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Hi all,

Second time posting here and I’m hoping you can help! Hopefully this is the right place to be for a troubleshooting question like this.

Im currently working from a recipe for Val Cushing’s Cone 6 Porcelain Slip. 

grolleg: 15%

tile 6: 15%

Epk: 3%

om4: 10%

flint: 25%

neph sy: 30%

gerstley borate: 2%

——————————

Sodium silicate: .0028%

calgon: .0005%

When I mixed this before, as now, I subbed the deflocculants with Darvan. I mixed it successfully then, but I’m currently have difficulty. 

I know that darvan generally subs for sodium silicate at 2-1, so bearing that in mind, that’s where I started. I kept adding darvan in small 5g increments up to 100g in a 10000g recipe. That seems like far too much, but maybe I’m wrong?

 I feel like I’m on the other side of that deflocculation bell curve some how and missed my mark. Every time I come back to the studio the slip is gelled thick but never settled and never develops a skin.  When I’ve added more darvan, the slip temporarily thins, but seems to rebound to being too thick the next day.

Can the casting process indicate overdeflocculation? If so, in what ways? What should I be looking out for other than settling and a skin in my bucket? Can anyone offer any advice on this recipe or deflocculants? I’ve been doing some hard research and testing and can’t nail it down.

My current gravity is measuring at 1.71, so I’m hesitant to add more water, since according to some other online sources I should be aiming between 1.7 and 1.8. I’m wondering if I should throw caution to the wind and say “this slip just needs more water than most”.

Ive mixed this slip before with great success two years ago. I have no clue what I did right then. Dumb luck haha. Any suggestions or advice would be greatly appreciated. Thanks!

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On 10/17/2019 at 9:12 PM, cmayse said:

Ive mixed this slip before with great success two years ago.

This could be the key to the problem. Darvan has a shelf life so it could just be the efficacy of it has deteriorated. 

On 10/17/2019 at 9:12 PM, cmayse said:

I know that darvan generally subs for sodium silicate at 2-1, so bearing that in mind, that’s where I started. I kept adding darvan in small 5g increments up to 100g in a 10000g recipe. That seems like far too much, but maybe I’m wrong?

I work it out to being 56 grams being the ballpark figure you would need so I think this probably confirms the Darvan is shot.

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2 minutes ago, cmayse said:

It’s shot? Does darvan expire!? 

Wow. I just read it has a two year shelf life. I’m going to feel pretty dopey if that has been my issue this whole time haha. 

Thanks for the advice

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Try a tiny bit of it in some fresh slip and see what it does. I'ld try about a cup of slip then add some Darvan drop by drop and stir it up and look for a change. I know they say 2 years but I've got some that is older than that and it still works but perhaps yours was getting old when you bought it. Does it have a manufacture date on it?

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Somewhere in the internet world is an article that describes what happens when you add not enough/ just right/ too much deflocc.  I'll see if I saved it.

Found it:  https://static1.squarespace.com/static/527ac372e4b0d4e47bb0e554/t/527fd7f1e4b0c046bfa9b90d/1384110065234/Dispersant+Addition+Procedures.pdf

Edited by Chilly
Typo

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On 10/19/2019 at 1:22 PM, Min said:

Try a tiny bit of it in some fresh slip and see what it does. I'ld try about a cup of slip then add some Darvan drop by drop and stir it up and look for a change. I know they say 2 years but I've got some that is older than that and it still works but perhaps yours was getting old when you bought it. Does it have a manufacture date on it?

Tried fresh Darvan and that didn’t work

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15 hours ago, Rae Reich said:

Almost sounds thixotropic. Try shaking the container without stirring and see if it liquifies.

It does become slightly more liquid when I stir. Not quite liquid enough. But it gels again within a literal couple minutes. 

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I have never used sodium silicate but when I use Darvan I have used 0.2% And 40% water, both of the weight of dry ingredients. That means I multiply the dry ingredients times .002 to get the amount of Darvan and .4 to get the amount of water. So for 10,000g of dry material I use 4000g water and 20g Darvan (add Darvan to water, mix well, then add dry material). This is just starting point and the day after I mix the slip I measure the specific gravity and observe the viscosity of the slip to see if it needs adjustment.  It sounds like you slip could possibly be over-deflocculated. Another thing to consider is the way gerstley borate gels, but I'm not sure 2% could have that much impact. 

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16 hours ago, cmayse said:

Tried fresh Darvan and that didn’t work

New slip and Darvan and it's not working?  

Symptoms of of over deflocculated slip from Digitalfire: "If the slip does not gel at all, or settles out in a layer on the bottom of the container then there is too much deflocculant. Too much deflocculant is also indicated by a thin slow cast, a wavy and gritty looking inner surface after draining, rapid formation of a skin on the slip after the mixer is stopped, poor mold release, splitting and cracking of the ware during drying and brittle ware."

From your first post: "Every time I come back to the studio the slip is gelled thick but never settled and never develops a skin."

What's the  viscosity of it? How are you mixing the slip ?

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