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oldlady

Translucent White Glaze. Can It Be Corrected?

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this glaze is the one used over slips to allow their color to show and the drawn lines show up white.  i use it a lot and love the effects.  it is very touchy, literally touchy.  if i touch the glaze it scars over and looks awful.  it is the reason i spray all glaze now.

 

the problem is that it contains zinc.  the thing that wrecks green slip and turns it grey. i am grateful for  dick white who explained why any of the mason stains that contain chrome do not work well when zinc is in the recipe.  he gave me the list and we noticed that almost every color has chrome.  who knew???

 

anyway, a different glaze expert suggested that since only 8.6 % zinc is in the recipe i might be able to eliminate it entirely.  i will test this but wonder if any of you with the glaze programs see any problem with doing this.  is there a substitute for it?  

 

i want so badly to have the green slip made with my new copper carb come out green.  is it possible?

 

 

 

charles counts translucent white glaze      cone 6

 

silica                         5.6

zinc                           8.6

EPK                          15.4

whiting                       18.8

neph sy                      51.6

 

thank you for looking.   see why i love it?

post-2431-0-98012500-1503440621_thumb.jpg

post-2431-0-98012500-1503440621_thumb.jpg

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I had been using a Bristol type glaze for several years, because it was semi matt, and seemed durable. However it is very finicky at temp, as it has to be just right to be semi matt, if over or under by 1/2 cone or so it is gone. It either is too matt, and not durable, or too shiny. I gave up on it, and went to a liner MC6 base glaze that contained tin oxide. This was not cost efficient, so I went to zirconium/tin equal amounts. Nice color good response to greens and all other, flashing if I want it. Much more curable. I would think you could take the base and use some opacifier to get the effect that you want.

 

 

best,

Pres

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Having messed around with a zinc-free cone 6 clear for the better part of the last 2 years, I have made a couple of observations.

 

1. They are unforgiving b@stards over dark areas. Bubbles abound, making things cloudy very easily.

 

2.Soak holds at the end of your firing are a must. You need to allow time for as many bubbles as possible to clear the melt.

 

3. The correct bisque temp is also a must, or in your once-firing case, be really aware of easing extra special slowly through the temperature zones below about 1472 F (800C). You want to allow as many of the carbonates and organic material as possible to be able to burn off thoroughly before the glaze starts to fuse and seal things off.

 

Zinc is more useful as a flux in smaller additions, and at mid to lower temps because it has a very wide range of temperature at which it stays in its liquid state. It starts to melt pretty low, and doesn't begin to vaporize until relatively high. It can make a glaze stay fluid for a long time in the kiln, giving glazes a good chance to clear bubbles (or form macro-crystals). In large quantities or given too much heat, it can make a glaze too runny, which is why it's not generally used a lot at high temperatures.

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Unfortunately your glazes Silica is not in the cone 6 range.  If you want to make some tests you can get it within the cone 6 range and probably get some more good glazes also.  I would suggest a 20 grid biaxial test.  If you are not sure how to make this test I could help you off line.  My email is hdwoodin@gmail.com

David

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Unfortunately your glazes Silica is not in the cone 6 range.  If you want to make some tests you can get it within the cone 6 range and probably get some more good glazes also.  I would suggest a 20 grid biaxial test.  If you are not sure how to make this test I could help you off line.  My email is [email protected]

David

 

Unfortunately that is probably why it is a Matt glaze.

 

best,

Pres

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callie,  so sorry you have so much trouble with bubbles.  i wondered what zinc adds to a glaze since so many recipes have it.  fluidity sounds important for a glaze, i hope my removing it does not cause more problems than i have now.

 

 

david, thank you, but i am confused.  (not an unusual condition.)  i do not have enough knowledge to understand why i need to adjust the silica.  the glaze is one that i have used since the 1970s.    charles counts used for many years before that and his apprentice, susan barnhart maslowski taught me to use it as she had put it on her work for many years as well.  we all fire(d) to cone 6 in electric kilns.

 

i am only trying to eliminate the zinc which changes green slip or underglaze to gray.  and you are right, i do not know what a 20 grid biaxial test is or what it would accomplish toward my goal.  can you help me understand?

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and you are right, i do not know what a 20 grid biaxial test is or what it would accomplish toward my goal.  can you help me understand?

Search this forum for threads on Ian Currie test tiles. I think this is very similar to what Dave is talking about. It is a good way to examine the effects of systematic changes in silica and alumina on a glaze.

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Another approach would be to use a green stain that doesn’t use chrome. If you are okay with using a cadmium inclusion stain there is a new green one available now. No chrome in the formula. Are you just brushing on the green slip and using on the outsides of pots?

 

US Pigments inclusion stain - https://uspigment.com/product/green-zr-si-cd-se-1352-green/

Edited by Min

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callie,  so sorry you have so much trouble with bubbles.  i wondered what zinc adds to a glaze since so many recipes have it.  fluidity sounds important for a glaze, i hope my removing it does not cause more problems than i have now.

 

 ?

It doesn't necessarily cause more problems, it just means you have to adjust your firing cycle so that you're letting the kiln's heat do what the missing zinc was doing previously. It's a different approach, that's all.

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I reread the original post and noticed that the glaze recipe resembles a clear Raku glaze that I use at school -  just a mix of Neph Sy and Gerstley Borate.  I add Zircopax to produce a white Raku glaze -- yes, it crazes in Raku.  A similar NS-GB based clear glaze has also been used at cone 3, cone 5, and cone 10 just by changing the ratio of NS/GB ( all in gas kilns). 

[i have also used straight GB as a clear glaze at cone 10 (gas and wood kilns only)]. 

 
In effect you would be substituting boron oxide for zinc oxide as the "flux".  With a bit of simple arithmetic the whiting amount can also be adjusted.  Or just do some line blends with the original recipe without zinc by adjusting the Whiting, GB, and Neph Sy levels until you get what you want as a clear at your firing temperature.  [a Currie type grid approach might be useful here also].
 
If you are up to the experimenting and testing for crazing and durability you might want to consider this approach.

 

N.B.: The intended functionality of my work is decorative, so some of the criteria of success - such as glaze crazing and durability - are likely to be different for a potter making food service items.
 
LT

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callie,  so sorry you have so much trouble with bubbles.  i wondered what zinc adds to a glaze since so many recipes have it.  fluidity sounds important for a glaze, i hope my removing it does not cause more problems than i have now.

 

 

david, thank you, but i am confused.  (not an unusual condition.)  i do not have enough knowledge to understand why i need to adjust the silica.  the glaze is one that i have used since the 1970s.    charles counts used for many years before that and his apprentice, susan barnhart maslowski taught me to use it as she had put it on her work for many years as well.  we all fire(d) to cone 6 in electric kilns.

 

i am only trying to eliminate the zinc which changes green slip or underglaze to gray.  and you are right, i do not know what a 20 grid biaxial test is or what it would accomplish toward my goal.  can you help me understand?

The glaze you now have is not a cone 6 glaze and you fire to cone 6, to make it a cone 6 glaze you need more silica in the glaze.

You could  try a  line blend to get the silica into the cone 6 range and keep the AL/Si ratio the same as you now have.  The easy way to do a line blend  is to mix   A    and  B  containers first (about 350 grams batch each

                                                                        NEPH 180.6      145.6

                                                                   Whiting      65.8        53.2

                                                                     Zinc         30.1        24.5

                                                                     EPK         53.9        89.6

                                                                    Silica`       19.6        37.5

If You have a 60 cc syringe you squirt the glaze into 7 small cups as follows:       1   2   3   4   5   6   7  cups

   each cup has this recipe in it

                                                                                                                             A 96  80  64 48 32 16 0

  Nep Sy  whiting Zinc  EPK Silica                                                                       B  0   16  32 48 64 80 96

1  51.6      18.8    8.6    15.4    5.6

2  49.9      18.2    8.3     17.1   6.5

3  48.3       17.6   8.1     18.8   7.3

4  46.6       17.9  7.8      20.5  8.2

5  44.9       16.4   7.5     22.2   9

6  43.3       15,8   7.3     23.9   9.9

7  41.6       15.2   7        25.6   10.7

You also could just make the above separated  batches.  they will start getting into cone 6 range at 3 on.  The 20 grid biaxial is a simplified curry blend, but it will have matte to shiney glazes at cone 6. The above line blend will be in the matte range you now like. Sorry for the confusion.

David

Edited by David Woodin

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goodness!   what a lot of responses!

 

thank you all for the suggestions.  i must get some more knowledge of chemistry whether i want to or not.  sounds like there may be a better way.

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I think Pres has a point about the silica ruining the matte effect of the glaze. Matte glazes don't generally have much in common with the limits found in an industrial glaze for say, bathroom fixtures and tiles. Usually they have an excess of flux or an excess of alumina.

 

Oldlady,

 

I thinm Min's advice to skirt around chrome glazes is sound, if possible and if you're fond of that particular glaze. Otherwise, I'd just hunt down a recipe without zinc. Mastering cone 6 glazes has a few, as does difitalfire. Line blends are a headache if you don't entirely know what you're looking for.

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Old Lady, you asked about the chemistry involving the zinc, and seem confused. I am not a glaze guru, far from it. However, over the years I have discovered from simple replacement and testing that you can not take zinc out of a glaze and replace it with another opacifier. Zinc acts as a flux in low to mid-range glazes and as an opacifier. Even though there may be an apparent flux in the recipe, Zinc lends some fluxing power also. This is why the glazes containing zinc are more difficult to make straight substitutions. So the glaze would have to be reformulated with more/different flux in order to be a proper glaze. However, when you remove the zinc, you basically have changed the entire glaze into another category of glaze.

 

best,

Pres

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this glaze is the one used over slips to allow their color to show and the drawn lines show up white.  i use it a lot and love the effects.  it is very touchy, literally touchy.  if i touch the glaze it scars over and looks awful.  it is the reason i spray all glaze now.

 

the problem is that it contains zinc.  the thing that wrecks green slip and turns it grey. i am grateful for  dick white who explained why any of the mason stains that contain chrome do not work well when zinc is in the recipe.  he gave me the list and we noticed that almost every color has chrome.  who knew???

 

anyway, a different glaze expert suggested that since only 8.6 % zinc is in the recipe i might be able to eliminate it entirely.  i will test this but wonder if any of you with the glaze programs see any problem with doing this.  is there a substitute for it?  

 

i want so badly to have the green slip made with my new copper carb come out green.  is it possible?

 

 

 

charles counts translucent white glaze      cone 6

 

silica                         5.6

zinc                           8.6

EPK                          15.4

whiting                       18.8

neph sy                      51.6

 

thank you for looking.   see why i love it?

You could take out the Zinc, it will still have the same AL/SI ratio, but it will be a different glaze and probably fire in the cone 7 range, it may be possible to use a frit to get more fluxing.  The new glaze brought up to 100% would be neph sy 56.5, whiting 20.6, EPK 16.9, silica 6.0 

David

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