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March 15, 2010

How to Transform Your Clay Body into a Casting Slip

by Paul Andrew Wandless Read Comments (20)

Ready-made casting slip is commercially available in all firing ranges, making it super convenient. But if you like to combine handbuilt or thrown elements in your slipcast pieces, it can be very difficult to find a casting slip with exact same shrinkage and absorption characteristics as your regular throwing or handbuilding body.

In today’s post, an excerpt from our new free download Ceramic Mold Making Techniques: Tips for Making Plaster Molds and Slip Casting Clay, Paul Wandless shows you how to make casting slip from your regular clay body. - Jennifer Harnetty, editor.

A common studio challenge is trying to keep the number of different clay bodies you have on hand to a minimum. Throwing, handbuilding, casting, and surface design techniques often require different clay bodies to suit the demands of how your work is created. Sometimes the clay bodies also need to be in different forms such as regular moist clay, slip or casting slip. If I’m handbuilding with a cone 6 clay body and want to add a slip-cast element to it, I’d be hard pressed to find a commercial cone 6 casting slip with the exact same shrinkage and absorption characteristics. A singular work made of multiple clay bodies can cause several problems. Disparate shrinkage rates, maturation points, iron content, etc., can all have a negative effect on the final outcome of the work when using different bodies. When possible, it’s always best to stick with the same body. My sculptural work combines moist clay for general construction and casting slip for volumetric elements or for use with various image transfer techniques to address the surface. My solution is to use the clay body I’m working with and turn it into the different types of casting slips I need. This assures that all the parts and surface treatments shrink the same, fire to the same cone, and all the glazes work the same on all the different parts. The great thing about this approach is its simplicity. All you need is a 5-gallon bucket, a drill with a paint mixer attachment, a deflocculant, and the clay body you want to use as a casting slip.

Preparing the Slip
The process of slaking clay to make it into slip is a simple one. Step one is to have a 5-gallon bucket filled 60-75% with your bone-dry clay then add water until it’s a few inches below the rim (figure 1). Let it slake for 24 hours then mix it by hand a little with a stick (figure 2). Once you’ve mixed the slip for about 5 minutes, use a drill with a paint mixer attachment to mix it into a smooth slurry, which should be the consistency of thick cream. Use the stirring stick to check the consistency and find any lumps of clay that didn’t slake down (figure 3). Keep mixing until the lumps are gone.

Measuring Specific Gravity
At this point, I normally fill a quart container with the slip to use as a joining or brushing slip. The rest I use for casting slip. You’ll need to measure the specific gravity of the slip to determine if the water to clay ratio is correct before deflocculating it with liquid sodium silicate or Darvan. To determine specific gravity, which is a comparison of the weight of 100 ml of water (which is 100 g and therefore has a specific gravity of 1) with the weight of 100 ml of whatever liquid or slip you are working with. First tare (zero out) the weight of a graduated measuring container on a scale then fill it with 100 ml of slip and see how much it weighs in grams. Once you know the weight, you divide it by the weight of an equivalent volume of water (100g). This works out, in essence, to moving the decimal point in your weight measurement to the left by two places to get the specific gravity. Example: 183 g = 1.83 specific gravity.

In general, a specific gravity of 1.80 to 1.85 provides good balance of fluidity and strength for most casting purposes. This number can vary, though, depending on exactly what is being cast. I’ve used it as low as 1.74 for volumetric casting and as high as 1.90 for casting flat slabs. Different clay bodies have different optimal specific gravities that are determined by good old-fashioned trial and error. Specific gravity is simply a tool for you to adjust your slip to meet the needs of your particular casting project. Once you find a number that works, write it down and stick with it for that clay body. If you have a specific gravity that is higher than 1.85 and you want to bring it down, add a little water (only a few drops at a time), to your slip, mix and re-test. If the specific gravity is lower than 1.80, let the slip sit 24 hours, skim the water off the top, remix, and measure the specific gravity again.

Deflocculating the Slip
Once you have the correct specific gravity, you need to add a deflocculant to the slip to make it more fluid so it can be poured. Typically just a few small drops of liquid sodium silicate or Darvan is needed to achieve the appropriate fluidity. Clay bodies with high iron content will require less deflocculant than others. Use the drill to mix the deflocculant into the slip and use a ladle or pitcher to scoop some of the slip out to see how it pours. The consistency of cream is what you’re going for so be careful not to over-deflocculate the slip, which will cause slow uneven drying and soft spots in the finished piece. Just add a few drops at a time to achieve fluidity and remember a little goes a long way with deflocculants.
Using What Works
I’ve done this process with commercially bought clay and with home made clay. I’ve also had the exact same success slaking down clay that started off as moist clay fresh out of the bag or as 3-day old bone dry clay. I keep a 5-gallon bucket around to throw all my scrap clay into and when it gets about 65-75% full, I turn it into casting slip. I also just slice up a 25-pound bag of commercial clay into 1-inch cubes and fill an empty bucket with them and use that to make my casting slip. I’m aware of the different opinions regarding initial water content of the clay and its effect on the speed of water absorption/saturation while slaking. In my personal experience, I’ve found if you let clay in any state of moisture slake for 24 hours, it all mixes up just fine. I suggest you simply slake your clay in the state of moisture that you typically have in the past.


Paul Andrew Wandless is a studio artist, workshop presenter, educator, and Vice President of the Potters Council. He authored the book Image Transfer On Clay and co-authored Alternative Kilns and Firing Techniques: Raku, Saggar, Pit & Barrel. His website is www.studio3artcompany.com and he can be emailed at paul@studio3artcompany.com.
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20 Comments

  1. Thomas | March 15th, 2010 at 1:14 pm

    This is an approach that I have never tried before in 46 years of clay! I used to make huge batches (500 gals) at a time with slip we formulated just for the casting process. This is an easy studio approach to the problem. Great idea!

  2. Judith | March 15th, 2010 at 1:37 pm

    Hi Paul, Thank you so much for your description to use my everyday clay for casting. That is great help. Judith, Geneva, Switzerland

  3. Helen | March 15th, 2010 at 2:55 pm

    I remember asking a technician at Potclays (UK) about combining casting clay with plastic clay. He said it was not advisable, even if the clay bodies were the same clay, because the deflocculent completely changes the way the clay behaves. He said I couldn’t expect them to behave in a compatible way even though they were the same clay. Any comments?

  4. Marina | March 15th, 2010 at 3:51 pm

    thank you , this confirms something I always wanted to try. Have you done this process with porcelain clay. Here in South Africa we struggle to get good porcelain and none of the suppliers are keen to make slip, so I will be giving this method a go and let you know my results.

  5. viva | March 15th, 2010 at 7:08 pm

    I have always made slip for my pieces out of the clay I’m using. I do this before I begin a series and make at least a gallon. I also make a quart or so of this slip into paper clay by adding 10 % or so of cellulose. I use this paper clay for repairs and additions to the dry greenware. I have never had a problem with this technique. Thanks for sharing this.

  6. paul | March 15th, 2010 at 9:46 pm

    I’m glad you found this information helpful. It’s a real simple way to make casting slip and I’ve been doing it this way for twenty years. When I was an undergraduate student, my professor Victor Spinski taught us to do this in beginning ceramics class.

    @ Helen - It is true that it’s not good idea to mix casting clay and regular clay due to how the deflocculent can effect the clay. That being said, I do mix casting clay and regular clay quite often when it’s the same body and all I do is cast with it. I don’t advise to mix two different clay bodies under any circumstance. I normally make sure it’s about 75% regular clay and 25% casting clay. This way its a small amount and I’m still able to get a pretty accurate Specific Gravity and then defloccuate for my needs.

    @ Marina - This process works with any clay body and I’ve done with just about every different kind of clay you can think of.

  7. paul | March 15th, 2010 at 9:54 pm

    Hi Viva- I’m glad you have been doing this as well. I add paper & nylon fibers on occasion if the project calls for added strength and flexibility in the green stage. This is one of those techniques that seem too simple to work, but it really does! I’ve never had any problems and I’m glad that you are able to reinforce that point through sharing your experience doing this technique.

  8. Karen | March 15th, 2010 at 10:11 pm

    What a great idea. I recycle all the clay and slip from my throwing classes and have often wondered if I could simply turn the slop into slip for the many molds I’ve inherited. Thanks for the great tip.

  9. Sue | March 15th, 2010 at 11:53 pm

    I have a dozen 2 year old solid bone dry blocks of clay with with indication of what they are on them ( inherited from previous teacher).
    Clay is hard to find here (in the ME) so we have to use what we can. How would you slake down a solid block to slip, soaking overnight doesn’t touch it,we cannot smash it it up, as we are inside a closed building with no exterior access,so dust is a problem, and its so hot here that the water evaporates faster than the slaking process, to say nothing of the smell.LOL To say we under difficult conditions is to simplify. Ideas welcome.
    Thanks for sharing.

  10. Justin | March 16th, 2010 at 7:25 am

    I have successfully used Calgon water softener. I taught a sculpture class at the high school level and we were able to get great results. I believe I used a cap full to about 4 gallons of our own slurry.

  11. Rietje | March 16th, 2010 at 10:15 am

    @ Sue :Bone dry clay can be soaked in water.Just take your time…. We used an old bakery mixer for dough. Trow in the blocks, add sufficient water and let them soak untill they fall apart.Turn on the machine and mix the clay. Get it out and wedge.
    Super if you have a lot of “leftovers” Be sure they are completely dry!
    Lots of succes.
    Riet van de Kreeke, Breda, Nederland

  12. Lisa | March 19th, 2010 at 8:46 am

    Hello i just did this yesterday if you ask your clay supplier about turning a particular clay body into a slip they may be able to give you a recipe

  13. metka | March 20th, 2010 at 1:39 pm

    @ Paul- you wrote that you add nylon fibres- I would really appreciate if can you explain a little bit more. I hear this for the first time.

    Thanks,
    Metka, Slovenija, EU

  14. Katie | March 20th, 2010 at 5:17 pm

    Thanks, I am a student at a university. This has been a good artical.
    Thank you.
    Katie

  15. Liz | March 23rd, 2010 at 1:12 pm

    Very helpful article, thanks! I’m relatively new to slipcasting and have so far only used pre-prepared slip. Now i’m in the process of slaking down my own clay to produce casting slip, and i’m curious about the deflocculating step: is it really necessary to add deflocculant if your slip has a uniform, even, creamy consistency on its own after mixing? If it seems sufficiently pourable, i hesitate to tamper with it.
    Thanks,
    Liz in Canada

  16. neil | March 31st, 2010 at 11:05 am

    Liz in Canada, adding deflocculent is not primarily to make the slip even and creamy but to reduce the amount of water in the slip and still keeping it fluid so the mold does not have to absorb lots of water before the required thickness pot wall is achieved.
    Sue in the ME, you will have to leave the clay to slack for a long time use containers with lids to cut out the evaporation. Neil Worcs England

  17. Sue | April 1st, 2010 at 4:25 am

    Thanks Neil , yes the evaporation is a real problem, we are only airconditioned during school hours and clays paints and everything dries out so much when temps reach the high 50s Celcius.Even winter days are in the 30s.
    Plastic buckets with lids have not worked they water evaporates within a week and long before the clay is rehydrated,leaving them open and adding water each day begets moulds and is dangerous.We do not have enough students to justify the expense of a pug mill.Am still trying to solve this one as clay is hard to find here.

    I have just today been offered granulated stoneware clay by a ceramic tile making company that slakes down to slip quite fast.Has anyone used this type of clay body? Can it be dried out on plaster batts to use in handbuilding?
    Thanks from Sue in ME

  18. Karen B | April 1st, 2010 at 7:05 am

    Sue, I suggest that you add a small amount of chlorine bleach to the water to stave off mold. Also, forgive me if I am wrong, but could it be that the water in the covered buckets containing your dried clay is being absorbed by the clay, rather than evaporating? Just add water each day, a few drops of bleach and cover.

  19. INA | April 6th, 2010 at 1:04 am

    This is great information,I never understood how to measure Specific Gravity and I dont have a hydrometer. However I have a problem/question about the Moulds that grow in the clay. I gave up pottery for a few years because after doing a large sculpture I had what appeared to be a dermititus or psoriasis on my finger tips Drs and Creams were of no use. I have wondered if it was caused by the mould growing in the clay. I have dozens of bags in varing stages of mould growing. Actually I thought aging the clay was supposed to be good for it. Has any one had a similar experience? Or any comments as to what I can do about the clay [toss it?] or my fingers. regard ina

  20. Nina | July 8th, 2010 at 3:15 pm

    Neil,
    I cant thank you enough for this tip! And all of you for your wonderful comments to add to it. Wonderful information. I am new to the process and will report back promptly with my findings. I will be taking 25 pound block and cutting into 1 inch pieces, drying etc. My QUESTION FOR NEIL is this:
    IF I have slop (slip and water) what is the process for drying that sufficiently ? Or can i just go with it and start mxing? Thank you again for your helpo
    The Muddy Diva

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