On-glaze colors are a highly popular ceramic decorating technique, especially for creating vibrant patterns on white dinnerware sets. The key feature of these colors is their low firing temperature (approximately 700–900 °C), which means they don't react much with the original glaze.
However, due to their limited firing temperature range, some colors are highly sensitive to high heat, especially red and magenta. If the temperature deviates by just 10–20 °C, the color may shift, change from its original hue, or even melt away completely. Therefore, ceramic artisans must always observe and test the results after each firing.
Preparation of On-Glaze Colors
For general on-glaze color preparation, the process begins by mixing 10–20% of high-temperature synthetic stain color with approximately 80–90% of frit fired at 600–750 °C.
- Grind all ingredients together, then fire them until they melt into various glossy colored glasses.
- Grind and wash the resulting colored glass again.
- Continue grinding until fine, with more than 90% of the pigment particles smaller than 8 microns.
The prepared color can be used to coat metals, print patterns on glass, or decorate glass bottles. However, most ceramic artisans do not typically mix their own on-glaze colors because the frit preparation process is quite complex and requires high precision.
Decal Application Technique and Ceramic Clay Body Factors
The most popular on-glaze decorating method is using ceramic decals, printing patterns, and then applying them to tableware products that have already been glazed and fired. These pieces are made from various clay bodies, including:
- Bone China
- Porcelain
- Vitreous China
- Stoneware
- Earthenware
Since the glazes for each type of clay body require different firing temperatures, the firing temperature for decorative on-glaze colors must be adjusted slightly according to the characteristics of that glaze layer.
Precautions for Using On-Glaze Colors
In addition to decorating white surfaces, many people also like to print on-glaze colors onto colored glazed products, but there are two important precautions:
-
Beware of using glazes containing copper oxide: Copper will absorb printed colors, turning them gray after firing. This is especially true for red and yellow, which can easily shift.
-
Avoid printing on dark glazed backgrounds: This is because the on-glaze color will be absorbed by the background color and become invisible, unless white is printed over it in several layers to achieve extra thickness.
Safety Standards and Ceramic Product Regulations
Because on-glaze colors are commonly used on food-contact items, there are strict laws and consumer protection measures regarding this type of decoration. This includes testing for the leaching of lead and cadmium.
-
Testing method: Submerge the item in a 4% acetic acid solution in a room temperature bath and in darkness for 24 hours.
-
Results measurement: The acetic acid solution is then tested for lead and cadmium content. The leached solution must be below the legal standard to pass safety criteria and be sold.
Modern On-Glaze Colors: Easier to Choose, Safer
Today's on-glaze colors have advanced significantly. Options are available for offset printing, allowing colors to be layered or mixed without adverse reactions after firing. Most of these are Zircon-based colors.
Furthermore, there are various choices, including low-lead on-glaze formulas, lead-free formulas, and even fast-firing colors. These advancements make ceramic creation easier and meet both aesthetic and safety requirements.
For anyone looking for ceramic materials, equipment, or advice on choosing colors and glazes, Pottery Clay is ready to be your creative partner.
(Reference: Ceramic Colors by Paijit Ingshiriwat)