Flocculation Problem / Advise needed

[Rick Wise]

I recently flocculated  my standard glazes with epsom salts for the first time.  Happily, there was a noticeable improvement in the way they covered the bisqueware and in their resistance to "hard panning".  In my enthusiasm, I then added some of the same epsom salt solution to my Ash Glaze even though  it never settles out much and did not really need flocculation.  To my surprise the only change has been that it now settles out much MORE than before.  In other words, my attempt to flocculate appears to have backfired with that one glaze.  (The glaze is just red earthen ware slip and wood ash, 50/50)

What happened!?!!

[Rick Wise]

Uh .... that would be "advice needed".

[liambesaw]

You cannot flocculate an ash glaze as far as I know.  Ash is full of calcium oxide CaO (caustic lime), and adding epsom salts MgSO4 in water creates a new insoluble salt, calcium sulfate aka plaster of paris.  This explains your new hardpanning precipitate 

[Rick Wise]

Ooops. 

Any way to "undo" this?

[liambesaw]

1 minute ago, Rick Wise said:

Ooops. 

Any way to "undo" this?

Can try sieving out the chunks

[liambesaw]

2 minutes ago, Rick Wise said:

Ooops. 

Any way to "undo" this?

Also this is assuming you used unwashed wood ash, not an ash substitute or washed wood ash, which will be mostly calcium carbonate instead.  

[Callie Beller Diesel]

You can definitely over-flocculate a wood ash glaze. They come already flocculated through the solubles, like the significant amounts of sodium and potassium that are also present. It’ll be worse if you’ve washed your ash first, but it’ll still happen with the washed stuff. When you added the Epsom salts to it, it’s now over-flocculated, hence the gelling and rapid settling. I’ve never seen wood ash turn into plaster, but I can’t claim to have seen everything.  You might be able to rescue it with a dab of deflocculant like Darvan, but I’d test it on a small quantity before committing to the whole bucket. You may find that the glaze takes a lot longer to dry if that works  

Flocculating glazes is only really necessary if they’re settling so rapidly you can’t get a good application. Usually any glazes like shinos or wood ash glazes, things with lots of clay and/or soluble ingredients don’t need it at all. 

[Rick Wise]

No, the ash was "washed" so most of the lime was gone.  But then "washed" is kind of relative, right?  There's always a little more lime to get out.

[Callie Beller Diesel]

You won’t hurt it by sieving it, and if there’s no chunkies, it’ll give you information either way. 

[liambesaw]

Yeah, and formation of calcium sulfate is just theoretical, I don't know if this reaction would take place in a glaze bucket, or if it would require heating or pressure.  

Plenty of calcium in washed ash, positive ions, flocculant.  Unwashed ash plenty of calcium, but plenty of sodium as well, may be tipped either way, and definitely prone to change over time.

[Callie Beller Diesel]

Was curious about the relative solubility of calcium sulfate, so checked Wikipedia. Apparently its very soluble when its cold, and gets less so as it's heated. It's kettle scale.

https://en.wikipedia.org/wiki/Calcium_sulfate

[liambesaw]

9 minutes ago, Callie Beller Diesel said:

Was curious about the relative solubility of calcium sulfate, so checked Wikipedia. Apparently its very soluble when its cold, and gets less so as it's heated. It's kettle scale.

https://en.wikipedia.org/wiki/Calcium_sulfate

Cool!  Would make a decent cold weather dessicant then!  I have a bucket of calcium chloride in every windowsill right for that exact reason