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Green Engobe going Grey under Clear Satin glaze


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#1 SmartsyArtsy

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Posted 01 October 2012 - 07:15 PM

Hi, have been reading and thinking and still cannot figure out why. I have an Engobe recipe that I have been using as white. Recently I added 2% Chrome Ox to a few hundred grams.
After bisque firing, the colour is a medium sage green. The Clear Satin Glaze shows off the white, but greys out-- almost a blue-grey over the green Engobe. Tthere is no zinc in the clear satin glaze which I otherwise like and use regularly.

I want a satin or mat finish, and have no problem testing other glazes, but without understanding if there is something obvious, I don't know where to start.

Engobe: EPK 25, Ball Clay 28, NephSy 16, silica 21, Borax 5, Talc 5. Add Bentonite 2, Zircopax 5, Chrome Ox 2

Clear Satin Glaze: Gerstley Borate 3, Mag Carb 3, Whiting 23, NephSy 23, EPK 20, Silica 20, 3124 9


Any insights are very much appreciated.
Thanks

#2 neilestrick

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Posted 02 October 2012 - 11:21 AM

First off, satin glazes will always be at least slightly milky, which will affect how clearly the engobe or underglaze underneath shows up. Cobalt can show up blue under a clear glaze, but will go purple as the glaze become more opaque. You may be getting a similar effect here. Second, oxides behave differently underneath a glaze than they do in a glaze. Sorry I can't say anything more specific than that. It could be that you just need more oxide, or try out a different clear.
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#3 SmartsyArtsy

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Posted 02 October 2012 - 11:51 AM

Thanks neil, the extent of color shift is surprising, not just the milky-ness I suspected would occur.

#4 JBaymore

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Posted 02 October 2012 - 01:13 PM

Chromium is affected by the presence of zirconium compounds. Makes it move toward gray tones. You have zirconium in the slip. Add in the fluxing of the overlying glaze and the pickup of both into the boundary layer, and likely that is the culprit. Try the base slip with the chrome but without the zircopax.

best,

................john
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#5 neilestrick

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Posted 02 October 2012 - 01:54 PM

Chromium is affected by the presence of zirconium compounds. Makes it move toward gray tones. You have zirconium in the slip. Add in the fluxing of the overlying glaze and the pickup of both into the boundary layer, and likely that is the culprit. Try the base slip with the chrome but without the zircopax.

best,

................john


Good to know! I rarely work with chrome.

That engobe shouldn't need the zircon anyway. It's plenty opaque as is.
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#6 TJR

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Posted 02 October 2012 - 04:36 PM

Hey, guys;
I avoid chrome as a colourant as it is so flat, like green paint on a fence.
I would suggest using 2% copper carb., and 2% chrome. This will soften it up a bit and give your colour some depth. As always, test,test,test. Right,Neil?
TJR.

#7 SmartsyArtsy

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Posted 02 October 2012 - 07:49 PM

John, I did wonder about that-- the zircopax and my list of things to try includes making up a new batch of engobe base, now without the zircopax. Working with chrome is new to me but I wanted a green without a bluish hue. I liked the color of the test, and hoped the clear satin would add just enough life to it. Thanks all, fwiw I'll include the results so far, and post the new result when fired.

The first time I tested the glaze over engobe, I had granular illmenite in the glaze, so that is what this pic shows. I did another test without the ilemnite, just to make sure it did not cause the color shift. It did not; no difference. Got the same color without the speckles.

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#8 JBaymore

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Posted 02 October 2012 - 08:27 PM

Chrome oxide is a difficult colorant to use for greens. As TJR said, it generally produces a very "flat" uniform color.... kinda' like paint. Adding a trace of copper or cobalt helps enliven it...... but that is gonna' "shift" it color-wise.

best,

................john
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#9 neilestrick

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Posted 03 October 2012 - 08:14 AM

Might be easier to get the green you want by using a stain.
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#10 TJR

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Posted 03 October 2012 - 11:18 AM

Smartsyartsy;
Actually, Neil is right, you can get very good colour with stains. This is a more expensive way to go.
Don't forget to make a series of tests-probably a line blend to see what percentage of stain you require.
TJR

#11 SmartsyArtsy

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Posted 03 October 2012 - 07:19 PM

Yes, another option. This piece I am working on is for a dear friend and there is a lot of deep blue glossy glaze (Blueberry). I want the leaves and parts of the background to contrast well. I've got a few options to test-- revise the Engobe, make up a couple of matte glazes, colour a sugary mat I already have,, and may buy a green stain. Everything I am learning here will be of good use for my other botanical tiles.

#12 SmartsyArtsy

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Posted 10 October 2012 - 06:45 PM

Today's CAD excerpt is about Chrome (John Britt)-- very timely considering I had a very disappointing view of my new chrome tests yesterday. Every one of my tests for adding chrome-- 2 new glaze bases, 2 familiar mat bases--- gave me brownish yuk. The engobe without the zircopax still came out blue/grey under the clear satin glaze.

I am digesting this excerpt and looking further into the mystery.

#13 neilestrick

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Posted 11 October 2012 - 07:55 PM

Sometimes just buying a commercial underglaze is a good way to go, too.
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#14 oldlady

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Posted 23 October 2012 - 09:31 AM

i have tried that with mixed results. i have several bowls ready for the 'empty bowl' event our guild supports. i fired two of them that had been covered in a green commercial underglaze, Velvet # 353 and carved. the matt, translucent glaze i sprayed over it made the finished bowl a lovely grey/blue. the very same green used under another glaze is screamingly green, overpowering the other colors i used on that particular bowl.

the difference is that the first glaze contains zinc. i have been looking for a great glaze to go over any color and found that zinc is not trustworthy. some colors just fade away when covered with a zinc bearing glaze.

i realize that the first post mentioned that there was no zinc in the glaze the poster used. what other ingredients cause the loss of color?
"putting you down does not raise me up."

#15 SmartsyArtsy

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Posted 24 October 2012 - 03:32 PM

oldlady, I have no sure answer for you, but I am still working on solving this problem. I am very curious what glaze went over your third bowl to result in the "screaming green". Would you post the recipe, and I will include it in my test.

#16 oldlady

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Posted 25 October 2012 - 11:46 AM

oldlady, I have no sure answer for you, but I am still working on solving this problem. I am very curious what glaze went over your third bowl to result in the "screaming green". Would you post the recipe, and I will include it in my test.



as j baymore notes below, none of these posts mention a cone number. i work only with cone 6 electric.


yes, i will send you that glaze. in re-reading the august 30 entry in my notebook, i find it said that the results were "crap", (or just not what i wanted). today, in looking carefully at the finished bowl with the screaming green leaves, i noticed that the soft colors, coral, pink, and purple in the flowers came out fine. the finish is very shiny. it also has unclosed bubbles because my kiln had a broken element. my notes say it was the glaze i got from the mason stain company technician named Vince. he said it would take any of the mason stains but not more than 10%. recipe follows in my format, using the smallest of the ingredients first and working to the most. (have you ever forgotten and left the 1000 gram weight on the scale when making a bucketful af glaze?)

EPK 10
whiting 13
silica 325 19
kona f4 26
frit 3124 32 (the actual recommendation was a fusion frit 3292 which i found i could not afford. i was told that 3124 would be close enough.)


as long as you are testing, and to tease those of you with a great interest in chemical interaction, i will include the following satin matt recipe which has no visible zinc in the list of ingredients but which turns green to grey. a lovely exterior glaze if you are not using green. i would not use it on an interior because of the terrible sound of a metal spoon scraping up the last of something in it.

EPK 20
Dolomite 20
Cornwall stone 60

i hope this helps, i would love to know your results. i still have 20 bowls due on nov 6. they are all sitting there in the greenware state waiting for me to do them right.
"putting you down does not raise me up."

#17 JBaymore

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Posted 25 October 2012 - 12:19 PM

i would not use it on an interior because of the terrible sound of a metal spoon scraping up the last of something in it.

EPK 20
Dolomite 20
Cornwall stone 60


What cone is this second glaze intended for?

If it is a cone 9 recipe, then there is another potential reason to not use it as a "liner" glaze: it is slightly undersupplied with silica (2.96 mols) for that cone range (which can support up to 5.75 mols). If it is for cone 6, then it is within, but to the low side of usual limits for silica (2.4-4.7 mols).

best,

.................john
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#18 oldlady

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Posted 25 October 2012 - 12:29 PM

sorry, i assumed, (yes, i know!) that all these previous posts were for cone 6 electric, which is all i do.
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#19 JBaymore

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Posted 25 October 2012 - 01:10 PM

sorry, i assumed, (yes, i know!) that all these previous posts were for cone 6 electric, which is all i do.


No problem whatsoever. I read back through the whole thread and couldn't see a cone range posted anywhere (I still could have missed it :rolleyes: ) ..... which is why I asked.

best,

................john
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Professor of Ceramics; New Hampshire Insitute of Art

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#20 SmartsyArtsy

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Posted 27 October 2012 - 10:44 PM

My clear satin mat is ^6




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