Post #684005 by Jürka on Sun, Jun 30, 2013 3:20 AM

On 2013-06-29 16:13, MadDogMike wrote:

On 2013-06-28 12:54, VanTiki wrote:
MadDog is (as always) correct...

Thanks Henrik, I have a bunch of useless info floating around in my head :lol:

Jürka, you say that everything comes out when you pour the slip out? That's what make me think there is a lack of deflocculant. Here in the US we have ready-made liquid slip available so I have never used dry slip mix. I looked up dry slip mix and the 2 I found DO NOT have the deflocculant added to the dry powder: From the Laguna site "To make liquid slip for casting into plaster molds, these bodies require mixing with water and deflocculants such as soda ash, sodium silicate or dispersal/Darvan". From the Byrne's site "just add the prescribed amount of water and deflocculants (we supply the soda ash and sodium silicate - No Charge)." You might check with your supplier and see if you can find out if it includes the deflocculant or not.

Other than that, the wall thickness would depend on -

1. The amount of water in the slip mix. I use my slip a little less thick than heavy cream or yogurt. Some people use a viscosity meter, others just "eyeball it"
2. The amount of time the slip is left in the mold. Probably 1 hour minimum, 2 hours usually gives a nice wall thickness.
3. The dryness of your plaster mold, it needs to be bone dry.

Some other factors could include

1. The thickness of your mold wall, should be at least 3 cm in the thinnest place (1 inch or more). From the pictures yours look fine.
2. The temperature and relative humidity when you are pouring
3. They say that the purity of the water you mix your slip with can matter and that too much chlorine can affect it. If your water is heavily chlorinated, you can let the water sit out in a big open bowl for a few days and most of the chlorine will dissapate. (old fish aquarium trick)

Hope that helps.
PS - Holy crap - glazes cost you an arm and a leg! :(

This is the instruction by seller of the clay: http://www.loovuspood.ee/files/Valumasside%20valmistamisjuhend.pdf
My clay is GM. It is necessary to include water and clay-powder and recommended formsil and plastificator. Those two I don't use (too expensive). No, I don't have problem that all the clay slips out. Thickness of wall looks alright...

I made a slip-cast yesterday. I put 1 kg powder to 1 litre of water. I think maybe I should make it just a little bit more liquid. But the biggest problem that happened was that I think the clay and water was not mixed propably. It looked alright, but after I slipped it out, I saw that walls were not solid, hard pieces or something... So, today I try another slip-cast and try to mix it longer with better instrument and maybe use little bit more water.

1. I try to measure everything because I want it to come out every time exactly the same. I eyeball the first time, measured, and next times I use same relation.
   All the yogurts are different! :D Some are like drink, others are like food, most of them somewhere between...
2. Min 1 hour??? Is it really necessary? I have read somewhere that half an hour is ok and that's how I've done it and had no problems. I'm afraid that if it's longer, it gets too stoned and would not come out at all.
3. My plaster mold has been drying for at least month, maybe li'l longer... I guess it should be enough? It looks dry I'd say. Lightweight.