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oceancity

Help me alter this glaze recipe...

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Hi! I am looking for some help with this glaze recipe. I want to get a nice even glossy turquoise. As it is the color is a light blue/seafoam green and I get some speckling when I use it on my terra cotta. 

What I’d like to get from this glaze is more of turquoise color and less speckling. 

My first instinct is to lessen the amount of red iron oxide or maybe remove it all together. That should remove the speckles I think  

Then if I increase the amount of copper carbonate I’ll get a deeper blue color. 

Does all of that sound like a good plan? 

And, if I reduce the amount of tin in the glaze will that also help get a deeper blue/turquoise color? 

Copper Light - Cone 6

Recipe:

Gerstley Borate - 19.7%

Zinc oxide - 4.7%

dolomite - 6.5%

neph  sy - 11.2%

spodumene - 29.9%

flint(silica) - 28%

Add in:

tin oxide - 7%

copper carbonate - 2%

red iron oxide - .75%

Edited by oceancity

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Yes, removing the iron will help get rid of the speckling. If you remove the tin, it won't be opaque, and the dark color of your clay will totally change the appearance. If it still isn't the right color after removing the iron, you could use a stain instead of the copper.

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@oceancity, if you want turquoise blue using copper in oxidation the base glaze needs to include fluxes mostly from the alkali and alkaline earth group. Have a look at this abbreviated Periodic Table,  glazes containing fluxes from the left hand side are going to promote that colour. Your recipe does have a lot of these fluxes so it might work to just remove the tin and iron from the glaze recipe. It's not going to be as crisp and clear a colour going over your red clay, might try putting a white slip over some test tiles and seeing the difference between the glaze over slip and red clay plus increasing the copper carbonate.

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Another idea to try ... if you want the glaze to be a “deeper blue” you can try adding a smidge of cobalt to the recipe. 

@oceancity Can you post a photo of the glaze as it currently looks? It will help us make suggestions for achieving turquoise. 

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Every time I try to post a photo it tells me they’re too big so here are links from my Instagram of the copper light glaze on white clay and then there is a honey pot that is a deep red clay body. The test tile is the current clay body I’m working with...

 

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I made a small batch this evening omitting the red iron oxide and I added a little bit of cobalt oxide. I’m hoping that gets me closer to the look the color I want. 

I’m going to leave the tin oxide in the recipe so it retains the opaque quality it has. 

When I have some more time in the studio I’ll make a couple different test batches with varying amounts of copper carb and cobalt 

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6 hours ago, High Bridge Pottery said:

Is it not coming from the terracotta?

It definitely speckles more with a darker clay. I think removing the red iron oxide will help to get rid of most of the speckling. But there will still be some unless I change something else to make the glaze more opaque. 

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Even going more opaque you're going to get some speckling from the clay. My question is, why are you using such a dark clay if you don't want the effects that come with it? I say either embrace the beautiful earthy tones that come with dark clay, or switch to a white clay if you want pure glaze color. And there's no rule that says you can't do both.

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3 hours ago, neilestrick said:

Even going more opaque you're going to get some speckling from the clay. My question is, why are you using such a dark clay if you don't want the effects that come with it? I say either embrace the beautiful earthy tones that come with dark clay, or switch to a white clay if you want pure glaze color. And there's no rule that says you can't do both.

I love the effects of dark clay and with other glazes it’s exactly what I want.  But I also really love the look of the dark clay with an opaque glaze over it, because I want to leave some clay bare and have that contrast.  I knew I probably couldn’t  get rid of all of it. 

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