Full Project – Development of colored Engobe composition for local structural clay products

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CHAPTER ONE

• INTRODUCTION

Engobe is a layer for covering the surface of the clay bodies with a different color. It gives a smooth surface to structural clay products e.g. Tiles, after application the color is provided to the Engobe composition by addition of small amount of metal oxides. Therefore the structural clay piece can have an aesthetic appearance with different colors. Engobe are sometimes also called as slip or as an intermediate layer between clay body and glaze. Gminshaws (1971), Engobe is usually applied as an underglaze. Glaze layer is applied on the Engobe layer as a final coating layer to impart a shiny appearance to the clay body. A low melting additive of frit is often incorporated into the Engobe mixture to assist earlier vitrification upon firing. There is often less proportion of frit in Engobe than in glaze. The latter is more glassy than the former. The amount of frit adjust the hue of the Engobes are less shiny than glaze. In other words, Engobes have a porous structure. Porous structure of the Engobe according to Zanger (2002), allows passage of gases that may form during firing of the ware. If the Engobe layer is completely vitreous than gases cannot escape and blow-holes can form,

marring the surface of the ceramic. It is always advisable that the body and the Engobe should be compatible together during both drying and firing. This shows the quality of the Engobe. In addition the firing temperature of the Engobe should be near the firing temperature of the clay body (substrate) in which it is covered secondly the color of the substrate should be covered completely. The color of the Engobe is affected mainly by the amount of the iron in the Engobe. So the raw materials for making an Engobe should be chosen according to their purities. According to Rhodes .D. (200), Engobe should covers the substrate well without peeling and surface defects are not desired for Engobes. Calcium oxides (CaO) is used in the form of Ca.mg (CO₃)₂ dolomite. CaCO_3­ is effective to get while color in the Engobe it provides a matt apperanle when used too much in the Engobe composition because of increasing the refractory inclination MgO, however, gives a shiny appearances.

1.1    LIMITATIONS

Engobe composition must be formulated to enable the following:

1. Cover the ceramic ware with suitable dense coating of the desired color,
2. Cling to the ceramic ware during the shrinkage.

• It should be able to vitrify or harden at a temperature similar to or some what cover them the maturing temperature of the ceramic ware on which they are used.

1. Survive under the glaze coating (under glaze) without peeling or dissolving into the glaze

Engobe are often used on a dark clay bodies to provide a bright, white background for glaze. A damp body when applied with Engobe tends to have bright, white background which may or may not be further glazed.

1.2    APPLICATION OF ENGOBE CERAMIC WARE

Engobe can be applied at three different stages of the ware namely leather hard (damp) bone dry and bisque. This is its major advantage over slips. It also contains filler which is lacking in slips thereby reducing shrinkage.

1. LEATHER HARD (DAMP): This is the best stage of applying Engobe as it permits the widest range of decorating technique (brushing, incising, inlaying stencil etc). The Engobe must have enough clay in it to shrink at the same temperature rate as the body.
2. BONE DRY: Engobe for bone dry application needs to have less shrinkage so that they adhere to the body.

• BISQUE: The Engobe for bisque application are more like under fired glazes. Usually, Engobes designed for plastic clay will not fit on bone dry or bisque clay and vice versa. They are made up of mixture of plastic and non-plastic ingredients. Example short and long clays.

1. PLASTIC INGREDIENTS: For white Engobes, the plastic ingredients of ball clay can be adjusted to get correct shrinkage.
2. NON PLASTIC INGREDIENTS: The non plastic ingredients are feldspar and quartz. For high temperature Engobes, frit is added to lower the vitrification point. Borax in small quantity is added to giver better dry strength and to fuse the other ingredient together. (Borax as a hardner).

Recipes for Engobe looks like those for glaze and slip with difference in clay compositions. Production and application on wares will continue to play important role in the ceramics industry at large as well as improving the potter’s quest for colored texture, also reduces the high cost of glazed raw materials as this slips like materials will also give if not the same thing as glazes.

1.3    STATEMENT OF THE PROBLEM

1. i) Body composition: for the fact that Engobe composition must have the same amount of shrinkage as the body, if the composition is not given a good fit, it will tend to flake or crack
2. ii) Lack of finance to purchase some needed materials such as some oxides like vanadium stain etc.

iii)     Lack of literature that treated Engobe formulation

1. iv) Power supply: unavailability of power supply to energize the kiln
2. v) Kiln firing temperature range: because of the kiln temperature range, some more trials will not be carried out as the kiln will not fire to those high temperature.
3. vi) Unavailability of modern testing instrument e.g. Dilatometer for measuring linear expansivity.

1.4    OBJECTIVE OF STUDY

The main objective of this research work is to develop a color Engobe compositions for local structural clay products.

Moreover, the Engobe produced is environmental friendly and have a leadless frit in it’s composition with other non harmful materials.

1.5    SCOPE OF STUDY

Although Engobe and glaze sometimes may be used for the same purpose but for this research, Engobe can be prepared to achieve an underglaze decoration. The recipes yielded an Engobe as undertaken in the study

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Full Project – Development of colored Engobe composition for local structural clay products