As the name implies, these are not glazes. They are decorative colors that need a clear glaze over them to provide the sealing and durability qualities only a glaze with its glass can provide. Technically, they are any coloring oxide or combination of oxides (such as stains) use alone or mixed with other media such as gum or clay, that are fired onto green or bisque ware. Practically speaking, one could call glazes the glass coating and underglazes a colored clay coating. This definition would include slips and engobes as underglazes. Additionally, colored pencils and chalks and terrasigllata would fall under this broad underglazes definition.

The reason for using underglazes is to achieve different effects than can readily be achieved with a glaze. Since underglazes do not melt, becoming fluid and moving on the ware, more precise designs can be applied and two colors can be butted next to one another without losing definition. Traditionally, they are formulated to be applied to greenware. The high clay content and lack of fluxing or melt means there is high shrinkage comparable to greenware and little binding or fusing to hold the underglaze on the ware. Thus, without any additions, they are likely to crack off in the firing, or before, if applied to bisque, especially if a thick coat is applied. Decorating on fragile greenware risks breakage of the ware through handling, however. Be sure to read the labels of any commercially produced underglaze to determine whether it can be used on bisque. If not, the addition of flux will help. A simple addition of 2 to 3 tablespoons of clear low fire glaze to a pint of underglaze will generally do.

Note that most underglazes can be used as majolica-like decorating colors painted over an unfired glaze. In addition, commercial underglazes for bisque and properly fluxed others can be used as traditional over glazes, applied to an already fired glaze and refired. This works best on flat horizontal surfaces, such as tile. To get underglazes to 'stick' to a vertical glazed surface, the following may help:: 1) warm the ware, 2) add gum to the underglaze, 3) replace some or all of the water in the underglaze with alcohol. Test first, of course.

Application to Greenware:

Underglazes should be applied on a well-sponged, dust-free surface. Use a soft, good quality sable or camel hair brush. When trying to achieve a water color effect, load brush with color and apply underglaze with one continuous stroke. For over-all solid coverage, it is important to lay down or scrub in first coat of underglaze. This will act as a foundation for the subsequent 2 or 3 coats, each coat should then be applied diagonally across the previous coat. Bisque fire Cone 05-04.

Application of Bisque:

Basically the same instructions given for greenware may be followed for use of underglaze on bisque. Care must be taken that bisque is not too porous as it will have a tendency to grab or absorb too much color and thus give you a build-up which could peel or chip off. Due to the porosity of bisque, you may find it helpful to dampen the bisque before decorating, however, best results are obtained on greenware.

OVERGLAZING

Overglazes take on finish of the underlying surface.  For example, a matte glaze surface will yield a soft matte Overglaze, while a gloss glaze will yield a bright shiny Overglaze.  

When applying overglazes it is critical that both the object and work area be clean and dry.  Any contamination by dust or moisture will interfere with the overglaze application resulting in defects.  Clean the glaze surface before application with alcohol and allow to dry completely.  Use a clean, dry, soft bristled brush.  Saturate the bristles with overglaze and work out the excess.  Use firm, even strokes one next to the other.  Apply as evenly as possible.  Allow to dry before firing.

During firing, kiln should be stacked loosely and well ventilated.  

The following applies to oil-based Overglazes.  

There should be no moisture in the brush or you can have bare spots in the fired finish.  For this reason, you should not clean your brushes in alcohol which contains water.  Instead use a brush cleaner.  It is best to keep one brush per color to be used just for that color.  Clean brushes immediately after use.  

Overglazes can be thickened by placing a small amount on a glazes surface and allowing some of the medium to evaporate.  Thickened overglaze can be re-thinned with Hanovia Essence or Gold Essence.

When using both a metallic and a luster color on the same piece, it is best to avoid contamination by firing them one at at time.  Fire the metallic first, then the color.

WATER PROOFING

How to Seal and Waterproof Ceramics

I often get asked this question.  First, if you are firing stoneware or porcelain,  Your best means of achieving a waterproof piece is to fire your clay body to the right temperature.  But there are cases where you cannot make your ware completely waterproof. 

For example, HIGH firing does not achieve high enough temperatures to make the clay waterproof.  Low fire clay also is not waterproof.  Both will leak over time, if you leave water in them.  This will probably happen even if you use glaze, because it is very difficult to get a perfect clay/glaze fit which will expand and contract together and not craze.  (I have a mug that is a cone 10 stoneware fired at cone 6; it is fine with cold water, but when I put hot water in it the bottom becomes damp.  What is happenng is that when heated, the glaze probably expands and exposes tiny cracks, the hot water seeps through, and then through the clay.  This will weaken the clay over time, and this piece definitely should not go through the dishwasher or in the microwave!)  

The most common things I have heard potters say they use for waterproofing are:

- Tung Oil (available in hardware or woodworking stores

- Thompson's Water Seal (available in hardware stores, used for sealing wood and concrete)

- Acrylic Floor Polish (ie Futura.)  (available in the grocery store.) 

Note that I would not use any of these for dinnerware.  Just for vases, flowerpots, etc.  And some people use them over glazes just to get a nice subtle shine.  And I have been known to use them on the bottom of pots to make them less likely to scratch furniture. 

Another reason potters want sealers is to protect painted work.  For example, people that use acrylic paints on bisque and want to protect the paint.  We do sell Duncan spray and brush-on sealers in flat and gloss, which will protect the paint from chipping and smudging, and give it a shine if you wish.  They are good for decorative pieces.  But these are not waterproof.  So if you want your piece to be waterproof, try one of the above products.  But since those items work by penetrating the clay, and they probably won’t penetrate through the acrylic paint, I would put it on the inside of the piece, then protect the painted outside with an acrylic spray sealer like the Duncan products (or similar items available in the hardware store.)  So for example, if you want to waterproof a flowerpot, put the tung oil, Thompson’s water seal, or acrylic floor polish on the inside of the pot.  Paint and seal the outside.

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basecoating

antiquing

drybrushing

stippling n spouge colouring

floating colours mixing colours

plaids

using accesessories