Engobe And Glaze

[njabeid]

2 hours ago, glazenerd said:

soluble salts that migrate out and form granules

Surprisingly, even after months of sitting there, after adding water and a good stir the engobe is quite smooth and left no residue in a #80 screen.

[liambesaw]

Had you ever poured water off the top when it was settled?  Just asking because those same soluble salts would be thrown out.

[njabeid]

25 minutes ago, liambesaw said:

Had you ever poured water off the top when it was settled?  Just asking because those same soluble salts would be thrown out.

No. I found it almost dry, added water, stirred well. I don't even think I strained it. I don't think I was ever so attentive, having forgotten those  before-the-summer disasters which had baffled me but I hadn't really processed, so was going through a routine that used to work well. I brushed on the first batch, and then sprayed the next, attributing the crawling to having perhaps applied it too thick. The results are the same. 

[glazenerd]

@njabeid

Could you please post your current engobe recipe?   If you have them; perhaps a picture of the old and new clay-unglazed.

Tom

[njabeid]

The engobe is :

kaolin 15, calcined kaolin 20, talc 10, Ferro frit 3110 10, nepheline syenite 10, silica 20, borax 5, zirconium silicate 10

The clays:  last year's was mostly a sort of kaolin that actually vitrifies at 1,150°C, with the addition of some other clay to prevent slumping, on the left in this picture of two bisqued pots (1,020°C) :

The one on the right is a reclaim mix with more red clay in it.

This year's blends are like this raw :

But after glaze firing they go very dark, obviously not vitrified :

This is very earthy earthenware, and the clear glaze shows a fine crackle. The thin engobe has tiny cracks on the edges.

I couldn't find any decent white clay in the market, hence the muddle. Now there is some rather inferior stuff, and I'm trying it; here it is green, mixed with two other clays:

Let's see whether it vitrifies and behaves better than the dark clay.

These are the raw materials I start out with:

The orange stuff looks like sand but it is actually sandy clay, and can be thrown on its own, like this bowl:

Sorry if that is all too long and boring. Thanks for your help.

[glazenerd]

Not too long, nor boring: all info that I need.

Judging from the wet to dry- dry to fired: your iron source is hematite instead of iron disulfide (pyrite). Pyrite would not fire that dark at lower temperatures.  Your original clay has 2-2.5% iron and the new has 5 or so % iron content. ( estimated).  I will run  an estimated body to see what kind of COE the clay would have vs. the engobe.  Apparently you have limited access to clay.

T

[glazenerd]

@njabeid

You claimed your post was long and boring: after reading this one you will retract that.

your engobe recipe falls into a hybrid of sorts. Engobes by application is a clay body slurry applied over the primary body. Engobes can have up to 50% clay, while many glazes contain up to 20% clay. At 35% , at best you are at the starting point of an engobe. 

After analyzing your recipe: alumina is 11.74% molar- which is low. In application, it causes weakness and mechanical issues in a clay body. Secondly, boron (borax) is not used as a flux in clay bodies because it is a secondary glass former, but it actually interferes with typical soda glass formations if levels get too high. I am sure the original recipe included it to ensure a lower temp melt. I also believe this engobe was formulated to match a stoneware body at cone 5-6.

Red bodied clay in the States either comes with higher clay% with lithia fluxes to create lower COE values (4.50), or lower clay% with higher flux levels that vitrify at cone 5-6. When you fire a cone 5-6 body to lower cone values (1170C/ 2066F) it results in higher COE values because vitrification does not occur.  All of that said: I would estimate the final COE of your clay in the 7.50-8.00 range. I have an extensive list of tested clay bodies supplied by a friend: from that I am assessing your body higher because I doubt any lithia is present. 

Your engobe calculates to 5.49 COE, and your clay body I estimated at 7.50, to 8.00.  I think this is where the typical COE grazing pattern is coming from.  Then you have the low alumina issue of  11.74% molar.  Finally, 10% zirco lowers the COE by 0.50%.  

45% kaolin (calcined), no talc, 20% frit. 3110, 10% Nep Sy, 10% silica, 5% borax, 10% zirco = 6.32 COE.  Alumina 15.61 SiAL 4.61

** you should not fire this much above your current peak because of the frit content. Test it first! It would also serve you to run shrinkage test bars on your clay so you can keep track of changes as you dig or blend. 

Tom

[njabeid]

Wow Tom, thanks. Lots of things I don't know. I'm in Mauritania, without a potter or a source of commercial supplies within a couple of thousand miles at least. Took up pottery after retirement and have laboured to learn and understand. I brought glaze ingredients, but source my clay locally. The white kaolin-ish stuff is silky rather like porcelain to throw, plastic but no strength. I had it analysed and it contains Fe and Ti. The golden sandy one is from Chinguetti, in the desert in the North of the country ; the muddy one is from the Senegal river valley. Very heavy & sticky. Shrinks too much, dries too fast on the edges. So a blend can work out.

 

[glazenerd]

You can run an absorption test on an unglazed test tile. I am sure Tony Hanson has the protocols floating around on his site somewhere. High absorption = no vitrification. No vitrification = high expansion (COE)

[njabeid]

3 minutes ago, glazenerd said:

I also believe this engobe was formulated to match a stoneware body at cone 5-6

Thanks a lot for the calculations. You are right above. This recipe was provided by Min when my complaint was that the Robin Hopper engobe I was using disappeared under or into the glaze (see beginning of the thread). 

You say "alumina is 11.74% molar- which is low. In application, it causes weakness and mechanical issues". Is it alumina, or the low alumina content, that causes issues?

I'm going to try to wrap my head around that, and do shrinkage test bars as advised by Tony Hansen. Learning and testing is slow, because I'm not making that much, and don't switch on the kiln for a couple of pots, but I can do one soon. I'm receiving a load of muddy clay tomorrow, so I'll make several blends of clays and test them all with the engobe. Otherwise I'll just drop the whole stencil/engobe thing! :-) In my second life in London I play with porcelain... 

Thanks again, and best wishes. I'll be back with the next episode.

[njabeid]

3 minutes ago, glazenerd said:

You can run an absorption test on an unglazed test tile. I am sure Tony Hanson has the protocols floating around on his site somewhere. High absorption = no vitrification. No vitrification = high expansion (COE)

My previous clay had practically no absorption and was practically vitrified, at 1,120°C. That had its own lot of head-scratching, because there are NO glaze recipes for that temperature. It took me a long time to develop my own recipes to fit. The turquoise one in the photos does fit nicely, but the clear glossy has crazed very slightly on these clays. It's the silly engobe. By the way, I was cleaning up the pots and sanding them, and the engobe is not really hard, softer than the clay and than the glaze, so it is not properly fired. I must adjust the recipe again, so that it doesn't disappear and doesn't crawl or explode. Aren't those strange little explosions?It looks as if there were bubbles under the engobe but I don't think there were any.

[glazenerd]

Low alumina causes the issues. In clay, low alumina contributes to slumping and reduced strength. Pieces tend to be moe fragile, there is also the effects on green strength. In glaze, alumina is increased for strength; primarily to reduce cutlery marks. Think of alumina as the skeleton, on which everything else hangs. It is the "bones" in clay and glaze.

3 hours ago, njabeid said:

Wow Tom, thanks. Lots of things I don't know. I'm in Mauritania, without a potter or a source of commercial supplies within a couple of thousand miles at least. Took up pottery after retirement and have laboured to learn and understand. I brought glaze ingredients, but source my clay locally. The white kaolin-ish stuff is silky rather like porcelain to throw, plastic but no strength. I had it analysed and it contains Fe and Ti. The golden sandy one is from Chinguetti, in the desert in the North of the country ; the muddy one is from the Senegal river valley. Very heavy & sticky. Shrinks too much, dries too fast on the edges. So a blend can work out.

 

The orange sandy clay has the iron and alumina: but little natural fluxes. The "whitish" has the fluxes and is fine grained: but a little alumina. The river clay most likely has a lot of humus, but should be high in potassium, sodium, and calcium. So I would look at a native clay mix of 60% sandy clay, 25% whitish clay and 15% river clay to start.

the engobe mixture will be drier because of the added kaolin and frit. You can add 5% of your whitish clay to help with drying issues. Given you location, I doubt you have ready access to bentonite or macaloid. 

Tom

[njabeid]

Tom, that's useful knowledge regarding clays. While we're here, why does the river clay dry like crazy on the edges, and crack enthusiastically? Is that small clay particle size?

I have some bentonite. Not macaloid. I have quite a nice glaze pantry. I'll try your mix.

Thanks

Nancy 

[Magnolia Mud Research]

Nancy,  
    
Have you fired separate test bars of each of the ingredients you have for clay body, engobe, and glaze recipes and examined each bar carefully?  The data from such a set would be useful for further decision making from calculations based on assumed elemental analyses. 
  
I suspect that the fine clay particles are part of the cause of the river clay cracking on drying, along with a reasonable probability that these fine clay particles  are in the 2:1  Smectite mineral group which will expand and/or shrink as water moves in and out of its flexible crystal structure.  Tom should have further insights.  
 
LT
 

[glazenerd]

I suspect that the fine clay particles are part of the cause of the river clay cracking on drying, along with a reasonable probability that these fine clay particles  are in the 2:1  Smectite mineral group which will expand and/or shrink as water moves in and out of its flexible crystal structure.  Tom should have further insights.  
correct LT. with one add...calcium. The weight comes from water absorption: bentonitites can absorb 15 times their weight in water. Which also plays a role in rapid dehydration. Not aware of the topography or vegetation in that region: but where heavy vegetation  is: potassium, calcium, and sodium is sure to follow.

the orange sandy clay is high in iron, sand, alumina and larger particle. At 60% you get the fired color.

the white clay is fine particle, higher cation exchange (plasticity), higher in natural fluxes. At 25% it will help with plasticity, and supply  part of the particle size distribution. (PSD)

the alluvial river clay should supply sub micron particles, and plasticity. 15% 

T
 

[Magnolia Mud Research]

Tom,

According to my mineral handbook, calcium bentonite does not swell in water,  whereas sodium bentonite does swell.  Haven't heard that potassium is a part of the bentonite series,

could be and it's OK with me if it does  . 

An interesting experiment is to make cones out of 100%  ceramic store bentonite (Na type usually) and fire them to see what happens.  The stuff in our glaze lab melts to nice shiny brown glaze at cone 10 in the two tests done recently.   Haven't gotten around to actually use it as an engobe on a vertical piece.  really should also try a cone at bisque temperature also.  

LT

[glazenerd]

LT

the way I phrased my response was misleading, given your response. I was only referencing that higher levels of calcium and potassium are commonly found in sedimentary clays. Calcium was referenced in relation to cation exchange: any clay with a CEC above 11 is classified as a "swelling" clay. Calcium is more beneficial to plasticity in clay bodies because it does not stretch the water membrane: even though calcium has lower CEC values. Swelling being defined as absorbing water into the platelets causing a change in particle size. Not to be confused with the gelatinous nature caused by the large covalent sodium ion stretching/ distorting the water membrane.

Nerd

[njabeid]

This is getting quite nerdy... Actually it's fascinating, because I had never seen or heard this discussion about clay before. You are using the language used for glazes, i.e. skeleton, fluxes, alumina, etc.. It makes a ton of sense. What seemed pie in the sky was knowing the composition of my clays, but you seem to guess just by looking at them. I did get the white clay tested (can't find the report now): it had mostly aluminium silicates, about 15% extra silica, about 2.5% Fe and some Ti. But the sample was cleaned up, whereas the lumps have darker bits so iron may be higher. Too expensive to test the other  clays.

So iron is a flux in clay, and so are the other oxides. Aha. Interesting about hematite and pyrite. Wouldn't pyrite convert to oxide in the kiln? Is that what yellow iron oxide is?

Is CEC = COE?

So many questions!!!

[glazenerd]

Nancy:

i wrote an article on " cation exchange" in the January 2018 edition of Ceramics Monthly. (CEC) cation exchange capacity is a numerical value assigned to clays that predict the level of plasticity they can impart. A high plasticity ball clay can have a CEC of 9.00 or so, and bentonites can be 50-150.  Plasticity is created by a negative ionic charge in the clay water film. 

Pyrite is FeS2. Or iron- disulfide. The chemically bond sulfur, along with sulfides from lignite coal particles can cause coring, bloating, or blistering in clay bodies. You can look on my page: think I wrote about in the May issue last year. Hematite and magnatite do reduce in a kiln. Iron is considered a flux in clay bodies. 

Clays form the same way, regardless of geography. They are influenced by the same natural mechanisms. There is some deviations if parent minerals are isolated to specific places. Clays in sub-tropical climates are also much different because of the acidic climate in which they form. All vegetation has potassium, calcium, and sodium: as it decays those are washed into surrounding soils and through clay beds: part of the reason sedimentary clays tend to be so plastic.

at one point I use to post lengthy sections on clay chemistry, stopped doing that. Then I started posting synopsis, but that generally requires me to explain why. 

Tom

[Hulk]

Glazenerd Tom,

Your lengthy explanations, synopses, and explanations are appreciated!

- Hulk Tom

 

[njabeid]

7 hours ago, glazenerd said:

i wrote an article on " cation exchange" in the January 2018 edition of Ceramics Monthly

Aha. I loved those techno files, although they were sometimes a wee bit technical. I'm reading that one again. Thanks! 

[Benzine]

I'm going to suggest, that this topic be moved to the "Alchemy" and "Wizardry" Boards, because I have no idea what kind of incantations ya'll are talking about...

[njabeid]

6 hours ago, Benzine said:

I'm going to suggest, that this topic be moved to the "Alchemy" and "Wizardry" Boards, because I have no idea what kind of incantations ya'll are talking about...

 Some of them are a little bit abstruse... It's going to take me some time to chew through them. I'm an Engineer, but not a Chemist.

[Babs]

Glazenerd is down that rabbit hole so deep he is about to emerge on the oppposite side of the Planet with a beautific smile on his face!

I read these posts and then when in my shed occasionally glimmers return.....

Then the "it depends" comes to the fore...

[glazenerd]

Babs:

" it depends" is what started my obsession. I fire crystalline glaze; and the same recipe changed every time I fired it on a different porcelain. I came to the conclusion that yes it does depend: depends upon the chemistry of each clay body. 

I now have access to the Americam Ceramic Society Journals: the rabbit hole has new furniture:) 

example: did you know when you measure the specific gravity of slip; you are actually measuring the effects of PH?

Tom