zinc and cobalt carbonate - solid gel

[Julie P]

I was doing a 36 tile colour blend test with an Emmanuel Cooper clear glaze (#141 - see pic) and 9 oxides including tin and zircon.

The glaze contains 4% zinc and I added 2% cobalt carbonate. There is too much bentonite but this is not key to the problem.

1. When I added cobalt carbonate the glaze gelled and became a complete solid. It did not do this with any other oxide/carbonate.
2. I have eliminated bentonite as the main cause - although there is too much in the original recipe - even with 0% bentonite it gels.
3. I have eliminated any issue with the feldspar - the same thing happens irrespective of whether it is a soda or potash spar.
4. I have tested with cobalt oxide and with copper carbonate (and 7 other oxides and combinations thereof) - this does not happen.
5. Reducing the zinc to 2% made the glaze less thick and reducing the cobalt carbonate to 1% likewise.

So, the only conclusion is that it is a zinc / cobalt carbonate reaction. Does any chemist out there have a sensible explanation for why this happens.

I can happily create the glaze I need using cobalt oxide, so please don't suggest dispersant, that is not my question.  I am intrigued by the chemistry - does anyone have any answers about the chemistry of these two materials which could explain this phenomenon?

[Hulk]

I don't mix any glazes containing both zinc oxide and cobalt carbonate, nor do/did the glazes available at the local JC ceramic lab, can' help there.

I am curious, though, very, bump.

...and wanted to say great notes, explanation, and testing to verify.

Are you measuring specific gravity? I don't measure water volume - maybe I should, hrrmm; I wet until close, by how it stirs, check SG, then dial it down to the specific from there, 1.39 to 1.47, depending on which glaze, for medium stoneware. Just curious how wet the samples are.

[Julie P]

For these tests I simply used the same water volume in each one, so by weight 100g/100ml. That was about right for the original glaze and I wanted to see the effect on viscosity of the different materials. 

[oldlady]

try starting out with the weight of the glaze ingredients at 100 and the weight of the water at 66%.   

[Min]

Anybody have time to mix up a 100 gram test and see if happens with their materials?  I'ld offer to do it but my studio is packed up while we get the house ready to put on the market. I'm wondering if your cobalt carb has something odd going on with it. If someone could test this and doesn't have FFF spar then Custer should be a close enough sub for the FFF for the purposes of this test.

Really doesn't make sense what is happening. Bristol glazes (contain zinc) can be made with cobalt carb as can macro crystalline glazes (which contain around 25% zinc) without the clotting you are experiencing. 

[neilestrick]

I've used cobalt carb in dozens and dozens of glazes over the years and have never seen this happen. What happens if you just put some of your cobalt carb in water by itself? The zinc in water by itself? What does your cobalt look like raw?

What does the bottom half of the notes say? I can't decipher that.

[Julie P]

20 minutes ago, neilestrick said:

I've used cobalt carb in dozens and dozens of glazes over the years and have never seen this happen. What happens if you just put some of your cobalt carb in water by itself? The zinc in water by itself? What does your cobalt look like raw?

What does the bottom half of the notes say? I can't decipher that.

The bottom notes show water content 4000g dry glaze in 4600g water SG 1.46. I also use cobalt carbonate in mamy glazes with no problem and this zinc glaze works fine with every other oxide and carbonate. I have tested 9 other oxides in this glazr. It is just the CoCO3 and ZnO together which produces this problem. 

[Bill Kielb]

1 minute ago, Julie P said:

It is just the CoCO3 and ZnO together which produces this problem. 

Nice work! Very interesting..

[neilestrick]

26 minutes ago, Julie P said:

I have tested 9 other oxides in this glazr. It is just the CoCO3 and ZnO together which produces this problem. 

Are you using epsom salts or any other soluble additives?

Have you tried it with distilled water?

[Hulk]

I'm still curious - Are you measuring specific gravity?

Sometime yesterday I thought*, oh, the weight of material is there, and the volume (and hence, weight) of added water, so I can math the SG, duh.
...and sometime still later, uhm, maybe not, for the resultant volume is still unknown.

2000 grams material, four liters water (4000g) hence 6000 grams; I'm not seeing where the 8600 comes from.
Not sure that matters, any road, SG is weight per unit volume, isn't it? The volume, well, that needs measurin'.

*perusing and participating in this forum gets me thinkin'!

[Bill Kielb]

36 minutes ago, Hulk said:

2000 grams material, four liters water (4000g) hence 6000 grams; I'm not seeing where the 8600

2600 g of premixed Co???? Nice catch! Always just measure 100ml of my final mix as well, eliminates oops errors for me. Actually have a tare weighted 50 ml syringe, so less accurate, but quick and easy.

[Hulk]

How well the glaze behaves for application, and the thickness o' the resultant film are important to me; once happy, I'm looking to repeat, hence the measuring, notes, etc.

I mix mix mix, measure out 100 ml and weigh, aye.

I liked (and still like) the syringe idea.

[Piedmont Pottery]

Out of curiosity, is your zinc oxide calcined or raw?

[Julie P]

Raw. I know calcined may solve the problem with CoCO3 but the issue is why it behaves differently with the CoCO3 compared with other oxides/carbonates.

[Piedmont Pottery]

Do you know the pH of the water you are using?

[Julie P]

13 hours ago, Piedmont Pottery said:

Do you know the pH of the water you are using?

It's tap water, has some calcium in it but not a lot. Would tge water react with the CoCO3 differently from other oxides?

[PeterH]

For what it's worth:

Cobalt Carbonate https://digitalfire.com/material/cobalt+carbonate
...
Supplies of this material often differ in the shade of the raw powder (lighter and darker). This is because available grades of Cobalt Carbonate are not actually CoCO3 but a mix of the carbonate and the hydroxide. Cobalt II carbonate theoretically would have the formula: CoCO3.3Co(OH)2.H2O.
...
Manufacturer information sheets often quote the percentage of Co instead of CoO (thus a lower amount in the range of 45-47%. Trace elements like Ni, Fe, Mn, Cu, Zn, Pb and Mg are quoted but none are in large enough amounts to worry about in ceramics. Na can be 0.5%. Products are sold based on Co content, higher purity grades may have only 1 or 2% higher Co content. Particle size can be about 2.5 microns.

[Julie P]

8 minutes ago, PeterH said:

For what it's worth:

Cobalt Carbonate https://digitalfire.com/material/cobalt+carbonate
...
Supplies of this material often differ in the shade of the raw powder (lighter and darker). This is because available grades of Cobalt Carbonate are not actually CoCO3 but a mix of the carbonate and the hydroxide. Cobalt II carbonate theoretically would have the formula: CoCO3.3Co(OH)2.H2O.
...
Manufacturer information sheets often quote the percentage of Co instead of CoO (thus a lower amount in the range of 45-47%. Trace elements like Ni, Fe, Mn, Cu, Zn, Pb and Mg are quoted but none are in large enough amounts to worry about in ceramics. Na can be 0.5%. Products are sold based on Co content, higher purity grades may have only 1 or 2% higher Co content. Particle size can be about 2.5 microns.

Thanks for that Peter, the colour isn't the problem, but I do wonder if these complexities affect the viscosity of the glaze depending on the supplier of the material

[Callie Beller Diesel]

Gimme till next week. I’m out of town at my last travelling show for the year, but I need to get back into the studio when I’m back and this seems like a random, curiosity inducing enough project to get me there. I haven’t bought any of those materials in the last couple of years, so we can see if it’s a new source/batch thing.

[Hulk]

I'm still curious - Are you measuring specific gravity?

[Piedmont Pottery]

5 hours ago, Julie P said:

It's tap water, has some calcium in it but not a lot. Would tge water react with the CoCO3 differently from other oxides?

If the water was acidic it would increase the solubility of the cobalt carbonate, but if you have calcium in the water it's probably on the basic side.  Have you used this batch of cobalt carbonate in other glaze formulations with normal results?

[Min]

2 hours ago, Hulk said:

I'm still curious - Are you measuring specific gravity?

Even though I'm not so sure it's related as the op is using the same ratio of dry glaze to water I too am curious how specific gravity is measured. 

On 11/16/2021 at 3:17 PM, Julie P said:

The bottom notes show water content 4000g dry glaze in 4600g water SG 1.46

Given the only clay in the recipe is bentonite I ran it through Insight to double check the alumina level. It's quite low for a durable high fire glaze, wouldn't hurt to have the silica higher also.

[Julie P]

3 hours ago, Hulk said:

I'm still curious - Are you measuring specific gravity?

I do measure specific gravity, every time I open a glaze bucket, but that is not the issue here as they all come from the same base glaze batch.

[Julie P]

2 hours ago, Piedmont Pottery said:

If the water was acidic it would increase the solubility of the cobalt carbonate, but if you have calcium in the water it's probably on the basic side.  Have you used this batch of cobalt carbonate in other glaze formulations with normal results?

That's interesting, and I'll pop it away for future reference, but no our water is  definitely on the 'hard' basic side. Thank you

[Min]

2 hours ago, Julie P said:

I do measure specific gravity, every time I open a glaze bucket, but that is not the issue here as they all come from the same base glaze batch.

Okay, good to know. So if your specific gravity (sg) is 1.46 that would include the green glaze in your first image? It gets that thick with 3 bentonite and the sg at 1.46? 

edit: did you remove some of the cobalt glaze from the container? Level looks much lower than the green glaze. I know it's a long shot but it's not just evaporation is it?