Pin holing -Revised Sapphire Blue Glaze

I've having major pin holing issues with  the Sapphire blue (revised) glaze recipe from Ron Roy's web site Frog Pond Pottery. It's happening both with a standard slow pre-programmed Bartlett firing & with a custom controlled cool/slow glaze firing. Should I be holding it longer @1900 &/or 1400 to let all the gasses release? It's only happening with the Sapphire glaze. Or should I create  a new custom firing schedule?
I  even bisqued my last greenware at ^04 and it still didn't help. It's happening on both hand built & thrown pieces mostly on the red body clays. Is it just a case of a bad fit of the glaze & Clay body?
Any help or suggestion would be greatly appreciated as it is a glaze that I love dearly!!!!

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Permalink Reply by Tom Anderson on July 18, 2018 at 6:07pm

Juanita:

you said " major pinhole" problem. Does that means numerous pinholes or large pinholes= blisters? A picture would certainly be helpful. Pin holes are most commonly from off gassing spars: mostly Nep Sy in the clay. However, you are using a red clay body: which means it could be a sulfide problem. Red and dark body clays equal higher iron, which in turn means high sulfide content. The solution for off gassing pinholes and the solution for sulfide blistering is not the same.

you can start here and check results. Use your current slow firing schedule, but ramp at 125F an hour from 2050 to 2230F. At 2050F the clay body starts to vitrify, and the porosity begins to close as glass forms. At 2000-2050F is also when spars reach their most active state: which also means increased off gassing. So going slow from 2050F to peak allows the body to mature, as it also allows the time for spars to off gas.

Now once you fire this schedule and you still have noticeable pinholes: then sulfides become the prime suspect. Sulfides mimic off gassing spars: but require a different schedule to resolve. Red and dark bodied clays are the most likely suspects to have sulfide issues. 

Tom

i have a blog floating around here someplace called " clay tips for potters." There is a lengthy post on firing red/ dark bodied stoneware. 

Permalink Reply by Tom Anderson on July 18, 2018 at 6:11pm

By the way: next time I talk to Ron I will tell him to fix his recipe. :)

Permalink Reply by Juanita Short on July 19, 2018 at 10:45am

Numerous tiny pinholes!!!! I really think it is coming from too thick of an application cause there's little to none where it is thin. Unfortunately, it's not blue enough when applied thinly.

I don't think there's an issue with the glaze recipe itself.

I shall look at my firing schedule and maybe re program. I fire a lot of semi-matte  & glossy glazes in the same load and don't  want to mess with the results I've been getting so far. 

I'll take a few photos & post if I can focus in on the pinholes without glare from the glossy surface 

Permalink Reply by Juanita Short on July 19, 2018 at 10:45am

I think the recipe is fine as it is

Permalink Reply by Tom Anderson on July 19, 2018 at 3:51pm

I was joking about Ron :)

Permalink Reply by George Lewter on July 20, 2018 at 7:00am
Permalink Reply by Juanita Short on July 20, 2018 at 9:52am

Just had a quick look at the blog. Looks very complex. I'll give it a thorough read when I can concentrate. Hot & muggy in my neck of the woods right now!!!!

George Lewter said:

Permalink Reply by Juanita Short on July 20, 2018 at 9:55am

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Permalink Reply by Juanita Short on July 20, 2018 at 9:57am

The glare from the glossy surface made it difficult to photograph the pinholes. They're tiny & all over

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