Outward bending on edge of slip-cast porcelain cup

[Jalte]

Hi

I'm making a slip-casted cup (porcelain, fired to around 1260°C (2300°F) , approx. 2mm (1/16 Inch) thick). 

Everything looks great but the edge are kind of warping/bending outwards and I don't know why. It first appears after it fired.

See photos below.

Anyone got any idea on what's going on?

Thanks!
Jalte

 

 

[Hulk]

Hi Jalte!

Good question, I don't know, just wanted to comment: I purposely impart a similar recurve for drinking vessels - mugs and tumblers. Two reasons, one, it's a place to "park" one's lower lip, and two, the tool burnishes that parking spot to make it extra smooth. The tool I'm using is a tiny metal spatula, about .625 inches wide with a rounded end.

 

[Mark C.]

Couple things that cave is good for the lip

The cause may be because its the cut edge of your mold and you work on it to smooth with sponge and water?

[Jalte]

As you both say yes maybe I should just embrace it (you got a point with the bottom lip); but in that case I guess it should be more prominent/just bigger — now it looks like a mistake / kind of mess up the curve.

Quote

The cause may be because its the cut edge of your mold and you work on it to smooth with sponge and water?

I use a knife to cut the edge and sponge afterwards yes. Maybe I should try to sharpen the knife (and be extra careful not to flex it). But I have to round the edge with sponge (I guess; can't really see an alternative (right?)).

Thank you so much for helping !

[neilestrick]

It looks like it's only glazed on the inside? That could be part of the problem. Does it happen when the whole piece is glazed?

[Jalte]

10 hours ago, neilestrick said:

It looks like it's only glazed on the inside? That could be part of the problem. Does it happen when the whole piece is glazed?

Yes; it is only glazed inside. Haven't tried  glazing  the whole cup so don't know. But I will try! (hope it's not that though; would like to keep only glazed inside (but if it is that, maybe I can try a different or thinner glaze) ). 

Thanks!

(will post update in here but have to change heating element in the kiln before and very much hobby project so will take months)

[Bill Kielb]

3 hours ago, Jalte said:

Yes; it is only glazed inside.

Always a bit iffy to  only glaze one side of cups / bowls. Generally glaze inside and up over the rim and down the outside for a bit just to have a nice surface to drink over and offset the  forces of the glaze inside on the claybody.. Under some conditions, folks find  interior single sided glazed items can fail  suddenly, especially while filling the  cup with hot coffee / tea, cold beverage etc.......

Glazes that are in very slight tension help keep the clay in compression and add significant strength to the entire piece when applied evenly inside and out.

[Callie Beller Diesel]

The effect Bill describes happens more frequently when the glaze fits, and doesn’t craze. Even if they don’t crack in such a dramatic fashion, they can still develop vertical or U shaped stress cracks a few months after they come out of the kiln. Be sure to put a few cups through a heat/ice water test, or a freeze/boiling water test to make sure this doesn’t happen.

[Mark C.]

Glazing the outside will make the cup stronger as well.

[Jalte]

Okay; Sounds like it's generally better to just glazing both sides. I'll try that and see if the bend disappears. It is very much a aesthetic decision and it will look better only glazed inside so that's still the goal. Bought this one eight year ago in Japan and use it often, so guess it's possible to make something durable only glazed inside (it's also pretty much the overall look I'm going for). 

Thanks!

[Mark C.]

That flare on the pot  in your above photo makes drinking easier than a straight up edge

[Jalte]

40 minutes ago, Mark C. said:

That flare on the pot  in your above photo makes drinking easier than a straight up edge

Yea that's maybe correct. It's not a necessity though; I have plenty of cups without and never thought about it (functionalisticly I think most common problems is to thin/hot surface (without handle) and how some cups directs the liquid back up (and out) when you pour it in — but that's just me). 

Thanks,

Jalte

[Jeff Longtin]

The slight flare that you see is from the process you employ to cut the edge of the cup. I presume you cut the cup while its in the mold. Is that correct? If you added an additional ring to the top of the mold, to create a "sprew" essentially, then you can remove the entire piece from the mold before you cut the edge. Once you remove the entire piece THEN cut the edge. I experienced the same problem years ago and this is how I solved it. 

It seems insignificant but what you're doing, when you cut the piece in the mold, is that you're aligning the particles in a way that they remember as the piece dries. By removing the piece entirely you eliminate this "clay memory".

[Jalte]

On 3/27/2021 at 4:58 PM, Jeff Longtin said:

The slight flare that you see is from the process you employ to cut the edge of the cup. I presume you cut the cup while its in the mold. Is that correct? If you added an additional ring to the top of the mold, to create a "sprew" essentially, then you can remove the entire piece from the mold before you cut the edge. Once you remove the entire piece THEN cut the edge. I experienced the same problem years ago and this is how I solved it. 

It seems insignificant but what you're doing, when you cut the piece in the mold, is that you're aligning the particles in a way that they remember as the piece dries. By removing the piece entirely you eliminate this "clay memory".

Yes that is what I do.

I'll try that; you think I can put in a ring with a little larger hole and cut that? Like bottom image (cutting the blue part after casting) or how is the normally done? The cup is not completely circular at the edge.

Thanks!

[Mark C.]

I would continue the same line up and not flare out myself-then cut the edge-maybe a slight dimple so you know where to cut exactly 

[Babs]

On 3/22/2021 at 12:10 AM, Jalte said:

Okay; Sounds like it's generally better to just glazing both sides. 

That vesssel is glazed on upper half of outside. Same glaze inside and out.

Just saying.

[Jeff Longtin]

Jalte,

That's exactly what I do. I hold an exacto blade horizontally and cut the lip at the edge. No more outward bending.

ps the added ring form also reduces distortion when I remove the cup from the mold.

[PeterH]

22 hours ago, Jalte said:

Yes that is what I do.

I'll try that; you think I can put in a ring with a little larger hole and cut that? Like bottom image (cutting the blue part after casting) or how is the normally done? The cup is not completely circular at the edge.

Thanks!

You might also try experimenting with adding some sort of removable cottle to your existing mould. When things firm up remove it (maybe cutting the "flat" on top of the mould), then cut the top with a wire. This way you can get a very uniform cut while only applying outward pressure.  A wire harp probably works best if you have one of the right span..

 

[Jalte]

Thanks for all your answers.

 

Quote

You might also try experimenting with adding some sort of removable cottle to your existing mould.

I'm not sure I understand this; removable cottle made out of plaster or are you thinking about something like a plastic sheet around existing mould while casting (like drawing below)?

Cutoff: existing plaster mould (blue), cast clay (red) and outer plastic sheet (yellow).

 

Quote

The slight flare that you see is from the process you employ to cut the edge of the cup.

I thought about this. Are we sure that actually what happens? It could also be while the clay dries the overflow stick to the top surface of the mould. See drawing below.

Close-up cutoff: plaster mould (blue), cast semi dried clay (red) and surface where clay stick to plaster (green).

Don't know how much of a difference it makes though; it will still help to extend the mould (upwards) and cut later.

But if it is this, I though about a maybe little unorthodox solution: why not cover the top surface of the mould with shellac, making the overflown clay not dry there? Or cutting a piece of plastic that fit exactly with the top surface you remove when all the clay has dripped of? 

Cover top mould surface (yellow) with shellac or plastic?

 

Thanks again!
Jalte

[PeterH]

2 hours ago, Jalte said:

Thanks for all your answers.

I'm not sure I understand this; removable cottle made out of plaster or are you thinking about something like a plastic sheet around existing mould while casting (like drawing below)?

...

In my case I was given a mould which ended at the top of the form. So I cast a plaster ring  to hold top-up slip and ensure that the vases lip was fully cast. Ending up with something like this ...

I just: placed the ring in position; cast; drained; cut against the top of the mould and the inside of the ring; removed ring plus adhering plaster; used a harp with outward pressure to cut the top of the vase; then wiped with a damp sponge (again with outward pressure).

This might be trickier for you, as you have less spare space on top of your existing mould. So I was suggesting you cobble something together (like a sheet of plastic) as a one-off experiment to see if this way of cutting the edge preserves your shape any better. 

PS If I was to do it again I would probably try shellacking the inside of the ring, but not the top of the mould.

PPS I suppose that this way of cutting an edge allows the edge to be cut while the cast is comparatively soft, as the form itself is still supported by the mould. Which may have advantages (e.g. the situation where the cast shrinks while the clay sticks to the top of the mould couldn't happen).

[Mark C.]

My suggestion is to make a new mold which just brings the sides straight up on same slow outward level-put a seal grove in mold where lip terminates (a cut line for cutting later) and this new  say 1 inch height will be where extra clay is held as mold lever drop. (just like Peters extra cottle ring does above)

This is not like your drawing at all where clay goes sideways.The mold continues on same plane as your glass straight up just an inch or two more.

This is extra (called a spew is standard practice in mold making )

mold need extra room at top to hold extra slip so when level drops there is always more -then you do not have to add more slip as you do now to a mold . Slip dries level drops but the reservoir holds the extra so no need to add more. very common in all good mold making.

hopefully I have made this clear and you understand it

[Chilly]

This commercially available mug mould (http://www.clay-king.com/pottery_wheels/hump_molds.html) shows the shape the sprue should be.  The red line shows where you use a Lucy tool or similar to cut away the excess slip and gives a really clean edge.  I have similar moulds, both commercial and home made and they all give a clean edge, and I've never noticed any slumping.  You have to be careful not to let the slip be too thick or you risk cutting into the actual mug.

[Jeff Longtin]

"Are we sure...?" I like your pessimism. (Of course we're never sure.) 

What you describe is correct: the horizontal surface does "catch" the clay. That's why I cut the sprew before the clay has a chance to shrink significantly. For some reason the cut edge doesn't stick and the form shrinks uniformly.

As far as your "unorthodox" solution is concerned, it is unorthodox, and it won't work. It leaves you with an edge that is still wet and very uneven. (But you might have better luck?)