White spots coming through blue overglaze

[NatashaMM]

Hello,

I am a novice and very happy to find this excellent forum!  Please can I ask for some advice:

I am trying to get to a kind of majolica using stoneware clay.  I used a toasted clay, glazed white using a commercial tin based dipping glaze, then decorated using a stain and glaze mixture.  I fired to 1200c.  The result is fine, bar little white dots coming through the blue.  These are not exactly unsightly but seem kind of annoying.

 I got the same result when I changed the blue to Amaco brush on cobalt glaze. 

So if it's not the blue causing the white dots, is it my glaze or my clay or the combination?!

 

[Callie Beller Diesel]

Could be that some gasses are still coming through from the clay body when the glazes are starting to flux. If that’s the case, a longer bisque could help eliminate them. If the glaze is healed properly, it’s only a visual flaw though, and it’s up to you if you find it problematic or not.

[neilestrick]

Are the dots smooth or raised? Do they seem to be in the glaze or coming up from the clay?

[NatashaMM]

They are smooth and glazed over.  They seem to be in the glaze.

[Min]

Probably from gasses coming through from the clay. What is your bisque firing schedule?

[PeterH]

What are the small white dots ringed in this picture?

[NatashaMM]

My bisque schedule is 60c per hour to 600c then 100c per hour to 1000c.

[NatashaMM]

PeterH, you are indeed observant!  I didn't really notice but there are fine pinholes in the glaze.  They don't go through to the clay.

[Min]

5 hours ago, NatashaMM said:

My bisque schedule is 60c per hour to 600c then 100c per hour to 1000c.

 

13 hours ago, NatashaMM said:

I fired to 1200c

With darker claybodies it's a good idea to slow the bisque down during certain temperature ranges to allow carbons and sulphur to burn out. Between 150-320C organic carbons burn out and then between 700 - 900C inorganic carbons. Sulphur burns out between 700 - 1150C. I'ld slow your bisque firing down and go a little hotter with it also, up to cone 04 (approx 1050C. This could help your pinhole issue also.

I don't know what controller you have and how many ramps you can schedule in it butIld try something along the lines of the schedule below. If you still have white spots and pinholes then you could add a hold time at 900C.

Candle at 100C if pots are not bone dry

100C an hour to 315C, no hold

120C an hour to 700C, no hold

50C an hour to 900C, no hold

200C an hour to 1,000C, no hold

42C an hour to 1050C, no hold.

 

[NatashaMM]

Thank you so much,@Min. 

I had assumed it's just the initial part of firing that is important so it's very interesting to hear what you say about the 700-900c range.

I will follow your advice next bisque. 

[neilestrick]

Slow down the end of the bisque and the end of the glaze firing.

[NatashaMM]

Thanks all.  I've now got a plan of action!

 

[Babs]

The pinholes are throughout your glaze surface, not just the stain so the advice will sort the problem