Soda, Glazes, And Wine

[Dancing Earth Creations]

I have two cups that have been fired in a soda kiln at cone 10-11 and used for drinking red wine (Baileys and School Bus.) The glazes have lost their luster and feel dry. The school bus cup might have been in a cooler spot in the kiln since it didn't have the gloss when it came out but the bailey cup looked good. Is there a reason why this is happening? I'm concerned about food safety issues.

Karen

[bciskepottery]

It sounds as if the liner glazes were not durable, perhap due to incomplete melting. The wine is acidic, which further wore on the finish. Depending on what oxides were used in the liner glaze, you could have a food safety/leeching issue.

[Dancing Earth Creations]

Thanks. I thought it might be that for the school bus--it looked odd when it came out but the Bailey's looked fine. I am in a community studio and I have no control over the firing. Recently I had a plate that was obviously underfired and they decided there was a cold spot in the kiln and won't load there anymore. We have a ceramic sale coming up and now I'm concerned about selling pieces that are not food safe. Fortunately I did not like how the school bus looked and stopped using it. Is there anyway to tell if a piece is underfired if it's not obvious? I've been testing the Woo Blue glaze but there is only so much wine I can drink. ;-)

Karen

 

 

 

 

[bciskepottery]

John Hesselberth and Ron Roy, in Mastering Cone 6 Glazes, devote an entire chapter to testing glazes for stability and fit. These are tests you can do at home or in a studio, like soaking a glazed item in vinegar to test resistance to acids. Other authors of glaze books describe similar tests a potter can -- and should -- do before selling wares to the public. Even though a person may say they have tested a glaze, you really need to do it yourself -- what may have worked in their kiln or firing schedule may not work in yours. Glaze authors often have different views of what is acceptable in terms of the ratios of alumina to silica; so, keep in mind there is no single correct answer nor is there a single standard for defining "food safe".

 

You can also send one of your wares to a professional testing laboratory. Will incur a cost, but if you are really in doubt, that is the only way to put yourself at ease.

 

Underfiring in a kiln will not be solved by declaring and avoiding cold spots. That only leads to inefficient loads and firings. They really need to figure out how to adjust the kiln or the firing schedule to get a more even firing. That cold spot could have been the result of stacking/loading and just limited to that specific firing or it could be the result of the firing schedule.

[Guest JBaymore]

That cold spot could have been the result of stacking/loading and just limited to that specific firing or it could be the result of the firing schedule.

 

 

Or the kiln design and construction.

 

best,

 

..................john

[Guest JBaymore]

Heavily soda fluxed glazes are not all that strong as far as glasses go. So that is likely part of the issue. You are likely getting a synergistic effect of the sodium getting introduced into a galze base that already is deficient in something or possibly already high in soda. Post the glaze recipes here and that will help people comment accurately about the issue.

 

Sodium silicate....a pure sodium fluxed glass........ is totall soluble in water. It is also called "water glass". It is a liquid.

 

There is a reason that industry quickly moved away from salt firing for wares that needed to be durable in harsh environments.

 

best,

 

.................john

[Dancing Earth Creations]

I can't do anything about the design of the kiln or loading and firing since I'm a student and work during the time they load and fire the kilns. I assumed they tested their glazes but I won't assume anymore. I am testing the glazes I like and won't use Baileys or School Bus in the soda again. I am curious about the chemistry of what is happening. I'd love to take a glaze chemistry class but that is during the day so I'm attempting to teach myself through books.

 

Here is the Bailey's Red recipe:

 

Custer Feldspar 18,800

Silica 5,200

EPK 5,200

Bone Ash 5,600

Talc 9,200

Lithium 1,000

Red Iron Oxide 3,200

 

Here's one I am hoping is safe:

Ranch Butter

Custer Feldspar 36.3

Silica 26.5

Whiting 8

EPK 5

Talc 6

Gerstly 13

Zinc Oxide 5.2

 

Zircopax 14.9

Rutile 2

 

Thanks for all your suggestions and help!

Karen

 

 

[Guest JBaymore]

Hi.

 

I finally had a moment to look at the formula (not recipe) for this glaze. Before the effects of the soda on the composition, it comes out looking about like this:

 

CaO .32

Li₂O .06

MgO .31

KNaO .14

Fe₂O₃ .09

P₂O₅ .08

Al₂O₃ .25

SiO₂ 1.98

 

It is important to note that this above molecular formula is before any additional sodium oxide is imparted into the melt from the introduction of soda into the kiln.

 

The upshot here is that as even a straight cone 9 glaze (non-soda fired) this is not a particularly good formula for functional ware. The alumina (Al₂O₃) content is well below the typical lower limits for this range (.38 to .75) , as is the silica (SiO₂) levels (3.0 - 5.75). The silica to alumina ratio is about 8:1, and the real target for this cone range is about 10:1 for durability.

 

Initial fixes for this glaze might be to try to raise the alumina and silica values and adjust them to a 10:1 ratio also, and do this without changing the visual character too much.

 

Now we get to the introduction of MORE sodium oxide into the formula of the melt from the soda introduction in the kiln. As the flux content goes UP (sodium oxide is a flux), the number of moles of both the alumina and the silica then goes DOWN. So a glaze sample fragment taken from the actual fired glaze likely will show an even lower Si:Al ratio, and lower molecular levels of those oxides. The heavier the soda deposit, the "worse" this glaze is getting as far as durability goes.

 

At the moment the actual glaze recipe's sodium oxide value is not all that high, limit-wise; about mid-range. However the CaO flux value is only about in mid range too. CaO is a good oxide to impart some durability in high fire glazes. It could stand to go up closer to the .65 limit. But as the sodium oxide content is increased from the kiln-introduced soda, the CaO value also goes down. Wrong direction. The MgO value is approaching limits, which is good for some part of durability, and this oxide probably along with the traces of Li₂O and P₂O₅ trace is imparting the main visual characteristics of the glaze. But again, as the sodium content goes up, all of these values go down.

 

This gets slightly more complicated. The sodium is not getting dispersed throughout the entire glaze thickness, but is tending to be most concentrated on the outer surface. So the molecular analysis of the outermost layers of the glaze will be even worse as far as durability goes, due to the higher concentration of soduim oxide as a dominant flux.

 

And soduim oxide fluxed glazes are not known for their durability. Soda fluxed glass is "soft". Add to this high sodium factor the low alumina and silica numbers... and you have the result that you might expect out of this glaze....... soft.

 

Hope this information is of help to you to help explain your (likely) accurate observations.

 

 

best,

 

...............john

[Guest JBaymore]

I assumed they tested their glazes but I won't assume anymore.

 

Unfortunately, there are far too many places that do not do this and/or have little accurate knowledge of this aspect of ceramics.

 

best,

 

................john

[Guest JBaymore]

In some ways your alternative glaze looks better as to the potential durability........ but it still has a low alumina value and some other potential concerns.

 

CaO 0.46

MgO 0.13

K₂O 0.13

Na₂O 0.07

ZnO 0.20

P₂O₅ trace

TiO₂ 0.07

Al₂O₃ 0.28

B₂O₃ 0.15

SiO₂ 3.32

Fe₂O₃ trace

ZrO₂ 0.26

 

The CaO figure is right up there toward a good level for durability. The silica content is also greatly improved, but is still toward the lower end of acceptable limits. However, the added boron content tends to change the potential durability in the wrong direction again. The ZnO is kinda' problematic in a fuel fired kiln, since if there is any early reduction, the zinc oxide tends to be reduced to pure zinc and then boils off (volatilizes) out of the glaze....so it is no longer present.

 

So this glaze is not an instant "bulletproof" solution either.

 

I'd be looking to get the alumina up, get the boron down, lose the zinc, and get some more silica in there.

 

best,

 

.................john

[Dancing Earth Creations]

Thanks John, I appreciate the time you took to analyze and explain. I will keep looking for a more durable glaze for the soda.

Karen

[Guest JBaymore]

No problem Karen.

 

I made one incorrect statement in the above....... as the soda inclusion from the atmospheric firing increases the sodium flux content of the melt.......... the silica alumina RATIO will not go down....... just the relative # of moles of both silica and alumina (in concert). The ratio will remain the same (# of moles of silica relative to the # of mols of alumina).

 

best,

 

..............john

[buckeye]

I've been testing the Woo Blue glaze but there is only so much wine I can drink. ;-)

 

 

I would suggest sending me the cup and a few cases of wine. Once I have tested the cup (and sobered up) I will send the cup back and a report of my findings