- Material sciences and production techniques -


Low fired earthenware clay which turns red terra-cotta during firing is important for the Finnish ceramic culture, because it is the only type of natural clay found in Finland. It is used in the brick factories and to a limited degree in the ceramic industry. The earthenware clay, which is used in the present investigation, has the strong red colour tone in the sintering point fired at 1020-1050C. One typical chemical analysis of Finnish earthenware clay is presented as Kultela Brick factory clay. 
There are wide changes in the colour palette of terra-cotta clay according to the application of alkali or lead glazes. The low temperature and the type of the earthenware clay, rich in iron (9%), reduce the choice of glaze rawmaterials. The principle reason for this investigation of lead-bearing glazes is that they are known to pick up a strong colour effect with different metal oxides in low fired temperatures. The present work is concerned with the changes in the release of lead from glazes and the solubility of the lead glazes used for application over the colored engobes. The following questions are of interest: 
- What kind of increasing effect on the solubility will be observed when comparing two different lead frits? 
- How will a colored engobe change the lead release of lead glazes and the commercial transparent glazes with low solubility ? The technical effects, such as firing time with soaking or control of the glaze layer, which also have some influence on the solubility of lead, are kept constant in this research. 

Lead glazes, which melt at 1020-1050C, have been investigated together with different engobes. Two different lead frits have been used as materials for lead oxides in the glazes. The glazes have been tested according to ISO 64886/1-1981 test methods. The results indicate that these lead glazes have a low lead release according to ISO 64886/2-1981 standard and to EEC directive 84/500. The low lead release glazes are tested over the different engobe compositions melted with borax frits or lead frits or when 5 % copper oxide has been added. Tests show an increasing amount of lead release. From the results it can be concluded that lead solubility is not stable if you use glazes to cover the engobe decoration. The different engobe compositions with lead or borax frits cause the lead release on the surface of lead glazes. 

1) Lead release changes according to the composition of the engobe under the lead bearing glazes. 
2) The permissible limit of lead release of ISO standard and EEC directive is exceeded with an addition of CuO. 
3) Soft borax frit P2964 in the engobe increases the lead release in lead glazes more than the lead bisilicate. 
4) Lead sesquisilicate frit increases the lead release more than the lead bisilicate in the same glaze composition. 
5) Copper colored engobe with soft borax frit instead of lead frit decreases the lead release of the lead glaze. 
6) The lower firing range commercial lead glaze has considerably lower lead release than the higher firing glaze. 
1) J. NORDYKE, Lead in the World of Ceramics,The American Ceramic Society, 1984. 

2) A. HORTLING, The Lead Release from Glazes used Lead Monosilicate, Dept. report of Ceramic and Glass Design, University of Art and Design, Helsinki, Finland, 1992 

3) F. SINGER, S.SINGER, Industrielle Keramik II Band,Sringer-Verlag, Berlin, Germany,1969 

4) Health and Safety in Ceramics, The Institute of Ceramics, Shelton House, Stoke road, Shelton, Stoke-on-Trent, St4 2DP,UK,1986 

5) Potterycrafts Ltd Catalogue for 1993/94, Campell Road, Stoke-on-Trent ST4 4ET, England 

6) ILZRO MANUAL 1 Ceramics, Lead Glazes For Dinnerware, International Lead Zink Research Organization, USA, 1971. 

7) Partek Industrial Minerals Oy, Finnish Flotation Feldspar, 25700 Kemiö, Finland, 1994 

8) ECC, English China Clay International Ltd, Raw Materials for the Ceramic Industry, John Keay House St. Austell Cornwall, England. 

9) W.LAWRENCE, R.P. WEST, Ceramic science for the potter, 2ed, Chilton Book Company, Pennsylvania, USA,1982 

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Airi Hortling, @
Senior Lecturer in the Department of Ceramics and Glass

University of Art and Design Helsinki UIAH
Department of Ceramics and Glass
Hämeentie 135 C 
FIN-00560 Helsinki, Finland 
phone: +358 9 75630505, fax: +358 9 75630223