A. Hortling, K.-M. Sirén
Interaction In Ceramics - Art, Design and Research, A. Valkonen (ed.), ISBN 951-9384-56-1, ISSN 0782-1778, UIAH, Helsinki 1993
 
 

THE EFFECTS OF FINNISH EARTHENWARE CLAY AND CALCIUM OXIDE ON THE COLOUR CHANGES IN STONEWARE GLAZES

Finnish Earthenware clay has a low-temperature composition firing. It contains a high iron content and produces after firing a reddish colour. On analysis Finnish Earthernware clay shows a close resemblance to the famous Albany Slip clay mined in New york State in the US. 

As a low-firing material, Finnish Earthenware clay is unseful to the ceramic artist for it's pliability. ln powder form it can be mixed also with other raw materials and fired as stoneware slipglazes such as Temmoku and Celadon (Koreana 1991). Calcium oxide mixed with iron oxide produces diverse reddish brown or yellow colours; in this case calcites such as whiting, dolomite and wollastonite are selected. The amount of Earthenware clay used constitutes between 50 to 90 per cent of the glaze. Furthermore, the glazes have been fired in oxidation and reduction atmospheres to test and compare them. The iron oxide changes the colour easily from brown to green, depending on kiln temperature and the amount of oxide. The addition of calcium oxide shades the colour towards yellow. The brown colour brought about by the iron turns through orange to greenish yellow at higher stoneware temperatures. 

Additionally, the evolution of the varied colourtones depends on glaze thickness during glazing: the thick layer is lighter than a thin one. The modifications in the glazes have been analysed by comparing the content of oxides in their empirical formulae. 

In the present work dolomite, wollastonite and whiting, which contain calcium carbonate, are used with low-firing red clay. These minerals are all mined in Finland, but because of their excessive particle size they are not generally used in ceramics but only in the construction industry. 

In this article the Finnish clay which burns red under firing is called Finn slip and the American clay is called Albany slip. Because of their similar compositions they also share very similar properties. The Finn slip contains 5-8 % iron oxide. At the Department of Ceramic and Glass Design of UIAH, applied research relating to low-firing clays has been carried out, and their suitability as stoneware glazes has been tested (Salmenhaara, 1968). In this work a low-firing clay from Somero. which contains 9.0 % iron oxide and starts to melt to a glaze at 1220šC, has been under investigation. The clay forms a nice glaze at 1260-1300šC (Oc. 9-2) Finn slip is close to that of the famous Albany slip clay. The Albany slip originates from a clay mine in New York State, in the US, which is closed today. But the Albany slip name is nowadays used to describe the common type of low-firing red clay, which melts to a glaze at stoneware temperature. Moreover, a synthetic Albany slip has been proposed by Rowan ( 1988). Besides other suitable clays have been proposed instead of Albany slip clay such as American Cerdar Heights Redart Clay (Table 1). 

In this study an old Albany slip delivered by The Pottery Supply House Ltd to Finland is used as a reference for the Finn slip (table 1, Salmenhaara 1964). The melting region for this Albany slip in between 1170-1190šC. 

Calcium oxide mixed with iron oxide gives different reddish brown or yellow colours. In this case three raw materials including calcium carbonate were chosen. Dolomite, wollastonite and whiting are used with the low-fired Finn slip clay. These minerals are all mined in Finland, but as explained above because of their over-large particle sizes they are not generally used in the ceramic but only in the contruction industry. 

The calcium carbonate, which is used in the tests is impure and comes from Maalarimestarit Ltd. Its exact composition is not known. Decomposition reaction: CaCO3 > (900šC) CaO + CO2 

The dolomite, which consists of calcium- and magnesium carbonate comes from Norway. 

Wollastonite is a rather rare calcium silicate mineral. The wollastonite found in Finland contains a small amount of iron oxide and melts at 1390-1410šC. 

The planning of glazes with these materials aims at obtaining yellow slip glazes, which have covering properties. The effects of raw materials on the colour and melting point of Finn slip clay in glaze mixtures will be investigated. 

The percentages of Finn slip in the glazes are 90, 80, 70, 60 and 50 %. The corresponding percentages of dolomite, wollastonite and whiting are 10, 20, 30, 40 and 50 %.
The glazes have been prepared at three different thicknesses because it is very typical for this type of the slip glaze to be influenced by thickness. The thickness affects the formation of the crystallized surface and the colour. When going from a thin to a thick glaze the colour changes from dark brown to yellow. 

Each mixture has been fired as a testpiece at three different temperatures. One of these firing methods has been wood-firing. The temperatures varied between, 1240-1260šC (Oc 8-9). The temperatures in the eleÇric kiln have been 1250 and 1300šC (Oc. 8 and I l).The rates of temperature rising have been 60 and 100šC per hour and the soaking at the final temperature lasted 10 minutes. The changes in the glazes have been analysed by comparing the varying content of the oxides in the different empirical formulaes. 

Typical for the slipglaze is their viscosity and their matt surface. The glazes, which are matt have covering properties and form tight yellow crystal-nets. The formation of the crystals depends on the degree of melting of the viscose glazes. A high content of crystals gives a light colour. Glazes that melt are transparent and the darkness of the glaze depends on the amount of iron oxide used. Calcium oxide and magnesium oxide affect the viscosity of the glaze. The colour change caused by ironoxide depends on the change of the ratio of calcium oxide and aluminium oxide. The ratio of iron oxide, calcium oxide and aluminium oxide effects the colour changes caused by iron oxide. Therefore the ratios of iron oxide, calcium oxide and aluminium oxide from the analyses of the test glazes. In the results iron oxide is compared to the other variable compounds, and it is observed that the ratio of aluminium oxide is constant and the ratio of calcium oxide changes.

The glaze series LPW I - LPW5 of Finn slip and wollastonite all melt and form even surfaces, which change from pale (LPWI) to transparent and shining(LPW5).Mixtures change from transparent and glossy to dark 'temmoku'.
In the glazes LPW I - LPW5 of Finn slip and dolomite, there is too much aluminium oxide compared to the amount of silica. A high content of magnesium oxide forms yellow crystals, which occur as a crystal cover on the surface of the glaze. 

The glaze series LPL I - LPL5 of Finn slip and the whiting contain 0.82 equivalents of CaO from the RO-group of the empirical formula. The glazes LPL4 and LPL5 do not form continuous surfaces,butareso-calledstructureglazes.Theratioofaluminium oxide and silica to calcium oxide in the empirical formula is too high to produce a smooth surface on the glaze. 

The colour caused by iron oxide depends on the changes in the ratio of calcium oxide to aluminium oxide. From the analyses of the testglazes we have calculated the ratio between iron oxide, calciumoxide and aluminiumoxide.lntheresultstheeffectofiron oxide has been compared with the effect of the other components. It can be concluded that the content of aluminium stays constant and the ratio number of calcium oxide changes. 

The colour caused by iron oxide depends on the changes in the ratio of calcium oxide to aluminium oxide. From the analyses of the test glazes the ratio between iron oxide, calcium oxide and aluminium oxide has been calculated. In the results ironoxide has been compared with the other components. From this can be concluded that the content of aluminium oxide stays constant and the ratio number of calcium oxide changes. 

When the ratio number of calcium oxide is higher than 2 the effect of the iron oxide on calcium oxide causes yellow colour changes. When the ratio of iron oxide and calcium oxide is below 2 the colours of the glazes are brown or brown-yellow. 

Through the stereomicroscope different crystallisation patterns of the testglazes have been observed. The least crystallisation is noted in the glaze mixtures containing Finn slip and wollastonite, which melt best. Under the microscope you can discover a weak crystal formation at the interfaces, where the clay body and the glaze have their contact surface. In the glazes containing Finn slip and dolomite, the crystallisation occurs at the surface. At the interface between the claybody and the glaze there is a different glass-phase. The crystals are yellow and lightens the dark colour of the glaze. In the glaze serie containing Finn slip and whiting a lot of yellow crystals are formed when calcium oxide reacts with silica. 

The analyses of the glaze series containing Finn slip and whiting, and Finn slip and wollastonite have been compared to the analysis of the Albany slip clay. Differences are seen in the concentrations of aluminium oxide and silica and in the amounts of calcium and iron oxide, which influence the melting of the surface. 

When Finn slip is applied as a glaze wollastonite should be used as one raw material, because it contains silica and calcium oxide. Silica influences the darkness of the glaze colour. Wollastonite alone cannot be used as a silica source and pure silica has to be added. The composition of a Finn slip corresponding to Albany slip should be 80 % Finn slip clay, 10 % wollastonite and 10 % silica. 

TABLE 2, TABLE 3A, TABLE 3B, TABLE 3C

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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