The audio file for this article was produced by the Ceramic Arts Network staff and not read by the author.
You don’t have to look for problems in ceramics. They will find you. And when they do, if you have the knowledge and necessary tools to problem solve, you can gain better control of your process and production.
Make vs. Buy
So, you want to make your own slip instead of purchasing prepared slip? Congratulations for wanting to take control of your work and process! This is a good thing, but I would offer a caveat, which is to commit to purchasing some low-tech
equipment and, if possible, to dedicate space for dry material storage and slip mixing. There are both positive and negative aspects for each choice.
Prepared Slip
Buying slip that has already been made with the correct amount of water and deflocculant is very convenient. No fuss, no muss, right? Prepared wet slip can be obtained in gallon-sized corrugated boxes lined with plastic bags, in gallon-sized
plastic jars with lids, and in five-gallon lidded pails. Prepared dry slip can be purchased by the pound, usually in 50-pound bags. Dry preparations come with mixing instructions that specify the correct amount of water and deflocculant.
Prepared slips are available for all temperature ranges and types of clay bodies, including earthenware, terra cotta, mid-range stoneware, whiteware, porcelain, and high-fire stoneware and porcelain. Catalog descriptions also include images
of fired test samples. Your clay supplier will often provide the specific gravity of their product. It would be wise to test your purchase prior to use and always screen the slip before you cast with it.
As these formulas are proprietary to each company, it is unlikely that they will provide you with the particular recipe. Convenience aside, you are also paying for their development costs, materials, labor, packaging, and of course, water,
and its added weight to ship. But most importantly, if you encounter problems or issues with their product, your ability to change the rheology is limited to adding small amounts of deflocculant to correct viscosity to a lower
number, or additional water to lower the specific gravity. If you purchase the same body as a dry mix, you can slowly raise the specific gravity and viscosity. Although these corrections might solve the issues that you encounter, there
are, of course, other problems that cannot be ameliorated by simply adding water or deflocculant. This being said, ceramic suppliers will stand behind their manufactured products, but only to the extent of replacing the material
and will not compensate you for your finished products. Purchasing prepared clay bodies and casting slips is always accompanied by assuming some risk.
Even if you purchase prepared slip, it is important to check specific gravity and viscosity before each casting session. Why? When you add slip that has been dumped from your casting molds, that slip is no longer exactly the same as the
parent slip in your blunger or bucket. So, keeping accurate records and testing for viscosity and specific gravity is part of the process of maintaining consistency as you slip cast.
Making Your Own Slip: Testing
First, making your own slip involves testing formulas and selecting the correct one based on rheological properties, costs, and availability of some extra space and minimal investment in equipment. Perhaps first on this list is a sense
of inquiry and a desire to learn about raw materials and how casting slips work. A testing procedure would involve these ten steps:
Determine your firing temperature.
Check with your supplier on the availability of raw materials.
Use a test sheet to design casting bodies and start with mixing a ten-pound test sample (which will yield approximately one gallon of test slip).
Set the specific gravity and viscosity.
On a flat plaster slab, pour out enough mixed slip to make a slab approximately 10cmÅ~10cm, and make four rectangular bars for shrinkage and absorption tests.
With the remaining slip, pour a few test casts of an object and record the casting properties.
Bisque fire the samples and then test your glazes on the sample castings for glaze fit.
Select the body that provides the best rheology, casting properties, and glaze fit.
Make ten gallons of this slip (100 pounds of dry materials). Set the proper specific gravity and viscosity.
Cast some of your work, then decide if this casting slip is indeed the one that you can commit to and successfully use in your practice. If you have mixing equipment for a larger batch, mix it up. If you are not satisfied with
the results from the ten-gallon batch, start a new round of testing. You have a minimal investment in materials, a larger investment in time, but you have increased your knowledge base appreciably.
Reclaiming Scrap: The Correct Way
Whatever method you choose for your casting slip requirements, you will always generate some scrap, mostly from trimming off the collar from the slip reservoir and fettling seams. You will also have some seconds from casting and handling
that can be reclaimed. Save all your scrap in a five-gallon bucket. When you are ready to reclaim, an unwise decision would be to add it directly back into your mixing tank and just let it mix up with the rest of your batch.
The correct procedure would be to add just enough water to cover your scrap bucket and let it slake, then mix well. Run the reclaim through a 40-mesh sieve. Set the specific gravity and viscosity, mix again, and then add that adjusted
reclaim to your tank.
SLIP-CASTING EQUIPMENT
Premade Slip
Some amount of set up and equipment is required even when using premade casting slip. Below are several methods to incorporate premade slip in your practice:
The simplest is to buy a five-gallon bucket of slip and use a Jiffy-type mixer with an electric drill. Then use plastic pitchers to fill your molds.
Purchase or build a casting table and fill the molds with slip from a pitcher.
Use a casting machine. This is a combination casting table, pump, and slip tank all in one unit. The tank has an add-on mixer but it is not designed for mixing slip from dry powder. Some casting machines use a rotating metal screw-type
impeller inside a rubber cup to pump the slip through a hose to a dispensing nozzle while others use an impeller-type pump. Bearings for both types do not have contact with liquid slip. All tanks are made from molded fiberglass.
Purchase a blunger and fill it with prepared slip.
Mixing Slip
Similarly, making your own casting slip will require a workflow to use it in your studio. Here are a few options:
Purchase an industrial-quality tank—fiberglass, polyethylene, or stainless steel—install a spigot, add a mixing device that clamps onto the side of the tank.
Purchase a blunger. Depending on the scale of your studio practice, blungers are available in different sizes and have an integral mixer and a pre-installed spigot on the side or at the bottom of the tank.
Build or purchase a casting table. For well under $1500–$2000, you can purchase a casting table and a blunger. Casting machines start at approximately $1000 and go higher depending on the size. There are many possibilities. I know a
large ceramic manufacturing company that has multiple 100-gallon tanks and purchases premade slip as it is more convenient and their space is at a premium. It all depends on your practice.
Q and A: Solving Casting-Slip Problems
Most of the problems encountered in slip casting are due to incorrect deflocculation. For example, specific gravity, viscosity, incorrect ratio of plastics to non-plastics, or selection of raw materials. Generally, most formulas will cast,
but the issue is casting correctly.
I have read, on a number of occasions, articles with procedures to make a casting slip from the trimmings of your throwing body (plastic body), or from blocks of throwing clay, or from a purchased dry mix throwing body. I have no doubt
that there may be ways to do this, but I question its efficacy. “Well, I do this all the time,” is an answer that I have heard on many occasions. Proper rheology depends on so many things, and an important one is the ratio between
plastic materials and non-plastic materials, discussed in Part 1 of this chapter.
If making a casting body from a plastic clay body works for you and provides the results you wish, then keep doing it. But, it is a work around that, to me, is technically incorrect. If you plan to slip cast with any degree of predictability
and consistency, buy a premade liquid slip for your firing temperature, or buy a premade dry mix and add the proper amount of water and deflocculant. Or develop, test, and mix your own formula.
Excerpted from The
Mold-Making Manual: The Art of Models, Molds, and Slip-cast Ceramics by Jonathan Kaplan. Published by The American Ceramic Society. Available in the Ceramic Arts Network Shop at ceramicartsnetwork.org/shop.
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The audio file for this article was produced by the Ceramic Arts Network staff and not read by the author.
You don’t have to look for problems in ceramics. They will find you. And when they do, if you have the knowledge and necessary tools to problem solve, you can gain better control of your process and production.
Make vs. Buy
So, you want to make your own slip instead of purchasing prepared slip? Congratulations for wanting to take control of your work and process! This is a good thing, but I would offer a caveat, which is to commit to purchasing some low-tech equipment and, if possible, to dedicate space for dry material storage and slip mixing. There are both positive and negative aspects for each choice.
Prepared Slip
Buying slip that has already been made with the correct amount of water and deflocculant is very convenient. No fuss, no muss, right? Prepared wet slip can be obtained in gallon-sized corrugated boxes lined with plastic bags, in gallon-sized plastic jars with lids, and in five-gallon lidded pails. Prepared dry slip can be purchased by the pound, usually in 50-pound bags. Dry preparations come with mixing instructions that specify the correct amount of water and deflocculant.
Prepared slips are available for all temperature ranges and types of clay bodies, including earthenware, terra cotta, mid-range stoneware, whiteware, porcelain, and high-fire stoneware and porcelain. Catalog descriptions also include images of fired test samples. Your clay supplier will often provide the specific gravity of their product. It would be wise to test your purchase prior to use and always screen the slip before you cast with it.
As these formulas are proprietary to each company, it is unlikely that they will provide you with the particular recipe. Convenience aside, you are also paying for their development costs, materials, labor, packaging, and of course, water, and its added weight to ship. But most importantly, if you encounter problems or issues with their product, your ability to change the rheology is limited to adding small amounts of deflocculant to correct viscosity to a lower number, or additional water to lower the specific gravity. If you purchase the same body as a dry mix, you can slowly raise the specific gravity and viscosity. Although these corrections might solve the issues that you encounter, there are, of course, other problems that cannot be ameliorated by simply adding water or deflocculant. This being said, ceramic suppliers will stand behind their manufactured products, but only to the extent of replacing the material and will not compensate you for your finished products. Purchasing prepared clay bodies and casting slips is always accompanied by assuming some risk.
Even if you purchase prepared slip, it is important to check specific gravity and viscosity before each casting session. Why? When you add slip that has been dumped from your casting molds, that slip is no longer exactly the same as the parent slip in your blunger or bucket. So, keeping accurate records and testing for viscosity and specific gravity is part of the process of maintaining consistency as you slip cast.
Making Your Own Slip: Testing
First, making your own slip involves testing formulas and selecting the correct one based on rheological properties, costs, and availability of some extra space and minimal investment in equipment. Perhaps first on this list is a sense of inquiry and a desire to learn about raw materials and how casting slips work. A testing procedure would involve these ten steps:
Reclaiming Scrap: The Correct Way
Whatever method you choose for your casting slip requirements, you will always generate some scrap, mostly from trimming off the collar from the slip reservoir and fettling seams. You will also have some seconds from casting and handling that can be reclaimed. Save all your scrap in a five-gallon bucket. When you are ready to reclaim, an unwise decision would be to add it directly back into your mixing tank and just let it mix up with the rest of your batch. The correct procedure would be to add just enough water to cover your scrap bucket and let it slake, then mix well. Run the reclaim through a 40-mesh sieve. Set the specific gravity and viscosity, mix again, and then add that adjusted reclaim to your tank.
SLIP-CASTING EQUIPMENT
Premade Slip
Some amount of set up and equipment is required even when using premade casting slip. Below are several methods to incorporate premade slip in your practice:
Mixing Slip
Similarly, making your own casting slip will require a workflow to use it in your studio. Here are a few options:
Q and A: Solving Casting-Slip Problems
Most of the problems encountered in slip casting are due to incorrect deflocculation. For example, specific gravity, viscosity, incorrect ratio of plastics to non-plastics, or selection of raw materials. Generally, most formulas will cast, but the issue is casting correctly.
I have read, on a number of occasions, articles with procedures to make a casting slip from the trimmings of your throwing body (plastic body), or from blocks of throwing clay, or from a purchased dry mix throwing body. I have no doubt that there may be ways to do this, but I question its efficacy. “Well, I do this all the time,” is an answer that I have heard on many occasions. Proper rheology depends on so many things, and an important one is the ratio between plastic materials and non-plastic materials, discussed in Part 1 of this chapter.
If making a casting body from a plastic clay body works for you and provides the results you wish, then keep doing it. But, it is a work around that, to me, is technically incorrect. If you plan to slip cast with any degree of predictability and consistency, buy a premade liquid slip for your firing temperature, or buy a premade dry mix and add the proper amount of water and deflocculant. Or develop, test, and mix your own formula.
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