njabeid

@PeterH, I believe the formula for sodium to work as a defloc is it needs to combine with calcium as in the following equation, Na2CO3 + Ca2+ = CaCO3 + 2Na. 
Simple enough to test sodium bicarb out as a clay slip defloc but I don't think it will work.
@Babs, for sure your schedule could work too. It's getting more and more expensive making pots with costs for just about everything going up isn't it?
edit: adding a link to a Britt video showing what sodium silicate does to a slip and how you would then add more dry clay to make a more "concentrated" slip. ie one having the same amount of water but more clay.   Video also shows how to use epsom salts to do the opposite and flocculate a slip. 
 

@Callie Beller Diesel Those are fantastic sources.  Still reading and digesting, but now I think I understand my problem pretty well.
I was stuck because I didn't think melted glaze would be an obstacle to degassing, but obviously the lower the melt the more barrier to degassing, as Min explained.
Good ol' Digitalfire's Tony Hansen, so thorough as always. 
 

@Rae Reich no, the runny glaze was extremely simple, no rutile. I do like to add a pinch of rutile though

@Min Thanks, that will be my next schedule. It doesn't look as if it can do any damage - it's the glaze firing that brings trouble.
Re soda ash, washing soda doesn't seem to be something the French use, and our commercial patterns here are French, Arab, or even Turkish nowadays. Amazon UK has lots, Amazon France only bicarb and caustic soda. Is that the only alternative to silicate? Let's see the results with just plain slip. I'm waiting to dry a couple of things to fill the kiln.
Regarding humidity, in this season here the problem is stopping things from going bone dry too quickly. It's crazy. It has been cloudy for three days, drizzle yesterday, and the r.h. is still 21%.  I'll do the candling anyway, just in case.
 

The schedule I linked above includes a 12 hour candle (low temp soak), if your pots are dry you don't need this. If you aren't sure pots are bone dry then to cut down on the length of time the kiln is running heat it up to 90C in the evening the day before you are going to fire then soak it for X amount of time, an hour should be plenty based on what you are making and where you live. Turn the kiln off after the soak and leave it until the morning. Residual heat should dry everything out just fine. This leaves you back up to 3 ramps for your bisque firing.
Perhaps something along the lines of 
Overnight as above then in the morning 1) 110C to 700C no soak. 2) 50C to 900C no soak.  3) 42C to 1000C no soak.
If you try this please don't try it with anything precious until you test it out.
 

How much of any given deflocculant you use depends on what else is in the clay body and what consistency you want to arrive at. If there’s any solubles in your clay/glaze/engobe, that will affect things. Although if there’s solubles, you usually don’t need a deflocculant. 
My suggestion would be to make a saturated solution of soda ash (say that 5 times fast!), and add it in tiny quantities, mixing for several minutes between additions until you get your desired results. 
 

@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:

 
 

@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;
:

@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:

 
 

@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;
:

So I'm going to assume there is some, maybe a lot, of stuff in your clay that probably needs burning out which hasn't been with your existing bisque schedule.
There are a couple temperature zones that carbons (both organic and inorganic) burn off plus there is also sulphur. Given that you have a maximum of 2 ramps (with holds) I would suggest that firing your bisque slowly through the 700C - 900C would be where I would try. Given that you end your bisque at 1000C I would slow down from 700C thru to 1000C. This would be your second ramp.
I don't know what the cost of electricity is in your area but I do recall you saying you have frequent power outages so it's going to be a balance between cost + liklihood a power outage verus the ideal rate for the crap to burn out of the clay. Have a look at this ramp schedule for really problem claybodies and adapt it to your circumstances and 2 ramp controller. If you can drop down to 40-50C / hr from 700C to finish I would try that. Other option would be to not slow it down that much but add a hold at around 800C. 
Re soda ash vs sodium bicarbonate, snippet below from Laguna Clay via Marjon Ceramics:
 
"SODA ASH Soda ash works with sodium silicate to aid in deflocculating. The correct combination of the two will give proper casting qualities. Soda ash works to dissolve lignite in clay. It works as a deflocculant, but if only soda ash is used as a deflocculant, your clay will become sticky. If soca ash is not stored in tightly closed containers, a chemical change occurs and it becomes sodium bicarbonate (baking soda). If this occurs it will then act as a flocculant (it will thicken) instead of as a deflocculant."  

I think you're probably dealing with a lot of inconsistencies in your materials. Especially the clay- it's a big unknown. I say simplify things as much as possible- if the engobe has been problematic for a long time, use a slip or something else instead, or start with a new recipe. Or even find a different approach to what you're trying to achieve in your work.

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
 

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

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. 

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

@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:

 
 

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. 

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.
 

@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?
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.

@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)

 

@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.

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

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?
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)
Recipe? 
 

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