and colorful, they will be chalky and dull. You also don't want to over-fire a clay body to accommodate a higher-temperature glaze. Over-firing clay can cause it to crack, melt, and/or stick to the kiln shelf. For beginners, we strongly recommend just picking a firing range and sticking with it for a while. Later on, after you've gotten your feet wet, you can start to branch out.
Nearly all commercial glazes are formulated for a brush-on application. To apply a brushing glaze, shake the bottle vigorously to mix the contents before opening. Wipe your bisque piece with a damp sponge to remove any dust. Apply 2-3 coats of glaze to the bisque surface, allowing each coat to dry in between. The first coat will take only a few minutes to dry. The second will take much longer (about 20 minutes). Be patient! The application should be fairly thick, like applying BBQ sauce to a rack of ribs. Alternate the direction of your brush strokes between coats. Most commercial glazes require 3 coats, but check the label to verify before you start. Some glazes, like clears, need fewer coats. Always read the application instructions on the bottle before beginning to glaze.
If you've taken a class in a high-fire studio, you've probably encountered this method of glaze application. Dipping glazes are typically mixed in large batches (3-5 gallons) and stored in lidded buckets. To apply dipping glaze to your piece, stir the glaze thoroughly to re-mix the ingredients. Apply wax resist to the bottom of your bisque piece to repel the glaze. Once the wax is dry, use your fingers or dipping tongs to submerge your bisque piece in the glaze bucket for 2-3 seconds. Remove the piece and allow excess glaze to drain/drip off the piece and back into the bucket. Once the dripping stops, set the piece aside to dry. Clean up the waxed bottom and any unwanted drips with a damp sponge.
Spraying glazes requires specialized equipment, most importantly a properly-rated respirator. Airborn silica dust is harmful to your lungs and should not be inhaled. To achieve an even application of glaze, place your piece on a banding wheel inside the spray booth or other designated area. Fill your spray gun with dipping glaze. Brushing glazes are too thick for this method of application. Commercial underglazes can be thinned with water for use in a spray gun, however be aware that you may need to apply an extra coat or two to achieve full opacity.
To apply the glaze, turn your piece on the banding wheel slowly, spraying glaze in quick, even passes side-to-side until the entire piece is coated. Heavier application will result in drips of glaze, which may or may not be your intent. Multiple coats are needed to achieve full coverage. Spraying glazes is a very gestural method and can provide a wide range of results depending on the artist wielding the spray gun.
Once you have a grasp of the basics of glazing, you might want to experiment with layering. By placing a few layers of one glaze atop a few layers of another, you can get a result that is wildly different from either one of the glazes individually. This is because the glazes contain chemical components that interact with one another during the firing. Independently, they may act one way, but when combined with each other, they create something new.
Though many commercial glaze manufacturers test glaze combinations and publish recommended layering guides, layering glazes is a relatively unpredictable process. It's important to approach this technique with an open mind and no expectations. Also be aware that combining glazes often means they will run/flow more during the firing, so it's a good idea to place a bit of scrap kiln shelf beneath your experimental pieces.