Engobe And Glaze

[Babs]

1120 what cone us that? 2 or 3?

[njabeid]

22 minutes ago, Babs said:

Misunderstanding..though should have guessed, I thought your clay was blown in from the Sahara:-)))) Aeolian clay. 

I really don't know. I'm looking for photos, and for some reason can't find them.

[njabeid]

27 minutes ago, Babs said:

1120 what cone us that? 2 or 3?

I have no cones here, but I suspect it's between 01 and 1. I fire at that temperature because the clay I was using vitrified at 1,150°C. Stuck to the shelf, broke with glassy fracture. I managed to make glazes to fit, and the engobe was working nicely too. Now I have a lovely clay blend I'm throwing, and once it has been engobe-d, fired, glazed and fired again I'll see how everything fits together.

[njabeid]

Next episode: bisque firing of pots made of my new blend,  (lovely)decorated with stencils, and most with Glazenerd's engobe, one with the old faulty one, to compare. I sprayed it because it is totally unbrushable, even with gum arabic added. It looks great if untouched: (the mug on the left is the old engobe, which is likely to crawl after glazing unless it really likes this new clay blend).

Result: Tom's engobe rubs right off! This is rubbed with a damp yellow sponge, as one does normally before glazing:

:-(

The question is : is there any chance that if I refrain from washing (or sponging), spray glaze on and fire the pots, the engobe and glaze will somehow combine and stick to the pots? or shall I just wash all the engobe off and start thinking of a different decoration strategy ?

It's all about temperature. I bisque fire at 1,000°C and glaze fire at 1,120°C.

[glazenerd]

With 20% frit and 5% borax- it wiped off like powder after bisq firing to 1000C? (1800F). Something is off, frit starts melting at 1475F. 

No, you should not give up your technique.

Tom

Edit: I went back and looked at your old recipe that bonded: 15% frit, 5% borax.

         The new recipe has 20% frit and 5% borax.  And it did not bond.  

[njabeid]

Would it be the 45% calcined kaolin just sitting and staring at the clay?

[glazenerd]

The old recipe had 35% kaolin, and the new recipe has 45% kaolin. Calcining removes molecular moisture to control shrinkage: but at 1000C; all kaolin is at the same stage regardless: metakaolin begins to convert to spinel at 1000C. 

The old recipe with 15% frit and 5% borax bonded at 1000C. The new recipe with 20% frit and 5% borax did not bond, and wiped off like powder. The more obvious answer would be: you forgot to add the frit, and perhaps the borax to the new recipe.

i make a low fire porcelain frit ware, that is pretty much a brick by 1050C. 

Tom

[njabeid]

I promise I didn't forget any ingredient. It was very gritty and hard to sieve through 80 mesh (the borax?), but all went through. I'm miffed too. The old engobe is bonded, just about, so the temperature was OK.

How about my question. Any chance it will all sinter and bond in a glaze firing? Or will it be an absolute mess.

[njabeid]

By the way, the bisque shrinkage of the new clay blend is 7%.

[glazenerd]

Nancy: sintering the kaolin only removes molecular moisture: it does not change any properties beside shrinkage. 

You added the frit,  so it "should" bond at 1120C. Should have already bonded.

[njabeid]

That wasn't 1120C, it was 1000 bisque firing.

[glazenerd]

7% shrinkage would be a typical ball clay value. That tells me a lot actually. Let me go back through the thread and see what the shrink rates were on prior blends.

t

[njabeid]

Let me recapitulate: I had an engobe formulated by Robin Hopper for cone 4-6 that used to work fine on my clay (1000°C bisque and 1120°C glaze) and under my glazes. Six months later the same batch of the same engobe is doing fairly well on new clay blends after bisque, (although it tends to fall off in places) but then under glaze it crawls like crazy and shows little bursts - so not bonding to clay. Glazes are OK. 

I have not been able to understand why the same workhorse engobe changed.

I need an engobe that will bond at 1000°C and not crawl at 1120°C. I think I'll run some small tests of my own, including making a bit of your recipe again just in case I did forget the frit, although I'm pretty positive I didn't, and Min's new suggestion. I'll mix some new 'old' recipe too. So frustrating, because it takes forever to throw, trim, decorate, fire, glaze, fire.

[glazenerd]

Nancy:  some info about clay- firing.

at 1000C; both the clay and engobe would be in an expanded state.  Let me explain, at 563C molecular moisture has pretty much been driven out of clay. Kaolin can have 10-14% molecular moisture: so it obviously reacts more to calcining. Ball clays, fire clays, chlorites, can have as little as 2% moisture and upwards of 6%. Bentonites, hectorites, and smecites are unique- I will skip them. So moisture is driven out by 563C, but at this exact temperature silica converts from alpha to beta phase: and it actually expands. Both things are going on at quartz inversion (563C) 

silica continues to expand up to 2050F, once you cross this mark: then it begins to shrink as vitrification starts. If you go back to your test bar shrinkage rates: you gave them at bisq 1000C and glaze 1120C. The differences are a direct reflection of the expanded state at 1000C, and as the body is beginning to contract at 1120C. 

The pops at 1000C are most likely dirt, or oil marks: and the engobe simply let go. There are other conditions that can cause that: but you are using frit as the primary flux ( pseudo), so they are unlikely. Again referring back to your test bars: the difference between bisq and glaze values. You then have to convert those values: your clay at glaze is X% and the engobe is Y%. The cracks in the original pieces is a direct result of the shrinkage values of the clay and the engobe. (COE).

a third variable is at play here. You said your original clay blend and  engobe worked for six months then changed. The white clay is alluvial: and should be consistent. The sandy clay should also be consistent. The river clay however is sedimentary: and is subject to change. The river clay is the unknown factor. Any location could be subject to changes in fluxes, silica, iron, and alumina. The fact that your new blend is 7% shrinkage, also tells me there will be a slight change in COE. Alumina, silica, and iron levels predict COE and shrinkage values: flux levels will also cause variations. 

tom

[njabeid]

Thanks so much for all that input and patience. I'll try to put it all together and work something out.

[Min]

Nancy, are you dissolving the borax in boiling water before adding it to the engobe?

[njabeid]

Uh, no, there's another thing I didn't know about. It does go through the sieve with some coaxing. At the moment I'm following Tony Hansen's instructions in the excellent article you recommended. I have made clay/engobe sandwiches with four different engobes, to be fired tomorrow, including your latest suggestion. I even had the right ingredients. Although I got no simple answers here I have learnt a lot. Monet and Hokusai both figured that by the time they reached the age of 100 they should be doing some pretty good work...

[Min]

3 hours ago, njabeid said:

Monet and Hokusai both figured that by the time they reached the age of 100 they should be doing some pretty good work...

Working with clay is definitely playing the long game. One of my daughters recently sent me a link to a series of videos essays from Adam Westbrook, he speaks of famous people in history and how we tend to gloss over the fact that many of them took years and years of slogging away at it before having a breakthrough. (I’m not suggesting that it will take you that long to get an engiobe to fit your claybody!) With our rapid fire lifestyles these days I think it’s easy to just get into a rhythm of hurry hurry hurry with the subconscious thought of getting results quickly. There is a comment made by many of the people on the forum who have been making pots for years, if not decades, when something screws up just get on with it and make another one. Getting to that calm state of mind where you can accept that working with ceramics will result in failures and challenges being part of the process can be exceedingly difficult or frustrating (and a bit of a moving target) but something to aim for.

 

[liambesaw]

So true @Min.  I hope I never master clay, how boring would it be then!  I'm all for instant gratification, but when I'm in the mess everything goes away and that need for instant success disappears.  It's like a version of being in the zone or something.  It still hurts to flop a pot just as you're putting the finishing touches on, but in my eyes it means you get to try again and this time hopefully you stop before it flops!

[njabeid]

So true. The bright side is that I just LOVE throwing, and don't really know what to do with finished pots afterwards, so I have no problem with the long slow learning curve, all the more so that I read once that the moment of mastering a skill, however small, provides a shot of dopamine, so I'm always a bit high on my small aha-s! I won't have time to get really masterful like you folks, but every little progress is a boost. It's quite strange to have got hooked on pottery late in life, and strange that it should be such fun, such a pleasure, but hey, why question? I'll look at that link.

[njabeid]

 

12 hours ago, Min said:

a series of videos essays from Adam Westbrook, h

Autotelic. Yes, exactly. As for the 16 years, there is the 10,000-hour theory.

[Min]

Another excellent resource is Daniel Rhodes book Clay and Glazes for the Potter, he wrote a very good chapter on engobes. Chapter 25 in this free online version of the text. It's a book worth picking up if you have the chance. 

[njabeid]

7 minutes ago, Min said:

Another excellent resource is Daniel Rhodes book Clay and Glazes for the Potter, he wrote a very good chapter on engobes. Chapter 25 in this free online version of the text. It's a book worth picking up if you have the chance. 

I have it, but for some reason when I was tinkering with engobes his didn't work for me. I'll go back to it. Maybe my silly firing temperature. By the way, another autotelic resource was provided by a potter friend in France :  https://artistryinaction.files.wordpress.com/2016/03/david_bayles_ted_orland_art_and_fear.pdf, and Grayson Perry's 'Playing to the Gallery'.

[njabeid]

Hi wonderful folks! After a long wait for a new kiln controller after mine died, I'm back with the engobes. My clay blend is lovely, so I have tested seven different engobes. For anyone interested, here are the results:

First I made the sandwich strips as per Tony Hansen, with mixed results;

Then I made several bowls and brushed on the engobes, each with a stencil and then, once bisque fired to 1,020°C, covered with a plain glossy transparent glaze.

After bisque:

They all look the same!

My aim is not to have brush marks, but rather a nice smooth surface. I usually spray (not enough work to mix a bucket big enough to dip) but the tests were brushed for practical reasons.

After glaze firing at 1,120°C:

Some are duplicated on two slightly different clay blends, with the same results. 

The best one so far is the one Min suggested, with 40 Kaolin and 40 Feldspar (MIN), and the one that consistently didn't work is Glazenerd's (GNE) (remixed making sure the frit was in it), with all the kaolin calcined.

It's a delicate balance to achieve, and I'm still on my way, but this discussion was invaluable, for which my thanks to all. Now I know so much more about engobes that I can work on them and get a perfect fit that doesn't disappear under the glaze.

[njabeid]

In real life the clay, engobe and glaze all fit together! Yay!

 

But the same glaze (supposed to be satin) is very glossy over the engobe and matt/satin on the bare clay saucers. The ad-hoc glaze turned out rather opaque and hid the stencil work.

Yes, there are some tiny bubbles. :-(

Thanks again!