Problem with royal blue underglaze (cobalt)

[Gazal]

Hi, I used royal blue underglaze on stoneware clay (greenware) then bisque fired it and then glazed it and fired it to 1250 C degree, and unfortunately I got some metalic bits, instead of nice blue, as if it was burnt or something, does anyone know why it's happened and how could it be fixed? thank you 

[Min]

Hi and welcome to the forum!

Could you give a bit more info about the underglaze you used? Firing range of it? Glaze info?

[Pres]

Your image shows some pinholing towards the center, and some other anomalies I can't make out. Could you post a picture with less reflections in it?  It may be that something in your transparent glaze interacted with the cobalt underglaze, not certain at this point.

 

best,

Pres 

[Babs]

That purpling crystals are somethkng I get with a glaze containing Magnesium Catbonate. The effect of different fluxes I guess. The cobalt may have been thicker there where it is occurring on your dish. Metallic usually comes grom overload of the metallic element in the underglaze

The bleeding is 

Bit thicker glaze?

Glaze and so underglaze moving at that temp?

[Gazal]

5 hours ago, Min said:

Hi and welcome to the forum!

Could you give a bit more info about the underglaze you used? Firing range of it? Glaze info?

Thank you Min,

sure, I've attached photos.

previously I used porcelain glaze and fired it to 1240C degrees and colour came out nice. this time I used stoneware clay and maybe thicker underglaze (not sure though) and so had to fire it to 1250C (glaze temp for stoneware clay).

would it be ok if I fire the same clay to 1230-1240C? and see what happens? 

I am new to this and have no clue how it can be fixed. have already painted heaps of greenware with the same underglaze and don't wanna go through what I went through last night opening the kiln. 

Thank you

[Gazal]

4 hours ago, Pres said:

Your image shows some pinholing towards the center, and some other anomalies I can't make out. Could you post a picture with less reflections in it?  It may be that something in your transparent glaze interacted with the cobalt underglaze, not certain at this point.

 

best,

Pres 

Hi Pres, thanks for your message. 

sure , are these better? I slow fired them, but the glaze has got some bubbles in it, it's not pin holes, they are in the glaze.

Do you know how could I fix the problem for both underglaze and glaze?

I used the same glaze and underglaze previously on porcelain clay and fired it to 1240C and result was better, nicer blue. 

what happens if i fire a clay to 10C degree lower than it's glaze tempreture (for example this stoneware should be fired to min 1250C, can I fire it to 1240C or even 1230C to get a nicer blue?)

[Gazal]

27 minutes ago, Babs said:

That purpling crystals are somethkng I get with a glaze containing Magnesium Catbonate. The effect of different fluxes I guess. The cobalt may have been thicker there where it is occurring on your dish. Metallic usually comes grom overload of the metallic element in the underglaze

The bleeding is 

Bit thicker glaze?

Glaze and so underglaze moving at that temp?

Thank you Baba,

I may have applied thicker layer of underglaze to get a more consistant colour accross the plates, last time I used different clay (porcelain /fired to 1240) and it seems that it worked better, but I am not sure, maybe i did not apply thick layers of underglaze.

Do you know how could I fix this issue? I have painted heaps of greenware with the same underglaze and don't wanna lose them. 

Thank you 

[Pres]

Pictures are better. Looks like there is something going on with the touch up areas of your underglaze where it may be thicker. Too bad(I hate to say this) but looks like testing would have been your only way out. I know. . . . lots of us get in a hurry wanting to see the finished pieces, but here with all the pieces done. . ... .?  At any rate try looking up the formula for the glaze. Post it here, maybe someone will have an idea. There are chemical reactions that cause purples, but certain conditions are usually needed for the kind of reactions you are getting. One is thicker underglaze coloring that may shiver or flake, interactions between the glaze and underglaze, and firing temps.

 

best,

Pres

[Bam2015]

22 minutes ago, Pres said:

Too bad(I hate to say this) but looks like testing would have been your only way out. I know. . . . lots of us get in a hurry wanting to see the finished pieces, but here with all the pieces done. . ... .? 

Ain't that the truth Pres? I do the same thing, hurry up and get to the finish line.

Gazal, I am not inferring that  you were racing to the finish line, that was just a little self-reflection. But, it will be interesting to find out what the problem was. I hope you will keep us posted.

Betty

[Min]

1 hour ago, Gazal said:

previously I used porcelain glaze and fired it to 1240C degrees and colour came out nice. this time I used stoneware clay and maybe thicker underglaze (not sure though) and so had to fire it to 1250C (glaze temp for stoneware clay).

would it be ok if I fire the same clay to 1230-1240C? and see what happens? 

If you have a look at the underglaze it says "For best colour response at 1280 we recommend low or no Zinc stoneware glazes." Then have a look at the glaze you are using, it contains zinc. "Midfire to Stoneware Clear Glaze containing approx 5% of Zinc and 5% Calcium" Even though you fired to 1250C and not the full 1280C this combined with what may be a heavy application would be my hunch as to why you got the colour response you did.

Which porcelain glaze did you use? Have a look at what the writeup for it, see if contained zinc, you can also look at the Safety Data Sheet (SDS) for it if it isn't clear.

For your second question, for functional wares you want to fire the clay to it's maturity to make pots that are as vitrified and strong as possible. I don't know what the maturity for your clay is so I can't answer that one for sure.

I know it seems very wasteful to fire a kiln with just test tiles but this is what I would do rather than taking a chance on a kiln load of pots. Ask your supplier if they can recommend  a zinc free clear for your clay and underglaze and make some test tiles. Try the underglaze with 1,2 and 3 coats then glaze with a thin, medium and heavy application and see what happens. Sometimes the unexpected happens and we don't care for it because it wasn't what we were going for but I think the purple areas look nice on your pot.

[Babs]

You may have unevenly dried greenware at the point of application and so inadvertently getting thick areas where the clay grabs the moist underglaze..

I spray the whole pot with a light mist of water before starting to apply underglaze.

It looks like you are brushing in lots of directions. Perhaps applying a resist to your flowers would enable you to then brush across your plates and get a more even application.

[Gazal]

On 10/27/2022 at 8:59 AM, Pres said:

Pictures are better. Looks like there is something going on with the touch up areas of your underglaze where it may be thicker. Too bad(I hate to say this) but looks like testing would have been your only way out. I know. . . . lots of us get in a hurry wanting to see the finished pieces, but here with all the pieces done. . ... .?  At any rate try looking up the formula for the glaze. Post it here, maybe someone will have an idea. There are chemical reactions that cause purples, but certain conditions are usually needed for the kind of reactions you are getting. One is thicker underglaze coloring that may shiver or flake, interactions between the glaze and underglaze, and firing temps.

 

best,

Pres

Thanks Press, I didn't have this issue before,  only happened to me last time , when I used different clay (stoneware instead of Porcelain) and I did not know I was gonna get this problem, so painted all other greenwares with the same glaze :(. 

 

[Gazal]

On 10/27/2022 at 9:22 AM, Bam2015 said:

Ain't that the truth Pres? I do the same thing, hurry up and get to the finish line.

Gazal, I am not inferring that  you were racing to the finish line, that was just a little self-reflection. But, it will be interesting to find out what the problem was. I hope you will keep us posted.

Betty

that's true! I didn't know changing the clay may cause this problem so I didn't do any test! lesson learnt! ... 

[Gazal]

On 10/27/2022 at 9:41 AM, Min said:

If you have a look at the underglaze it says "For best colour response at 1280 we recommend low or no Zinc stoneware glazes." Then have a look at the glaze you are using, it contains zinc. "Midfire to Stoneware Clear Glaze containing approx 5% of Zinc and 5% Calcium" Even though you fired to 1250C and not the full 1280C this combined with what may be a heavy application would be my hunch as to why you got the colour response you did.

Which porcelain glaze did you use? Have a look at what the writeup for it, see if contained zinc, you can also look at the Safety Data Sheet (SDS) for it if it isn't clear.

For your second question, for functional wares you want to fire the clay to it's maturity to make pots that are as vitrified and strong as possible. I don't know what the maturity for your clay is so I can't answer that one for sure.

I know it seems very wasteful to fire a kiln with just test tiles but this is what I would do rather than taking a chance on a kiln load of pots. Ask your supplier if they can recommend  a zinc free clear for your clay and underglaze and make some test tiles. Try the underglaze with 1,2 and 3 coats then glaze with a thin, medium and heavy application and see what happens. Sometimes the unexpected happens and we don't care for it because it wasn't what we were going for but I think the purple areas look nice on your pot.

Thank you Min, it was very helpful! 

I just bought the underglaze and as I had used it with no problem before, I thought there won't be any issues, but don't know what glaze I had used, plus my clay was different this time! What happened was very disappointing, but I am glad I learnt something so important from it. 

thank you for your time and Help! 

 

 

[Babs]

That blue can be problematic. I used to mix a bit of frit into it, you'd have to test, as it doesn't meld with the glaze at certain thicknesses 

[Gazal]

Thank you everyone, I have tried this underglaze on different clays and it works fine on mid-fire and earthenware clays. 

[Gazal]

On 10/27/2022 at 2:41 PM, Babs said:

You may have unevenly dried greenware at the point of application and so inadvertently getting thick areas where the clay grabs the moist underglaze..

I spray the whole pot with a light mist of water before starting to apply underglaze.

It looks like you are brushing in lots of directions. Perhaps applying a resist to your flowers would enable you to then brush across your plates and get a more even application.

thank you Babs, do you know a good resist?

[Babs]

 @Gazal                   Given you're using Walkers , in Oz?, wax resist , I use unbranded, decanted by my supplier I guess. Clear but if that is a prob, add a drop of food colour. Coat your brush with green liquid detergent, dip in resist and paint. Wash brush out immed when finished with the resist.

Or if you want to go in again and colour your flowers, latex allows you to peel it off after you apply the underglaze to add colour to the flowers.

Both products need to dry before brushing over them. Clean brushes immediately.

Spritz the whole pot with water for even application

Rims dry faster so will grab more underglaze.

Good luck

Edited December 12, 2022 by Babs