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njabeid

Engobe And Glaze

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In a remote location, self-taught and still learning, I use local clay that vitrifies at 1,150°C and make my own glazes. Not many recipes around for this temperature. To cover the red clay I use engobe made following Robin Hopper's recipe.  Pots are bisque fired at 1,020°C, then glazed and fired again at 1,120°C. Problem: all my glazes (particularly the glossy transparent one based on Ferro Frit 3124) seem to dissolve the engobe, making it patchy and translucid. I tried 5 other engobe recipes with essentially the same result. Is there any way to prevent this, or is it inevitable at this rather odd half-way temperature?

Thanks for any ideas.

 

Bisqued items:

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After glaze firing :

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post-63688-0-75717800-1491146654_thumb.jpg

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I don't think the glaze is dissolving the engobe, I think it's just the red clay coming through because the engobe isn't applied thickly enough. Second picture, mug on right, you can see the clay through the engobe at bisque stage. Do a test piece with 1, 2 and 3 layers of engobe and see if you still have a problem. I like the red clay showing through the way it is now but if you are looking for solid coverage then thicker engobe should do it. Could add some zircopax to the engobe to make it whiter so you might not need as thick a layer to cover the red clay.

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I don't use engobe but I do a lot of work with dark clay covered with white slip and I agree with Min. Try making samples with increasing coats of the engobe to find the right coverage for your dark clay. 

 

Another option might be to put less water in the engobe, as long as that doesn't create other problems for you. I know if I allow my slip to become thicker I have to apply much less to cover even my darkest clay. 

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I really like the look you have achieved. I have never been a fan of solid white backgrounds. Your background reminds me on a skiff of snow. A thicker application would fully opacify the background. What you have now is more interesting in my opinion.

 

Jed

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OP i have experimented a lot with white engobe and slip both on greenware AND bisqueware. i've actually worked my way down to look like yours. it was easy for me to get super white. but i prefer your look so that took some time experimenting. 

 

i agree with all the voices that shared here. i've tried layers, less water and deflocullated slip. the best white coverage i got was with deflocculated slip but i had to apply that to pretty wet greenware to get it to stick well. it was a porcelaineous slip which looks super wonderful white in ^04 but changes to off white at ^5 with or without clear glaze.

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the photo shows two very nice pieces, i cannot see a problem but you do so you are right to ask the question.  min is an expert in glazing, so try her suggestions. good luck, i also like the look of the finished pieces. 

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Thank you all for those great answers. I'll try them out, but I have had the same problem with a much paler kaolin clay (beige fired to 1,120°C).

 

The clay coming through or the glaze melting the engobe? There is a double interface, and I'm not sure the glaze won't fall off if I make the engobe much thicker. I'll take on board your appreciation of the snowy look - except that sometimes I really would like to have a solid background, e.g. for some cups I'm trying to make with a special logo, for which I need pale green and pale blue but only get spotty results. I'll try thicker, hot chocolate, yes.

 

The recipe I use is:

kaolin   20

calcined kaolin  10

talc   25

Ferro Frit 3110   15

Silica   15

Borax   5

Zirconium silicate   10

 

With all that in, it should stay opaque.

 

Pretty, you say your slip also changes with temperature. I wonder whether it is just impossible to get a smooth result with glaze on top, even though Robin Hopper says this engobe holds up to a high cone.

 

Again thank you. This is the first time I find my way into this forum, but now I know where to go! So much to learn.

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but I have had the same problem with a much paler kaolin clay (beige fired to 1,120°C).

If you applying it in the same manner then this makes sense, I’m guessing the engobe is too thin on that clay also.

 

The clay coming through or the glaze melting the engobe? 

Think of the engobe as a layer of paper. Your current thickness would be like a sheet of tissue paper, you will see some of the red clay through this. What you are aiming for is a sheet of paper thick enough to not see the clay beneath it but not so thick that it cracks. The engobe isn't melting.

 

There is a double interface, and I'm not sure the glaze won't fall off if I make the engobe much thicker. 

The fluxes in the engobe are supposed to bond to the clay, then in turn the glaze will bond with that. 

 

 

The recipe I use is:

kaolin   20

calcined kaolin  10

talc   25

Ferro Frit 3110   15

Silica   15

Borax   5

Zirconium silicate   10

 

With all that in, it should stay opaque.

Yup, it already has zircon in it so should be nice and white.

 

 I wonder whether it is just impossible to get a smooth result with glaze on top, even though Robin Hopper says this engobe holds up to a high cone.

 

Hopper notes this version of the engobe is good from cone 04 thru cone 3. There are slightly modified versions for mid and high fire. The glaze will be as smooth as what it’s applied to. Are you spraying the engobe on some of the pieces? I'm wondering if that might be where the problem with texture is from?

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Thanks Min. By "smooth" I mean not spotty, i.e. as smooth after glaze firing as before. I have tried dipping, brushing, spraying, pouring, sponging... but the glaze just highlights any irregularity. My pouring is dismal, brushing even worse, sponging splotchy. Spraying is the method that gives the most even coating, but obviously there are still thicker dots. Dipping is nice, but being alone and learning I don't always have a big enough bucket.

 

If the engobe is like a sheet of paper, why does it become "thinner" - i.e. translucide - at a higher temperature? My impression that the glaze is dissolving the engobe is due to the fact that the transparent glaze does go misty.

 

By the way, I'm in Mauritania, so thousands of miles from pottery resources, and the inspiration for these particular pieces is a particular style of house painting in a place called Walata, which is very white (even though I am doing it in negative, not yet having made a stencil for white-on-clay):

post-63688-0-41017000-1491253818_thumb.jpg

 

I'll definitely run some tests with different thicknesses, and at the same time learn to appreciate my faded look.

 

Thanks again!

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What lovely inspiration for your work!

 

Could you post your glaze recipe, perhaps the misty look is from boron clouding from the glaze, have you noticed the misty look more where the glaze is thicker or thinner? A picture of the misty part of the glaze would help too.

 

The engobe should be okay up to approx 1170C (if you fire really slowly then a bit lower than that) and you are going to 1120 right? As I'm sure you have noticed clay looks a different colour when fired to a lower temperature than a higher one. I'm thinking that what you are describing as translucent is the clay colour becoming darker when fired to 1120C than your bisque temp and thus showing through the glaze more. 

 

If you are not sure then try the Hopper recipe for cone 4-6 on a test piece, apply it the same way you are now and I think you will find you get the same effect.

 Cone 4-6  Kaolin  15  Calcined Kaolin 20  Talc  10  Calcium Borate (or Frit 3110)  10  Nepheline Syenite  10 Silica  20  Borax  5  Zircopax  10  Total 100%

 

I think you are just going to have to keep experimenting with application of the engobe. For spraying I would use a thin consistency but put on multiple coats.  For the bowls, I would put them back on the wheel and with the wheel spinning brush the slip on. Brush marks are part of the design. Mugs are going to be a pain to do either of those ways so I would try and dip those, shouldn't need to large a pail to do those in. Just don't over saturate the piece or it will likely split.

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Thank you!

 

My glossy transparent recipe is: Ferro Frit 3124 - 60, nepheline syenite - 10, wollastonite - 10, kaolin (A polywhite) 10, silica - 10. It is simple and works nicely. It definitely mists a bit where it is thicker. 

 

Here is a plate with pathetic brushwork (trying not to unstick the stencil instead of doing it on the wheel which looks pathetic anyway in my hands), and you can see the mist in the middle. Is it thicker there??

 

post-63688-0-31604100-1491318455_thumb.jpg

 

Yes, I'm firing up to 1120C. I tried a couple of firings at 1099C because my teapots were dunting and I wondered about cristobalite, but now I'm back at 1120. And yes, my clays all darken considerably with temperature.

 

This is the 'red' clay - green, bisqued and glazed:

 

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and this is the kaolin clay bisqued with engobe on, and two different finished pieces, one with opaque satin blue glaze:

 

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My feeling is that engobe should stay opaque and not show the clay through it, and that long ago I tried thicker engobe and it all fell off taking the glaze with it! But my learning curve has been a roller coaster, starting at low fire. I had more or less given up on engobes and tried opaque glazes, and that is why I didn't dip the cups - the inside is poured opaque satin, vs. the glossy outside. But since I got a die-cutter and tackled the Walata designs, engobe (and its frustrations) is back on the agenda. Here is a cup with brushed engobe instead of sprayed   (:-/) (also showing some spotty blue and green engobe).

 

post-63688-0-56186500-1491321724_thumb.jpg

 

I'll try the recipe you quoted, and try thicker engobe, that will keep me out of mischief for a while. 

 

Thanks a lot, have a lovely day.

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If your engobe is shivering when thicker then I would either change that recipe or the clay. There is a super article of fitting slips and engobes to clay here. Slip recipe I like is called Fish Sauce, I have used it on red clay (^6 and ^04), it is definitely opaque and very white. It can be applied thickly without cracking. If you want to give it a go while you do the other test this is the recipe. Your work is lovely.

 

Fish Sauce ^04-^10
Minspar 23.50
Grolleg Kaolin 43.60
Pyrophyllite 7.80
Bentonite 9.50 (or substitute 6.3 macaloid if your bentonite has specks)
Silica 15.60
100.00

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Thanks again! Alas! the only ingredients I have for Fish Sauce are bentonite and silica. I'll look up some equivalents and definitely try it.

 

I suppose once one puts down a thick layer of engobe or slip one gets into the realm of COEs. After many tests and trials I have managed to make some glazes that fit my clay, including a really nice strontium one (satin and quite opaque)... 

 

It's all wonderful fun, a surprise retirement hobby as satisfying as it is frustrating. I would love to see your work (Avatar is tantalising) but you are very discreet here.

 

Cheers! :-)

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post-63688-0-98922700-1495223346_thumb.jpg

 

Following various trials, and having read the excellent Digitalfire article Min recommended (info I had sought but not found, particularly on the double interface), here is a vastly improved result: (for some reason the phone camera distorts things - the plates are actually round and entered), the old plate on the left and the new one on the right. Thicker engobe did shiver off some pitchers, but the Hopper cone 4-6 engobe seems to hang on here. The glaze is a bit cloudy in the centre, I suspect due to thickness.

 

Very happy and grateful for the help!

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After that discussion nearly a year ago, thanks particularly to Min's input I was able to make a number of nice pots with stencils and engobe, and even had a show and sold them!

IMG_4282.jpg.170f63c980352c5d863ee044f886697d.jpg

Then something went wrong. Some pots started showing crawling in the engobe/glaze, and some spectacular blow-outs :

IMG_4374.jpg.496c0fad62e3cbaa8f3145f62f950302.jpgIMG_5458.jpg.90105285535da6a44663bebe918d71e1.jpg

 I was away in Europe after that for 6 months, happily potting in an open studio with proper clays and glazes (no engobes or stencils)

When I came back I couldn't find my usual clay and had to try some new local muds, and using the same engobe I mixed last year I have two glaze firings full of faults - crawled engobe under non-crawled glaze, and little blow-outs again.

IMG_5423.jpg.4a9253b3fa9612d98fa036844c3cd95f.jpgIMG_5421.jpg.666a19b2169ef337b05810f2a3ef57f9.jpgIMG_5416.jpg.7fbfb8359a9aba810c86f29fc053791a.jpg

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It is obviously due to the engobe, as the pots with none under the glaze show no faults. Where the engobe has crawled the glaze is not cracked, but the blow-outs obviously have damaged the glaze.  I'm miffed. I'm pretty sure all these pots were covered with the same batch of Robin Hopper cone 4-6 engobe, some brushed, some sprayed, quite thick as before. The stencils are not good either, due likely to the rough sandy clay.

Any ideas? Thanks !

 

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Possibly need to mix engobe very thick then use a deflocculant. Eg sodium silicate or Darvan to get it to a brushable consistency so it dries with less shrinkage on your pot.

Rough sandy clay. You could try smoothing area to be stencilled with a rib before taking from wheel.

Or applying terra sigellata to pot for a smoother feel

 

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"Rough sandy clay" is most likely just sand. However, I am seeing issues that make me question that. I see a deeper color; which means more iron in the new clay. "Rough" can also come from higher alumina and. Iron content.  Your "blowouts" suggests the natural fluxes in the old clay were higher than in the new: clay/ interface bond failure. I see pinholes in one sample: also a new issue.

The easiest way to help solve this issue is to run your glaze on a calculator, and check the flux levels, and more importantly the COE. If the glaze fits, then dial in the engobe to the flux levels and COE of the glaze. It will not be exact, but head in that direction.I would also calcine all the kaolin/clay used in the engobe.

Tom

Edited by glazenerd
Info added.

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Thanks for the interesting advice. Unfortunately I can't get any sodium silicate or Darvan here, and I'm not very good at calculators. But I'll try all of those things, although I can't quite figure out what you mean by "dial in the engobe to the flux levels and COE of the glaze". The glazes seem quite happy on the engobe, and on the bare clay, but the engobe obviously doesn't fit the clays (two different mixes).

I find it odd that the engobe should bisque-fire well (it only flaked off on one pot that was still leather hard but maybe a bit wet when I sprayed it on) and then should crawl like that in the glaze firing, but not taking the glaze with it.

Glazenerd suggests a bond failure for the 'blowouts'. Is that enough to explain the shape? And why should the same batch of engobe change, even with the old clay?

I'll try terra sig and mix a new batch with more calcined kaolin, and try again, although it feels a bit like Sisyphus, starting at the bottom over and over again... sigh.

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 but the engobe obviously doesn't fit the clays (two different mixes).

Exactly: I think there are several problems going on together. I found it interesting that the engobe is fracturing in a very distinct crazing pattern commonly associated with COE issues. Although other samples suggests delamination of the engobe, which could be poor application, oil/ dirt on the surface, but also bond failure. 

Except for one piece they made it through the bisq fire-  that one piece would be oil/ dirt or application issues. 

You asked why the engobe would change? How long has it sat since you first mixed it. Flux (spars) have soluble salts that migrate out and form granules- seen any evidence of that?

back to clay ( my area) low fire clay bodies do not vitrify in the traditional sense; which means the metakaolin (clay) is in an expanded state. With that comes COE values ranging from 7.00 up to 9.50. The engobe recipes shown have significant talc additions: which is used to lower thermal expansion, and thereby lower COE.  Summed up: your engobe is shrinking at a much higher rate than the clay. Which is where I think the COE graze pattern, the bond failure, and the cracks are coming from. From what I understood: previous engobe issues had been corrected.

One thing I would do because high iron in the clay is involved: is crack a piece open and look for black (glass) coring. If there are enough sulfides in the clay, this could easily occur. Doubt it is occurring, but needs to be ruled out.

Nerd

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May be that your pots are slightly too dry when engobe is being applied  also.  Try light spray of water before spraying on engobe as well as making a thickened slip ising less water. Thd engobe would have to be quite fluid to be sprayed on...

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2 hours ago, njabeid said:

And why should the same batch of engobe change, even with the old clay?

 

Because it contains borax which is soluble and also Ferro 3110 which is somewhat soluble. Try sieving the glaze and see if you get any little gritty bits left in the sieve. 

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54 minutes ago, Min said:

Because it contains borax which is soluble and also Ferro 3110 which is somewhat soluble. Try sieving the glaze and see if you get any little gritty bits left in the sieve. 

There is only a bit left, but I sieved it and got no residue in the #80 sieve or the bucket. It is smooth. If there were gritty bits would it be crystals settled out of the dissolved borax? If so, what effect would the lower borax content have on the engobe? 

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