Engobe Questions

[Polydeuces]

Hey y'all,

Working with some cone 6 stoneware, interested in getting into the world of engobe. I've done quite a bit of research, but some things still seem relatively unclear. Here's a list of questions, I'd really appreciate some insight on the topic.

1. When is the best time to apply engobe? When the piece is leather hard? And, it needs to be thick?

2. What kind of surface is to be expected from common engobe recipes? I see a lot of slipware with glossy surfaces, but my assumption is that they were either A) fired in an atmospheric kiln or B ) coated in a clear glaze.

     2a) Because of the materials commonly present in engobe recipes, is it difficult to glaze over? Is it essentially an underglaze?

3. Anything particular about obtaining particular colors? I was considering trying some mixes out using iron oxide and a few Mason stains.

 

Thanks for your help~

[Sputty]

.

[Polydeuces]

Yes! Thank you for your answers. Stains need to be applied in high proportions...  I guess that makes sense, being that it's mostly clay. It's all the zazz of fluxes & glass that makes stains do what they do in small amounts, amirite?

For clarity, I'm mostly interested in playing with slip/engobe/underglaze decoration & sgraffito which in my 4 year self-taught pottery journey, I've never really messed with. I've always liked the look of slipware, and the techniques involved, but something about it has made me squeamish. I sort of see it as a jumping off point for more artistic expression, which feels slightly lacking as I've been favoring  a more utilitarian look. Aside from that,  the forum thread you offered is sort of the general confusion I stumble into on a routine basis, when I'm really just looking for techniques of preparation & application.

Additional question: Would it be easier to achieve a black engobe from a stain, or from a mixture of cobalt & iron? (would that do it?)

 

[Babs]

Body stain.

Do searches in these forums

[Sputty]

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[glazenerd]

Just now, Sputty said:

 

Good for any cone. Get a Ball Clay low in iron, and you have a good base for adding stains. Mix (and sieve through 100's mesh) to a creamy consistency.

Bangs head on kiln lid.  85% clay content? - faints!  Okay, my cardiac event has passed- onward!

[Sputty]

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[Tyler Miller]

1 hour ago, Sputty said:

Not sure I understand your sudden fibrillation...

I've used this slip from cone 04 to cone 8, with no problems whatsoever. Hopper used it all the way up to cone 12, equally with no problems.

What am I missing?

Nothing at all.

I do feel the need to say that the former link of the two French engobe sites is very much earthenware only.  Immediately apparent to a French speaker, but maybe not to a non-fluent, or non-speaking browser in the context of a discussion on cone 6 engobes.  At cone 6 that much talc and frit might not play so nice.

The text of the second link looks very familiar...

[Sputty]

.

[Tyler Miller]

Thanks for the clarification.

[Min]

7 hours ago, Sputty said:

In my slippy days, I used to use the following as an all-purpose engobe, courtesy of Robin Hopper:

Ball Clay - 75

China Clay - 10

Silica - 10

Potash Feldspar - 5

This would be his slip recipe for mocha diffusion, nice and alkaline so it works well for that process. 3 basic engobe recipes from the late Robin Hopper here along with some slip decoration application methods.

[neilestrick]

2 hours ago, Sputty said:

Not sure I understand your sudden fibrillation...

I've used this slip from cone 04 to cone 8, with no problems whatsoever. Hopper used it all the way up to cone 12, equally with no problems.

What am I missing?

I would only use that recipe at cone 10.

[Sputty]

.

[neilestrick]

35 minutes ago, Sputty said:

Why? I've used it at cone 04, and it was perfect. No issues whatsoever. Took to the pot beautifully, fired beautifully, took glaze beautifully. Utterly reliable whenever and wherever I've used it, at any temp I've tried.

 

It is (or was) his slip for doing pretty much anything with (including mocha):

SLIP-SLIDIN'- AWAY!

For me, using a cone 10 slip at cone 6 is no different than using a cone 10 clay body at cone 6. It's not the best way to do it- at some point it won't work. There is no such thing as a 'works with everything' situation in ceramics.

[glazenerd]

Sputty: 

perhaps using a paint analogy will work.

1. Terra sig..one coat: flexible, will move with the surface. Will show some of the surface under it.

2. Slip: 2 coats: will move less, but some with the surface under it. Will hide some but not all of the surface under it.

3.engobe 3 coats: will not move with the surface under it ( surface tension) will hide everything under it. Engobe is more akin to plaster over drywall. Essentially engobe is a  thin coat of clay over another clay. That said: clay chemistry comes into play.

shrogren (PHD) study: cristabolite formulation exponential if spar level under 10%. Ron Roy did the dilameter testing in this study.

checks pulse.. I'm good... 

[Pres]

5 hours ago, neilestrick said:

The reason we don't use cone 4-10 bodies here is because most cone 10 bodies will weep at cones 4-6. To say that your method has worked for you doesn't negate that fact. Search the forums here, we have people posting about weeping almost weekly. If it works for you, then great, keep at it. No one is telling you that it doesn't work for you. We're simply saying that it's not the best practice for people in general.

I take offense at the assertion that maybe 'we don't want the hassle of thinking, and want everything laid out for us just so'. Let's leave personal attacks out of this.

Years ago as and experienced, but naive potter, I used a 6-10 body from Standard Ceramics. This was a nice off white clay that took glazes well, and with spraying stains and glazes over a bristol glaze that matured at ^6 which I fired to a hard cone 6 seemed to work well. A few years went by, and I found that most of the pots I used personally started to weep, craze, and after some time glaze would lift from the piece, spalling.  Surfaces had become dull also. In the end, I had been involved in other things and when got back to ceramics, I rethought the entire clay firing range, glaze fit problem. Since then post made 15 years ago are still as they came out of the kiln after heavy use, microwave, oven and dishwasher use. They do not weep, even when not glaze on the bottom of bowls and other items.

This slip discussion has been subverted as some do, I would appreciate a return to the subject with emphasis on the engobe situation. It seems that the engobes have been covered well, and presently there is little to add. Most that I see seems to be in the realm of personal agendas. I will be splitting off some of these strands into a separate area on clay body firing ranges. Those of us interested in functional ware may have a completely different viewpoint than those of you into sculpture, but we all need to understand preference and difference.

 

best,

Pres

[Babs]

Well said Pres and Neil.

The poster's question  can be addressed without the need to destroy another person's contribution, or push a personal agenda or petty one up manship vendetta.

In ceramics testing for self or researching further the info. presented as solutions is in the poster's hands

Unless there is an obvious danger to public or potter within the soln.

Then a moderator steps in.

[glazenerd]

Deuces:

the better way to look at engobes is effect: what effect are you after?

Engobes are used as a coating that completely mask the clay body under them. Some examples would be a porcelain engobe over a stoneware body: often used in crystalline glazes. Perhaps a porcelain over a dark bodies stoneware so glazes are clearer. Often times potters use one primary clay body, and use engobes to change things up without having to stock multiple clay bodies.

engobes are used for carving; a stained engobe over porcelain: then carved through for effect.  White engobe (Zircopax or titanium) over a dark stoneware: then carved. At cone 04: white earthenware over terra cotta, or vice versa, carved, layered, etc.

The easiest way to use an engobe is to just mix it from your current clay. Add body stains, colorant oxides, etc. Porcelain takes stains, produces better whites than stoneware. Second option would be to select a stoneware and porcelain bodies with nearly equal COE values. EX: stoneware COE at 5.65 and porcelain at 5.75.  Use one for the body and one as the engobe: buy quantities accordingly. Then you do not have to deal with recipes at all. Engobes are more about coating than anything else.

once you get a few miles down the road with it: experiment! The fun stuff.if you want to do some intricate detailing that requires time. Dry the engobe clay completely, powder it down ( outside with a mask) then add 2% bentonite, blend dry, and add water slowly until you hit  the creamy paste state. 2% bentonite will  buy you extended carving time before it dries.

if you want super white use porcelain : add up to 10% zirco. Add 2-3% feldspar to counteract the properties of the zirco.  The easiest engobe is the body you are using, or using a second body in the same cone range, with a closely matching COE. Makes life much easier.

t

[Polydeuces]

Hey y'all, thanks for the responses!

Despite arguments & debates, I find it all very educational and appreciate multiple viewpoints—so thanks for chiming in!  It's plain to see that there's a passionate bunch here

On 7/18/2018 at 8:39 PM, glazenerd said:

Deuces:

the better way to look at engobes is effect: what effect are you after? 

 

I'm going for a layer that totally masks the clay body underneath. I like the look of the clay body (working with Highwater's Brownstone—fired in reduction it's beautiful and toasty) but would like to add some variation. Perhaps irrelevant, but some of the "Why" behind it: I've experienced some frustration within the variables of colorful & interesting glazes, so I'm intending to move forward by experimenting with something a little more "controlled," and going with simpler, easier, more reliable glazes suitable for functional ware.

I'm really just interested in exploring the uses of them. I had made a couple personal attempts by simply following recipes, and really had no idea what I was doing insofar as the baselines of "how thick" and "when I do I apply?"—hence my post. I'd like to try painting motifs & banding, sgraffito, or simply providing a more suitable canvas for certain glazes that I'd still like to use from time to time.

I like deeper colors & muted earth tones, but I think I'd like to experiment with some whites or oxide-laden slips, too.

I guess if I'm going with a dark body, I would probably want to layer a white/porcelain engobe on top, and then used a white/porcelain engobe with whatever stains on top of that? If I used a black body stain on an otherwise dark engobe, and threw a clear or translucent glaze on top—would it show? My understanding was that the addition of flux to the engobe brightened the coloration.

Thanks for all the tips!

 

 

[Polydeuces]

Here's an additional question concerning the topic of "under the glaze."  It just occurred to me, and is kind of side-reel from engobe but in the same ballpark perhaps.

If I were to apply a fairly thick mixture, let's say something like

25 ball clay

55 whiting

10 cobalt carbonate

10 rutile

to a piece that was leather hard—let's say it was painted somewhere on the surface, and then it dried & went through bisque, and was glazed & fired again—would the flux in the Whiting still be present & active in the glaze fire, and would it react to the glaze applied on top? Or would this mixture vitrify at bisque temp?

I guess my reason for asking is, it would seem a rather convenient way to influence the behavior of an otherwise simple glaze. That said, I've only reached the tip of the iceberg regarding my understanding of ceramic chemistry & the interaction of kiln-firing.

[Babs]

With  10% cobalt and rutileit's gonna do something

[Min]

12 hours ago, Polydeuces said:

I guess if I'm going with a dark body, I would probably want to layer a white/porcelain engobe on top, and then used a white/porcelain engobe with whatever stains on top of that? If I used a black body stain on an otherwise dark engobe, and threw a clear or translucent glaze on top—would it show?

Yes to the first question. Or to keep things simpler while you are starting off down this road apply an engobe, could be made white with 10% zircopax,  then when that has dried apply underglaze on top of that. Second question, would have to try it and see. 

I think the first step would be to find a slip or engobe that will work with your clay. I would suggest making up some test pieces and try out a few slip or engobe recipes with and without zircopax and see how they behave / look on your clay. Apply them at the leatherhard stage, try dipping and brushing them on. One coat over the entire test piece, two coats on the top half. Fire and glaze them and see what you get. The engobes / slips should be about the consistency of pancake batter. Once you get an engobe / slip that you like then start playing with the colourants. This link that Sputty provided in an earlier post had a lot of colour tests, you could use that as a jumping off point.

 

[glazenerd]

Deuces:

Instead of an engobe discussion: perhaps an oxide discussion is in order. Iron, magnesium, and titanium  are reactive to temperature. In electric there are color shifts, and even more notable in reduction firing. So when you are preparing engobes, bear in mind that these metallic oxides are going to create color hues, more so in reduction firings.

here is an oxide chart of sorts: it will give you some direction:

the round test tile is pure porcelain: less than 0.10% iron! magnesium! and titanium.

bar 1. Titanium bar 2-3 iron/ magnesium bars 4-5-6 titanium, magnesium, then iron 6-7-8-9 titanium, magnesium, and iron levels increase. It takes surprising little of these oxides to change color hue. If you are going to dive into the engobe rabbit hole: then calculating/ tracking metallic oxides will become  essential. 

A basic high white porcelain: 50% grolleg, 20% 325m silica, 30% potassium spar. ( no Nep Sy please) Nep Sy in clay = pinholes.

cone six by the way. If you fire to cone 10 - 50% grolleg, 25% 200m silica, 25% potassium.  These are basically the old Coleman recipes.

for now just use 2-3% calcium bentonite as a plasticizer: then ball clays later.

now you get to study ball clays: how much iron, magnesium, and titanium do each add? Do your research.

tom

[Polydeuces]

23 hours ago, glazenerd said:

Deuces:

Instead of an engobe discussion: perhaps an oxide discussion is in order. Iron, magnesium, and titanium  are reactive to temperature. In electric there are color shifts, and even more notable in reduction firing. So when you are preparing engobes, bear in mind that these metallic oxides are going to create color hues, more so in reduction firings.

here is an oxide chart of sorts: it will give you some direction:

the round test tile is pure porcelain: less than 0.10% iron! magnesium! and titanium.

bar 1. Titanium bar 2-3 iron/ magnesium bars 4-5-6 titanium, magnesium, then iron 6-7-8-9 titanium, magnesium, and iron levels increase. It takes surprising little of these oxides to change color hue. If you are going to dive into the engobe rabbit hole: then calculating/ tracking metallic oxides will become  essential. 

A basic high white porcelain: 50% grolleg, 20% 325m silica, 30% potassium spar. ( no Nep Sy please) Nep Sy in clay = pinholes.

cone six by the way. If you fire to cone 10 - 50% grolleg, 25% 200m silica, 25% potassium.  These are basically the old Coleman recipes.

for now just use 2-3% calcium bentonite as a plasticizer: then ball clays later.

now you get to study ball clays: how much iron, magnesium, and titanium do each add? Do your research.

tom

Whoa! That's awesome—thanks for that! I'm going to need to study this a few times, but this will be a great jump-off for additional testing. Thanks!

[Tamas]

I can heartily recommend John Mathieson's book Techniques Using Slips. It is from a UK perspective, but still a very good overview, and it gives you insight how different ceramic artists use slips at different firing temperatures and atmospheres. Tons of recipes, too.

 

Link to the book on Amazon