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Iron oxide over glaze (majolica style) - food safe?


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Hello!

I am experimenting with different sorts of oxide decoration, and I like to brush on the oxides on the outside of my glazes. The glazes are food safe, and the oxide seem to be melting/blending in nicely with the glaze. When i feel the texture, it is smooth all the way. Does this usually mean that the oxides are sealed and the vessel is food safe? Iron might not be the most sinister oxide, but I am planning to also use cobalt and manganese ++, and I of course want to make sure that the products are safe. Does anyone know any tests or any signs that they might be? 

I could use oxides and seal with a clear glaze, but as I usually use dark clays, this would not work so good. Also, I am so charmed by the dark clay/white glaze combination. The glaze reacts nicely with the clay, and I am very happy with the oxide patterns. Would be very grateful for any help!

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Should be fine, copper being the exception.  Copper hates becoming part of glaze.  But I know iron, manganese and cobalt all love entering the glaze melt.  You'll want to experiment with the strength of oxide used though, you want enough to be effective as a colorant, but not so much that it doesn't enter the glaze and instead becomes metallic upon firing.  

As far as food safety, cobalt and chrome are the ones I'd be sure to really water down or not use at all since they're potentially toxic and 100% incorporation isn't likely.  Iron and manganese less toxic once fired, and not water soluble.

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Van Gilder posted a series of washes a while back that you might find of interest. Of course testing would be recommended.

Wash Cone 6-10    
Material Empirical 5 10
OM-4 25 125 250
Nepheline Syenite 25 125 250
       
Blue      
Red Iron Oxide 25 125 250
Cobalt Oxide 25 125 250
Greeen   0 0
Chrome Oxide 50 250 500
Red/ Brown   0 0
Red Iron Oxide 50 250 500
Black Wash   0 0
Red Iron Oxide 16.25 81.25 162.5
Cobalt Oxide 16.25 81.25 162.5
Manganese Dioxide 16.25 81.25 162.5
Rutile   0  
Red Iron Oxide 30 150 300
Rutile 30 150 300
Gerstley Borate 30 150 300
Total   500 1000
Bill Van Gilder 2012  

 

 

 

best,

Pres

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Thaks so much to both of you!

One question though: how does one know that the oxide properly enters the glaze? On my tests, most of the pieces come out smooth and even, so I guess that is a success. Sometimes especially the iron crackles: it gets crusty. My guess is that the iron wasn't diluted enough. But I have not seen anything come out metallic, does it look in a special way?

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11 minutes ago, Earthandwater said:

Sometimes especially the iron crackles: it gets crusty. My guess is that the iron wasn't diluted enough.

Add a bit of frit or gerstley borate to help it melt, it will take care of the crustiness. Iron is refractory in oxidation. You would need to add some flux (frit or gerstley borate) to chrome also, it's extremely refractory.

 

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1 hour ago, Pres said:

Van Gilder posted a series of washes a while back

Pres,

my brain still in slow this week; what are the units for Van Gilder's mixtures?  

I guessing that everything is by weight and the base is half and half, and the "others are grams based on a 100 or 250 or 500 grams of the base. 
How far off am I this time?   

LT
 

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These are based on empirical formulas, , and are considered  washes. They should have water added to match the consistency of you needs When I mix them up, I use use grams, and end up with around 750 grams in most cases, using the second column not the third.

 

 

best,

Pres

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Thanks everyone! I will add some frits and check out Van Gilder, it looks very promising!  I really appreciate your help! One last question: Quote liambesaw: you want enough to be effective as a colorant, but not so much that it doesn't enter the glaze and instead becomes metallic upon firing

What does this part about metallic mean? I have som vessels that is decorated with cobalt, and some places this cobalt is almost purple and "shiny" maybe a little metallic in the colour (but smooth to touch). Does this mean that it has not entered the glaze properly?

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57 minutes ago, Earthandwater said:

Thanks everyone! I will add some frits and check out Van Gilder, it looks very promising!  I really appreciate your help! One last question: Quote liambesaw: you want enough to be effective as a colorant, but not so much that it doesn't enter the glaze and instead becomes metallic upon firing

What does this part about metallic mean? I have som vessels that is decorated with cobalt, and some places this cobalt is almost purple and "shiny" maybe a little metallic in the colour (but smooth to touch). Does this mean that it has not entered the glaze properly?

That's correct, if it has gone metallic looking it has not become part of the glaze. Imagine the glaze as a sponge.  It will suck up cobalt until it's full and then the remaining cobalt will puddle on the surface.

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22 hours ago, liambesaw said:

That's correct, if it has gone metallic looking it has not become part of the glaze. Imagine the glaze as a sponge.  It will suck up cobalt until it's full and then the remaining cobalt will puddle on the surface.

Does it get sucked up or just is more dense and sinks in the melt? Is there any force that says it has to become fully encased. Don’t know, just asking.

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41 minutes ago, Bill Kielb said:

Does it get sucked up or just is more dense and sinks in the melt? Is there any force that says it has to become fully encased. Don’t know, just asking.

Gravity could cause it to sink, but not onto a vertical surface.  It would sink down if that were the case, right?

 

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2 hours ago, Bill Kielb said:

@liambesaw
I think, was just asking actually if it was a density thing or solubility or both or something else. Don’t know, have not studied at all. I was hoping you knew. 
 

Happy thanksgiving eve!

Well im guessing it's a mixture of all sorts of things.  Solubility for sure, probably also compounding with other chemicals, and of course simply free particles trapped in glass.

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I see. Thanks so much for all the help! But, when you write "gone metallic" - does that mean that the blue has a somewhat shine to it, or that it crusts and puddle on the outside of the glaze? When i stroke my fingers over it, everything is perfectly smooth, but some places the cobalt has a lilac ish, "metallic" colour. Is there some way to test this? I have heard of a lemon test, will it work on cobalt as well? Sorry for all of my questions, but i really don't want to poison anyone:)

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Yes a lemon test will tell you if it's really bad.  It won't test if cobalt leaches out though.  Here is a photo of what I consider "gone metallic".  This is iron, so while it might not be the tastiest surface to eat from, it is harmless to 99.9% of people.

When cobalt is over saturated it tends to turn black and crusty.  Copper turns black rimmed with red and green, chrome is black and crusty, iron and manganese typically melt into place and turn metallic.  They all will show metallic a little bit around the edges when theres way too much oxide.

Let's see... Rutile will boil into a bubbly yellow/orange crust, titanium goes to a white crust.  Those are the only colorants I've tested.

 

20200916093252_IMG_0517_copy_1500x1000.jpg

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8 hours ago, Earthandwater said:

I have heard of a lemon test, will it work on cobalt as well? Sorry for all of my questions, but i really don't want to poison anyone:)

Although the lemon test is assumed to be the threat it generally is far less so than caustic soaps and hot water. So dishwasher, chemical corrosion (leaching) cutlery, along with  daily wear are all difficult to predict without specifically testing for them appropriately. Putting lemon juice or a cut lemon on something is often not really scientifically revealing.

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Than you so much everyone! I  do not have a photo: smashed the vessel long ago. But I am about to start again, so I´ll post if it ever happens!

In the meantime I have used commercial underglazes and transparent glaze coat. Not my favourite, but it works, sometimes. But now I though I would check if these underglazes can be applied majolica style (if I dilute them first). I guess these all are safe? Much testing will follow:)

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if you buy some new colors, try Stroke and Coat from duncan.  oh, i think they changed their name.    i use them on my cone 6 pieces after spraying white glaze on the interiors of bowls.   look at my gallery for the flowers inside bowls for Empty Bowl suppers.   they work because thay are glazes, not underglaze.

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As I understand it, glaze in the melt has surface tension (kind of a barrier surface). If the oxide/wash is not absorbed partway into the glaze when applied, a few things could happen. .. .metallic surface, shivering of the oxide surface with sharp edges, or metallic oxides running over the glaze surface.

 

best,

Pres

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