Friday, 31 July 2009

Quad blend for Cobalt green

BLC.Blue Green
PotashFeldspar 40.00
Dolomite 9.00
Whiting 15.00
Silica 11.00
NZ China clay 25.00

Cobalt carb 1.00
Rutile 8.00

Unity Formula:
KNO 0.224
CaO 0.647
MgO 0.129

Al2O3 0.509
SiO2 2.648

The overall trend of the grid shows that as the Alumina (Amphoteric)decreases from Corner A towards corners B,C,D the Texture and surface quality becomes more glassy as the silica increases until it reaches a Silica overload at Corner D and there the surface becomes rough and sugary but still reasonably shiny. There was an almost visible diagonal divide from Corners B and C with the drier more matt surfaces closer to A and the shinier glossy surfaces closer to D..The driest most matt tile is Corner A T1 the colour is dull and grey green and the surface is scratchy to touch…The tiles become progressively less matt T21 and T26 have pink crystals in a green background where the glaze has moved and become more shiny and fluid towards Corner C T31 become beige and cream colored with some skyblue crystals in the molten glaze.

Colour wise also the deeper more interesting colours,blues around T18 and T19 Pinks around were present moving towards D as the silica content increased also and Alumina decreased. T23,24,25,28,29,30,34 and 35.T35 where the silica levels were highest the Pink became metallic where the glaze was thick and where thin, rough,sugary and grainy and a watery shine and dark green. I think that the pink comes from Cobalt when it is in an alkaline glaze and Rutile tends not to produce colour when in an alkaline glaze(Steve Ogden, potters palette)hence the colour changes as the flux oxides become more predominant at the reduction of Silica and Alumina.

The Alkali flux balance can change in the glaze melt as the silica (Acids) levels change in the Quad blends.posssibly changing the PH levels and affecting the colourant responses.

There were some very interesting colour variations caused by a change in the balance of flux and Al:Si mols around 14,15,18 and 19,allowing the cobalt to become blue,suppressing the rutile influencing factor…perhaps!!! I found that the Al;Si ratio was not necessarily a a guideline indicator to glaze quality or colour as I found two tiles with similar Al:Si ratios T8 and T31 both of which have an Al:Si ratio of 6.049. Tile 8 has a higher clay content with Al @ 0.601 and Si @ 3.637 and it had a semi matt dull dead looking grey green colour while tile 31 has no clay or silica added in the recipe and its Al mols sit @ 0.232 and Silica @ 0.401 and has the same Al:si ratio, its surface is the most fluid and has a semi matt crystalline surface which has run at the back forming sky blue glass with pinks and beige..As for colour I have 2 tiles 14 and 15 with Al:Si of 8.457 and 10.914 which are green/blue but have higher Alumina and lower silica.and showing no sign of pink and I have tiles 23 and 24 which are going green/pink. But have lower alumina levels. And higher silica levels.

Also tile 19 and Tile 28 have similar Al:Si ratios of 9.484 and 9.384,one is pink and one is blue.

So the more consistent guideline is the actual amount of mols of Alumina and Silica

Tile 18 would make an interesting stable functional glaze giving nice semi translucent forest greens and royal blues.depending on the thickness of the glaze with a tinge of pink as well. And would be worth pursuing with some line blends.etc. Also for non functional glazes it would be fun to do some tests on tiles 31-34, trying to get the pinks in oxidation and safe non volatile colour as well.The best rutile/Cobalt greens,11,12* 16,21 and 22.

The clay content also made handlebility easier and the glaze became consistently more difficult as the Alumina decreased. Not just in suspension of the glaze for mixing but also in application as well.

My overall thoughts are that the Alumina levels are the ones that control the colour the most as the only white beige tiles were in the area of 31 where there was no added clay and 26 where there was low clay.that may have some thing to do with alkalinity of the fluxes and the ph of the glazes but that hasn’t been tested here.The response of colouring oxides in a glaze melt may be different when taken up by Al2O3 and SiO2 available and different from the flux dominated overload around tile 31.

The Flux overloads of tiles 31 may allow the colourant oxides to precipitate out of the glaze melt as the glaze cools and a possibility of crystal seeding where there is an overload.

No comments:

Post a Comment