njabeid
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Posts posted by njabeid
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1 minute ago, Bill Kielb said:
It seems to have worked here. Maybe reset your equipment
What do you mean by reset? If it worked this time maybe the problem is at the other end? I have been trying to send you all un update, and finally got it to upload photos and actually look as if it had been submitted.
So odd. See what happens to this one.
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Hello, why can't I post? I have written stuff twice, and it all disappears
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@Bill Kielb sorry, I’m afraid I don’t use cones here, The discussion has been more about pushing the firing time to get rid of the carbon etc. It looks positive, combined with a change in the white coating.
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@Min I added 10% zirconium silicate.. yes, these mugs are better, so way to go. But I’m not sure about the buff colour of the slip. I’ll go back a step and try the new slower firing schedule with different combinations. Not sure about the pinholed interiors though - the slower firing should have reduced degassing, instead there are now pinholes where there were none. Interestingly there are far fewer and smaller pinholes in the coffee cups with no slip, same glaze inside.
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OK, so the firing is done, and results and feelings are mixed. Somehow I feel lost and have forgotten where I was going.
Anyway, the cups and mugs were fired with Min's schedule, i.e. 110C to 700C no soak. 2) 50C to 900C no soak. 3) 42C to 1000C no soak. They all have the homemade slip ("K-slip).
1) K-slip brushed on while still damp, dipped in GT1 (my glossy transparent). No pinholes but cracks at the bottom, and a piece of slip missing.
2) K-Slip sprayed while damp, sprayed GTI + 0.3% CoCO3 . Kinda OK
3) K-slip + 10% WrSiO2 sprayed damp, and sprayed with GT1 + 3% CoCO3. (Inside is a poured commercial clear with added opacifiers. I was using it up. Lots of pinholes inside in nearly all. Something went very wrong with the pouring in this one.) There's a big kind of burst bubble and one pinhole.
4) K-slip + zirconium sprayed dry, and sprayed GT1. Definitely the best, albeit not any whiter, but very pinhole inside.
5) K-slip with 0.5% CoCO3 sprayed dry, and dipped GT1.
6) I'm afraid I missed writing down the slip part, but it was applied dry and sprayed with GT1 + 0.3 CoCO3. Clearly visible two little blow-outs; a few pinholes.
Coffee cups:
One was dipped in K-slip while leather hard ... big mistake. It almost melted pathetically to a puddle, handle cracked, but the result the front cup in the picture is better than all the others, which were brushed with a hake brush; big mistake: I touched up the edges with the wrong glaze. All sprayed GT1 on the outside and poured opaque stuff inside. Better just have the same inside and out. Even waxed, edges are tricky with two glazes.
So - conclusions:
1) The slower firing is good, I'll stick to it. But we changed two variables.
2) There is still a small issue with cobalt. My spray gun is a cheap thing, and makes the slip blotchy.
3) Slip works, but is far from white . Do I like the buff colour? It is too coloured to take colour kindly, but I'm going to try adding some yellow iron oxide to push it a bit further into a more interesting look. Maybe try something besides cobalt.
4) I didn't dip any dry pots - must check whether the stencil will stay on.
5) I wonder why the interiors pinholes so badly. Need to make the surface smoother.
There's more to work on, but thanks to you all I have learnt a lot and am on a track to improvement.
These cheered me up a bit though:
Thanks!!
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@Babs I have one, it’s super useful, but the rate of drying is mind-bending, Including one’s skin!
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It's the Sahara. Look up Nouakchott.
Yesterday I wrote a post that doesn't seem to have been uploaded. It was about the ongoing tests. Various methods of applying the home-made slip, and various glazing methods and additions are all now cooling in the kiln. I'm very excited and hope it will not be too disappointing. True, several variables are changed at the same time (firing schedule, slip, additions, application methods, but it will be interesting and open up new avenues of research.
This was the bisque fired pots, looking good.
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12 percent r.h. today!
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7 hours ago, Babs said:
Epsom salts flocculates . Is that what you are after.
Soda ash and sodium silicate deflocs..just saying
Thanks! I'm aware of that. Just responding to Min's mention of Epsom salts. It's the deflocs I'm after now. Let's see how the slip holds up - maybe I won't need to deflocculate.
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@PeterH yes, my mind was wandering along the lines of a chemical way to get rid of the carbon / organic stuff, thinking « caustic soda, but not practical for one’s hands »
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@Rae Reich no, the runny glaze was extremely simple, no rutile. I do like to add a pinch of rutile though
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@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.
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Great explanations. Actually the question is how much sodium carbonate should one add to slip to deflocculate it?
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On 1/8/2023 at 1:19 AM, Min said:
I am never ever going to complain about the time it takes me to pick up supplies!
Even in the UK there are glitches: I placed an order of stuff I had forgotten, and could send to London to be brought by a traveler, and it was delivered yesterday. Strikes + Xmas… traveler already flew. The post here is a joke. My cousin in Canada insists on sending Xmas cards - sometimes they arrive at Easter, now and then the following Xmas!
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@Min sorry I was slow to absorb your previous post. Now, with all the precious advice from all, and the sources recommended by Callie, here is a summary of what I have learned:
1) Problems with the body clay - it has all the flaws in the book, so need to clean it up in bisque firing to prevent degassing, by slow firing. More about that anon.
2) Problems with the engobe - zircon hampers adherence to the body, talc produces gas, so either the recipe must be tweaked (the first attempt removing borax and borax frit, decreasing FF3110 and increasing zircon resulted in smaller more numerous flaws), or slip is used instead. Tests are ongoing with the only whitish clay available. A suggestion to colour the engobe or slip is the next round of tests. If this slip works, even though it isn’t white I’ll adopt it.
3) Problems with the glaze - although the problems occur when there are colouring oxides in the glaze, it seems to be caused by the lower glaze melting temperature, something I hadn’t understood. My glazes behave well on their own but don’t heal over an exploded engobe. Interestingly Tony Hansen says ZnO, SrO and Li2O can have a dramatic effect on glaze flow, which affects pinhole healing. May explain my observation that strontium in glazes seems to reduce problems.
So to the firing schedule. The one you sent has 6 ramps and takes 26 hours!. Our electricity isn’t cheap but at least we don’t have many power cuts now in winter and they are short, so that’s not a worry. My current schedule totals 10:40 hrs. With my two segments I could for instance:
- @80 to 700, then @60 to 1000, total 13:40 hrs, (soak at the end?)
- @80 to 700, then @50 to 1000, total 14:40 hrs, or
- @80 to 800, hold 1 hr (?), @80 to 1000, total 13 hrs or more with a longer soak
which do you think would be best?
maybe I should also modify my glaze firing schedule to slow it at the end? Now it is …
I just went to check and discovered there are actually 3segments! ( modify the above schedules?)
@80 to 90, 1 hour hold, @180 to 600, @120 to 1100, 20’ hold.
Should I take the @180 up to 900 then slow right down?
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On 12/31/2022 at 2:01 PM, Rae Reich said:
Possibly irrelevant: the super-fluxed copper glaze screams ‘rutile’ to me.
@Rae ReichLooking back on the thread, I’m intrigued by this observation. Can you elaborate please?
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@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.
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@neilestrick Thanks. I think I'm doing all of those at once now...
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@Magnolia Mud Research I wonder how much that would be per weight of slip or of dry clay before making the slip?
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@Callie Beller Diesel Thanks, I'll check all that out. I'm a DigitalFire fan/client already, Tony is incredible and I look up Insight for glaze calculation.
One problem is that here I have a small electric kiln and I got a new controller that only does two simple programs (2 segments, with holds possible for each), and so can't do a slow cool. Looking closely at the mugs, the pinholes are tiny blow-outs, so I'll work on your suggestions. I bisque fire @80°C to 600 then 120°C to 1000°C. How slow or fast is that? I end with a hold for glaze firing, but hadn't gone into details about the clay.
However, (obstinately) the problems are all with engobe, and more specifically with oxides in a glaze over engobe.
There are treasures of advice in this thread, and I hope to work on all of it and come back with some really nice results.
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@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;
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- Kelly in AK and Rae Reich
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2
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Problems with engobe - again
in Clay and Glaze Chemistry
Posted · Edited by njabeid
Oh well, here's another try.
Dear Wise potters, thanks! You were right: after tweaking the engobe recipe, using local whitish clay as slip, slowing the bisque firing and also slowing the glaze firing now my pots have only some very small pinholes.
That involved some protracted procrastination, while I indulged in other techniques, and then 'forgot' to add cobalt to the glaze. That's now done.
Three cheers! and thanks again.