Glaze & Underglaze
Glaze is not like paint.
Glaze is composed of minerals similar in composition to clay that when fired melt into a hard, glass-like surface that helps to seal the clay. The glaze materials are mixed with water and applied to the ware and then fired to melt (flux) and become fused to the clay.
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glaze
Glazes must be fired in a kiln to the appropriate temperature. Usually, glaze is applied to ceramics that have been through a first (or bisque) firing.
Some glazes are meant to be decorative, rather than functional. Check the labeling of your glaze to ensure it’s meant for the purpose you intend (for example, food-safe glazes on the insides of dishes). Some glazes are glossy and shiny, some are dry or matte, some are in between and satiny. Some glazes run while others are stiff.
Glaze melts and can flow when fired, so any detailed applications will probably look mushy and will interact with the glaze underneath. In other words, you can't use glaze like paint because it melts and flows. If you want to make detailed images on your clay, you will want to use underglaze.
underglaze
Underglaze is liquid clay with colorants added to it. Since they are made of the same materials as clay they do not melt like a glaze and remain in place when fired.
Underglazes (also called slips or engobes) can be applied to unfired clay (greenware) and to fired clay (bisqueware).
When applied according to the manufacturer's directions underglazes are opaque and can be used for detailed imagery. Underglazes require a food-safe approved clear or translucent glaze applied and fired over them to become sealed and food-safe.
applying glaze
Glaze can be applied in a number of ways, usually with a brush but some glazes are meant to be applied by dipping or pouring, or even sponging.
Brushing a glaze:
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Use a soft long-bristled brush
Apply 3 fluid coats, allowing the glaze to dry between each coat
Hold the ware in the glaze bucket long enough to build up a thick coat of glaze (usually 3-5 seconds or 1.3-1.5mm thick)
Dipping a glaze:
When glazing, make sure to leave the part of the clay that will touch the kiln shelf unglazed -- or put the piece on a stilt or bisque “cookie” that will hold it up off the kiln shelf.
Learn more about applying glazes by watching our video on the Top 5 Glaze Tips & Tricks
glaze safety
The Art and Creative Materials Institute, Inc. (ACMI) certified product seals (AP Approved Product and CL Cautionary Labeling) indicate that these products have been evaluated by a qualified toxicologist and are labeled in accordance with federal and state laws.
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"Toxic / non-toxic" refers to the unfired glaze
"Food-safe / non-food-safe" refers to the fired glaze
The best practices for glazing safely are to keep
food and beverages well away from your glazing area and to clean up thoroughly after glazing with water. Ingesting and/or inhaling glaze materials is an unhealthy practice. Always thoroughly wash your hands of all glaze.
Once fired, most glazes are safe for food surfaces. Look for the food-safe label on commercial glazes, and fire according to the directions on the jar. We recommend using a single food-safe glaze on surfaces that will come in contact with food or beverages.
AP Seal - Glazes with an AP seal are rated for use by people of all ages
CL Seal - Glazes with the CL seal can be safely used by people ages 13 and older, they should not be used by children 12 or under.
They should be handled with care by all ages.