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tomhumf

Why is glaze blue

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Materials Amt
BPS Soda Feldspar 33.00
Quartz 26.00
BPS Calcium Borate Frit 14.00
China Clay 14.00
Talc 9.00
Bone Ash 4.00

Total:100.00 (g)

+ 7% titanium dioxide

Hi, 

This recipe was meant to be a pure white! I did a test tile with very similar glaze which looked white. I'm wondering why it has gone so blue. It was fired in gas kiln in a fairly neutral atmosphere as far as I know. 

It was sat next to some cobalt glazed pots, is it possible it took the colour from them? Otherwise maybe it's the iron in the clay which is causing this?

And then how to get a purer white, seems like 7% titanium is fairly high already and I wanted to see some of the body break through... just not the blue colour. 

 

1540398941876.jpg

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Yes -my guess it absorbed the blue from its blue pot neighbors -since they where cobalt which is a strong colorant and can drift. 

If this was easy everyone would be doing it.-Oh wait everyone is doing it.

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Why is there Titanium Ox in a white glaze anyway?

My comment was based on the main formula-I did not see that addition below-I do think Min is onto the answer-but cobalt pots can drift blue on pots next to them as well.

Mins answer explains the interior blue which is more rare from any drifting cobalt.

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That's a titanium blue, like Min said. Cobalt will fume and jump to other pots, but it doesn't give that pearl blue color, and would only have given a blush at the spot where the pots were close together. The titanium is acting as an opacifier, as well as causing the blue color. If you don't want it to go blue, try using a different opacifier, like tin oxide or zircopax/superpax.

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If you put that over a red clay body or slip, the effect will be more pronounced. Usually if you’re using titanium as an opacifer, you get a white or a cream that would look innocuous enough alone, but would react interestingly with other glazes, or as you’ve found, react with the clay underneath. 

If you think this look is something you’d like to play with but want a whiter white, try a 50/50 split of titanium and zircopax.  If you just want a flat white, use either tin or zircopax.

Also, if you’re going to be testing various shades of white, make some test pots to do it on. Standard test tiles are too small to give you an accurate representation of what will happen on an actual piece, and things like this crop up as surprises. 

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Thanks all, yes I've used titanium dioxide as an opacifier with other base glazes in electric kilns and it has just given a pure white. I will experiment with zirc and tin. 

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