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Problems with engobe - again


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On 1/10/2023 at 10:35 AM, Callie Beller Diesel said:

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. 

This is how I do it. I add the soda ash solution to slip that’s thick like yogurt or pudding and stop when it’s fluid like cream. Adding too much will make it get thicker again. As Callie said, washing soda is often soda ash with other additives/detergents in it so be careful of that one. 

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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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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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3 hours ago, njabeid said:

For some reason my posts seem to be disappearing.

@BabsThanks, I have one - it's just a bit Mario Bros working with clay drying around the edges in minutes. And hands!

@Min I'm being optimistic. You never know, something could work out. Anyway, my pots never ever turn out the same as test tiles.

You may have to become nocturnal.

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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:







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20 hours ago, njabeid said:

5) I wonder why the interiors pinholes so badly. Need to make the surface smoother.


My hunch would be that given there is less air circulation inside the pots during the bisque firing less of the problem materials in the clay had a chance to off-gas. 

Looks like these mug tests are more successful than the previous ones. Local white clay + opacifier on damp clay seems the most successful of the lot with slip only on  the outside better than the ones with slip inside. Is that about right? How much opacifier did you add to the slip?

20 hours ago, njabeid said:

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

If you had a thicker slip laydown with the dipped slip mug then my takeaway from this would be to either dip when the mugs are still damp or deflocculate the slip and brush it on the damp pots.


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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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A little color theory - lavender corrects for too much yellow, so try adding a bit of talc to your cobalt. Talc makes cobalt go lavender. In very small doses, you might be able to color-correct your cream colored slip to appear whiter. Myself, I like the cream, but if it’s not your vision …

I just love your camels!:wub: 
(Maybe a bit longer soak at the end of firing would heal those bubbles. )

Please invest in cones!!! Or maybe an earlier form of testing - draw rings?

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8 hours ago, njabeid said:

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

I would definitely find the lower temperature limit of this. To me it’s definitely melting and even running a bit so firing a cone lower in temp may be just the thing that reduces your pinholes and improves your color rendition. It’s not uncommon for some glazes to have a fired surface tension such that they won’t heal. Drop and hold schedules generally help solve this issue.

Fading color, movement, pinholes, I would figure a way to test glaze fire at a lower temp and look for a trend.

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  • 1 month later...
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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Posted (edited)

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.





Edited by njabeid
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