Slip won't stick

[Pir]

Hi, I scooped up some clay exposed via erosion on a Lake Erie gorge, under which sits shale. It's been sitting (wet) for 2 or 3 years (the idea being that it could become more plastic?). I made a slurry/slip and applied it to greenware, but it fell right off when dry.

Can I add something to make it stick?

Would it be something refractory, i.e., with alumina? EPK or ball clay?

Thanks for ideas.

Pir

[Babs]

How damp was your pot?

Fell straight off...how soon? Dried and cracked off?

[oldlady]

have you ever tested the clay to find out how hot to fire it?   is the pot you tried to cover with this slip made of a different clay?  there are lots of things that need to be just right in order to use a found clay.

[Pir]

Thanks oldlady and Babs,

I knew I'd forget some details! The pot was leather hard, not sticky but nowhere near bone; it is a Laguna standard buff cone 6. 

I dipped it, I probably covered it to let it slow dry, and maybe two days later half the slip coating had neatly slipped off (or did when I lifted the plastic), and the rest fell away at the slightest touch.  (I was trying "mocha diffusion," too, but I doubt that would've made a difference).

I haven't fired this clay yet. Just unsure how to go about it (and I don't own a kiln), but mostly b/c I've been so engrossed in so many other pottery/glaze things.

I'm going to slowly test it, just thought there might be some easy fix (haha).

Sorry, too many details this time.

[Babs]

Maybe try deflocing itwith a bit of Darvan. This means less water in slip as what you describe is that the slip is shrinking much greater than your pot as they dry.

Spritz your pot all over, take your thick slip, add a few drops of Darvan, or sodium silicate till it becomes runny. Drop by drop there!

Apply slip to pot by brushing, maye the dip gave you a very thick layer of slip.

Keep testing  it's fun. Could hit a sweet spot..

[Pir]

Darvan to the rescue again? I'll give it a shot, when I get some Darvan. Will try with sodium siliacate in the meantime.

Thanks!

[Kelly in AK]

Deflocculate, can’t hurt to try a sample that way. Thin some way down too and test by layers. Try mixing a bit 1/2 & 1/2 with your clay body and see how that fits.

Sticking upon application and drying is only part one of the fun. More to come! 

[Pir]

Good idea, thanks Kelly!

[Pir]

Hi, a follow-up question:

I added drops of sodium silicate to my slurry of found clay and it actually made it thicker!

Then I tried with a white slip: 15-20 drips made 1 cup of this slip get thinner (Simple Slip from Britt book, but don't have recipe with me).

Then I tried with a high ball clay slip: after 7 drips it got really thinner.

Then I tried with a white glaze with something like 50% Gerstley Borate... and it got thicker! 

Does anyone know why, and how to predict when, sodium silicate will thin vs thicken? 

Thanks, Pir

Edited October 5, 2022 by Pir

[Min]

1 hour ago, Pir said:

Does anyone know why, and how to predict when, sodium silicate will thin vs thicken? 

When you add too much of it you will get pudding.

[Pir]

27 minutes ago, Min said:

When you add too much of it you will get pudding.

Ah, okay.  I didn't notice that this particular slurry had become thin at all, before it thickened. I did pour off the top later of water from this slip before adding the sodium silicate--I wonder if that made any effect? Does the slurry need some minimum quantity of water? And I still wonder about the glaze with 50% G. Borate; that too never seemed to thin out, but at some point it did indeed, become pudding.

Does sodium silicate just not work with some ingredients?

[Callie Beller Diesel]

Ceramic folks talk about deflocculation differently than, say, a sanitation engineer would, and we are a bit inaccurate in our usage of the term. A fully deflocculated slip in the scientific sense is one that settles out quite harshly, because all sizes of particles are free to move through a solution, so wind up at the bottom of the container faster. What we are after in a casting slip or a glaze is controlled flocculation, but it’s longer to explain that to beginners or people who think they hate science. 

Sodium silicate is an unsubtle tool for adjusting a glaze or decorating slip. If your slurry went straight to pudding rather than flowing, you might have added too much without mixing thoroughly enough first to see the fluid state, or your clay could simply be one that’s already prone to gelling. Darvan may serve you better, because it’s got a wider tolerance for you accidentally adding a bit too much. Remember to only add it a drop at a time to your test cup, and mix for several minutes with some form of blender before adding more. Since you’re testing a found clay, you’re going to want to record how much you had to use to get repeatable results. 

If you’re using sodium silicate on a glaze with 50% gerstley borate, it will indeed send it straight to the pudding state. Darvan is your friend. 

If you want more reading, here’s the digitalfire article on deflocculation in casting slip. If you scroll waaay to the bottom, there’s loads of handy links to a nice deep research rabbit hole if that’s your thing. The same website also has info on testing wild clay.

[Pir]

Thanks so much for that detailed explanation, Callie.

I was whisking, only briefly, between drops of sodium silicate; will try blender next time. And very good to know about the gel-inducing gerstley borate glaze (was very curious).

I've never scrolled that far on that great Digitalfire page!, but will now.

Thanks,

Pir