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Purpose Of Sodium Silicate In Slip Casting


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#1 Wyndham

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Posted 02 August 2014 - 10:26 AM

I'm wondering about the use of sodium silicate in slip casting. Is it to help release the firm slip from the mold?

Here's my problem. I have some plaster plate  hump molds that if I use slabbed clay draped and formed, even with Mea's help with cornstarch(which works very well), slabs can retain some memory and rims warp slightly .

What I would like to do is create a mud that can be jiggered onto a hump plate mold, yet release easily on drying.

This is where I thought sodium silicate might come into the process, allowing the mold to release the clay.

I seem to remember a potter using this method but that was 30 yrs ago and is no longer around, so I have no idea if he used sodium silicate or any other release agent in the mud or on the mold.

The clay, as I remember, was firm enough to stay without puddling out or releasing any water as is sat and jiggered smoothy over the mold.

I feel I'm missing a key step or ingredient to make this work

Thanks.

Wyndham

.

 



#2 Tyler Miller

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Posted 02 August 2014 - 10:58 AM

I thought sodium silicate was just a deflocculant to keep the clay particles in suspension in the slip, like darvan etc.  I'm not sure it facilitates any kind of mold release.



#3 Wyndham

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Posted 02 August 2014 - 11:16 AM

I have no idea the function, but now that you mention it, it sounds reasonable.

Wyndham



#4 JBaymore

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Posted 02 August 2014 - 11:28 AM

It allows you to create a flowing viscous slurry with the minimum amount of water due to the defloculation.  If you make slip without it... all the casts will crack and warp.

 

There are better defloculants than sodium sil... like Darvan #7.

 

You don't need mold release agents in slip casting.

 

best,

 

.....................john


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#5 Wyndham

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Posted 02 August 2014 - 01:08 PM

John, I'm trying to jigger plates with a thick slip or soft clay but the clay wants to stick to the plaster and not release or it sticks in one spot and not another and warps the plate.

I have tried successfully, with a slab rolled round of clay dusted with cornstarch but the slab rolling imparts a memory that that shows up later as a warped or waffled rim, slight but noticable.

I was grasping at straws thinking the sodium silicate would aid in the release.

Somewhere between these 2 extremes is bound to be a sweet spot for jiggering plates

Thanks wyndham



#6 JBaymore

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Posted 02 August 2014 - 02:02 PM

Sounds like your working plate molds are screwed up.  Or not DRY.  For jiggering they have to be a certain level of dry between each repetition.  If they are wet.... it sticks and will crack also.

 

Sticking in PLACES seems to indicate that the original plaster was not mixed well... and the density of the mold is uneven in areas.  Common issue.

 

best,

 

...................john


John Baymore
Immediate Past President; Potters Council
Professor of Ceramics; New Hampshire Insitute of Art

http://www.JohnBaymore.com

#7 Fielding

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Posted 02 August 2014 - 03:29 PM

Have you thought of using a blast of compressed air to release the clay body?  I have a recurring interest in making large  concave dishes about 24"-30" in diameter.  To that end, I have collected a number of satellite dishes with the intention of making a plaster reverse for a hump mold.  The advantage of the hump mold is that it would allow me to throw a foot ring in the process, so that the piece could be placed on a table or hung on a wall.  Naturally, there is a cracking problem if the piece is left on the hump too long. 

 

I spoke with a potter at a show who had successfully done this, and I wish I could re-call his name so as not to plaguarize.  He said that he had a compressor and used an air nozzle to shoot air between the mold and the clay body.  He said the air  released the piece quite easily.  Those air nozzles I believe limit the PSI to about 30 pounds.  Let me know if you can try that--I am all about being a copy cat.

 

As far as warping, the clay memory is a problem for me too.  I have been told that very slow controlled drying would help.  Thus if you made a corresponding slump mold as a drying tool, you could line it with paper and then wrap the thing for a long nap.  

 

Another problem is that my studio fires to cone 10, which is awful hot for a flat piece, so some warpage may not be avoidable.   When I throw dishes, I use a large plywood bat 3/4" thick that I varnished.  I thought the rigidity of that plywood is better than using a plastic bat which is prone to flex.  I think that does work better. 

 

Let me know what you think.

Fielding



#8 Wyndham

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Posted 02 August 2014 - 04:29 PM

I have heard of the compressed air but have yet to try it.

I was reflecting on what John had said about plaster issues and since I need to make more molds anyway, I may start getting better at that then trey the air.

Thanks Wyndham



#9 Mug

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Posted 04 August 2014 - 07:08 AM

John had mentioned an uneven mold.

I'm not sure if it will help, but I have recently been making some slip cast molds. In school I was taught to use the inaccurate island method of mixing gypsum products. I have found this method to be ok for waist molds, but it makes for an uneven slip mold. I now measure out the mix and it makes a mold that casts more even.






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