PPP: Using Decorative Engobes
CharacteristicsEngobes can contain clay(s), fluxes, fillers and other materials, depending on the requirements of the coating. As previously mentioned, the terms slips and engobes have been used interchangeably over the years. In the past, however, slips were referred to as a suspension of only clay and water.2 For example, Albany slip, a once common high-iron-content clay, was used to coat utilitarian stoneware pottery when combined with water to produce brown/green glazes in wood and salt kilns.
An engobe is not as glasslike as a glaze, but it can be slightly more vitreous than the clay body it covers. Engobes contain many of the same raw materials found in both clay bodies and glazes, but they are used in different ratios. They can be applied to ceramic pieces that are in the leather-hard state (and shrink more than engobes), or applied to bisque ware (requiring an engobe that shrinks less).
When using either type of engobe, the critical goal is to achieve a compatible fit with the clay body. In some ways, engobes can be more difficult to use than glazes. While glazes only have to match the clay body, engobes have to shrink at compatible rates when applied to the clay body and must also fit the covering glaze.
ExamplesThe Zam White Engobe, Easy Engobe and Tom White Engobe are designed to fit leather-hard moist clay and can be fired from c/06 (1828°F) to c/12 (2383°F). They can be used in oxidation, reduction, salt/soda and wood kiln firing atmospheres, but colors may vary depending on temperature, clay body color, application thickness, and kiln atmosphere.
It is always best to test the engobes on your own clay body to ensure a compatible fit with the underlying clay body and the covering glaze. Whether the clay body is wet, leather hard, bone dry or a bisque application, the clay body should be free of dust and surface particles before applying the engobe. In wet to leather-hard applications of engobe, bisque fire the test tile; after that, spray, dip or brush the covering glaze. Place the test tile in the kiln and fire to the appropriate glaze temperature. (Keep in mind that small test kilns may not produce accurate results due to their faster rates of heating and cooling, and lower thermal mass.)
Often, the potter's clay body can be the basis for an engobe. In many situations, the engobe will fit consistently as it shrinks at a similar rate as the underlying clay body. However, if the clay body is dark in fired color, a lighter engobe color would not be as easy to achieve from the same clay body.
To prepare, weigh the dry materials to total 100 g, adding approximately 90 g of water. Place the wet mixture through an 80-mesh sieve before applying to leather-hard clay. The engobe must be the correct consistency for the intended method of application. It should be homogeneous, free of coarse particles and air bubbles, and remain in suspension.
Optionally, two to four drops of Darvan 7, Darvan 811 or sodium silicate can be added to one quart of engobe during the mixing procedure. These sodium-based compounds deflocculate the engobe, which takes the place of some of the water and reduces shrinkage to help the engobe form a mechanical bond to the underlying clay body.
Potential UsesEngobes offer potters an alternative method of introducing color and texture to the ceramic surface. They can be used as accent colors on unglazed clay, or a glaze coating can be applied over the engobe. In addition, engobes can be used to mask the color of the underlying clay body or to introduce new color variations that can interact with covering glazes.
Engobes can also respond to the kiln atmosphere in reduction, wood, salt and soda-fired kilns. In some instances, engobes can be applied thickly, resulting in a raised surface that can add another dimension to the ceramic form. It is always amazing how many "old techniques" can be rediscovered and used on contemporary ceramics.
References1. McColm, Ian J., Dictionary of Ceramic Science and Engineering, Plenum Press, New York and London, 1984, p. 114.
2. Cushing, Val M., Cushing's Handbook, third edition, p. 25.
About the AuthorJeff Zamek started Ceramics Consulting Services in 1980. He works with individual potters, ceramics companies and industry, offering technical advice on clays, glazes, kilns, raw materials, ceramic toxicology, and product development. In addition to Pottery Production Practices and the PPP HotSpot, he is a regular contributor to Ceramics Monthly, Pottery Making Illustrated, Clay Times, Studio Potter, Ceramics Technical and Craft Horizons. Jeff's books, The Potter's Studio Clay & Glaze Handbook, What Every Potter Should Know and Safety in the Ceramics Studio, along with The Potters Health & Safety Questionnaire, are available from Jeff Zamek/Ceramics Consulting Services. For additional information, call (413) 527-7337 or visit his website at www.jeffzamek.com.
SIDEBARZam White Engobe
Grolleg kaolin 25
Kentucky OM #4 ball clay 10
Nepheline syenite 270x 14
Flint 200x 10
Soda ash 5
Ferro frit #3195 10
Vee Gum CER 2
Kentucky OM #4 ball clay 70
Custer feldspar 20
Flint 200x 10
Tom White Engobe
Helmer kaolin 60
Grolleg kaolin 25
Nepheline syenite 270x 15
Mason stain #6404 8%
Spectrum stain #2044 10%
Spectrum stain #2083 10%
Spectrum stain #2004 12%
Spectrum stain #2033 10%