Problems with engobe - again

[njabeid]

@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.

[njabeid]

@Magnolia Mud Research I wonder how much that would be per weight of slip or of dry clay before making the slip?

 

[PeterH]

42 minutes ago, Magnolia Mud Research said:

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 mixture.  

LT
 

Are you sure?

[neilestrick]

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.

[njabeid]

@neilestrick Thanks. I think I'm doing all of those at once now...

[Babs]

Ha, one bag of that clay would make a lot of slip, just saying!-)

Trade routes for centuries throughout , would make that authentic, right?

Handluggage, right. For your face mask beauty treatment.

[njabeid]

@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. 

 

[Min]

1 hour ago, njabeid said:

I bisque fire @80°C to 600 then 120°C to 1000°C. How slow or fast is that?

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."

 

[njabeid]

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?

Edited January 10, 2023 by njabeid

[njabeid]

@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?

 

[njabeid]

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!

[Magnolia Mud Research]

20 hours ago, njabeid said:

I wonder how much that would be per weight of slip or of dry clay before making the slip?

my assumption is that you are looking for the amount of sodium in the slip.  the amount of sodium in:  sodium bicarbonate is Na-H-CO3  (molar mass = 84).  sodium carbonate is Na-Na-CO3 (molar mass =106) therefore:  

to get the same amount of sodium you need to 168 grams of baking soda to get the same amount of sodium if you used 106 grams of sodium carbonate. 
In other words you need 1.58 times of sodium bicarb instead of than the amount of sodium carbonate in your slurry recipe. 

All of this is based on the assumption that both the sodium carbonate and the sodium bicarbonate are "dry" solids; that is they have no water attached to the molecule.  Sodium carbonate is more likely to absorb water than baking soda but both can /and will/ pickup water and still be a "dry" solid.  Both are white powders that will absorb water vapor from the air if not stored in a sealed container.   

I have no useful insight if the reason for using sodium carbonate is adjust the pH of a slurry to control suspension of the solids.  I seldom care about suspension as I apply "paste" instead of "slip" on surfaces.  I add the sodium to get the effect of the sodium in the fired surface.  

Regarding @PeterH question: yes. long ago I began using a water spray of sodium carbonate as a means of controlling the fired effect of surfaces.   
I ran out of sodium carbonate but had baking soda in the cupboard and  used it instead; saved a trip to the store and the results were equivalent.  I have been using baking soda more often as it is less damaging to my skin and is available at the grocery store near home.  

A footnote: once the sodium carbonates are dissolved in water they are no longer "sodium carbonates" but are sodium ions and carbonate ions.  the pH is determined by the total collections of ions in the liquid phase.  Soluble ingredients other than the sodium carbonates also must be considered.  

LT

[njabeid]

Great explanations. Actually the question is how much sodium carbonate should one add to slip to deflocculate it?

[Min]

2 hours ago, njabeid said:

- @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?

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.

 

[Callie Beller Diesel]

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. 

 

[njabeid]

@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.

 

Edited January 10, 2023 by njabeid

[Babs]

@Min Not having read your posts closely, would a firing 100deg/ hr to 600   6hrs

Then slow through to 800°, say, 80° /hr 2hrs. 20mins ish

with a  soak

then 150°/hr to 1000°C   1hr 20min

which gives a more economical firing schedule.

Just thinking of a compromise to lessen electricity usage 

[Rae Reich]

7 hours ago, njabeid said:

@Rae ReichLooking back on the thread, I’m intrigued by this observation. Can you elaborate please?

Rutile on a runny clear glaze often creates those opaque falls of cream/pink/blue. Thought you might have done some decorating with it. Does your runny copper glaze have rutile in it?

Edited January 10, 2023 by Rae Reich

[PeterH]

3 hours ago, Magnolia Mud Research said:

Regarding @PeterH question: yes. long ago I began using a water spray of sodium carbonate as a means of controlling the fired effect of surfaces.   
I ran out of sodium carbonate but had baking soda in the cupboard and  used it instead; saved a trip to the store and the results were equivalent.  I have been using baking soda more often as it is less damaging to my skin and is available at the grocery store near home.  

We seem to be talking at cross purposes. I was commenting on the use of sodium carbonate/bicarbonate for adjusting the rheology of a clay slip. While you seem to be adding sodium carbonate/bicarbonate to the surface of a pot then heating it: decomposing them to sodium oxide which eventually forms part of a "salt glaze".

Min (with a like from Callie) commented on another thread
How was your soda ash stored? If not kept sealed up in an air tight container it will change from sodium carbonate into sodium bicarbonate, which is a flocculant.
... and I was happy to take her/their word for it.

PS
This point has since amplified by quoting from http://www.marjonceramics.com/pages/Tips/slipmaking.htm
Soda ash works to dissolve lignite in clay.
... I've no idea of the details of this mechanism, which sounds more complex than the simple addition of Na+ & CO3-- ions to the liquid. I've only got as far as https://patents.google.com/patent/US3325537A/en
The humic acid content of lignite may be solubilized by treatment with alkalies, such as sodium hydroxide and sodium carbonate.

 

[njabeid]

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

[njabeid]

@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 »

[Min]

@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. 

 

Edited January 11, 2023 by Min
added a link and last paragraph

[Callie Beller Diesel]

Products that are sold specifically as washing soda usually have other ingredients added to them that will really mess with your glaze bucket, so it might not be the end of the world.

Can you get your hands on some Dispex from any of the places you get materials from? Because that would be the ideal stuff for the job, and a little goes a long way. Even if someone has to bring a 1l bottle in their checked bag, it’ll last you a looooong time. 

[njabeid]

@Min

I always use Epsom salts, but so far have not felt the need to deflocculate anything. I'm sure I'll find something.

[Babs]

Epsom salts flocculates  . Is that what you are after.

Soda ash and sodium silicate deflocs..just saying