FORMULATION OF BASE ENGOBE FOR APPLICATION ON CLAYWARE

Abstract

This experimental research was based on the formulation of base engobe utilizing locally available raw materials with established recipes. Where the fired colour of the ceramic body is dark and the colour of the glaze is light, or where it is necessary to equalize differences in stress between body and glaze, then an engobe is required as a compensating layer. Engobe is a specially composed ceramic slip with the mixture of clays, fluxes and filler which coats the substrate with a permanent opaque layer. The core materials used were kaolin and ball clay that fires white with minimal level of iron content. All the materials used including broken borosilicate glasses, feldspar, Calcium carbonate were processed and milled to a fine particle size up to 63µm. Samples of the core materials were characterized using X-ray Fluorescence Spectroscopy (XRF). The result clearly showed that the amount of silica present in the clay, feldspar and cullet was predominant than other oxides present in the same material. The initial compositions of the base engobe produced using 20% silica with established recipes were seen peeling off wares at drying stage. This indicated that both the engobe and the substrate were far being compactable possibly due to the higher silica content; this called for adjustment of the recipes to suit studio pottery. Other batches were therefore prepared with 10% and 5% less of silica content. An abrasion test was conducted on these fired test samples respectively and the batch composed of 5% silica was found to be more compactable than the rest. Thermal Coefficient of Expansion was also carried out which revealed the expansion between the substrate, engobe and the glaze and the extent to which these three (engobe, glaze and clay body) were compactable. This study established that the locally sourced materials can be used to produce base engobe for clayware body base on adjustment of established recipes. The use of local kiln may not be completely reliable talking of firing white engobe as it affects the colour. This could be connected to the fact that the firing was carried out under reduction atmosphere where combustion of fuel is incomplete leading to shortage of air causing mixture of carbon with the glaze resulting in colour defect. So it is advisable to employ the use of electric kiln in the firing of engobe wares to bring out the best colour result.