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Ginny C

Slip On Greenware

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I keep seeing photos of potters dipping a finished pot, greenware stage, into a bucket of slip.  I'm wondering what the various reasons for doing this are.  Obviously, a white slip over brown or red clay allows a nice result when incising a pattern or design. And of course designs can be put into the slip while it's still wet, with a finger.  But are there other reasons to use an overall slip? 

 

Just curious!

 

 

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You can get a better color response from some glazes on a white slip than a dark body clay.

 

I do this a lot when I work with black clay. Although I haven't worked with it in a long time. If I used a dark body then a white slip over the pot, it would have nice color, but still that rich undertone, and I could always scratch the white slightly to make it look the way I wanted. I should really go back to that work. I enjoyed dipping and pouring entire pots in slip.

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The slip is often a much higher quality material than the underlying clay body, with correspondingly better overall results for the fired piece.  

 

Porcelain slip, for instance, will usually be:

 

  • cleaner (less iron, titanium and other trace contaminants),
  • finer (smaller average mesh size of particles) and
  • smoother (better application and lay down)

than the underlying handbuilding or thowing clay body you are using in the making. 

 

This kind of a material gives an exceptionally good surface on which to apply glaze.  Simply put, the glaze just looks better in almost every way.   Colors are stronger, truer and more vibrant against the whiter backdrop (important when testing glazes).  Glaze surfaces are much more even and smooth, particularly when examined in any kind of decent lighting, enhancing their functional appeal.

 

Of course this presupposes that is the kind of aesthetic you are looking for.  You could just as easily use a dirty, irony slip and get completely different results.  All about what you want!  :rolleyes:  

LeeU and High Bridge Pottery like this

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So can any slip be used on any clay, or is this a really tricky thing depending on expansion rates etc? Does the slip need to be chemically formulated or will throwing slip do? For example - I have a bag of porcelain clay (powder), could I add water to make slip and apply it to a white stoneware and/or earthenware body, to get the whiteness and smoothness of porcelain. Does the addition if slip in this way, change the firing temperature of the piece - at the bisque stage and at glaze stage?

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Celia, Yes, use your porcelain powdered clay, mix with water, allow to soak for a while and use a blender or immersion blender to incorporate.  Sieving is a good idea.  Thin with sodium silicate or not depending on the look you want to achieve.  Be certain to match the cone requirement of both clays.  As mentioned, the glaze colors are brighter over the porcelain slip than over stoneware. 

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and, celia, apply the slip at the right time, after the wet sheen has left the clay when thrown or when the handmade piece is stiff enough but not really leather or ;)  cheddar-cheese hard.

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Or if your like me and have like 10 random bags of different white clays and porcelains sitting around your studio that you never use. Cut the pugged clay into chunks, throw it in a bucket fill it to the top of the clay with water. Let it sit like 2-3 days, take a drill mixer and mix it up, instant slip. If its too dry add more water, if its to wet leave it open to dry. Easy way to use any extra clay you dont work with anymore. 

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Celia, if the slip and clay body have similar shrinkage, as well as the firing range, it shouldn’t crack or pop off the pot when fired. Also, the glaze now has to fit both the slip and the clay body so if you don’t want crazing or shivering COE's should be similar also.

 

If you are dipping the outside then I would pour the inside first, let it dry for an hour or so then dip the outside, if the pot gets too wet it is prone to splitting apart. (been there, done that) Dip it quickly so it doesn’t soak up too much water, also, having a nice substantial foot ring to hold onto while dipping the pot into the slip helps too. Brushing is safer but I think you get a more even coverage with dipping (if that's what you are going for).

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As long as the shrinkage rate (both dry and fired) are close (say within a percent or two), you should be okay. Ideal application time varies from clay to clay. I apply my porcelain slip to stoneware at any point before I attach handles, and I prefer to do that at the cheese hard stage so the mug doesn't distort. Push it and see what happens.

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If you only want to slip the interior, then I apply a thin coat of water to the outside and foot, flip it over and brush the slip onto the interior of the bowl, on the wheel.  Needs to be the right moisture content of pot as indicated otherwise it may look ok after a bisque and bloat away from the pot after the glaze firing. FInd if I use Sodium silicate to thin to the "right" thickness for application it is a better fit.

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I have throwing slip I want to use (on greenware of the same clay) but I don't have any sodium silicate...does anyone know if adding vinegar & Karo syrup (like for the mending agent called  "Spooze") would help with adhesion? 

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Sodium Silicate prob not necessary, I use it because it eases the fit of the two IMO. Tas staed above the stage of leather hard piece is crucial. I moisten the  whole bowl with a brush lightly watered ,on the wheel, then apply the slip where I want it.

Have a  go, good luck.

Some people apply the slip when the pot has just been thrown also.

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I forgot to 'follow' is thread when I posted my question - have just picked it up again.

All very logical responses, but matching the shrinkage rates and cones of both clays assumes these are all known factors! Need to do some research here - especially as the bag of porcelain was a 'freebie'.

Many thanks for all the above info!

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Another reason is some clays throw way better than others and the form you want is going to take a really great throwing body but the surface you want is say a white porcelain. Throw the pot from that groggy throwing body and cover with a pure white slip.

All the above posts cover the shrinkage comparability issues well so I will not get into that. But you can see many reasons for wanting another surface other than the clay you are working with.

GiselleNo5 likes this

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I do this ALL the time. I love working with dark speckly and otherwise interesting clay, but I also want bright vibrant color from my glazes. I use just regular old B-Mix slip over various other Laguna Cone 5 clay bodies.

 

I've never had a problem with bloating, chipping, flaking, nothing. It's funny, I started doing this almost a year ago (I thought I was being SO original too) and I didn't know all the things that could go wrong or I would have been too afraid to try it. The fearlessness of inexperience has worked out and I'm glad I took the risk! 

 

Usually the day after throwing, I trim, add handles, and THEN apply the slip to any desired areas. Otherwise it turns to mush. Adding the slip at that point also will slow down your drying process by a day or two ... I've used this to my advantage when I allowed a piece to get a wee bit too dry for handles. A coat of slip to the inside of the mug, put it in a bin and the next day it's perfect. :D 

 

Pugaboo and LeeU like this

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Using Sodium Silicate means the slip becomes more fluid with less water than would have to be used to get to that state. This means there will be less shrinkage due to water evaporation.

Slip is silkier.imo

You use a very little, add drop by drop.

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I loved using slip, but haven't in a while because I found it was a pain to set up. I think I bought some already made, but then had to sieve it, and it got funky fast. Is there a simple way to make/keep slip workable? (Sorry for the hijack, but thought this might be a question others have, too!) 

Nancy

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