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@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:

@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.

@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 ...

@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.

@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.

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

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..

@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.

Wow, thank you all for pitching in with excellent advice. However, my question is: why does this happen when there are oxides in the glaze and not when there are none? Photos attached of when things go well. As I mentioned, iron is flawless, and higher cobalt percentage seems to be better. My engobe fits both the clay and the clear glaze, and shows no flaws when it is applied on its own. I am in Mauritania, all my raw materials have to be imported (usually brought in a traveller's suitcase), no local suppliers. The mix of clays works @glazenerd There is no problem with the clay, a blend of three very different clays, never had calcium issues, and I make glazes and engobe to fit. No flaking. @neilestrick Amazon is not an option I'm afraid. It's not a fit issue, because everything fits when there are no oxides in the glaze. @Min 1) Correct 2) Electric kiln with badly fitting bungs and door, bisque 1000°C, glaze 1100°C short soak, both with no bungs up to 600°C 3) If I thought the issue was the clay I would keep trying, but I have worked out a blend of three clays that isn't as nice as porcelain but makes quite good pots. 4) Correct, and this is what is so odd. I notice now that it started ages ago. I pulled out my test tiles, and noticed that many of them have loads of issues over engobe, even with previous engobe recipes and various glazes. I'm afraid I don't use cones here. I should, I do in London. Ah, my engobe is at least some months old. One of my problems is that I am here for a few months, then spend months in London so I forget things from one stay to another, and this engobe is definitely at least eight months old. Oppur si muove ... It works fine without the oxides, so it's not the engobe. @Kelly in AKI'll try those suggestions, thanks. This blue one has engobe and stencil, but I made a mistake and put too much cobalt - turned out gorgeous. This one had copper and cobalt, pitted badly, but I liked it so much I added a layer of glaze and refired it, and saved it - except for one tiny blow-out in the second firing.

By the way, although it says I'm in London, just now I'm in warmer Nouakchott.

Hello Madeleine, lovely to see you again. Yes, I do fire to 1100°C, but I thought it was somewhere between 01-1.

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....

Happy holidays, and warm wishes to all for a wonderful New Year! A couple of years ago I posted about engobes, but I couldn't retrieve the thread. Different question this time. Using locally found clays, firing at 1100°C, I spray engobe over stencils before bisque firing, then add glaze. The engobe is my own laboriously developed recipe (kaolin 15, calcined kaolin 15, Silica 20, Zirconium carbonate 10, Feldspar 20, Boron frit 5, talc 10, Ferro frit 3110 10, nepheline syenite 10, Borax 5). Don't ask me how this recipe got so complicated, I have forgotten. I tested a dozen different engobes, and all disappeared into the glaze. This one is the best and doesn't usually fall off either. The usual glaze is my glossy clear workhorse (Ferro Frit 3124 60, Nepheline Sy. 15, wollastonite 12, kaolin 10, silica 12 and lithium carbonate 1). This all works well, although the engobe does get faded by the glaze but with no incidents, 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. Nothing is wrong where the clay was covered by the stencil. Iron seems safe, and much more cobalt (1%) also seems OK. Besides, in my last firing I tried an alkaline copper glaze (2% CuCO3), and it washed out the engobe completely into dark blue, while staying green on the stencilled shapes. It all ran like crazy too. Photos attached, of a plate with the plain combo and of various faults. Can anyone think of an explanation? I'm baffled. Why should the oxides have this effect? Thanks! Finished plate that was particularly lucky: Bisqued ware These are messily stencilled over the glaze, but it shows how the engobe gets lost in the clear glaze. This shows a blow-out, a Robin Hopper engobe and clear glaze with a wee bit of cobalt. (2019) A lot of activity happens on the edge of the engobe, but not only. The engobe is mottled when I spray it on still-damp clay. The runny copper one, same set of cups, same stencil, same clay and engobe.