Problems with engobe - again

[njabeid]

@Min Yes, good to know there's zinc in it, but really I seldom use it.

I'm tweaking the alkaline glaze  (10% TiO2 so far) hoping to get the bright colours without the runs.

Let's be patient until I get through the firings with new tests. I was throwing some new stuff as I don't like to fire a half-empty kiln. I haven't really heard a theory about why oxides would go through an engobe layer and blow it out - or in, looking like a bubble burst. It makes no sense.

Have a nice party, hopefully with family and friends.. 

[njabeid]

OK, here we are. I have run the tests but am not much the wiser.

I made a new batch of engobe (kaolin 15, calcined kaolin 15, silica 20, ZrCO3 15, potash feldspar 20, talc 10,FF 3310 15, neph.sy 10), sprayed it on 6 mugs, bisque and then applied:

Mug 1 - sprayed GT1 (my glossy transparent)

Mug 2 - dipped GT1

Mug 3 - sprayed GTI+ CuCO3 2%

 

Mug 4 - sprayed GT1 + CoCO3 0.3%

 

Mug 5 - sprayed GT1 + RIO 5%

Mug 6 - sprayed GT1 + CoCO3 1%

 

As you can see,  there are still problems, even with the midnight camel and even the clear ones, more with the sprayed one than the dipped one, but nearly nothing with the iron. Some of the stencils are pathetic, and I did wax the rims but still got runs. Bother.

Removing the borax and boron frit and decreasing the 3110  has made a difference, but has only changed the shape of the faults; not eliminated them.

There are no (big) blow-outs though. I suppose that's progress. Are those holes just pitting, or pinholes, or tiny blow-outs?

My conclusion is that I should avoid engobe, but I'm frustrated not knowing the reason of these strange effects.

Cobalt and copper are fluxes, so they should increase the glaze's COE, which should result in crazing, not in crawling.

 

 

Edited January 6, 2023 by njabeid

[njabeid]

Besides the mug tests, I tried applying a slip made of one of the clays I blend. This one is nearly kaolin, very white before firing, and the results are actually very encouraging, although it doesn't fire white. 

I made two test bowls and tried all sorts of combinations (sprayed, brushed and dipped slip on leather hard or bone dry clay), then sprayed, poured, dipped glaze, clear and with cobalt. I didn't try stencils, but there are no visible flaws. Problem: it looks best dipped, but I'm not sure stencils will stay on in a dip. More testing required. Although it isn't white I think it can work well enough for what I want to achieve.

The problem is that camels can be done by sponging colour as an overglaze through a negative stencil, kind of majolica-style, but I can't do that with the more complex traditional patterns. As you can see, brushing doesn't work well. The slip needs to be thick.

[njabeid]

Meanwhile I pulled out some older test tiles. All my test tiles before 2017 disappeared in a house move. Bother. So I only have the more recent ones.

Interestingly practically all the copper and cobalt tests had disasters on the engobe side - I don't know why I hadn't 'noticed' this - I suppose I had given up on engobe and/or was looking at what the glazes do on bare clay. Anyway, again interestingly, practically all the glazes that showed no flaws (not photographed) involved some strontium, sometimes in one glaze, sometimes in layered combinations, whether over or under.

Various iron oxide glazes. The two problem ones are Linda Bloomfield recipes, the second on the left my version of a "sparkle" (Glazy?)

Copper glaze tests, front and back.

Cobalt

 

 

[Rae Reich]

I like the color of your homemade slip. Try using different brushes to apply it. When I worked with a slip of porcelain throwing clay I learned to apply it with a large floppy house-painting brush in a thick single coat, on the turning wheel or a banding wheel- Anything thinner would melt into transparency. For your mugs, that would mean putting the stencil on freshly thrown clay and coating with slip while on the wheel, removing the stencil when slip sets and adding the handle when leather hard.

[neilestrick]

Have you tried coloring the engobe instead of the glaze?

Is there texture in the surface of the clay, like lines or pits from trimming?

Are there any pits/holes in the glaze surface after applying the glaze, that could be rubbed out before firing?

[Min]

I can't help but think that it's the body gassing out during glaze fire. Gas is able to go through the thinner and more fluid glaze and heal any blisters / pins but same gasses coming through the engobe are disrupting the engobe and it isn't able to heal over. 

Peeling engobe on the test tiles is a separate issue, not a good bond between the engobe and the body.

2 hours ago, njabeid said:

Cobalt and copper are fluxes, so they should increase the glaze's COE, which should result in crazing, not in crawling.

Copper and cobalt do both act as fluxes but both will actually reduce crazing.

[njabeid]

@Rae Reich I'll try that, although as you say the slip goes transparent and I find it hard to get an even coat, even with a soft hake brush; as you can see in the bowl. 

@neilestrick Excellent points. I did use commercial coloured engobe and I think (forgotten) there never was a problem. I'm going to try putting colour in the engobe/slip, and see what happens.

The surface of the clay is not always super smooth, although I try to smooth it. I also rub out the pinholes in the dry glaze - maybe should put more effort into that.

@Min You must have hit the nail on the head, but why does this happen with oxides in the glaze and not with plain clear? My bugbear. 

10 hours ago, Min said:

Peeling engobe on the test tiles is a separate issue, not a good bond between the engobe and the body.

So why is the bond better when strontium is in the picture? Maybe I should try adding some strontium to the engobe? I think I'll carry on testing with the new slip, and only use engobe when I want white white.

10 hours ago, Min said:

Copper and cobalt do both act as fluxes but both will actually reduce crazing.

That's interesting!! I guess I need about 50 years to wrap my head around all this glaze chemistry stuff. 

[PeterH]

43 minutes ago, njabeid said:

That's interesting!! I guess I need about 50 years to wrap my head around all this glaze chemistry stuff. 

You may find this of interest

How to Increase or Reduce Crazing in a Glaze
How to Adjust Crackle Without Changing the Overall Appearance of the Glaze
https://ceramicartsnetwork.org/daily/article/How-to-Increase-or-Reduce-Crazing-in-a-Glaze

[Min]

5 hours ago, njabeid said:

although as you say the slip goes transparent and I find it hard to get an even coat, even with a soft hake brush; as you can see in the bowl. 

Have you tried making this slip whiter by adding zircon? If not I would try a small test using the same amount of zircon as you put in your engobe. Also, any chance you have a deflocculant? Sodium silicate by any chance? Would this be available to you in Mauritania? If you add a deflocculant then less water goes into the slip to create the same brushing consistency therefore you can create a more "concentrated" slip so to speak. I'm not seeing the blowouts with this slip, my hunch is it isn't as vitrified or at least not as tight when the body is outgassing. 

Have you tried a slower bisque schedule to allow more impurities to burn out?

5 hours ago, njabeid said:

but why does this happen with oxides in the glaze and not with plain clear?

Are these from the same batch of cobalt and copper carbonates as you use in England? I not what would happen if I mailed you out a tiny amount of mine to try? (going back to Rae's suggestion that they might be contaminated)

5 hours ago, njabeid said:

So why is the bond better when strontium is in the picture?

Recipe? 

 

[njabeid]

@Min I'm trying some zircon in the slip. And some strontium in the engobe, out of curiosity, and some oxide in the engobe too. No time to make anything but tests! 

I have no sodium silicate here, but can find a bit to try. The slip does look good on those tests, even with cobalt in the glaze. 

It's very kind of you to offer to send some oxides - on one hand they might arrive in June 2024, and on the other hand the oxides are the same ones I have been using for years. They are from Ceradel in France (except the new copper on the green mugs in in the latest tests). What could be the influence of contamination? Attached are photos of the latest pots with copper and cobalt glazes with no engobe. The problem is the engobe, not the oxides themselves.

 

The blue bowl is the test with cobalt over slip, no problem.

The strontium observation concerns a whole bunch of tests with various recipes and combinations, but all had some Sr involved. I make a "strontium base" that works well for me; (FF3124 - 30, Neph s. - 30, SrCO3  - 15, LiCO - 3 5, wollastonite - 5, kaolin - 5, silica  - 10). There is an opaque version, a blue one, a green one, etc, and I find it works well as a bottom layer under another glaze.

[njabeid]

@PeterH Thanks. It's super useful, but crazing isn't the subject of this discussion - crawling is more to the point, sort of.

 The snowflake glaze in the article is gobsmacking. Oddly one of my dishes has a small snowflake crackle that should absolutely not be there. It was supposed to be a turquoise (alkaline, copper) glaze, over engobe with a stencil, and it turned out colourless with this crackle.  Impossible at 1100°C but there it is. The snowflakes aren't spectacular like Joe Thompson's or a French potter I saw in Paris once, but they are definitely there.

(The close-up is enhanced and you can see the crystals)

 

[Babs]

22 hours ago, njabeid said:

Besides the mug tests, I tried applying a slip made of one of the clays I blend. This one is nearly kaolin, very white before firing, and the results are actually very encouraging, although it doesn't fire white. 

I made two test bowls and tried all sorts of combinations (sprayed, brushed and dipped slip on leather hard or bone dry clay), then sprayed, poured, dipped glaze, clear and with cobalt. I didn't try stencils, but there are no visible flaws. Problem: it looks best dipped, but I'm not sure stencils will stay on in a dip. More testing required. Although it isn't white I think it can work well enough for what I want to achieve.

The problem is that camels can be done by sponging colour as an overglaze through a negative stencil, kind of majolica-style, but I can't do that with the more complex traditional patterns. As you can see, brushing doesn't work well. The slip needs to be thick.

For the slip to fire whiter, you need to apply it a lot thicker. Applying slip to dry pot could / will cause a bloating of the slip away from clay body on firing.

Your stencils will stick to the clay at leatherhard stage , depending on what they are made of.

You could try a white firing porcelainous body as a slip .Need to use sodium silicate and test a few tiles for thickness and fit.

 

[njabeid]

@Babs Good advice. But the slip I'm trying is a local sort-of-kaolin that contains iron and titanium and won't fire white, even made into a pot. I'm trying it applied damp and dry, but it might just stay on as it is one of the three clays I blend to get workable stuff.

My stencils are made of thin cardstock and they stick to damp clay but also to dry clay if I wet them.

Lots of testing ...

[Babs]

Lots of slipware artists in UK atm, some very generous with their methods, check them out.

[Callie Beller Diesel]

I do a bunch of work that involves combining a red clay and white slip, and what I’ve found is that when you apply it and how much water is in the slip does make a big difference in how well the 2 stick to each other. But my white clay has a lot less flux in it than yours does. I also found that firing for my clay body solved a huge boatload of glaze problems.

My inclination would be to try 3 things and see what works best. The first two are adding more opacifier to your homebrew white, and doing a bit of a line blend with your most recent iteration of the original engobe, decreasing the frit 3110 in increments of 2%.

I think what’s happening is that the engobe is melting early enough that it’s keeping your clay body from being able to offgas properly, leaving some residuals that are still trying to break loose in the glaze. Despite what they used to tell us years ago, colouring oxides do have an effect on the melt, and cobalt and copper both get moving kinda early, slowing down the release of those residuals. Iron is also melting, but it can also act as a fining agent, and it releases bubbles more readily. 

A third suggestion would be to slow your bisque down, and let it soak without pushing any cones over. That should allow further off-gassing without changing the porosity of your bisque.

I don’t know if you have access to a machine like a Cricut, but if you make your stencils out of reusable vinyl, it could make them more “dip friendly.” Or maybe spraying the backs with some kind of adhesive or starch before applying them. 

[njabeid]

@Callie Beller Diesel

Thanks, you and Min agree on degassing, and your explanation about the oxides makes sense. My bisque firing schedule rises at 80°C up to 600 and then 120 up to 1000°C. My controller is very limited. Should I go even slower? Or just add a soak?

3 minutes ago, Callie Beller Diesel said:

I also found that firing for my clay body solved a huge boatload of glaze problems.

Not sure I understand what this means specifically. Is it the slow firing?

I have started modifying the engobe, and testing my clay slip.

I do make my stencils with a Silhouette Studio, but found that vinyl just doesn't adhere, adhesive or not. Maybe that was on dry clay. I don't dip much because my small studio doesn't have big buckets of glazes, but I could make enough white stuff to dip.  

So many things to try, thanks to all yous wonderful advice.

[Babs]

One at a time and take copious notes and photos. Test tiles or shot sized cylinders may save a lot of kiln space

[Min]

5 hours ago, njabeid said:

...on one hand they might arrive in June 2024

I am never ever going to complain about the time it takes me to pick up supplies!

On 12/29/2022 at 7:07 AM, njabeid said:

...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.

Misinterpreted what you wrote in your first post snippet above, thought there might be an issue with your cobalt and or copper but you've cleared that possibility up.

2 hours ago, njabeid said:

My bisque firing schedule rises at 80°C up to 600 and then 120 up to 1000°C. My controller is very limited. Should I go even slower? Or just add a soak?

In an ideal world you would slow it down during specific temp ranges. Is your controller limited to 2 ramps?

[Callie Beller Diesel]

What I mean by firing for my clay body is that instead of trying to finesse the glaze, I bisque with an eye towards a clean burn off: slow and thorough, and as Min mentioned, paying attention to a couple of specific zones. I also will go slowly in the first part of my glaze fire (up to about 700C) just to hedge bets. I fire my particular clay body hotter than the manufacturer recommends because otherwise it doesn’t meet my porosity standards. Rather than firing to a higher temperature, but I do a drop and hold schedule and let heat work push the cones over so my clay doesn’t bloat. 

I started doing all of this because I was trying to get a clear glaze to actually look clear over a red clay body. The clear I was using looked great over white clay bodies and I had no problems with it even over the darkest underglazes, but I was having a heck of a time with micro bubbles over just the red clay. I’d already tweaked glaze recipe and application problems, so I started doing all the firing adjustments above. In the process I noticed that it improved the appearance of my other glazes, and gave me more consistent results all around. 

Some helpful links are this digitalfire article on pinholing, and this video/article from an industrial ceramics site. Note that the last one assumes that the reader is working in an industrial setting, and adjusting the firing cycle is one of the least practical solutions. Because we’re working on a much smaller scale than that, the firing cycle is one of the first things we can look at. The digitalfire article is a good starting point for troubleshooting, and has lots of links to other helpful stuff. You’ve already gone through a bunch of it, but there are some other things it mentions, and digitalfire is a good resource to know about in any case. 

[Kelly in AK]

11 hours ago, njabeid said:

I have no sodium silicate here

Soda ash (sodium carbonate) is also a deflocculant, possibly easier to come by. You can actually make it by putting baking soda in the oven, but I suggest you try and find some first. People use it in swimming pools to adjust ph and fiber artists use it in dyeing. 

The long bisque firing is a good plan. I know you use local clay, if you have any hand in the processing it may be worthwhile to get more coarse particles out. Those are like little gas factories, as well as providing a path for gasses to escape.

 I agree with @Min about trying zircopax in your slip to whiten it. It will also make it less susceptible to getting soaked up by the glaze.

 I see from those test tiles you’ve been through a lot! Sometimes it feels like a miracle any of this stuff works at all. 

[njabeid]

@Kelly in AK I think soda ash is the (nearly) one ingredient I haven't got, but I'll go along to the shops and have a look. I do have bicarb, so I could bake that.

I do all the processing myself, with some manual help, and the clay is sieved and fairly smooth. The coarse particles are really sand. I love the white clay, it feels like porcelain, but it has no bones and keeps going pear-shaped, hence the blend.  I suppose I should just steer clear of engobe...

I am deep in tests, one of which includes zircopax in the slip. Let's see. I'll be back with the results.

Yes, I fell into the glaze rabbit-hole because I hate hearing 'ping!' and having crazed glazes, so I mix my own but I'm a slow learner. It definitely is miraculous, fascinating and absorbing. In London I get glorious colours on porcelain but here it's a bit frustrating, even more so because I go away for months and forget where I was in my endeavours the previous season, and start the same mistakes all over. A double life comes at a price.

These are the clays I start with:

White nearly-kaolin found in the market,

Muddy clay from the Senegal River valley:

Sandy clay from Chinguetti out in the desert:

 

 

[Babs]

That's where your notes would help, eh?¹

[Magnolia Mud Research]

3 hours ago, njabeid said:

I think soda ash is the (nearly) one ingredient I haven't got, but I'll go along to the shops and have a look. I do have bicarb, so I could bake that.

no need to "bake" the bicarb,  just add more of the bicarb to equal the amount of sodium that you want to have in the mixtrue.  

LT
 

[njabeid]

@BabsI do write everything down, and take photos of all the pots, and make test tiles so I expect to see the result of the procedures written down. What would help is going through the entire archive every time I come back.... 

Some of my London pots, my glazes too, but on white commercial clay. Goes to show something or other;

: