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Glaze For Satin/matte Dinnerware Safe Porcelain Black Clay


moh

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Hello,

I've recently been experimenting with black Mason Stains (6660 and 6650) 6.25% on porcelain body ^5 oxidation.

 

Having looked at about a hundred options in the local ceramic store, I've yet to figure out how to make a surface like in the attached pic.

I love the texture of porcelain clay and would like to leave it the unglazed feeling, but also want to make it dinnerware safe, AND get that jet black color. post-79579-0-77925400-1476837293_thumb.png

 

Any hints?

 

 

post-79579-0-77925400-1476837293_thumb.png

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Possibly black stain was wedged into clay body and fired with no glaze.

 

John Britt has a satin black (John's 10x)in his Cone 6 Glaze book that uses 10 different ingredients, each 10%, plus 5% black mason stain 6600. I use it as a liner glaze for vases.

 

On the black mason stain, be aware some use cobalt that can give a blue tint to the black; others do not use cobalt.

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Matte and food safe is not all that difficult. Matte and jet black and food safe is much more difficult. Matte and jet black and food safe and durable enough for dinnerware becomes even more difficult.

 

For a durable, food safe matte glaze, I would look for a magnesium matte that's also got a decent amount of alumina, then start testing to get the black. You're probably going to have to saturate it with iron (12% or more), plus some cobalt. But even then it's not going to be like the surface in your photo. That looks more like an underglaze type of finish that's probably more colorant than glass.

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So would a satin black be food safe?

 

I understand some people assume any matte can carry inherent risks of trapping food particles over time, but what would be in the black specially that makes it un-food safe?

 

In my studio, the professor claims all the glazes are food safe, including the mattes. With over 30 glazes available to use, I find it very hard to believe that they are all "food safe". I have asked her why she claims the mattes are safe, and she says because there are no harmful materials in them, and it would take a very long time to stain and trap enough bacteria to really make you sick. While I want to trust her, and have tested many of the matte glazes myself, unfortunately they vary from firing to firing.

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I am not even sure that black pot has been glazed. Any glaze I have seen that's close is 40-50% clay and on the border between glaze and slip. It is always going to be abrasive on cutlery and probably unhygienic because that's part of the lovely surface.

 

Blacks generally mean there is a higher percentage of colouring oxides so more to leach out into your food. How much actually makes it into your body I don't know, never seen any experiments. If you are not using any toxic metals then it can be classed as food safe in that area even if the surface is not exactly hygienic.

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Moh, I don't think that piece is glazed at all. I think you're right in that the colour comes from additions to the clay body, but I don't think the base is porcelain, it looks like stoneware. Something about the way the light is hitting that pot makes me think there's a lot of iron involved, not just Mason stain. I concur with Neil, you're not going to make that texture durable enough to eat off of on a regular basis, even if it's not likely to be immediately hazardous. I don't know if that piece pictured will hold water without weeping, or if mild acids won't discolour it. Glass within the clay body is what makes ware durable, waterproof, resistant to acid wear...and shiny.

 

That kind of leads into Sydney's questions: if your shop glazes aren't formulated with anything toxic, you're not going to poison yourself with anything, and your instructor is correct about the unlikelihood of giving yourself salmonella from the crazing. But that doesn't mean a matte glaze will be a durable one. Depending on the mechanism used to obtain the matte, the glaze won't have enough glass formed to be durable over time, or it'll be susceptible to staining. Mattes are famous for cutlery marking, and discolouring when you leave a cup of coffee or tea sitting overnight. A matte glaze that is actually an under fired shiny glaze (usually indicated by a glaze that varies a lot between firings), will be especially susceptible to all these problems.

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That kind of leads into Sydney's questions: if your shop glazes aren't formulated with anything toxic, you're not going to poison yourself with anything, and your instructor is correct about the unlikelihood of giving yourself salmonella from the crazing.

 

Yes and no. It's possible to overload a glaze with metallic oxides that would otherwise be safe at a lower percentage. Copper, cobalt, and even iron can leach if the content gets too high, or if the glaze is poorly formulated, but are perfectly safe at a lower content in a good glaze.

 

A matte glaze can trap bacteria, but a dishwasher will usually take care of it. As Diesel mentioned, there are several ways to get a matte surface, and under-firing a glossy glaze is the worst way. Poor durability, more likely to leach, etc. High alumina mattes are generally quite durable and can have good glass formation, same with magnesium mattes. I like magnesium mattes because you get your mattness through crystal growth, which means it can be runny and matte at the same time.

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Thank you Callie and Neil.

 

I bet that after 30 years of teaching a ceramics class has allowed my professor to ensure the students safety. If I could brainwash everyone I give my pots to that a matte glaze on the inside is only use for dry foods (breads, fruits, cheese etc...) then I would not have to worry. I start to think twice if people want to drink coffee out of a turquoise matte mug or store molasses in a white matte jar! Yet I see people everywhere doing just that, even the potters themselves advertise it. Maybe they are not as concerned about the long term as I am... 

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Thank you Callie and Neil.

 

 If I could brainwash everyone I give my pots to that a matte glaze on the inside is only use for dry foods (breads, fruits, cheese etc...) 

 

Easy solution: pierce so they can't hold liquids.  Think bread basket, not bread bowl.

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