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Ceramic Glazes
When working with ceramics, the final creation ends with a glaze. Ceramic and pottery glazes are liquid coatings applied to fired clay objects to add color, texture, and a protective finish. The glaze fuses to the surface of the pottery during a second firing, creating a glassy and durable coating. Here at Blick, we source a diverse range of glazes from top brands.
Factors to Consider When Selecting Glazes
- Firing Temperature: To ensure optimal results, it is important to choose a glaze with a firing range that matches the clay’s firing range.
- Clay Body: Consider the porosity, surface texture, and composition of the clay, as some glazes adhere better to certain types of clay bodies.
- Desired Finish: Common glaze finishes include glossy, matte, satin, and crystalline.
- Functionality: If your piece is meant to be food-safe, microwave-safe, or similar, double-check that your glaze meets the criteria.
Underglazes
Underglaze is essentially a type of paint designed for ceramic applications. While not strictly necessary, underglazes come in a variety of colors and are used to decorate your piece. You can even use sgraffito tools to scratch through a layer of underglaze and reveal the clay body.
Firing Process
- Apply underglaze to the unfired, dry clay piece. To achieve deeper color, apply 2-3 coats of underglaze.
- Once the underglaze is dry, place the clay piece in a kiln and bisque fire it to harden the clay body. Bisque firing refers to the first firing of a ceramic piece and it prepares the clay for glazing.
- When the piece cools, apply a layer of glaze (clear or colored) over the underglaze.
- Place the clay piece in the kiln once again. Glaze fire the piece to melt the glaze and fuse it with the clay and underglaze. This adds a final protective layer and completely seals the surface.
Firing Temperatures
- Low-Fire Glazes: Low-fire glazes are commonly used for earthenware and mature around cone 06 to cone 04.
- Mid to High-Fire Glazes: Mid to high-fire glazes are commonly used for stoneware and porcelain. These glazes mature around cone 5 to cone 10.
Testing Your Glaze
To ensure you’ve selected the right glaze, test a small sample before applying it to your ceramic pieces. Testing helps you understand how the glaze will interact with your specific clay body and firing conditions. Additionally, testing helps to visualize any special effects you may be trying out with the glaze. When layered, glazes can often create different visual effects and colors. Small batch testing is a great way to experiment with these techniques without sacrificing a large project.
Frequently Asked Questions
How do I know if a glaze will work with the clay I have?
It's crucial that clay and glazes be matched for firing temperature. Both low- and mid/high-fire clays can bisque at cone 04, but the recommended glaze firing range will be different for each. Glazes need to mature in the recommended glaze firing range for the clay. Fire the glaze too low and it won't mature, too high and it may discolor or flow too much. Fire the clay too low and it won't vitrify enough to be durable, too hot and the clay will soften and distort or may even melt. Also, we always recommend firing test tiles to verify that the glaze is a good "fit" for the clay. Most of the time commercial glazes and clays work together, but it's still possible for them to not have the same rate of expansion and shrinkage, which can result in glaze crazing or shivering.
Can I fire hotter than the glaze states?
Many glazes can fire hotter than the specified firing range, but the glaze will typically flow more, and certain colors may fade, darken, or otherwise change in appearance. Consult the manufacturer's guidelines to determine whether the glaze you have in mind can take a hotter firing and be prepared to fire test tiles to make sure you know what to expect before applying to actual ware.
Can I use a glaze that fires hotter or cooler than my clay is intended for?
This is not a good idea. Firing the clay hotter than the recommended range can destabilize or even melt clay that's meant for lower temperatures. Firing the glaze too low will not provide the heat necessary for it to mature to its expected appearance. The clay and glaze should be matched for firing temperature.
What's the difference between Food Safe and Dinnerware Safe?
"Dinnerware safe" means the glazed surface meets food safety standards when it is properly applied and fired. These glazes have a smooth, durable, and crack-free surface and are suitable for food and utensil use.
"Food safe" means the glaze does not contain harmful substances (such as lead or cadmium) and has been tested to ensure it doesn't release toxic materials when in contact with food. It does not meet standards for regular food use.
If neither of these standards are mentioned, the glaze should be used for decorative applications only.
Can all glazes be used on dinnerware?
No. Only glazes that state "dinnerware safe" are appropriate for regular food and utensil use. Please verify prior to purchasing that the glaze is acceptable for dinnerware if that is your intended use.
Will the clay body affect the appearance of the fired glaze?
Absolutely. It's always a good idea to fire test tiles with your specific clay to see the results you can expect before applying to your finished piece. The photos on our website will typically only show you the results with a single clay body.
My glaze is not the color I expected out of the jar. Should an Aqua glaze really look red in liquid form?
Ceramic glazes can look very different in liquid form compared to the fired result. Don't be alarmed if the liquid color is quite different.
I don't find the glaze color I'm looking for. Can I mix glazes to get different colors?
Sometimes you can. Look at the product description for the glaze to see if it notes that colors can be mixed. Otherwise, just know that any mixtures will be experimental; yellow+blue may not give you green for example, and take meticulous notes so you can reproduce any favorable outcome.