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annacondagh

stains carbonates in glazes

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I am a novice and want to start mixing my own glazes.  I am starting simple ie mixing stains with transparent glazes so its a simple enough process so far. I do however have copper carbonate and colbalt carbonate. My understanding is the colbalt carbonate if added in a 2 - 5% mix will make a straightforward blue. But what of the copper carbonate can I add that to a transparent glaze without any other additives to make a glaze? I understand food safe issues but its not relevant for what I want to do. 

Thanks

Anna

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7 hours ago, annacondagh said:

the colbalt carbonate if added in a 2 - 5% mix will make a straightforward blue

Don't need that much cobalt carbonate to make blue. 1/2% up to 2% at the very most will give you blue. Many people find that just using cobalt gives too garish a blue so they tone it down by adding some iron oxide (2 - 4% ballpark amount) and/or a small amount of manganese dioxide.  If your base glaze contains a lot of magnesia then the blue will shift towards more of a lilac colour. (high magnesia content from large amounts of dolomite or talc or magnesium carbonate in the glaze)

Some stiff glazes have bubbles from  copper carbonate additions. While the copper is gassing off during the firing it leaves bubbles trapped in the glaze resulting in a glaze that looks underfired. Don't know how fluid / stiff your glaze is but just doing a test tile will give you this information. 1/2% up to 3% copper carbonate is a good place to start testing with.

Like Babs said, try blends of both the copper and cobalt too.

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Thank you both so much for your responses, it is such a generous thing to give your time and share your wisdom and it is greatly appreciated.  I worked like a mad scientist on your suggestions and can't wait to fire up the kiln for the test tiles this weekend.

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