Slip casting straight walls

[Nicola21]

Hi All

I just began slip casting block forms from stoneware slip and it has been going well for my first time. I’m having the problem that while they are drying, the edges are warping and no longer staying square. 
I left them to dry in the mould for 4 hours before demolding as I thought there may be thicker clay on the edge below, so the walls are ‘sinking’ as it dries. 
 

I am using both red and white slip, and it happens a lot more with the red. Does anyone have any ideas? Should I let them dry longer in the mold so it is supported until it is firmer? Or is that not the problem? The red slip is thicker than the white, if that makes a difference?

thank you! 

[Jeff Longtin]

Hello Nicola,

Welcome.  Not quite sure in what way your clay is warping? Do you have a picture? 

Questions that come to mind: are you trying to cast solid blocks and they're coming out with warped dimensions and/or are you casting into an open or an enclosed mold?

The fact that the problem is greater, with thick slip, suggests that a vacuum is being formed and pulling the walls inward. In such situations thinning the slip will help.

 

 

[Nicola21]

Hi Jeff,

Thanks for your reply! Yes, photos are a much better idea. 

These photos should show  the pieces. The closed face that is sitting on the table will be the top face of the piece, and the open edge is the bottom. These open edges are going out of square as they dry - I've drawn on a photo to show. My theory is that the wall are sinking into the corner below (I've circled in another photo), perhaps it is still too soft to support the weight when I have taken the pieces out? But I am not sure. 

Great to know about the thick slip, thank you. I think that will help. 

[PeterH]

Does the slip glug as you pour it out?

How do the warped sections relate to the section of the mould  you pour from?

[Nicola21]

It does glug a bit. Is that a good or bad thing?

This is the mould - I trim the excess down to get the open edges of the piece.  Would leaving the piece in the mold longer help?  So that it is more rigid and can support the walls? Or is that not the problem?

[neilestrick]

Getting a flat sides to stay straight is difficult no matter how you make them- cast, slab built, etc. Do you dry them with the opening facing up or down? It could be that the open edge is drying faster than the rest, and therefore pulling away from the mold sooner and warping. Have you tried letting them dry upside down so it sets up more evenly? I'd leave them in the mold as long as possible. You may need to make them thicker, or you may even need a support wall across the middle to hold everything straight.

Open bottoms are problematic, as the walls tend to catch on the shelf and limit movement of the piece as it expands and contracts during the firing. This can result in warping and/or cracking. To prevent that, you'll need to fire them on a slab. If you're going above low fire temps you may still get some warping, though, so attaching a slab to the bottom would be necessary. I would test fire one on a slab before doing a whole batch.

[PeterH]

21 minutes ago, Nicola21 said:

It does glug a bit. Is that a good or bad thing?

 

Possibly a bad thing. When it glugs it creates a  vacuum, which tries to pull the casting off the mould. Large flat surfaces would be especially susceptible to this.

Does pouring more slowly/gently reduce the glugging?

Does the glugging happen very early in the pouring? If so try bailing/decanting out some of the slip before pouring, as the gurgling  might relate to the shape around the top of the mould.

PS Does your slip gel-up in the mould at all?

[Years ago I had a batch of commercial slip that gelled-up in the mould and made pouring out difficult (the mould was for a narrow-necked vase).  I found gently agitating the slip in the mould (without touching the sides) before pouring out reduced the issues. Not a long-term solution of course.]

[Jeff Longtin]

The pictures help alot Nicola, thanks.

In the first picture you indicate where the form is "sinking". This is indicative of too much deflocculant in the clay. (Perhaps?) A first step would be to thin the slip. A possible second step is to add more clay AND water. (The "glug" sound is also an indicator of too much deflocculant. A properly balanced slip should smoothly flow out of the mold.)

The clay walls are warping because they are weak. After you trim the sprew, so you can lift off the mold piece, leave the excess clay in place.  Only cut it once the clay is leather hard. (Or are you doing that already?) If you cut out the opening immediately, after you pull off the mold piece, the clay walls will warp inward. Leaving the clay in place will (should) allow the clay wall to retain its shape.

Casting thicker would help.

[Nicola21]

Hi All,

Thank you so much for all your replies! It has been a relief to have some more experienced thoughts help out. 

I have done a few more casts today with the following changes:
- thinned out the slip. Especially the red as it was like custard straight out of the container. It may have to be thinned further
- left the molds upside down to dry & for longer (5-6 hours)
- not trimming the excess clay until the following morning. This was definitely dragging the clay and pulling the walls. 
- bit thicker wall

I still have to let them dry with the open edges up, but I have also tented them in thin plastic so they dry more evenly. I will report back with how these ones went!