Help me diagnose this problem?

[dangerouspottery]

Hi friends -

long story short, I'm on the hunt for a matte white glaze to use on porcelain mugs and bowls in cone 6 electric/oxidation. I really like a stony/eggshell surface, but I have been running into a bad scratching/staining situation with the glaze i currently use -- plus, some of my pots have a slow leak, even when this glaze is only on the outside of the pot. part of the problem is that the origins of this recipe are unknown - i got it from the art school where i take classes.

so i have 2 questions: is it possible to diagnose my problem just by looking at the recipe? and is there a way to adapt this recipe to make it work on functional pots?

Here is the recipe - at my art school, we call it Mullen White.

Kona  60

Spodumene 10

EPK 10

Dolomite 16

Zinc Ox.   2

Titanium   2

I really appreciate any help or suggestions you can provide. Alternatively, if anyone has a successful recipe for a matte white in cone 6 oxidation, I'd love to try a new recipe!  (I have mixed up some VC71 in the past, but it's too satin for my taste...)

thank you in advance,

karen

[Min]

The more you edge toward a dry stony matte the more cutlery marking you will get. Stony mattes are fine on the outside of pots if cutlery marking is an issue. For surfaces that will be used for cutlery a glaze that has more gloss will mark less. You need to find a balance between the two that works. 

Basically if you take a clear glaze and do a glaze line blend with it and your matte you should find the sweet spot where it is no marking and yet isn’t a gloss either. If you haven’t done one before a line blend is super easy to do and gives you a bunch of test samples with just 2 glazes.

In the chart below A would be your dry matte glaze and B a clear gloss. You would weigh out 200 grams of dry base for each and add the exact same amount of water to both. Then use a teaspoon, tablespoon, syringe, whatever, to scoop out the parts of each glaze to blend together. For example #4 would be 3 parts of matte plus 3 parts of gloss. The #1 and #7 are the control base glazes. Glaze and fire the test tiles the same as your pots are fired.  After doing this test you can then fine tune it if needed. (I like the G2934 matte from this page.) 

Re leaking, this has come up here a fair bit lately. If your clay is either under fired or has a high absorption rate it will likely leak. As you are using porcelain it should be nice and tight and not doing that. What clay are you using? Hopefully it's not a wide firing range one. Also, are witness cones used to verify firing? There is more on absorption here. 

I just plunked your recipe into Insight,  calcium and magnesium are not high enough to be matte from those and I don't think the alumina is high enough for a stony alumina matte so it leaves me thinking it is matte because it is underfired.

 

[dangerouspottery]

Thanks, Min. I will try the line blend as you suggested. If I find that #3 is my winning formula, do I just continue to mix both glazes as normal and then combine in the proper proportions?

Re: my clay -- I am using Standard 551 VP Porcelain at cone 6, and my cones are indicating that i'm hitting between ^6 and ^7. And pots fired using other glazes aren't leaking, which leads me to believe the glaze is the issue here.

I appreciate your insight and the recommendation of G2934. I'll try that one as well.

 

[Min]

If you find one of the blends work then just combine the two recipes in the appropriate ratios. So, if #3 works you would combine 66.6 Glaze A with  33.3 of Glaze B in a new recipe to total 100. (glaze calc programs can do this super fast) You could change the ratios to 10:0, 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8, 1:9, 0:10 or any combination you like.

hmmmm, I just looked up that Standard 551 and they list the absorption at 0 point 3 so it really shouldn't leak. Glaze should not be relied upon to seal a claybody. I used to think that also but it doesn't work that way.  Pinholes, crazing, micro fissures in the glaze are going to let water through. For functional pots that will contain liquids the clay without any glaze should not leak.  I would run the tests that were outlined in that other thread I posted and measure what the absorption is with your firing methods. Something screwy going on if you are getting leaking with this clay fired to maturity.

 

[neilestrick]

A magnesium matte can give you greater durability than a glaze with a low silica:alumina ratio. You can also adjust the matteness by changing the cooling cycle- slower cooling=more matte.

[High Bridge Pottery]

I have found the beautiful buttery egg shell look comes with cutlery marking. I don't like to say they are underfired but they are not completely melted and the way they look comes from that rough surface. It seems to walk the border between clay and glaze. It can be fired slightly hotter and not mark but then loses some of its charm.