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Delving into the realms of mixing my own glazes I found a Greg Daly recipe that covered a range of firing temperatures and was the sort of turquoise I love. On a white stoneware test tile (bisqued to 1000oC), glazed to 1074oC it was a lovely bright matt turquoise. I decided to use this for the interior of a set of 7 bowls a friend asked me to make as gifts for old college friends, at a reunion.

I made up a 2 litre batch to the same recipe but with 1% bentonite added. (After reading various posts on CAD!) The specific gravity was 1.64. Not sure of this significance of this and I left it as it was, as it seemed ok for pouring to me.

The only other differences between the test tile and the bowls were that the bowls (v thinly rolled - about 1/10" final thickness, and hump moulded) were bisqued to 1100oC and glazed fired to 1120oC. The outsides had a very dilute manganese wash and I poured the insides for 10 seconds.

 

This is the recipe

Neph Sy 60; Barium Carb 30; Lithium Carb 5; Kaolin 5

+ Bentonite 1; Copper Carb 4

The test tile and finished bowl photos are attached. Not sure how clearly the colour difference will show. The bowls were much darker, less bright than the test tile and were slightly more towards satin than matt which is very finely crazed (barely perceptible). I do actually think the colour became somewhat brighter as the pieces cooled, if this makes sense to anyone?

 

So, from my explanation above, can anyone suggest the most likely reason for the difference in the final colour?

Glaze thickness? Firing temperature? Bentonite? Manganese seeping through from outside?

 

Also, if I want to experiment around this basic recipe, what approach would be worth taking? Which of the ingredients would it be logical to adjust. I have books explaining line blends and triaxials, but some pointers as to how to do that with an existing recipe would be much appreciated.

Now I've fulfilled my friend's brief - (she was delighted) I have time to spend on glaze testing.post-13648-0-35733100-1431693286_thumb.jpg

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post-13648-0-35733100-1431693286_thumb.jpg

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Barium blues are difficult to duplicate-- the color is unusually clean and intense.  You could try substituting strontium, but the blue will not be quite as vivid.

 

You might look into glaze stains.  Mason makes a turquoise that is pretty good in the right base glaze.

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A hazard message is not reassuring when the bowl could last beyond one owner. Use the glaze on the outside but not for contact with food.I have a glaze similar. It looks great on carved vases. (outside of vase). Have you done a slice of lemon test on it? I think you would see a noticeable change in color. You can use strontium carbonate to sub for barium but not 1:1. 

I'd have to look it up. 

Marcia

 

http://ceramicartsdaily.org/ceramic-glaze-recipes/glaze-chemistry-ceramic-glaze-recipes-2/leaving-bariumville-replacing-barium-carbonate-in-cone-10-glazes/

 

http://community.ceramicartsdaily.org/topic/1064-substituting-strontium-carb-for-barium-carb/

 

http://cone6pots.ning.com/forum/topics/pete-s-weathered-bronze-green

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I'm getting more worried as the day goes on. I need to look back at the book I got the recipe from as I'm intrigued to see if there was any warning there. I'm afraid I made an assumption that something which appeared in a standard book on glazing (bought only recently, so not a historic item and recommended on CAD no less) wouldn't be so hazardous - or would at least have a clear warning on the recipe pages. This recipe appears many times in the book, for different temperature firings and differing amounts of copper oxide.

These are very small commemorative bowls which in my mind were never intended to be functional as far as food goes, so the whole notion of food-safety did not enter my mind! I anticipate, that if they are put to any practical use it will be for putting rings or earrings in but more likely they'll just sit on a shelf.

I'll be sure not to use it on the inside of anything in future! As these bowls have today gone to the college reunion, my only option is to remake them using a different glaze.

I've gone from feeling very pleased with myself - first time I've made a 'set' of anything and first time I've mixed my own glaze, to feeling very deflated. Perhaps I should repost this under the 'what discourages you and how do you get over it" thread!

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Don't be discouraged.  This is something you wouldn't know about without a fair amount of glaze experience, I think.  If this is your first time to mix your own glaze, it's an understandable error.  Commercial glazes need to be non-toxic, so if that's what you've used before, no wonder it didn't occur to you.

 

Just tell your friend to say that these are keepsakes, and shouldn't be used for food.

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Thanks for that Ray - I've texted her already!

The whole process - developing an idea, trying different clays, moulds, texturing methods, decorating techniques, as well as mixing several glazes and testing on tiles has been really valuable, so I'll add this to the list of things I've learnt. It has also prompted me to do a bit more reading around the subject. I've read the Safety Data Sheet and the ATDSR public health statement already. I've got some blue and green glaze stains, so I'll 'play' with those.

Thanks for your input Marcia - I'll do the lemon test on it - another first as I've never made anything remotely functional, nor mixed my own glaze, so never had to do this before.

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Don't beat yourself up over this, you learned something and in the grand scheme of things thats what counts. There are a gazillion horrible glaze recipes out there, some will say they are not food safe but most won't. 

 

3 part article on barium here:

 

http://www.claytimes.com/articles/barium1.html

 

http://www.claytimes.com/articles/barium2.html

 

http://www.claytimes.com/articles/barium3.html

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.........

 

bisqued to 1000oC), glazed to 1074oC it was a lovely bright matt turquoise.........

 

bisqued to 1100oC and glazed fired to 1120oC........

 

 

 

The test tile and finished bowl photos are attached. Not sure how clearly the colour difference will show. The bowls were much darker, less bright than the test tile and were slightly more towards satin than matt which is very finely crazed (barely perceptible). I do actually think the colour became somewhat brighter as the pieces cooled, if this makes sense to anyone?

 

 

So, from my explanation above, can anyone suggest the most likely reason for the difference in the final colour?

 

attachicon.gifimage.jpgattachicon.gifimage.jpg

Bisque and glaze temps are different, so I would be surprised if you had got the same result !
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There were reasons for the firing differences - not worth explaining here Ann.

Given that I'd added the bentonite and used different glazing methods which could result in different glaze thickness, as well as the firing temperatures, there were several differences which may have been the cause. I guessed some of the forum members might know which of these was the most likely culprit.

So far only the bentonite is innocent!

I'm going to research some other recipes now - in fact there was one I tested at the same time - hence the 1074oC firing temp, which wasn't a million miles from the result of the Daly one. I'll have a look at that one - to see if it has any dangerous components before using it on some other test pieces.

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Your Barium glazes can be used on other pots not suited to food situations but will still be a hazard to you when mixing and handling them.

beautiful blues.

I thought Strontium was no safe substitute for the food container.... Anyone want to elaborate on that?

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Thanks Babs. I was very careful - gloves, mask etc. so not completely incompetent!

As I've said in my previous posts on this, these bowls really didn't fall into the foodware category in my mind. But I'll keep it for exteriors of anything in future!

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Strontium is not toxic.  It is not the same as the radioactive isotope.

 

But many folks would say that the color response is not quite as vivid as barium.  Strontium can be obtained in frits as well as strontium carbonate.

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What are safe levels of any of the glazes that have a toxic carbonate in them? I realize this isn't a reason to assume they are safe, but what are normal levels.

 

Like I have a few glazes I am wanting to test. One has 5% barium carbonate. I wasn't planning on using them on the inside of the cup, but on the outside of the cup down from the lip a good bit. Maybe I should rethink my plans. 

 

My plan is to send a cup off to that lab that does leech testing once I get my glaze combinations down that I plan to use for most of my pots.

 

Right now all my pots have the same glaze on the inside that contains 0 toxic ingredients. It is a beautiful cream white that has brown hues in it. However it is pretty boring on the outside of the pots. So I was looking at a few glazes with some toxic ingredients to get colors that I haven't worked with yet.

 

Stuff like: Lithium Carbonate, Zinc Oxide, Cobalt Oxide, Barium Carbonate. All of these are less than 10% in the recipe. But I am still concerned about them, this is the first time I have mixed glazes with toxic ingredients. 

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It really isn't a matter of amounts, though as in Celia's glaze, 30% of barium can't be made non-toxic in a normal glaze.  It's whether the toxic elements are safely contained within the glassy matrix of the glaze, and how stable that glass is when attacked by acids or other agents that might degrade it.

 

For pots that are to be used for food, I would always use a liner glaze that contains no toxic elements.  It's one less thing to worry about.

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I do use a liner glaze that has 0 toxic ingredients for the insides of my bowls and mugs and plates etc. 

 

I guess the main thing is that if I am using glazes that contain toxic elements on the outsides of my pots should I have anything to worry about. These are the surfaces that food doesn't touch unless someone was eating off the side of a mug or something? 

 

I guess the main thing is to keep any toxic ingredients away from the rim where the lips might be placed as well. 

 

Ceramics. SO much to learn in one lifetime.

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Well, it's a hard thing to talk about, because it's possible to come up with all sorts of scenarios where toxic materials leaching from the outside of a piece might somehow get into the user's body.  But as a general thing (and I emphasize that this is only my opinion) I don't believe that very many glazes are dangerous if used on the exterior when they contain only small amounts of toxic elements, well involved in a good melt, and not in a glaze with a weak structure.  

 

As an aside, this is where potters who are working at higher temps have an advantage, since high range glazes can be made of simple non-toxic materials and still be very beautiful.

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That is good to know. My liner glaze, even with 0 toxic ingredients. I tested it vs drano, lemon and ive been running one cup in the dishwasher every day for about 3 weeks now. No signs of wear yet. I am pretty satisfied with it, knowing now that there isn't really a reason to worry about it makes me ready to move onto my exterior glazes. I am a little worried about those but I will figure out. 

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Grype,

 

there are sooo many beautiful glazes out there.  Why risk using a potential toxic one?

 

I was looking through John Britt's book on cone 6 glazes.  Pinnell Strontium Matte is very similar in color, but matte.  You may want to try it.

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