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Hi all:

I've been playing with defloculated slip as decoration. We have a prepared bucket for use in the studio and I've been shown how to dissolve some epsom salt in a bit of water and then drip it into a cup of slip to thicken it up.
But the cup preparation only achieves the thickness of creamy yogurt - I was hoping for something closer to that of a plaster. As suggested, I added more defloculated slip to the cup and it did thicken it up some but never got past that creamy consistency. I tried adding some epsom salt crystals directly to the mix bit it didn't seem to thicken it up any further. The instructor said that the defloculated slip was fairly old and he may have to add sodium silicate to revive the slip.
I don't completely understand the chemistry but was hoping for some suggestions to thicken up the texture of the slip I want to use. At least until the bucket of slip is reincarnated...

- Jeff
 

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Epsom salts work as a flocculant so what’s happening is it’s making the slip look thicker but it’s not changing the ratio of solids to water in the slip. To make a thickened slip for trailing or raised slip decoration you need a slip that is low in water content so it doesn't crack or flatten out while drying.

Darvan is a deflocculant, as is sodium silicate (which is usually used in combination with soda ash). Adding a deflocculant will make the slip “thinner” and more watery looking so you then can add dry claybody to thicken it up. 

Add some Darvan to the slip so you can increase the clay content in the slip without increasing the water content. You just need a tiny bit of Darvan, depending on the amount of slip you have, for a cup of slip a few drops up to 1/4 teaspoon of Darvan should be enough. Give the slip a stir after adding the Darvan and the slip will go "thin" and watery looking. Now add as much powdered dry claybody as you need to get it to the thickness you want.

Darvan causes the clay particles to repel each other so even though you haven’t added any more water the slip will be “thinner”. With epsom salts the opposite happens, the clay particles are attracted to each other therefore the slip will thicken. You can use sodium silicate plus soda ash in place of Darvan. (same principle for using a flocculant or deflocculant in glazes)

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Thanks Liam and Min - I think I'm wrapping my head around the chemistry a bit better with your explanations. What I realize now is that by adding more epsom salt solution or more defloculated slip wasn't working because I wasn't changing the ratio of clay to water.

Questions:
- If I need to add powdered clay body to get to a thicker slip, can't I just add it to the creamy slip I've just created? Do I have to defloculate again, ie with Darvan, before I add additional powder?
- I don't  believe we have any powdered clay body on hand at our studio - can I just pulverize some greenware I have?

Thanks again for the education - really fascinated by the details of what we create!

- Jeff

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At this point, If you need a very thick slip the consistency of plaster and have no powdered clay to adjust your existing batch, I’d just slake down some of your greenware and not add any flocculant or  deflocculant. Keep the water level low. There comes a point where if you’ve added too many things to adjust your consistency, you’re better off starting from scratch. 

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3 hours ago, JeffK said:

If I need to add powdered clay body to get to a thicker slip, can't I just add it to the creamy slip I've just created? Do I have to defloculate again, ie with Darvan, before I add additional powder?
- I don't  believe we have any powdered clay body on hand at our studio - can I just pulverize some greenware I have?

Yes and yes.

edit for clarity: yes, add some Darvan plus pulverized clay to the existing bucket of slip.

Edited by Min

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One thing I will add to the discussion about flocculating and deflocculating a slip (or glaze) is the chemical reactions are reversible. If you mistakenly over-flocculate, you can just add some deflocculant to thin it out again, and vice versa. In your case of wanting a thickened decorative slip, the issue is complicated by shrinkage on drying. A slip that has been flocculated to thicken it so it stands up on the surface of the vessel still has the same clay-to-water balance, and will shrink and crack as it dries. It is better to create a slip that has more clay in the same amount of water. The problem is that you can't put enough clay into the same amount of plain water to accomplish this as it becomes too thick as mud to stir. You then deflocculate the slip and it instantly turns runny, and now you can add more dry clay. You must add dry clay, not more slip. You are trying to raise the amount of clay in the existing water, and adding more slip adds more water at the same time as it adds clay.  So, as Callie suggests, take some dried greenware made from that same clay, crush it to powder (inside a doubled plastic bag to control the dust) and add it to the developing slip until it finally is the thickness you want. Be sure to sieve it to get all the lumps out.

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Okay - I'm getting confused.  You've all given me some great insight into the chemistry. But that's getting lost in the sauce - so-to-speak.
We have a 5 gallon bucket of defloculated slip in the studio. I don't touch the chemistry of that. That's up to one of the studio managers. What I can do for myself  is take that defloculated slip, put some into a pint container, add epsom salt solution, and thicken it up. But it's not as thick as I want.

I can then take that creamy slip I just created in that pint container, add some clay powder (that I can get from pulverized greenware I have handy) and thicken the creamy slip to what I want. Correct?

- Jeff

Edited by JeffK
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Jeff,

Take that deflocculated slip, that your studio has, that part is correct.  But DO NOT add the epsom salt, which will just flocculate it, reversing what you want to do.  All that is doing, is giving the impression it is thicker.  It is still the same ratio of water to clay, which is why it will shrink a lot, as that water evaporates off.  

Instead, just take some bone dry pieces, of whatever claybody is used to make the deflocculated slip, crush it up into a powder, and add it to that slip, until it gets to the thickness you desire.  Because you are adding more clay to the ratio, it shrinks way less, and is also less likely to crack.  This is also why potters use this as a "joining slip" for connecting pieces.  

But once again, do not add the epsom salt!

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Trimmings from the foot turning  process gives sem i dried, quick to really dry, source to add to the slip. Can be run over with rolling pin after being placed in plastic bag for instant stuff!

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OK - got it now. Defloculated slip, NO epsom salt. Add powdered clay. Get thick consistency I want.

Thank you all for your patience - so much to learn!

Now I'm off to the lab...er...studio! :)

- Jeff

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Another way to achieve the same thing as adding powdered clay (if you don't have any) is to evaporate some of the water from the deflocculated slip. A wide, low container will give more surface area to evaporate the water and achieve the desired consistency. 

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7 hours ago, tinbucket said:

Another way to achieve the same thing as adding powdered clay (if you don't have any) is to evaporate some of the water from the deflocculated slip. A wide, low container will give more surface area to evaporate the water and achieve the desired consistency. 

All tips and resources greatly appreciated!

- Jeff

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So in the studio the other day, I crushed up some unfired greenware and turned it into powder. I then took about a half cup of deflocculated slip and started adding powdered clay to it. Mixed it as best as I could but ended up with a watery lumpy mess. Just for the heck of it, I flocculated the slip with some epsom salts and tried adding the powdered clay to that. Same lumpy result.  Could be that I need to find a better way to mix the powdered clay in but I wonder if this is becoming far too labor intensive - maybe there's another way to achieve that "plaster" type consistency. Might try the last suggestion at letting some water evaporate out of the flocculated slip.

Ah well - back to the pottery lab....

- Jeff

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2 hours ago, JeffK said:

So in the studio the other day, I crushed up some unfired greenware and turned it into powder. I then took about a half cup of deflocculated slip and started adding powdered clay to it. Mixed it as best as I could but ended up with a watery lumpy mess. Just for the heck of it, I flocculated the slip with some epsom salts and tried adding the powdered clay to that. Same lumpy result.  Could be that I need to find a better way to mix the powdered clay in but I wonder if this is becoming far too labor intensive - maybe there's another way to achieve that "plaster" type consistency. Might try the last suggestion at letting some water evaporate out of the flocculated slip.

Ah well - back to the pottery lab....

- Jeff

When I make slip, I've found that covering the crushed, dry clay with water, then leaving it untouched for several hours is the best way for it to absorb (slake down).  The more water, the better.  

That doesn't help you, but I'd try adding it to the existing slip and walk away. Don't stir, touch, poke.  Don't even look at it.  Just let it do it's thing, leave it alone overnight.  Then see what happens.

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@JeffK, how did you mix the powdered clay in? An immersion (stick) blender works well if you have one of those. If the powdered clay contained chunks then it's going to need to sit for a few hours. Maybe a video would help show the process.

 

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1 hour ago, Chilly said:

When I make slip, I've found that covering the crushed, dry clay with water, then leaving it untouched for several hours is the best way for it to absorb (slake down).  The more water, the better.  

That doesn't help you, but I'd try adding it to the existing slip and walk away. Don't stir, touch, poke.  Don't even look at it.  Just let it do it's thing, leave it alone overnight.  Then see what happens.

I'm trying to keep the water down to a minimum or not at all. But this is a possibility! Thanks!

- Jeff

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@Min - extremely helpful video. That thick bucket he has at the end of the video is what I'm trying for. What I tried to do was just create a small amount - about 8 oz - in a cup to  see the result. I was using my hand to do the mixing and that might be part of the problem.  That stick blender would work but don't have one available right now. But will try again and let you know results. Thanks again for the help!

- Jeff

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