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About Underglazes: Article or Project

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Underglazes are paints that simply go UNDER a glaze. There are primarily two kinds of under glazes . One, designed primarily to go on greenware and another designed to go on bisque. Underglazes can be translucent or opaque. Even though opaque underglazes need multiple coats in order to obtain complete opaqueness, they are not as controllable for translucent effects. Some have more color pigment and less clay. Others have more clay and less color pigment.

An underglaze designed to go on greenware, is a clay base suspended in a liquid with color pigment and chemicals that produce a color when subjected to heat. Because it has a clay base, it bonds well with the greenware. Most usually, you do not see the final color until after it is fired. There are some companies who add a dye or coloring agent to the ‘paint’ so you can visually discern it from other paints and some companies try to color the paint similar to what it will look like when fired. These dyes however, fire out and are replaced by the true chemical/color pigment. These added dyes are helpful to discern where you have applied which paint to your item. Being clay base, these paints readily soak into your greenware and become one with the piece.

The second type of underglaze, designed to apply to bisque, is a synthetic base with chemicals and color pigment and it lays on top of the bisque, requiring the glaze to permanently affix it into place. As with the clay base paints, the true coloring is not visible until after it is fired. Often, these underglazes when applied to greenware, will not bond as well and have been known to chip or flake off after firing especially if applied too thick.

It is important to know your paints and how they interact with your greenware, bisque and firing. Just because something is called underglaze, does not mean it will suit your purpose.

NOTE: Some Texture Glazes are actually underglazes with specks of glaze (glaze crystals) mixed in to give texture contrast both tactile and visual. Because of the addition of glass particles, they are referred to as glazes. Some can be used for greenware application and some for bisque only. It is necessary to stilt your items when using texture glazes as the bits of glaze can adhere to the kiln shelves. Therefore, it is not recommended to apply them to the bottoms of greenware as the ware may warp during firing if stilted. Again, KNOW YOUR PRODUCT - read the instructions and labels.

When EPA and CPA got involved in the production of paints, the USA manufacturers had to change many of their formulas to come into compliance. The result has been the loss of many great products, but has also spurred the creativity and development of many new ones. In recent years, manufacturers have come up with formulas that work well on both bisque and greenware. In fact, some of them have glaze ingredients and they actually shine somewhat without a glaze over them. Also, colors that were hard to achieve previously have become improved and much more vivid. This development has brought about a resurgence of interest in working with fired finishes that pretty much fell to the wayside during the stain and dry brushing craze.

 

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