Slip Casting Onto Dried Clay

[sungod3k]

Hi,

 

Im a 3d printer by trade and working on printing direct claymodels.

To make the surface more appealing I thought about covering the dried models with slip and leaving it there. The dired clay should have similar water absorbing potential as plaster of paris I assume and once fired clay and slip should be bonded enough to make permanent object.

 

Any ideas or suggestions about this proposed techique?

 

Cheers

[Chris Campbell]

More info please ...

Sounds like you are trying to layer slip over dry clay ... Letting it dry ... and assuming they will bond permanently?

Where does the plaster of Paris fit in?

How dry will the first object be when you try to cover it with slip?

Is the slip the same composition as the original form or some other type of clay?

 

Everything matters ... and chances of success rise when the slip is the same as the original clay.

[sungod3k]

 

 

Sounds like you are trying to layer slip over dry clay ... Letting it dry ... and assuming they will bond permanently?

Yes exactly. The plaster is just a comparison to dried clay not part of the process. 

 

How dry? as dry as necessary. To achieve the water absorption (comparable to plaster) I´d think it would to be rather dry.

 

Composition: assuming that the powdered clay for the slip is some form of normal clay, I would certainly not try to match it as much as possible.

[Benzine]

Chris, it sounds like they were just using plaster as a point of comparison.

 

My question is, are you talking about dried clay, or fired/ bisqued clay.  Bisqued clay is porous and will absorb some water, but slip will likely crack and fall off, due to it shrinking.  Slip on an unfired piece also has a chance of it cracking as well.  Ideally, slip is applied around the leather hard stage, at the latest.  That way it shrinks along with the clay body itself.

[sungod3k]

I thought about unfired, just dried clay.

If you say slip tends to crack while drying, how do the normal forms/molds handle that? or would the cracking also happen on plaster molds if you would leave them on permanently?

[Benzine]

With normal plaster molds, the slip is poured into them. So the clay pulls away from the mold, as it dries. The slip would crack, if you poured it over the plaster, and let it dry.

 

What exactly are you looking for in the surface? Do you want it smoother, a different color, etc?

[Callie Beller Diesel]

I'm assuming you're using clay powder mixed with some kind of resin or glue in your printer?

 

Typically when you want to apply slip to something, you want to do it when the item isn't too dry at all yet. The reason being that clay shrinks as it dries, and the more water content, the more this effect is pronounced. If the pot and the slip shrink at different rates, ie the wet slip looses more water (and shrinks more) than the dry pot, the slip can crack and pop off. The trick is to have them dry at roughly the same rate. Slip will certainly separate from plaster as it dries. in slip casting, the clay shrinks away from the walls of the mould, making release much easier.

[PeterH]

Im a 3d printer by trade and working on printing direct claymodels.

To make the surface more appealing I thought about covering the dried models with slip and leaving it there.

 

I'm having problems understanding what you are trying to do, can you elaborate a bit?

- What's wrong with the as-printed surface: body-colour, body-texture, 3d-printing artefacts, ...?

- Are simply wanting to coat your models with a more decorative finish, or trying to add extra 3d-printed detail?

- Are you 3d-printing simple shapes or ones with lots of fine detail [or even worse thin parts, eg wargaming models].

- Presumably you biscuit-fire the models, do you glaze them as well?

 

For a decorative finish on moderately sized objects without fine detail you might simply dip [or paint] the object with a

slip/engobe/underglaze appropriate to its state of dryness [or do so after biscuit firing]. 

[sungod3k]

@benzine: Aha that makes sense and yes the 3d print leaves layerlines 1-2mm thick. I want to smooth them over.

 

@Dieselclay: Not necessarily, powder can be used just with water but also normal clay that is watered down a bit for smoother consistency. No resin as well.

 

@PeterH: The layerlines are the biggest issue. Im trying to smooth out the surface to make it easier to wash. Im mostly aiming fir dishware so little cravesses would attract bacteria and goo. Filling them in with slip was the aim and I would think glaze would be applied later for sure.

 

Engobe I hadnt heard of that. I have to add to the trial and error list. I theory glaze could also fill in the layers but I think that would be a bit to thin. I guess I just have to keep the slip to a rather thin coat (1-3mm) to keep cracking down. For cups for example only the inner side would be important, so if I manage to match the drying rates it shouldnt pull away to much and leave me with the desired effect only on the inside.

 

Or I sell the cracks as feature and not a but

[Tim T]

As you probably know, the word slip is used in 2 ways in pottery:

- slip as in slip casting is when a slurry of clay is poured into a plaster mould, left for a few minutes for the layer next to the plaster to start drying out, and then the rest poured out to make your clay object. This isn't relevant to you

- slip as in slipware is when wet clay (often with additives) is poured or painted over a nearly dry piece of pottery before it is fired, either for decoration or, historically, to make Western earthenware look like porcelain before we know how to do porcelain. Once the slip is applied and the pot has dried again, it is then fired and glazed as normal. This is what you want.

 

Engobe can also be a bit of a slippery term - some people use it for a coating that is somewhere between a slip and a glaze (I think this is the original meaning), whilst others use it as being synonymous with the 2nd meaning of slip given above.

 

Assuming you are happy with the colour of your clay, I suggest the easiest starting point is to water it down to a thick cream consistency and apply that. Use of a different clay may cause problems due to different shrinkage rates between the clays. People do this at different stages of drying of the item, but if it is too wet when you apply the slip the clay may get too wet and collapse, or if too dry the water will make it expand a lot and then cracks may form - I generally do it a bit before the clay gets to look very dry and matt, but it is firm enough to handle without deforming or leaving fingerprints.

[sungod3k]

Aha, I wasnt aware of the two meanings but your argumentation sounds good.