Sunday, April 7, 2013

How I mix my slip.

I spent the weekend house cleaning, making up for the last three months of frenzied sculpting.  It was amazing how much of the contents of my studio had become scattered throughout my house.  Everything is back in place now, which feels freeing.  On top of all the cleaning I managed to make time to mix myself a double batch of casting slip to get ready for another round of slip casting tableware.  I got another check in the mail yesterday from one of my galleries and it made me realize that I really need to get some more work out.  (I like that extra money coming in.)

Since I mixed the slip I thought I would let you know how I make my casting slip.  I got the recipe several years ago from Ceramics Monthly (November 2008, p.57).


 
In case you can't read this it says:

UARTS CLAY BODY

Nepheline Syenite ............................23%
Grolleg (English Kaolin) .................55%
Flint (silica) ......................................22%
                                                         100%
Add Bentonite or Macaloid ..............1%

For cone 10 use Custer Feldspar instead of nepheline syenite.

Mixing a Batch of Casting Slip from UArts Clay Body
 The water should be no more than 50% of the total weight of the dry materials.  Darvan should be 0.4% of the total weight.  Bentonite should be 0.5% of the total weight - up to 1% if desired.  A batch of 10,000 grams of dry material, when mixed with water, makes about 5 gallons of slip.

Step 1
Blunge water and Darvan with a heavy-duty drill and a squirrel cage or jiffy mixer.  Add the Bentonite and continue blunging until thoroughly mixed.

Water ..................................... 5000g
Darvan 7.....................................40g
Bentonite ....................................50g 
Step 2
Add dry materials while continuing to mix:

Grolleg (English Kaolin)...............................5500g
Nepheline Syenite..........................................2300g
Flint (silica)...................................................2200g

After mixing thoroughly add up to 18g (0.2%) more Darvan (no more than 0.6% total) as needed.  Small amounts of water may also be added to improve viscosity (use drops of water at a time and go slow).

Step 3
Check the specific gravity of the slip.  Specific gravity is the measure of the density of a substance.  For liquids, we compare the density to water.  Weigh a specific volume of slip and divide that weight by the weight of the same volume of water.  The ideal result should be 1.7-1.8.  This means that the slip  is 1.7-1.8 times as dense as water.  Most good slips are in the 1.75-1.78 range.

Step 4
Let the slip stand overnight (or continue mixing overnight if you have the equipment to do so).  Mix once again the next day and recheck specific gravity before using.  It is possible to use the slip right away, but waiting helps to insure all particles are thoroughly wet.  As you mix, try to avoid causing a multitude of air bubbles by going too fast - the bubbles can get trapped in your slip and reappear in your castings.

I was very excited when I found this recipe, because most of the previous information I had found was either not so specific or designed for people using industrial scale equipment.  I have since also found Andrew Martin's The Essential Guide to Mold Making and Slip Casting to be extremely helpful with trouble shooting.

So the first step is to weigh the water. I've tried to shortcut around this by figuring out the volume, but it really does work out better if you take the time to weigh the water.



The next step is to add the Darvan 7 and Bentonite.  I like to leave the Bentonite to absorb water for at least an  hour.  Bentonite swells as it absorbs water, and I find that I end up with chunks of Bentonite throughout my slip if I do not do this.

Bentonite added
ready for blunging


 After this you are ready to add the dry materials.  I had been discouraged from the idea of trying to mix casting slip for quite awhile because I was under the impression that you needed a special slip mixer.  While it would be nice, it is quite possible to do all of this with a standard drill and a paint mixing bit.  It just takes a little more time.

don't forget to wear a good dust mask when using any dry ceramic materials

While the recipe doesn't call for it I find that I like my slip a lot better if I sieve it after I mix it.  I run mine through an 80 mesh sieve twice.





When you are done the slip should look like this, the webbing formed between your fingers is a very important indicator that you have successfully made good slip.




Checking the specific gravity sounds scary and overly scientific, but it really is quite easy, and then you know you have a good batch of slip.  I have a plastic container that I weighed 100g of water in and marked a line where that water reached on the container.  I can now just add slip to that line and weigh it, then divide by 100 to get the specific gravity.  A graduated cylinder would be a more exact way to do it, but this seems to work fine for me.




Slip mixing 101 concluded.

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