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An Introduction to Spectrum Underglazes for Ceramics

Not quite a glaze and not quite a slip, underglazes allow you to color your work without worrying about how it will look after firing. It’s easier to control than overglazing as well, and the results will often match what you see unfired.

Spectrum’s 500 Series underglaze colors are all lead free, dinnerware safe and A/P non toxic when covered with a clear glaze. They are usable at nearly any temperature range and can be applied to leather hard greenware or bisque.

Basic Ingredients

Underglazes are paints that are applied to a ceramic surface before it is covered with a transparent glaze and fired in the kiln. They can have a velvet-like matte finish when used without a glaze or they can be made glossy by applying a clear glaze over them. They are made with pigments derived from oxides.

Underglazing is a decorative technique that allows for intricate brushwork and patterns on the surface of your ceramics. It is a way to create a surface decoration that is food safe and durable when covered with a clear glaze. Essential pottery supplies like this underglazes, are available in a wide range of colors and can be mixed with each other or with other underglazes to achieve varying intensities of color and transparency. Underglazes are lead free, non-toxic and dinnerware safe when covered by a dinnerware safe clear glaze

Unlike pure clay body stains, underglazes do not bond with the pottery surfaces they are painted on. They are formulated into a recipe of materials with a base medium to impart the necessary properties to be compatible with a variety of clay bodies. Commercial underglazes contain a variety of additives to keep frit and stain suspended and to make the underglaze paintable.

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These orange underglazes on this bowl were not properly fluxed (melted) so that they adhere to the body. This is a common problem with many commercial underglazes. The underglaze also has a high gum content that seals the surface of the bisque and inhibits its ability to pull in glaze water during dipping. These slow absorption rates cause the underglaze to dry with a rough, flaky surface that can generate bare spots. The solution is to decorate ware in the leather hard or dry (bisque) state or to use an underglaze that has a lower gum content.

Preparation

Whether you are creating intricate details on a functional mug, painting a landscape on a decorative vase or exploring abstract patterns on a sculptural piece, underglazes allow you to express your artistic identity with each brushstroke. As a slip or pigment, spectrum underglazes can be applied in a variety of ways including brushing, dipping and spraying, and they can be covered with transparent glazes or left unglazed for an organic textured surface.

To make underglazes you will need to combine frit, kaolin, silica and metal oxides in the proportions specified by the color recipe. Mix dry ingredients together thoroughly and then add water slowly while continuously mixing until a smooth slurry consistency is achieved. Use a ball mill or mortar and pestle for smaller batch sizes to ensure that the particles are well dispersed.

Pure ceramic stain powders don’t melt at typical pottery temperatures and do not bond with clay body surfaces. This is why underglaze manufacturers dilute the stains with a medium, a ‘base glaze’, to impart these properties.

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To create a high-fired underglaze you will need to incorporate high percentages of stain. These diluted colors are very bright and very expensive to produce, which is why commercial underglazes are often double or triple the cost of a comparable transparent glaze from the same manufacturer. The top mug on the left was painted with V-326 underglaze and the bottom mug with Amaco LG-10 underglaze, both glazed and fired at cone 04. Both are dinnerware safe when covered with a clear glaze.

Application

Whether you are brushing, dipping, or spraying your ceramic creations, you can add intricate patterns and vibrant color with underglazes. These underglazes are essentially colored slips or pigments that are applied to greenware or bisqueware before the final glaze firing. The underglazes are easily applied using a wide variety of methods such as painting, brushing, and sgraffito techniques.

Before you begin creating your underglazes, prepare your workspace by setting out your raw materials and equipment in a well-ventilated area. Always wear appropriate safety gear when working with raw materials and mixing chemicals. Read and follow all instructions provided by your manufacturer when handling and mixing your ingredients.

The preparation of your underglazes can take several days, so be sure to plan ahead! Once you have mixed your colors and created a consistent consistency, store them in airtight containers. It is a good idea to conduct test firings with your underglazes before applying them to your work to ensure the best results.

Spectrum offers a full palette of rich, vibrant colors that are opaque with 3 coats, semi-opaque with 2 coats and semi-transparent with 1 coat. These underglazes are lead free, dinnerware safe and A/P non-toxic when covered with a dinnerware safe clear glaze. They can be applied to greenware, bisque and raw clay and fired at nearly any temperature range.

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Firing

Underglaze colors look very different before firing than they do after. This is one of the biggest challenges with brush decorated pottery because it can be difficult to see what the decoration will actually look like until after the kiln fires and the glaze covers it.

Commercial underglazes mix stains with a recipe of materials to give them ceramic properties. Pure stains do not melt at typical pottery temperatures so they can’t bond with clay body surfaces. This causes problems when the stains are too potent or not mixed properly.

Spectrum 500 series underglazes are lead free, dinnerware safe and A/P non-toxic when covered with a clear glaze. These underglazes can be applied to a dry bisque or greenware. They are designed for use at a wide temperature range and can be used on oxidation or reduction fired clay bodies.

Spectrum’s 600 series one-stroke underglazes are a more concentrated version of the underglazes in the 500 series. They are designed to provide opaque coverage with a single brush stroke and they work great under our Glossy and Milky Clear cone 6 mid-range glaze recipes. These underglazes are also dinnerware safe when covered with a clear glaze that is compatible with your firing temperature.

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