New Pinholes, Old Glaze

[Chantay 106]

Half of the kiln I unloaded yesterday was glazed with the Laguna Morrocan Sand Glaze Cappuccino.  I fired to cone 5.  I have used this glaze numerous times before and not had this problem.  Same clay.   Only difference was new kiln.  I didn't have in witness cones (ran out, waiting on shipment) but I have fired this glaze to cone 6 before without problem.  I let cool naturally.  This kiln takes longer to cool, about 12 hours from shut off, than my old kiln but I wouldn't think that is what caused the problem. All the other glazes in the firing looked good. I plan to refire and do a slow reduction in temp.  Any other suggestions? 

 

 

[bciskepottery 959]

Those are not pinholes but blisters or burst bubbles or craters, generally caused by overheating. Likely the new kiln fires hotter than the old one. There might be something in that particular glaze that reacts more to overheating than the other glazes. Maybe try slowing down the rise in temperature for the last 200 degrees or so of your firing schedule.

[Chantay 106]

bruce,  OK, I can see that it might of over fired.  But, do you think refiring, making sure not to exceed cone 5, and slowing down the climb rate will fix these?  I have around 9 pieces that look like this.

[TJR 361]

Yes, you can refire those pieces. I agree that they are not pinholes but gas bubbles. Slow down your firing, and they may disappear.

TKR.

[Babs 919]

May help to soak it when you reach the temp required.

[ayjay 121]

I've had this problem occur a couple of times, slowing down the rate of firing towards the top end and a longer soak certainly does help.

 

I've not refired any of my pieces; should I wish to, is it advisable to try and get some more glaze in the area of the holes or just leave it to seal over?

[Norm Stuart 82]

The answer depends on how fluid the glaze is at the peak temperature.  If the glaze created this sort of problem initially you could almost always assume that the glaze isn't that fluid, so apply more glaze in those areas should help as the glaze won't be fluid enough to anneal the damage on reheating.

 

When people have had problems with the application of a clear glaze, I've suggested applying a thin layer of clear glaze of a lower maturity cone.  In the refire these two glazes combine with the original glaze and they end up with a perfectly smooth surface due to the extra flux in the lower-fire clear glaze.

 

 

I've had this problem occur a couple of times, slowing down the rate of firing towards the top end and a longer soak certainly does help.

 

I've not refired any of my pieces; should I wish to, is it advisable to try and get some more glaze in the area of the holes or just leave it to seal over?

[ayjay 121]

 

The answer depends on how fluid the glaze is at the peak temperature.  If the glaze created this sort of problem initially you could almost always assume that the glaze isn't that fluid, so apply more glaze in those areas should help as the glaze won't be fluid enough to anneal the damage on reheating.

 

When people have had problems with the application of a clear glaze, I've suggested applying a thin layer of clear glaze of a lower maturity cone.  In the refire these two glazes combine with the original glaze and they end up with a perfectly smooth surface due to the extra flux in the lower-fire clear glaze.

 

 

Thank you Norm.

[Chantay 106]

Thanks for the suggestions everyone. I'm hoping slowing down the climb rate a nd cooling speed will fix the problem. I didn't have problems with this glaze in the past. Will have to wait for warmer weather. Kiln is in the garage. Was only 27 F  today. Kids home for the third day. I am thankful they still like being outside and playing in the snow.