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Iron Speckles


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I have 2 glazes with the same colourants, 1 results in a amber colour with no iron speckles, the other is full of speckles. Same claybody, firing schedule used for both glazes. I know I could probably ball mill the speckled one but I'm wondering what is causing the iron to agglomerate in the glaze? ^6 electric, low coe glazes.

 

no iron speckles recipe:                                                

Tea                                                                           
Silica 23.00                                                                   
EP Kaolin 14.00
Nepheline Syenite 10.00
Frit 3195 16.60
3134 4.80
Australian Spodumene 15.00
Dolomite 15.00

Bentonite 2.00
Manganese Dioxide 3.00
Iron Oxide Red 2.00
105.40

 

Speckles

Silica 34.50
Wollastonite 5.00
EP Kaolin 23.00
Minspar  12.00
Gerstley Borate 22.00
Talc 3.00
Dolomite 3.00
Nepheline Syenite 2.90

Manganese Dioxide 3.00

Iron Oxide Red 2.00

 

 

I don't think the higher silica levels in the second recipe is the culprit, I'm thinking the lithia in the first recipe might have something to do with it? Boron differences?

Any thoughts most welcome.

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Interesting, got any pictures?

 

I have no real ideas 

 

My though process would be the first glaze melt is managing to incorporate all the iron and manganese into the melt and the second glaze is not. Can't back this up with much information, just a guess.

 

It could be manganese speckles and not iron.

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I love the speckle :D

 

It could be to do with the high silica/borate. Doing some crude calculations you seem to be at the upper limits of cone 6 glazes for both. "Crystals are normally silicate or borate compounds, thus SiO2 and B2O3 need to be present in significant amounts." http://digitalfire.com/4sight/glossary/glossary_devitrification_crystallization.html

 

Now is the speckle a crystal, I do not know. To me it would come from adding some granular manganese but seeing as you are using the same for both glazes I am stumped.

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Wallastonite can have many impurities,such as garnet as mentioned in digitalfire. Could this be the source of your specks? I would try a different source of this ingredient.

 

That's my guess.

 

Jed

 

Thanks for the idea but I don't think it's the wollastonite as I use some from the same batch in another glaze with no specks. I just got the last bag of the old supply from my supplier, don't know how different the new stuff will be.

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It is the talc in the glaze.

David

 

Even though the MgO supplied to both recipes is exactly the same? I will omit the talc and replace with more dolomite and adjust the silica and calcium levels and see. Thanks for the thought. It seems the consensus is it could be anything and everything except the silica, spar or kaolin. Lots of fun. I think I might just scrap the speckled glaze and start over but I would like to know the cause of the specks for glaze testing going forward. Or to look at the other way around, why there are no specks in the first glaze.

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Sorry, before looking at the pictures I though you were talking about teadust. MGO will produce teadust in some glazes.  The speckles glaze is on the AL/SI outer limits and B203 is quite high.  Both glazes use a lot of different materials.  They both could be modified so that you would not have to stock so many materials. Neither is in the food safe limits. I am not much help.

David

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Hi David,

 

Which limit figures are you using that have them both coming up outside the limits? Using Insight and the Hesselberth/Roy limits plus the Green/Cooper ones the silica and boron are high in the speckled one, I have the first one coming within the limits.  That second one, the Speckles, is my version of G1215U Low Expansion from Tony Hansen at Digitalfire where I removed the 3249 frit and rebalanced the formula. I used the original G1215U for a few years but that frit is so expensive I have redone the recipe without it. Without the colourants the 2 versions are very close. I have never considered high silica levels to be a problem if there is a good melt, as Hansen is okay with these silica and boron levels I wasn't concerned with them being what they are.This is what I get for the unity formulas:

 

Tea:

CaO 0.40*
Li2O 0.11*
MgO 0.25*
K2O 0.02*
Na2O 0.12*
P2O5 0.00*
TiO2 0.00
Al2O3 0.40
B2O3 0.21
SiO2 2.80
Fe2O3 0.04
MnO 0.00*
MnO2 0.10*

 

and the Speckled one:

CaO 0.51*
MgO 0.22*
K2O 0.03*
Na2O 0.12*
P2O5 0.00*
TiO2 0.00
Al2O3 0.42
B2O3 0.31
SiO2 3.81
Fe2O3 0.05
MnO2 0.13*

 

Yup, if I used a lot of the first glaze I would rework the recipe but haven't bothered. I use all those materials in other glazes so it's no bigs. 

 

Thanks for your thoughts,

Madeleine

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does the speckle occur without the colourants ie in the base glaze? or without the iron etc....

 

base glaze is crystal clear, it's a terrific low coe base which I modified from this one: http://digitalfire.com/4sight/recipes/low_expansion_glossy_clear_cone_6_45.html

(it will likely shiver or dunt on a lot of stonewares so if anyone tries it please test for this)

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Have you tried both glazes with different colourants to see what happens?

 

I think it could be the high b2o3 with the silica and high cao crystallizing something out the melt. Could try reducing the b2o3 and adding some other flux. 

 

Lots of speculation in here but great ideas to test out.

 

Why do you want this other glaze to 'work'?

 

Interesting to see the pic on digital fire with mason stains, do the browns there have a slight speckle? I can see something. Maybe removing the frit has enhanced it along with using raw oxides. Have you tried the recipe with the frit before you rebalanced it?

gofbisenov.jpg

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Hi High Bridge,

 

I'm mostly trying to figure out the "why" of this. I've had other glaze tests speckle and have often wondered why some do and some don't.

 

I'll add a test tile with original G1215U plus colourants in the next glaze fire, I hadn't done that since I'm trying to get away from using the expensive frit that it uses but it might answer the question. 

 

Glaze testing sure takes up a lot of time doesn't it?

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Probably some of my materials are not up to date..  Tea using Matrix, all within limits, Green and Cooper in limits, Food safe Alumina  okay, Silica too high, Insight Alumina too high SI okay MGO high KNO low B203 low.  Speckles, Matrix AL okay Si on the edge of being too high, Food Safe AL too high Si on the edge, Green and Cooper B203 on the edge of being high, Insight AL and Si high B203 low MGO high KNO low.  So it is best to use what you are satisfied with.

David

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It was mixed and applied to test tiles straight away. I mix the glaze up then put through a 80 mesh sieve.

 

Speckles were not apparent in glaze slurry that is why I am wondering if the iron is agglomerating during the firing? I've always thought RIO is very fine particled, the speckles are fairly large.

 

I think the only way I'm going to solve this is change one material at a time as see what happens. Crazy at home right now so this might get pushed onto the back burner as it's more about the theory than practice.

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I remember an organic chem lab I had to take. I made vanilla crystals which normally are large and white. Mine were small and green. The size came from being too much in a hurry. I can buy that....But the green color? The mass spectrometer showed copper in the mix. It turned out that I used an old chrome plated scoop to measure ans stir my soln. The chrome had worn off and the copper metal was exposed to the acidic solution. The result...green crystals.

 

It comes to mind every time I have an unusual outcome. When it comes to glaze making, it is ever present on my mind.

 

Just reflections.

 

 

Jed

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