Small bubble on the hard porcelain glaze

[Mhg]

I have got problem on the white hard porcelain ware .there is one or more bubbles on glaze .It is fired CO atmosphere .It is not pinholes.But the other kiln products are ok.I made many trials for this kiln.Unfortunatelly I havent got  exact solution for this defect sometimes it is decreasing sometimes it is increasing I didn't find the reason Is there any body to give advise?

[Denice]

You need to post your firing schedule so someone can help you?  Denice

[Min]

When I enlarge your image it looks like glaze blisters but it also looks like there are healed over pinholes / blisters also. Under or over firing and / or  glaze viscosity would be my first guesses. Could you post a closeup of one or two of the blisters?

[Mhg]

On 8/12/2018 at 7:30 PM, Denice said:

You need to post your firing schedule so someone can help you?  Denice

Kiln time 5 hours 15 dk reduction start 1190 celcius degree biscuit temperature 980 celcius degree

[Mhg]

On 8/12/2018 at 10:45 PM, Min said:

When I enlarge your image it looks like glaze blisters but it also looks like there are healed over pinholes / blisters also. Under or over firing and / or  glaze viscosity would be my first guesses. Could you post a closeup of one or two of the blisters?

 

[Mhg]

4 minutes ago, Mhg said:

 

 

[Babs]

Almost looks like it's a clay defect i.e. a blemish in the clay surface.

5 hrs is a fast firing.

Do you add a soak at the end??

[Mhg]

10 hours ago, Babs said:

Almost looks like it's a clay defect i.e. a blemish in the clay surface.

5 hrs is a fast firing.

Do you add a soak at the end??

There are 4 kiln they are using same raw material this kiln create these kind of problems also the problem is fluctuating by hours and hours is getting well or worse when i look kiln temperature and CO value there is no significant differance soaking time 2 hours i cant understand gas bubble is coming from body or from glaze or generating in the kiln .May the CO reduction region cause this problem I try many things no exact solution

[Babs]

So you are not firing your own work? Don' understand the " soak" at just over 1000d.

Quite a low bisque happening.

Fast rise at end of firing..can this be slowed for this kiln/glaze?

Where are the defect pots situated in ref. to flame path?

[glazenerd]

In the USA, sodium is the primary flux used in clay bodies. Sodium produces smaller pinholes, but many more times more numerous. In the UK, both sodium and potassium are used as fluxes. Potassium is a more abundant resource. Potassium produces larger pinholes/ blisters; but much fewer in population. Given it is a porcelain body, I would be looking at potassium off-gassing. Ramping faster at the end of the firing is contributing to this issue. Sodium begins it most volatile at 1120C, and potassium just slightly higher. It is at this same temperature (roughly) that the pores of the clay begins to seal up. Ramping fast at this segment tramps escaping spars, increasing pinhole/blister issues.

general rule of thumb: sodium produces small pinholes, but heavily populated. Potassium produces larger pinholes to blisters, but far less populated. Sodium issues below.

 

 

potassium shows like this. Potassium will often leave a ring of discoloration around it.