Slips,  Glazes  and  Wet  Surface  Techniques  

Slip

What is slip? Slip is a liquid clay body containing clays, flux and filler. It is used for joining 2 pieces of clay together and for decoration. Colorants are added to slips to give it color for decoration. When fired slips have a dull surface – just like your bare clay body. In most cases you want to apply a glaze on top of the slip.

Studio Slip Studio slips are the buckets of slip found in the studio. They are liquid clay with colorants added for color.

Underglaze Underglazes are commercial slips that come in small jars. They are liquid clay with colorants added for color. Do not let the term “glaze” in the name fool you. These are not glazes and will not have a satin or gloss finish when fired.

Engobe An engobe is a slip. It is called a vitreous slip because it fluxes (melts) a bit. This means it is a bit shiny and does not require a glaze on top. The studio does not have these.

Firing Slips are formulated for specific temperatures. Never over-fire a slip. Slips fired too hot may turn to glass and melt off your work. Over-fire means you are firing to a temperature above its maturation – firing too hot. Slips may be under fired (firing to a temperature lower than maturation) but may flake off because they never fully melted.

Application Apply in layers allowing each layer to dry before adding the next. Depending on consistency of slip, apply 1-3 layers (brushed). If a slip is on too thin it will look “sketchy”. If too thick it may flake off. Slips are formulated for a specific stage of clay. Most of the time slips are applied at the leatherhard stage, but formulas for bone dry and bisque also exist. Be sure you are putting the right slip on your piece. If you do not match the appropriate slip to the wet/dry state of your object, it may flake off before or after the firing. Look at the labels.

Studio Slip

It is best to apply the studio slip at leatherhard. Be sure you are reading the labels on the buckets and using the correct slip at the correct time. Most of the studio slips are for wet/leatherhard but a few are for bone dry/bisque. Aside from the black, studio slips give pastel or duller colors than underglazes. Studio slips are less forgiving than commercial slips (underglaze) and a little harder to apply. You must fire them to the temperature stated on the label.

Underglaze An underglaze can be used on wet, leatherhard, bone dry and bisque. They are much more stable and easier to brush on. Underglazes have very vibrant colors. In ceramics colors like bright red, orange and purple are impossible to achieve without an underglaze. If you want brighter colors plan to purchase some underglazes.

Underglazes can be fired at different temperatures. The label may say fire to cone O4 but many will go as high as cone 10. Some of the underglazes will loose their color at the higher temperatures. If you purchase an underglaze, many of the Velvet Underglazes” by Amaco will go to cone 10. See their glaze chart on their website at www.amaco.com

Wet Surface Technique

Wet surface techniques refer to any surface application done while the clay is still wet. See end of document for examples of the following techniques.

Brushing Slip decoration adds depth and interest to an object. It can be used to change the color of your clay body or to add designs to your surface. Designs can be brushed on using a paint brush. Underglazes are much better for this application because they flow much smoother. Slip decoration can be applied at wet, leatherhard, bone dry and bisque depending on the type of slip or underglaze you are using. Read the label.

Scrafitto Scrafitto means “scratch the surface”. Using a needle tool, pencil or ball tipped tool, a design is drawn into the surface. For a clean line it is crucial that the clay be leatherhard. When drawing into the surface burrs of clay will form at the edge of the line. DO NOT REMOVE. Once the work becomes bone dry you can remove burrs by a finger or brush. Removing them too soon results in the burs going back into the line. For a greater effect, apply decorating slip to the surface and scrafitto through the colored slip.

Carving When carving, clay is being removed from the surface. A wire or ribbon tool is used to scoop out the clay. For a clean carve it is crucial that the clay be leatherhard. For a greater effect, apply decorating slip to the surface and carve through the colored slip.

Mishima Mishima is a technique of inlaying slip, underglaze, or even clay into a clay body. By first carving or stamping a design into the surface, slip is then added into the carved surface. Once the slip firms up a bit, the surface around the carving is scraped with a rib leaving behind the colored slip in the carved design. Mishima must be done at leatherhard.

Slip trailing Slip trailing leaves a raised surface. A slip trailer or something with a small nozzle like a hair-coloring bottle is filled with decorating slip. Similar to decorating a cake, the slip is squeezed through the nozzel onto the surface.The consistency of the slip is crucial to the success of the decoration. If it is too thick it won’t come out of the bottle. If it is too thin it will not create a raised line. Slip trailing should be performed at wet or leatherhard.

Stamping Stamping occurs when an object or texture is imprinted into the clay. Anything with a raised or recessed surface can be used to stamp into clay. This must be done at the wet state of clay. Stamping results in a beautiful surface once glaze is applied

Glaze

What is glaze? Glaze is the name of the “liquid clay/glass” coating applied to bisque ware to create the desired surface effect. Glazes are primarily silica (glass) combined with other material to get the desired color and surface texture.

It has been said that there are no “bad” glazes, only poor application of the glaze being used. Also that 80% of the success ratio in glazing is due to application. Since glaze is not an air temperature pigment, the fired color and surface quality is emitted when the glaze recipe (chemical components) are heated to the correct temperature for maturing the melt of the materials in the mixture. Glazes come in different colors, surface qualities and textures. It is important to be aware of the character of the glaze you are using. Some glazes are very active and may run off the ceramic ware. Others are not food safe and cannot be applied to utilitarian ware. Be sure to read labels on test tile board. It is important to know if the clay body can be seen through the glaze. It is a waste of time to apply painted slip designs if the glaze is going to obstruct it.

• Transparent: Light can pass though. The clay body and any designs can be seen. Good for all wet surface design.

• Transluscent: Light can pass but it is obscured. The glaze will be a bit cloudy. Changes in color can be seen but intricate details and designs may be lost. Good for carving and stamping.

• Opaque: Light does not pass through. The clay body and any decoration will be lost. Good for carving and stamping.

Overglaze

Over-glazes are applied after the glaze firing and over the glaze. Once an over-glaze is applied, the work is fired again at a much lower temperature (022-016). Some artists will fire their work 5-10 times all the while building their surface with over-glazes. Types of overglazes: China Paint, Enamel, Luster

Test tile

Test tile A test tile is a sample of the fired glaze on a piece of clay. Although the tiles give the artist an idea of what the glaze will look like, there are many factors that may alter the appearance of the glaze. Some of these include:

• Improper measurement of an ingredient when making the glaze.

• Not fully mixing the liquid glaze before application. The glaze separates and all the good stuff falls to the bottom. You must mix well.

• Contamination of the liquid glaze. This is often caused by dipping an object into one glaze and not waiting until it is completely dry before dipping into another.

• Improper application of the glaze to the ware. Usually too much or too little glaze.

• Placement of ceramic ware in the kiln. Test tiles take on many shapes and sizes. It is up to the artist to decide what their test tile should look like based on their needs. Every test tile should have an area of texture and at least two levels of thickness. A test tile should be made with the clay the artist is using. The clay body greatly affects the glaze.

Clay bodies

Firing

All glazes are formulated for a specific firing temperature. It is crucial that the glaze reach that temperature without being under or over-fired. An over-fired glaze may loose its color and run off the piece and onto the shelves making a huge mess. An under-fired glaze may not melt at all and become bisqued on. This is usually not attractive. It is dry and lumpy.

Application It is important to have an adequate thickness of the coating of glaze applied to the piece. On the average the glaze coating should be the thickness of a playing card or approximately 1/32 of an inch. Thickness of applied glaze can be checked when the glaze dries completely (five minutes on the average) by scratching through the glaze with your fingernail. This will reveal the thickness of the wall of glaze.

Techniques You can ally glaze a number of ways. Here are a few: Dipping

Spraying

Pouring Brushing

Sponging

Considerations Considerations when you are applying glaze: The thickness of the glaze liquid will vary and must be taken into consideration when glazing. If the glaze is applied too thick then your glaze may run or flake off. Too thin and the work may look as though it was never glazed or the color may not be what you expected. The length of time that the ware is dipped/poured into or over the ware will determine thickness. Submerging ware in a glaze for 10 seconds will be thicker than 5 seconds. The thicker the wall of the piece the more glaze it will absorb. Conversely, thin walled pieces can become over saturated with the glaze liquid. This will cause thinner application and problems in drying the glaze on the surface. Allow each layer to dry before applying the next. If the glaze is applied too thick then your glaze may run or flake off. Too thin and the work may look as though it was never glazed or the color may not be what you expected.

Brushing

Scrafitto

Carving

Mishima

Slip Trailing

Stamping