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

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.

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.
 
 

@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.. 

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....

@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...
 
 

@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...
 
 

@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...
 
 

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!!

@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.

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?
I think those areas are where the engobe hasn't bonded well with the body.
Google translate has Spanish for engobe as slip.
 

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

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?

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.

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.
 

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.