Problems with engobe - again

[njabeid]

Happy holidays, and warm wishes to all for a wonderful New Year!

A couple of years ago I posted about engobes, but I couldn't retrieve the thread. Different question this time.

Using locally found clays, firing at 1100°C, I spray engobe over stencils before bisque firing, then add glaze. The engobe is my own laboriously developed recipe (kaolin 15, calcined kaolin 15, Silica 20, Zirconium carbonate 10, Feldspar 20, Boron frit 5, talc 10, Ferro frit 3110 10, nepheline syenite 10, Borax 5). Don't ask me how this recipe got so complicated, I have forgotten.  I tested a dozen different engobes, and all disappeared into the glaze. This one is the best and doesn't usually fall off either.

The usual glaze is my glossy clear workhorse (Ferro Frit 3124 60, Nepheline Sy. 15, wollastonite 12, kaolin 10, silica 12 and lithium carbonate 1).

This all works well, although the engobe does get faded by the glaze but with no incidents, but any addition of copper oxide (2%) or cobalt carbonate  (0.3 - 0.5%) - or for that matter any other glaze with those oxides in - results in disaster: crawling, pinholes, and some odd holes that look as if something blew up under the engobe, folding out the engobe+glaze. Nothing is wrong where the clay was covered by the stencil. Iron seems safe, and much more cobalt (1%) also seems OK.

Besides, in my last firing I tried an alkaline copper glaze (2% CuCO3), and it washed out the engobe completely into dark blue, while staying green on the stencilled shapes. It all ran like crazy too.

Photos attached, of a plate with the plain combo and of various faults.

Can anyone think of an explanation? I'm baffled. Why should the oxides have this effect?

Thanks!

Finished plate that was particularly lucky:

Bisqued ware

These are messily stencilled over the glaze, but it shows how the engobe gets lost in the clear glaze.

This shows a blow-out, a Robin Hopper engobe and clear glaze with a wee bit of cobalt. (2019)

A lot of activity happens on the edge of the engobe, but not only. The engobe is mottled when I spray it on still-damp clay.

The runny copper one, same set of cups, same stencil, same clay and engobe.

[njabeid]

Still thinking. Why do I insist on applying an engobe, which contains similar ingredients to glaze (and may be the reason why it dissolves in the glaze)?  Why not just plain slip?

1) I have no white clay where I am, only imported raw materials.  Maybe I'll try some plain slips concocted from those, but maybe I already did, as many a pot was lost when the glaze fell off with the white coat.

2) I wonder whether in Spain, where I had my very first pottery lessons, 'engobe' is the word for slip?

All this because I want to apply these local patterns and haven't found a way to make negative stencils that can be used over the glaze....

[Min]

Hi Nancy,

I found the original thread on how you came up with the engobe recipe, its here: https://community.ceramicartsdaily.org/topic/16020-engobe-and-glaze/.

Just to confirm you are glaze firing to 1100C approx cone 03-02?

2 hours ago, njabeid said:

folding out the engobe+glaze

I think those areas are where the engobe hasn't bonded well with the body.

39 minutes ago, njabeid said:

I wonder whether in Spain, where I had my very first pottery lessons, 'engobe' is the word for slip?

 

Google translate has Spanish for engobe as slip.

 

[njabeid]

Hello  Madeleine, lovely to see you again. Yes, I do fire to 1100°C, but I thought it was somewhere between 01-1. 

[njabeid]

By the way, although it says I'm in London, just now I'm in warmer Nouakchott.

[glazenerd]

The clay is locally sourced (wild) clay 1. Without additives. Wild clay typically comes in at a lower COE of commercial clay. 2.  Zoom in on the flaked areas, and you will see white flakes. Those flakes are most likely calcium based; IE lime pop. Those specks could also be silica based minerals- unmelted silica has a COE well above 10. At the moment; calcium tops my “probable” list.

Tom

[neilestrick]

Can you buy any commercial glaze products where you are via the internet? Commercial underglazes may solve the problem, like Amaco or Speedball products. They are available on Amazon if that's an option where you are.

I think the wild clay could definitely be the problem, but I'm not seeing popouts in all of the images so It may just be a fit issue. I'm also wondering if the borax in your recipe isn't part of the problem, because isn't borax soluble?

[Min]

To sum up is this correct?

1 ) pinholes, craters, blisters, engobe peeling back etc show up with the engobe areas but not with just glaze over bare clay

2 ) glaze firing hotter than bisque firing (what is your bisque firing schedule and is the kiln vented somehow?)

3) no alternative claybody available to you

4) adding copper ox or cobalt carb results in flaws over the engobe but not over bare clay (except when a larger amount of cobalt carb was used)

When did this problem start? What changes have happened? Does your kiln fire fairly evenly or are there areas that get much hotter? Any chance you use cones?

@neilestrick, borax was used by Daniel Rhodes in every engobe recipe he used, low, med or high fire. He found it crucial to ensure the engobe bond with the claybody during the early part of the firing. Yup, borax is definitely soluble so large amounts of slip are not mixed; just what can be used in the short term. That being said perhaps it was the lack of low melting boron frits at the time that led him to use borax. Could possibly increase the 3110 and remove the borax.

edit: I think Neil's underglaze idea is a really good one if you can get it.

Edited December 29, 2022 by Min

[Kelly in AK]

I see a slip fit issue here. (I’ve never been clear about when a slip becomes an engobe, or the other way around, by the way. For that matter, why aren't underglazes called engobes?) Your clay is going through a dramatic shrinkage as it vitrifies at around cone 02. The slip is doing its own thing, looks like it’s shrunk quite a bit more from the crackle patterns evident. These are happening at different stages of the firing. The glaze has to fit all. 

My gut feeling is straight borax is not helping you, leaching out of the slip and into the body and glaze, exacerbating fit problems.

The “do it yourself” ethos I pride myself on has its limits. While I would never give up on a lovely locally sourced clay, I would give a commercial white underglaze a try here.

Not sure I see any lime pops as I’m familiar with them. I see strong glaze to slip interface and weak slip to clay interface.

A slip/engobe recipe tested for cones4-6 on midrange stoneware may not be suitable for you firing range. Slips designed in the 04-02 range may be more promising. More zircopax in the slip will reduce the amount of slip absorbed by the glaze. A little less flux to raise its maturity.

 

[Bill Kielb]

1 hour ago, Kelly in AK said:

I’ve never been clear about when a slip becomes an engobe, or the other way around, by the way. For that matter,

I learned as: “ Slips are predominantly liquefied clay; they usually are applied on wet to dry greenware. Engobes usually have a lower clay content and also can be used on bisque-fired ware. The word slip generally is used to describe any clay in liquid form. All slips and engobes can be colored with oxides, carbonates and stains.”

Here or ceramics network.org……. Somewhere! Locally I find folks more often than not refer to engobes and use on bisque but observe it is hard to define where one stops and another starts.

Edited December 30, 2022 by Bill Kielb

[njabeid]

Wow, thank you all for pitching in with excellent advice.

However, my question is: why does this happen when there are oxides in the glaze and not when there are none? Photos attached of when things go well. As I mentioned, iron is flawless, and higher cobalt percentage seems to be better.

My engobe fits both the clay and the clear glaze, and shows no flaws when it is applied on its own. 

I am in Mauritania, all my raw materials have to be imported (usually brought in a traveller's suitcase), no local suppliers. The mix of clays works

@glazenerd There is no problem with the clay, a blend of three very different clays, never had calcium issues, and I make glazes and engobe to fit. No flaking.

@neilestrick Amazon is not an option I'm afraid. It's not a fit issue, because everything fits when there are no oxides in the glaze.

@Min 

1) Correct

2) Electric kiln with badly fitting bungs and door, bisque 1000°C, glaze 1100°C short soak, both with no bungs up to 600°C

3) If I thought the issue was the clay I would keep trying, but I have worked out a blend of three clays that isn't as nice as porcelain but makes quite good pots.

4) Correct, and this is what is so odd.

I notice now that it started ages ago. I pulled out my test tiles, and noticed that many of them have loads of issues over engobe, even with previous engobe recipes and various glazes. I'm afraid I don't use cones here. I should, I do in London.

12 hours ago, Min said:

borax is definitely soluble so large amounts of slip are not mixed; just what can be used in the short term.

Ah, my engobe is at least some months old. One of my problems is that I am here for a few months, then spend months in London so I forget things from one stay to another, and this engobe is definitely at least eight months old. Oppur si muove ... It works fine without the oxides, so it's not the engobe.

@Kelly in AKI'll try those suggestions, thanks.

 

This blue one has engobe and stencil, but I made a mistake and put too much cobalt - turned out gorgeous.

 

This one had copper and cobalt, pitted badly, but I liked it so much I added a layer of glaze and refired it, and saved it - except for one tiny blow-out in the second firing.

 

[njabeid]

4 hours ago, Kelly in AK said:

it vitrifies at around cone 02.

@Kelly in AKActually this clay will not vitrify.  One of the clays in the blend does vitrify at 1150°C (about cone 1).  Shrinkage is not spectacular - about 10% I think. You are right about the fit, and I am going to tweak my recipe as you advise. The issue is that this engobe has no problem until I put some oxide in it.

[Min]

Lots of good thoughts and info here. So, what we do know is that both copper and cobalt act as fluxes. But, that one pot with the higher cobalt amount came out fine. The cup with the really fluid glaze with the camel looks like the glaze was very fluid. Overfluxed or overfired?

Refiring with extra glaze fixes the pinholes and pits, this is interesting.

One constant is the areas over the engobe are an issue. Seems logical to try an engobe without borax. If this engobe works until it doesn't either it's not aged well or I'ld try it with increased 3110 and zero borax. Or, is it the aged engobe that is the issue and not fresh?

+1 for increasing the zirconium to decrease the faded engobe areas. (even though they look nice IMHO)

 

[njabeid]

Thanks for understanding the problem so well.  I have made a new batch with a tweaked recipe, with no borax or borax frit, more 3110 and Zircopax, and  sprayed it on six mugs (with camel stencils). After bisque firing I'll glaze two each with the same glaze plain, with copper and with cobalt. See what happens. Maybe I'll make it more varied - one with RIO and one with 1% CoCO3.  I'll be back with the results and photos in a few days.

I remember the discussion about the fading. I did go along with that - sometimes it fades and sometimes it doesn't. :-/

The very fluid glaze is an alkaline glaze with FF 3110 - 45, Gerstley Borate - 45, silica 5 and kaolin 5, plus 2% CuCO3.  It totally dissolves the engobe. Is that just  too much flux? I don't think it was overtired, as it was the same firing as the other pots. I have made some with more clay and silica, and should do a bit of line blending, as I like the bright turquoise from alkaline glazes.

Happy New Year!!

[Babs]

If another firing sorts some of the prob, slow down your last hour of fitinv and soak at end.

@Hulk sorted some issues.

Spraying entire pot with asater prior to applying slip can help with a better evenness if application.

Have you tried a white clay slip instead of engobe?

Slip is different, as you stated engobe translates to slip in Sp. So perhaps try a slip.

I use a white stoneware on a midfiring iron rich clay successfully.

A "second" from my old stuff, don't make these anymore. White is reflection.

[Min]

@njabeid, in your original engobe which boron frit are you using? Is the kaolin you are using Grolleg or ? Any idea of the type of feldspar?

Without knowing the type of feldspar I just went with a hypothetical analysis. Have a look at the alumina levels of your 2 glazes. Also look at the silica : alumina ratios, this also shows how low the alumina is in your high sodium glaze. The "workhorse" clear adds to 110, is this correct? I would lower the boron and increase the alumina to start with and see how this affects glaze fluidity. I would then work on increasing silica levels to a point where the glaze is still glossy. Low silica levels means less glaze durability.

 

 

 

 

[Kelly in AK]

3 hours ago, njabeid said:

The very fluid glaze is an alkaline glaze with FF 3110 - 45, Gerstley Borate - 45, silica 5 and kaolin 5, plus 2% CuCO3.  It totally dissolves the engobe. Is that just  too much flux?

Yeah, that’s a lotta flux! Like Min just pointed out, it’s probably not a very durable glaze. Not good for the insides of functional pots. It was pretty fascinating how it came out on the piece you posted though. I think more clay in that recipe wouldn’t hurt.

Your work is gorgeous, by the way. 

[Rae Reich]

Are you always using from the same supply of cobalt carb and copper carb? Is your higher-concentrate cobalt mixed from the same supply? I’m wondering if there’s some kind of contamination in them.  

Do you add anything to them besides water, to keep the oxides in suspension? Have you considered making ‘control’ batches using bottled or purified water, in case that’s the problem?
Fixing your base glaze might not answer why the oxides act so strangely on it when that same base, without oxides, doesn’t crawl or blow out. Oxides alone don’t generally have that effect, as you know, and they aren’t behaving badly on the raw clay. 
Possibly irrelevant: the super-fluxed copper glaze screams ‘rutile’ to me.

[glazenerd]

Njabeid:

The reason we are looking at so many variables; I do not think any of us have seen 2% copper addition cause such a reaction. 

Tom

[Rae Reich]

23 minutes ago, glazenerd said:

do not think any of us have seen 2% copper addition cause such a reaction. 

It’s the .02-.03% additions that are the problem, not the 2%.

[glazenerd]

19 minutes ago, Rae Reich said:

It’s the .02-.03% additions that are the problem, not the 2%.

TY- I misread it. 

[oldlady]

babs,  i think the 2 typos may need clarification.    in the one about firing  "fitinv" and spraying "asater"  leave me wondring if there is a product in australia that i am unaware of.    or is my old age showing again.  so many words are used differently today.

[njabeid]

@Babs Second firings have usually resulted in split pots so I'm quite reluctant to try. That particular one was a happy fluke, and I did it because the glaze was a bit thin in the middle.

I'll try spraying water. I have tried applying to still-damp clay and to bone-dry, the target being getting clean stencil marks.

The problem is finding white slip here. I am setting up a trial with simply one of the clays in my blend, a sort of kaolin which is white when raw but fires tan.  We'll see.

Incidentally I  see your location is "Timbuktoo". That's actually quite near me. I was surprised when I came here 52 years ago that what I thought was at the other end of the world was actually only 1,200 km down the road from Nouakchott. ;-)

@Min The boron frit is from Scarva, Borax Frit 1201. I only use it rarely. The kaolin is kinda "china clay". The feldspar is potassium feldspar from a Spanish supplier. I don't usually use feldspars here, at this rather low temperature, but I have a bucket of this. I'll look at your suggestions but there is no feldspar in the alkaline glaze. 

@Kelly in AKThanks! :-) you made my day. My London pots are at www.wanderlust-ceramics.com. I lead a double life, even in pottery!

@Rae Reich Yes, always the same little jars of oxides. I'm happy that you are all as baffled as me - I don't need to be too embarrassed to have asked the question. As for the alkaline glaze, I tried adding 10% TiCO3 and have a nice result, less glossy, more opaque, worth developing because still runny.

@glazenerd Rae answered correctly. I raised two issues: one is the glaze/engobe problem, that happens with 0.2-0.3% CoCO3, the other is the highly alkaline glaze with 2% CuCO3, raised because it gobbles up all the engobe and ends up dark blue.

@oldlady I figured "asater" means water, Didn't even notice fitinv, which no doubt means firing. 

Lovely kind potters, warm wishes for a really good New Year. May it lavish on each and all of you everything you hope and wish for. Happy potting (and glazing)!

Discussion continues next year...

 

 

[Rae Reich]

I think you should buy a small quantity of fresh cobalt carb and copper carb and test with those. 

Another puzzle is why doesn’t the 2% cobalt in the base glaze cause the same faults? While larger percentages of cobalt usually do tend to overwhelm other colorants, is it  possible that it also got a better melt/meld with your base glaze and that’s why you didn’t have the popping and crawling away from the clay beneath? Is cobalt a bit of a flux? Never thought it was …

ps, I like your decorating style and what you’re trying to do. Beautiful effects, especially within the limits of available materials 

Edited December 31, 2022 by Rae Reich

[Min]

1 hour ago, njabeid said:

The boron frit is from Scarva, Borax Frit 1201. I only use it rarely. The kaolin is kinda "china clay". The feldspar is potassium feldspar from a Spanish supplier. I don't usually use feldspars here, at this rather low temperature, but I have a bucket of this. I'll look at your suggestions but there is no feldspar in the alkaline glaze. 

Borax frit 1201 is a high zinc frit, good to know.

23 hours ago, njabeid said:

The very fluid glaze is an alkaline glaze with FF 3110 - 45, Gerstley Borate - 45, silica 5 and kaolin 5, plus 2% CuCO3.  It totally dissolves the engobe. Is that just  too much flux?

Yup, but it's in your engobe isn't it? Sorry, I had 3 things I was thinking about, engobe + both glazes, was looking at your engobe chem but didn't post the chart. For sure too much flux and not enough alumina in your fluid glaze.