[INYA]

hello from Europe...


I am starting slipcasting stoneware and I have a question: why are (some of) the inner surfaces of my cups uneven?

Where the slip touches the plaster the surface is great and on the other side it is kind of rough. This is not happening every time and I really can`t think of the reason.
My procedure is: filling the mold, waiting for about 15 minutes, slowly pouring out the rest of the slip, leave the mold at the 45 degree angle for 2 minutes, turning the mold.

When I start pouring out the slip I notice that the surface is weird...

My problem is that I can`t smooth out the surface (since the casting slip is not very "workable"), I painted two cups with engobes for tests to see how this looks like covered with glaze but I would rather see if I can solve this problem in some other way.


Another thing-- how thick do you think should stoneware tableware be? I am making cups, bisquits box, cake stand, teapot...


thanks in advance!

btw this forum is just the best!

[Mark C.]

I had a slip business for 12 years so I'll chime in here
I'm not as familiar with stoneware slip as we did porcelain aromatherapy lamps about 15-20 thousand of them.I never want to see another one.

Wall thickness variations are all about keeping mold full during the process and using dry molds.
We filled the molds then as they dry some refill them again before slip drops below collar level (the fill spout of mold) before the form starts.
If the molds are all dry they all dry at same rate-When wall thickness is what we wanted we dump them UPSIDE down not at a 45 angle so all slip gets drained evenly
You should make them as thick as needed for strength-Its not something I can answer on the net.Cut one in 1/2 . Time the slip in mold to learn this also with a plastic tool you can cut into slip on collar as its setting to judge this-dry molds back to compete dry state before pouring again.
As far as rough on the inside vs outside I think that is the granular nature of stoneware??Its smooth where it lies against the mold and not as smooth inside (I'm guessing here)
Stoneware is a rougher surface then say a white porcelain-and more sponging will bring out a rougher surface still-you could scrape the surface but thats a lot of work.
It would be easier to slip cast with a smoother body than stone ware if thats what you want in the end a smooth surface.Coating will do that but its more work.
Mark

[INYA]

20.000! I can`t imagine making the molds!


I tried slipcasting before and if I turned the mold upside too soon I got "stalactites" form the bottom. That is why I turn them to 45 angle and after 1-2 minutes I turn them up (so the slip could even out at the bottom) and leave it to dry.
By the rough surface I don`t mean just the surface- it almost looks like I would take a thicker paint and splash it against the wall. I will take a photo of that next time.
The funny thing is it didn`t happen with all the molds -for example small cup was awful and the big one was fine, so it is not the stoneware surface that bothers me...

Thanks!

[Lucille Oka]

...."When I start pouring out the slip I notice that the surface is weird...
My problem is that I can`t smooth out the surface (since the casting slip is not very "workable"),....

What do you mean the surface is 'weird'?

[INYA]

Lucille Oka, on 07 April 2012 - 10:16 AM, said:

...."When I start pouring out the slip I notice that the surface is weird...
My problem is that I can`t smooth out the surface (since the casting slip is not very "workable"),....

What do you mean the surface is 'weird'?

I mean: it almost looks like I would take a thicker paint and splash it against the wall. The surface is uneven, it has levels.
I will take a photo of that tomorrow when I get back to my studio.

[Lucille Oka]

[/quote]

I mean: it almost looks like I would take a thicker paint and splash it against the wall. The surface is uneven, it has levels.
I will take a photo of that tomorrow when I get back to my studio.
[/quote]


Is your slip a commercial slip or did you mix it yourself? Does this happen all of the time or after several pours from your slip container? Is the slip properly mixed? It might be a tad too thick. Also when you pour be sure there is no hesitation in your pour out and don't shake the slip out of the mold.
Thickness of the cast is judged and tested by making a little cut/dent in the clay spare; a fettling knife or needle tool will work just fine.

[INYA]

Lucille Oka, on 07 April 2012 - 11:26 AM, said:

I mean: it almost looks like I would take a thicker paint and splash it against the wall. The surface is uneven, it has levels.
I will take a photo of that tomorrow when I get back to my studio.
[/quote]


Is your slip a commercial slip or did you mix it yourself? Does this happen all of the time or after several pours from your slip container? Is the slip properly mixed? It might be a tad too thick. Also when you pour be sure there is no hesitation in your pour out and don't shake the slip out of the mold.
Thickness of the cast is judged and tested by making a little cut/dent in the clay spare; a fettling knife or needle tool will work just fine.
[/quote]

It is a commercial slip -I buy all the ingredients in the shop and mix it myself. It is german stoneware (company Witgert) but the recipe that came with the dry body was not correct. It was written to add 20 litres of water (+175 grams of specific electrolyte) to 50 kilograms of clay. This should weight 1670 grams/liter (supposed to be the best). I mixed it correctly and got 1830 grams/liter. So I started adding water. Now I am at 1700 grams/liter and sometimes it casts beautifully and sometimes it is just weird.

About casting thickness- how much do you think it should be? I know how to determine how much it is... How much do you think is a good stoneware cup?

[INYA]

I uploaded few pictures of this strange surface...I hope you can see what is the problem (it`s taken with my phone)

Attached File(s)

•  1.jpg (176.92K)
  Number of downloads: 48

[Mark C.]

It looks like the slip is to thick or is missing something and leaving those marks-Thats my guess
The slip should be defoculated as well-here we used a product called darvon
If the slip is mixed right it will not leave stalactites when turned over.
Mark

[Diana Ferreira]

I am also a caster of slip. At one stage I made my own slip from clay.

Did you strain your slip? I always use a sieve and strain the slip into my pouring container just before I slipcast.

If you have over-flocculated your clay, it will clot. I suspect that that is what happened here. Pour a little bit of your slip into a container (say about 250 ml - 500 ml). Add a drop or 2 of normal vinegar to it. and stir it. See if it becomes more fluid.

I also do the above when I suspect that my slip is not correctly flocculated. I will then add a drop of my flocculant to the mix, and see if it becomes more fluid, or becomes thick and sluggish.

This way you do not destroy a whole bucket of slip :-)

Kind regards
Diana

[INYA]

Every cast is strained with a sieve. I use a sieve directly over the mold.
The slip was deflocculated with deflocculant recommended by the producer of the slip. Yes I heard of Darvan...maybe I should order some ...
I really don`t like it when company instructions are wrong. Who would thought -Germans do just the same )

Ok, tomorrow I will a try both possible solutions-adding vinegar and adding more water.

Funny thing is that this surface does not happen every time. I am guessing- maybe I poorly mixed (I am mixing with hand operated paint/glue mixer) or I took the upper (less dense) part of the slip at the time that the cast was ok...

[Riorose]

I had the same problem and in fact I think I posted it here in the forums. I made my own stoneware casting slip from some smooth stoneware. The first casting was smooth on the outside and inside but by the time i casted the second the inside appeared sandy. I reasoned it must have been the fine grog in the stoneware and maybe had something to do with the increasing moisture in the plaster mold.I switched to a commercial slip but it only fired to cone 04, hardly a stoneware. Does anyone have any ideas? one thing though, the stoneware from my own casting slip did not distort in a cone 7 fire. porcelain castings make me bonkers with distortion.

[Diana Ferreira]

Inya, do not add more water! First check to see if you can rectify the problem with your flocculant.
too much water will thin out your clay-body, and it could cause more shrinkage of your work. Also, it will water-log your molds, and you will just get a few castings out, before you have to dry it out. I use a fan heater, but if you do not have that, or live in an area that is quite moist, it could take days for your molds to dry.

[Lucille Oka]

About casting thickness- how much do you think it should be? I know how to determine how much it is... How much do you think is a good stoneware cup?


[/quote]


This question tells me that you have probably never made a cup on your own. You have jumped the gun if that is the case. There are some steps in ceramics that shouldn't be skipped.

It is like trying to read without being able to identify the sounds of the alphabet.

Make a cup to the thickness you desire use your own stoneware cup as a gauge for the thickness of your reproductions.

[INYA]

@Diana
I intended to add water to small amount of slip and cast one piece, thanks for the warning

@Lucille
I have made a cup before, but it was long time ago (8 years) and it was earthenware (throwing). The battle there was to get it as thin as possible since I was not a professional.
I thought maybe someone has a guideline, I am casting app 3 mm now and we will see haw that goes... I have only one stoneware cup in my cupboard but it is thrown, it is quite thick (at least 5 mm at the thickest part). It is super resistant I have it for 10 years without a chip or anything.

@Riorose
I don`have a groged body, so my surface is definitely not connected with that. Stoneware for cone04, hmm. Was that one of those fast sintering bodies?
I have three different porcelain bodies at home and they are all warping. Now I heard my problem is most likely in wrong firing cycle.

[Diana Ferreira]

Inya, I forgot to add, if you weigh a litre of slip, it should be in the region of about 1.8 kg.

And, ignore advice about 'correct' thickness. What is right for you, might not be right for someone else. And there is a big difference between casted work, and thrown work. And honestly, do you want work that is as thick as the cheap imported stoneware pieces you can buy in a shop?I used to cast my stoneware as thin as porcelain. Since I like thin ware. The black clay that I now use, is also about 2 - 3 mm thick, depending on the item. I only glaze the inside of my work, so I do sometimes get warping of my work in the kiln. But that is my risk that I take.

Play and enjoy!

[INYA]

Ok, I resolved this one out.
If anybody would like to know a little bit of water and everything is ok.

[Joanie]

INYA, on 10 April 2012 - 09:31 AM, said:

Ok, I resolved this one out.
If anybody would like to know a little bit of water and everything is ok.

I am glad everything worked out ok. Don't get discouraged with certain remarks. That's why I don't say a lot here. I do learn a lot though and enjoy this site, but there are certain members I don't care to read what they have to say. Know what I mean?

[INYA]

I know what you mean
but I am very resilient )


The most important thing is that we learn a lot...

[Mark C.]

INYA, on 10 April 2012 - 09:31 AM, said:

Ok, I resolved this one out.
If anybody would like to know a little bit of water and everything is ok.

Thanks you for letting us know-it really helps everyone when an solution is achieved and we all learn what it was. Even if our advice is wrong
Did the roughness go away as well??
Thanks for following thru.
Mark