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Magic Ingredient / Simple Ingredient Change To Turn Cone 10 Glazes To Cone 6?


PottaFella

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A friend has a number of mixed glazes for 1280c (with soak this works out at about cone 10), is there something that could be added to reduce their firing temp towards cone 6?

 

If not possible, longer term, would swapping the Potash Feldspar with Potash Soda  Soda Feldspar likely reduce its firing temp?

 

The type of glaze we are dealing with is:

 

Pot Feldspar 50

China Clay  22.5

Dolomite  22.5

Whiting  5

 

+ various oxides depending on colour

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Some key differences between cone 6 and cone 10 glazes include: Boron is a glass former that melts at a significantly lower temperature than silica, and so is used as if it is a flux. The silica and alumina levels (as measured by molar parts) are slightly lower. And there is a tilt towards the alkaline fluxes rather than the alkaline earths, together with a tilt towards sodium as a more powerful flux vs. potassium, and some usage of lithium. There is no single magical material that instantly cause a cone 10 glaze to turn into a cone 6 glaze. Glaze materials tend to be complex, each causing multiple changes in a glaze composition.

 

Gerstley Borate, as Marcia suggests, is a boron source. Sometimes nepheline syenite can be substituted for a feldspar to bring more sodium flux into the glaze. Sometimes a few percent of lithium carbonate can be added for additional fluxing. Sometimes a few percent of kaolin or ball clay can be removed to slightly reduce the silica and alumina to cone 6 levels. But all these ideas require testing and adjustment, as Marcia noted.

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Here is an example of a glaze I converted to ^6 from ^10 back in 1980 when we moved to a new facility and I decided to convert to ^6 to reduce costs.

When you add 10% to 100 , it becomes 9% when the total is back to 100.

SELSOR Semi matt ^6 Reduction 

EPK       20.3    

Dolomite 17.7

Neph. Syen. 33.6

Silica 15.9

Whiting     3.5

Ger, Borate 9

  100

Takes colors well

Blue/lavender  1% cobalt Carb

Light green   1.5% Nickel Carb

1.5% Rutile

Lt. Blue gray  .5% cobalt carb.

2.5% rutile

gray  2.5 rutile +2.5 nickel carb.

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