Clear glazes, slips and underglaze tests.
Some time later, I succeeded in taking photos of actual pots. The bottles and tiny glaze test bowls were an attempt to find a useful clear glaze that would work well over underglaze colours and slips at Cone 9, which is the maturing temperature of the clay body I am using. Clear glazes are often not as clear as what you might like them to be, and can be milky, or can adversely affect some colours. For instance chrome green can turn brown if it comes into contact with zinc. I realised that chromium oxide was probably used in the commercially available green underglaze that I had, so I firstly looked for clear glaze recipes that contained just feldspar, calcium carbonate, silica and china clay or ball clay. As an experiment I widened the selection of clears to include one that contained a small amount of zinc oxide so that I could see for myself what would happen when it was put over chrome green underglaze. I also tested some celadon* glazes that I had made up for the wood fired kiln, as some looked to be very suitable and I wanted to see if the small amount of iron in the glazes would add a hint of colour and interest when the glazes were fired in oxidation in the electric kiln.
These two bottles in the photo above were painted with the same underglaze colours but had different clear glazes put over them. You may notice that the painting of the flower on bottle on the right looks rather faded and has lost most of the green colour from the leaves and stem, this is due to the glaze containing a small amount of zinc oxide and it not playing nicely with the green underglaze.
All was not straight forward when it came to putting a glaze over an iron bearing slip. I had expected these bottles to have had a warm brown background, not the streaky grey that you see here. What I think has happened is that the iron has lost some of its oxygen when at high temperature and has turned to its black state, but has been unable to re-oxidise and regain the red of red iron oxide when cooling due to being sealed under a coat of clear glaze.
These two glazes proved reasonably clear when fired to cone 9, with the one on the left - actually a celadon recipe - being clearer than the "clear" glaze recipe that was used on the samples and bottle on the right. Both bottles had a manganese bearing slip under the glaze from the top of the bottle to the shoulders, the example on the left is more satisfactory than the one on the right.
Both these bottles had the same iron bearing slip from top to shoulder, but have different celadon glaze recipes. The one on the left has turned out beautifully and looks very similar to some antique ink bottles that we have. I also like the one on the right, but you can see that the colour is quite different, almost certainly as a result of the glaze preventing re-oxidation of the iron bearing slip.
As I write this I am supervising a firing of more tests. I made up a selection of my own underglazes, some coloured with metal oxides and others with commercial stains, and I have used them under and over the more promising of the glazes that came out of this first round of tests.
The most promising "clear" glaze thus far is a very simple celadon recipe that fires without annoying cloudiness at cone 9 and fits the clay body quite well.
Potash Feldspar 25
China Clay 25
+ 0.75 Black Iron Oxide
(I have seen this recipe attributed to Leach, but am not sure if that is Bernard or his son David.)
*Authentic celadon glazes derive their grey, green or pale blue colouring from a trace of iron oxide that is in the glaze. These glazes have to be fired in a reduction atmosphere (one where oxygen is depleted) in order to develop their celadon colour. If fired in oxidation they most likely will yield pale honey colours or a slight warm grey.
The word "celadon" is often used these days to describe pale green or blue-green glazes that have been fired in oxidation in an electric kiln and use commercial glaze stains to achieve the colour.