Speckled buff pinholes

[garyl]

I'm pretty new to pottery, just started this summer and am enjoying it.

I tried a glaze from glazy - studio white - https://glazy.org/recipes/7973

I made a batch, and used it with no zircopax on a speckled buff piece, laguna #60 clay. It has several pinholes right on a speck.

I added the 10% zircopax and used it on a bowl, and the glaze turned out fine, with no pinholes. Both pieces were bisqued and fired the same.

I used a very slow bisque, about 20 hours to cone 04.

The glaze firing was to cone 6, it went to 2200, held for 10 minutes, dropped to 2100 and held for an hour, then turned off. The witness cones confirmed the temps.

I'd like to find a clear glaze that doesn't pinhole on the speckled buff clay. It looks like the zircopax does something to the glaze so that it doesn't speckle. How can I get a clear that works the same?

 

Thanks

Gary

 

 

 

 

[Min]

Glaze looks really fluid, it's pulling those manganese speckles in the claybody a fair bit. I'm thinking the bowl form versus the flat bottomed form is contributing to the success of the white glazed pot. With a fluid glaze and gravity working on your side (with a curved rather than flat pot) the pinholes have an easier time healing over.

Have you fired the clear glaze on a bowl form with success and/or fired the white glaze on a flat bottomed pot? If the white is okay on a flat bottomed piece then its probably worth a try adding silica to the clear to bring the level up to the same as it is with the zircopax white version. (zircopax contains zircon + silica). Bumping it up to 22.90 and leaving everything else the same will balance it out so the silica level is the same in the clear as in the white formula. If you try this just mix up a small test batch to try it.

Might have to tweak your drop and hold schedule, could help soaking at a lower temperature. Probably don't need the one hour soak, 20 minutes should be enough. Saves costs and wear and tear on your kiln. Also, is your 20 hour bisque including a candling (slow and low soak at the beginning)? That is a heck of a long bisque schedule if not.

BTW, might want to check the durability of this glaze. Nothing nasty to leach out as you have them now but if you add colourants I would check for leaching. Glaze only has bentonite as the clay component of the recipe, alumina is on the low side. (it's a good idea to get plenty of silica and alumina into a glaze, one of the things that contributes to durability)

Welcome to the forum.

[garyl]

Thanks for answering Min. I'm learning about glazes, I took the cone 6 course from ceramic materials workshop which helped a lot.

My Potter's Dictionary just came in yesterday, so looking up surface tension and viscosity, it looks like zirconium increases viscosity, that may be the issue. I'll run some tests.

I'm looking for a clear that's functional and works well on speckled clay with and without added stains and also want to learn as much as I can.

The long bisque did add candling, and was an attempt to get rid of the pinholes. It's not needed for this issue.

Thanks

 

[Lucia Matos]

21 minutes ago, garyl said:

Thanks for answering Min. I'm learning about glazes, I took the cone 6 course from ceramic materials workshop which helped a lot.

My Potter's Dictionary just came in yesterday, so looking up surface tension and viscosity, it looks like zirconium increases viscosity, that may be the issue. I'll run some tests.

I'm looking for a clear that's functional and works well on speckled clay with and without added stains and also want to learn as much as I can.

The long bisque did add candling, and was an attempt to get rid of the pinholes. It's not needed for this issue.

Thanks

 

Hi Garyl, 

I am totally novice too, but also took the online course with Matt and Rose Katz. I took the Understanding Glazes course, lectures portion only.  I will add the labs and live hangouts portion next semester.

They have a great podcast now called For Flux Sake and coincidently  in the episode they released this week, he talks a lot about pinholes. I think you will like to listen to it. It is very educational and entertaining at the same time. He talks about the relationship between pinholes, glaze application and bisque temperature. 

Best wishes on your clay adventures!