Sunday, 24 February 2013

Post 77 In search of brown - line blend trials

I haven't yet taken the time to experiment with chrome in my glazes, but that is very close now. It seems that with the right additions it might be possible in an oxidation firing, to achieve dark reds, good green and zinc chromate browns.

In the mean time I've still been working with iron oxide and then manganese to test the limits. I'm entirely comfortable using iron oxide but not so the manganese and so if I can get away with it then that would be good. Mind you same thing for chrome actually. I did a trial recently of a couple of line blends.

The first one used two recipes for additives but both with the same base glaze - my mostly used Nepheline Syenite Post recipe.
On the LHS is 7-1 with 2% RIO, 5% Yellow Ochre, 3% Titanium Dioxide and 3% Tin Oxide.  The colour has more yellow tones than red and would be a good strong colour for a homogeneous application on a pot, to simulate the tonality of some of the Japanese and Chinese unglazed pots.
On the RHS is 7-6, a iron brown brown with 10% RIO and 7% Zinc Oxide. Zinc tends to encourage a deeper expression of the other colourants and it's done that here. I've found it quite challenging to get any red tones out of an oxidation firing of RIO in most but a very few recipes. Sankey's iron red is one of those and it is a very reliable glaze if you want a good glossy surface. I've yet to try to turn it satin. I'll save that for my search ffor red.

Anyway these are two good candidates for a line blend.
As per normal the second tile from the right, 7-2, has 80% of 7-1 and 20% of 7-6, etc etc

The results show the power of the zinc in small amounts. 7-2 is getting close to a useful colour.

The next set was an attempt to create a staining slip that I can apply to bisque ware, to apply a dark red brown colouration and at the same time retain much of the texture of unglazed ceramic.
 The underlying slip recipe is 50% Nepheline Syenite and 50% Eckalite.
Tile 7-7 has an addition of 50% RIO and 7-12, 50% manganese oxide. Once again the tiles in between are composed with 20% steps between the two on the outside.

The slips were brush applied, just a quick single coat. Texturally they are all quite matte, as you would expect. The underlying slip recipe will give a unity ratio of about 3. Of course you need to get to about 6 for a satin and 8 for a gloss. The colours show the dominance of the manganese impact. 7-7 which has RIO alone has a good red tone but the smallest amount of manganese takes it to almost black.
The clay content of the slip made it very good to apply to get an even coating with a single coat; very nice to use on bisque and performed well through the cone 6 firing with no cracking or delamination. This test has been a good start with clear massages about colourant levels for the next series; I think to bring back the level of RIO to 20% and Mn to 5%. This should undershoot, but that's the best second step for a trial.

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