Sunday, 29 November 2009
This test was set up because I suspect that the Alkali/Acid ratio in this test is only part of the equation. A 1.4 ratio can produce Greens and Blues from Cobalt and Titanium depending on the levels of Alumina.According to Frank Hamer “Ampohterics can act as an acid or alkali depending on the base formula and they are an interface between the two”.
This test has Alumina Decreasing from A to B and Alumina increasing from B to C .
KNO increasing from A to C and Calcium increasing from C to A whilst silica and Colourants remain constant.
I had hoped to show that by leaving the Acid /Alkali ratio of 1:4 it would change a Shiny Cobalt green glaze to Blue. Generally the surface quality goes from shiny translucent towards opaque there are considerable pinholing problems as it moves towards 16 and 21. and adjustment to Titanium levels with correspondent Cobalt levels ie 1.20 CoO and 5.60 TiO2 and a possible change in CaO source like wollastonite etc might help also experimenting with firing etc.
Corner A to C….I have found that CaO is an important factor in Cobalt Green glazes and I had hoped also to show that be exchanging KNO for Calcium that it would have an affect on colour too.@ A to C it has changed from a lovely transparent green to transparent blue and finally to a semi shiny Opaque Blue with Pink tinges.
Corner A to B this line blend has Alumina decreasing from A to B again a shiny translucent green tile @ A 1 progressing to translucent Blue to an Opaque Sky Blue with a rough sugary surface.
Corner B to C.
This line Blend has the lowest alumina levels at B and increasing to C. B has the same KNO and CaO levels as A but CaO decreases towards c and KNO increases towards C which give C the highest KNO and Lowest CaO at C21. this line blend changes in colour from a bright opaque sky blue at B to a paler opaque blue at C which is the palest colour of the whole test..
My conclusion for this test is that at an Acid/Alkali ratio of 1 to 4 shiny translucent greens can be produced but the critical factor is how the Amphoterics will behave in the glaze matrix.
Too much KNO and not enough CaO turn it from Green to a pale Blue.
With the right balance of flux CaO predominant,the reduction of Alumina will affect the colour as it decreases and also turns the green to blue.
Even with an acid /Alkali ratio of 1:4..I can say with the right balance of Flux and the wrong balance of alumina you still wont produce green and with the wrong balance of flux and the right balance of alumina you still wont produce green.
Hence it is not just the Flux:Silica Balance..it is also the Flux ,Amphoterics and Colourant balance needed to produce Cobalt green..and there is a small window of opportunity for Shiny translucent Cobalt Iitanium Green I suspect.