Pinholes: brush-on Western Matte Glaze

[Virginia Murphy Pottery]

I just took a load out of the kiln and noticed a ton of pinholes (and maybe a couple of blisters) and could use some advice about what I did wrong.

• Using B-Mix and B-mix with grog (cone 5)
• I bisque slow fired to 04
• I cleaned my pieces with a damp sponge or/or dipped in bucket of water before and let dry completely before  glazing
• I used brush on glaze (Western white matte cone 5/6). I did the recommended number of coats. I also let it completely dry between coats.
• But what I did do that I'm concerned could have contributed to the pinholes -  After the first coat of glaze was dry (overnight), I wiped away the brush strokes hard with my fingers and I think some glaze dust was left on the piece when I added the 2nd coat of glaze. Do you know if that could have created the pinholes? or maybe brush on matte glaze is just finicky. I should mention that I did not have this problem with the pieces I added to the same firing w/my Amaco brush-on Iron Lustre (however the Amaco was not a matte glaze, so maybe thats why. I don't know).
• Then I glaze fired slow to cone 5. Skutt 818-3 electric kiln. No formal vent. top peep always left out. I prop open the kiln with the kiln brick as Skutt recommends until it reaches 1000 and then remove brick and close lid.  Test cone's indicate that it was fired properly (though a little on the hot side). I opened up the glaze kiln firing at 188 degrees F.

I've got some Laguna Oatmeal dry by the pound glaze on order, so going forward, I can dip pieces instead, which will hopefully be less finnicky...fingers crossed. 

 

Edited December 17, 2023 by Virginia Murphy Pottery

[Mark C.]

When it comes to pinholes (which I have been fighting for over a year now in my gas kilns)

I have only one suggestion and that is a soak at end of fire.Its easy with an electric kiln harder with gas.

My last 3 kiln loads I have soakes (slowed down and held the top end temp for 10-20 minutes and all my pinhole issues went away.

a soak is whats needed since your bisque ware is clean

 

[Bill Kielb]

Interesting, my first experience with pinholes in glazes were with those very glazes, I believe. So much so that I decided to learn a bit about glazes. Mark has found a solution for his, so definitely test that. What I can share is some glazes will actually pinhole more the hotter you make them. For these compositions, the drop and hold firing schedule seems effective. Digital fire has a decent bit about this. For glazes affected this way a common thought is by composition their fired surface tension does not allow the pinholes to heal. For those, dropping temperature a bit allowing them to thicken slightly and holding allows the glaze to flow back together and heal the pinholes. My first experience with oatmeal was super frustrating with top end soaks and holds which for that glaze / clay combination made things worse. If a top end soak or hold seems to make things worse then I suggest reading about and trying (testing) a drop and hold schedule to see if that works for you.

[akilpots]

i've had some success with some glazes doing a drop and hold but with other the pinholes still prevail. i just finished matt katz's understanding glazes course and he has a compelling theory that the particle packing in some glazes or clay bodies is the ultimate source of the pinhole problem and that adding a small amount of darvan to help the particles pack better will eliminate the pin holing problem. i have yet to test it on my own but the data and experiments were compelling.

[Bill Kielb]

50 minutes ago, akilpots said:

i have yet to test it on my own but the data and experiments were compelling.

Just an add

I have entertained this theory and I think he wanted to call it “packed bed porosity” as I recall. A nice theory that may have value, but I have never been able to confirm with confidence in several trials.

[akilpots]

yeah thats right he calls it "packed bed porosity"

[Mark C.]

I will try the darvan theory. The thing is this glaze made from all old glaze materials just started about 14 months ago. I have had three other potters call me about pinholes in past year. My guess is for 49 years almasot no pinholes now pinholes  its not a new particle packing issue.

[Hulk]

Hi Virginia Murphy Pottery, welcome to the Forum!

Hard to tell if the craters, smaller pits, or both are open all the way to the clay?

Second question, are you seeing any pattern? ...looks like all three pitted/crater pieces have more defects up/away from the foot.
If the glaze layer is consistent, maybe the thicker portions didn't pit/crater as much?

The last ware, with the red glaze, that came out well?

My first guess would be drop and hold*, as Bill mentioned.
Here's a Tony Hansen article on the subject:
Pinholing (digitalfire.com)

Dropping 100°F and then holding for a half hour or more has really helped with multiple issues in my Studio...

[Virginia Murphy Pottery]

8 hours ago, Bill Kielb said:

Interesting, my first experience with pinholes in glazes were with those very glazes, I believe. So much so that I decided to learn a bit about glazes. Mark has found a solution for his, so definitely test that. What I can share is some glazes will actually pinhole more the hotter you make them. For these compositions, the drop and hold firing schedule seems effective. Digital fire has a decent bit about this. For glazes affected this way a common thought is by composition their fired surface tension does not allow the pinholes to heal. For those, dropping temperature a bit allowing them to thicken slightly and holding allows the glaze to flow back together and heal the pinholes. My first experience with oatmeal was super frustrating with top end soaks and holds which for that glaze / clay combination made things worse. If a top end soak or hold seems to make things worse then I suggest reading about and trying (testing) a drop and hold schedule to see if that works for you.

Thanks Bill. Yea, I was wondering if it was the western matte glaze, because the amaco iron lustre came out fine.  The western was actually super thick and I had to water it down a bit. I wonder if that could have added to the issue.  Thanks for the heads up on the oatmeal glaze. Do you have a recommendation for a similar color glaze that’s a bit more predictable? 

[Virginia Murphy Pottery]

3 hours ago, Hulk said:

Hi Virginia Murphy Pottery, welcome to the Forum!

Hard to tell if the craters, smaller pits, or both are open all the way to the clay?

Second question, are you seeing any pattern? ...looks like all three pitted/crater pieces have more defects up/away from the foot.
If the glaze layer is consistent, maybe the thicker portions didn't pit/crater as much?

The last ware, with the red glaze, that came out well?

My first guess would be drop and hold*, as Bill mentioned.
Here's a Tony Hansen article on the subject:
Pinholing (digitalfire.com)

Dropping 100°F and then holding for a half hour or more has really helped with multiple issues in my Studio...

Thanks for the reply Hulk! I can’t tell if the holes are making it all the way through unfortunately.

Yes maybe it’s a bit thicker at the foot. could be a thickness problem I suppose.

yea, that’s the amaco iron lustre i mentioned.  It was applied the same as the western glaze, and it had no problems. So maybe it’s a glaze thing. Not sure. 
 

that link is suuuuper helpful! Thanks so much!

[Virginia Murphy Pottery]

10 hours ago, Mark C. said:

When it comes to pinholes (which I have been fighting for over a year now in my gas kilns)

I have only one suggestion and that is a soak at end of fire.Its easy with an electric kiln harder with gas.

My last 3 kiln loads I have soakes (slowed down and held the top end temp for 10-20 minutes and all my pinhole issues went away.

a soak is whats needed since your bisque ware is clean

 

Will def try that!

you think the fact that there was loose glaze powder on the pieces between coats could have caused any issues? From rubbing off brushstrokes between coats?

[Bill Kielb]

6 hours ago, Virginia Murphy Pottery said:

The western was actually super thick and I had to water it down a bit. I wonder if that could have added to the issue.  Thanks for the heads up on the oatmeal glaze. Do you have a recommendation for a similar color glaze that’s a bit more predictable? 

It’s been so long, the only thing I remember is the aggravating pinholes and premixed glazes that were ridiculously thick on occasion. What I do remember is we always fired to cone 6 which in the end seemed a bit hot for some of the Amaco glazes especially adding a soak. Their published firing schedule is a fast ending segment of 270 degrees per hour in the lasts 200 degrees putting them at Orton cone 5-1/2 to cone 6. Most of our kilns fully loaded could not achieve that speed at the very top end so even more frustration. Good or bad It drove us to learn more about glazes and abandon all the commercial stuff.

[Virginia Murphy Pottery]

3 hours ago, Bill Kielb said:

It’s been so long, the only thing I remember is the aggravating pinholes and premixed glazes that were ridiculously thick on occasion. What I do remember is we always fired to cone 6 which in the end seemed a bit hot for some of the Amaco glazes especially adding a soak. Their published firing schedule is a fast ending segment of 270 degrees per hour in the lasts 200 degrees putting them at Orton cone 5-1/2 to cone 6. Most of our kilns fully loaded could not achieve that speed at the very top end so even more frustration. Good or bad It drove us to learn more about glazes and abandon all the commercial stuff.

Super helpful! Thanks!

[Callie Beller Diesel]

From your image, the bubbles look like they’re still trapped under the glaze somewhat, or have barely popped and haven’t healed over. In this case, I support getting a little more heat work into your glaze fire at the end, whether it’s going half a cone hotter or a drop and hold. Depending on the mechanism used to achieve a matte, some of them can either be a bit more refractory, or they’re just a bit less viscous than gloss recipes. 

[Virginia Murphy Pottery]

Thanks Callie! I’ll try one of those next time.