Like most things in ceramics, there are many ways to source casting slip. If making a slip yourself is of interest, the following discussion covers the basics, materials theory, and more.
Defining the Terms
Casting Rate: The time required by a casting slip to build up a sufficient clay wall thickness in the mold.
Deflocculant: Electrolytes that supply sodium ions. Chemically, electrolytes are substances that become ions in solution and acquire the capacity to conduct electricity.
Loss On Ignition (LOI): Refers to the burning out of carbonaceous materials and the driving off of chemically bound water present in all clays during the early stages of firing. LOI that does not burn off successfully can be responsible for many annoying glaze defects.
Newtonian Fluids: A fluid that has a constant viscosity, or thickness, even when force is applied.
Rheology: The study of how fluids flow, in this case, the array of characteristics that ceramic slurries exhibit (viscosity, thixotropy, etc.) over an entire range of applied shear conditions.
Shear: A force that makes one surface of a substance move over another parallel surface.
Specific Gravity: The weight of a volume of liquid divided by the weight of the same volume of water. This is sometimes referred to as relative density.
Thixotropy: The property of a slurry’s viscosity to decrease when subjected to shear, followed by a gradual increase in the viscosity when the shear is stopped. It is time dependent.
Viscosity: Resistance to flow of any fluid, caused by internal friction. High-viscosity fluids are thick (think cold honey) and low-viscosity fluids are thin (think water).
The Framework Behind Building a Slip-Casting Body
Building a successful slip-casting body requires the proper selection of materials and an understanding of the ratio between the plastic ingredients (clays) and non-plastic materials (silica, feldspar, talc, pyrophyllite, and sometimes frits). This ratio should always be 50/50, but I have seen formulas that vary slightly. Selection of the correct materials could be determined by availability, cost, and the temperature of your firing. Successful casting bodies use the correct amount of water and deflocculant, which will give a proper specific gravity and viscosity. Proper mixing and then testing your slip for the correct rheology, adjusting them if necessary, will provide you with a casting body that works for you.
Casting slips are counterintuitive to throwing bodies. To understand slip-casting bodies, we need to understand that they require more water than throwing bodies and a deflocculant. Here are some major things to know:
Casting slips require more water than a throwing body.
Casting slips are an aqueous suspension of materials that do not settle out.
Although a great many of the clays we have all grown to love and work with can be successfully converted into a slip-casting body, there are clays produced specifically for slip casting. These clays have larger particle sizes, or a blend of particle sizes, which promote permeability and greater packing densities. Some clays deflocculate better than others.
A properly formulated slip-casting body will have its own particular specific gravity (also known as density or weight) and its own particular viscosity (deformation under a given shear). Viscosity can also be thought of as the resistance to flow under a particular shear.
Casting slips are non-Newtonian fluids and are dependent on shear to flow. Think ketchup.
A Newtonian fluid is shear independent. Water is a perfect example of a Newtonian Fluid.
Casting slips have very specific ratios between the plastic ingredients, the clays, and the non-plastic ingredients, silica, feldspar, and pyrophyllite. These ratios are different than those for throwing bodies or other plastic bodies.
The amount of water and the amount of deflocculant determine the specific gravity and viscosity of the casting body. They are crucial to maintaining an effective casting rate.
Slip casting works by the controlled absorption of water through a build-up of a permeable clay wall and into the plaster mold.
The most important thing that you can do whether you buy premade slip (wet or dry) or make your own casting slip, is to always sieve the wet slip before you pour it into the molds.
The 50/50 Ratio: Plastics/Non-Plastics
The goal in building a casting body is to use enough water to allow the dry materials to enter into an aqueous suspension, and then by adding a deflocculant, achieve the required rheological properties. The way to successfully achieve this is to have half the slip formula contain plastic ingredients (clays) and the remaining half, non-plastics, silica, feldspar, talc, pyrophyllite. Non-plastic ingredients can also be referred to as filler. This 50/50 ratio is the gold standard. There are, of course, exceptions to the rule, and the ones I have encountered stay pretty close to the 50/50, deviating by no more than five parts on either side to always add up to 100 parts. The one true exception I have worked with is a low-fire white casting body (cone 06–04), which is 60 parts plastic materials and 40 parts non-plastic materials (60 parts ball clay : 40 parts talc).
Water
Depending on the plastic ingredients and their metrics that yield an ideal casting body for you, the amount of water can vary from 28% to 40%. So, in a 100-pound dry mix that yields 10 gallons of liquid slip, the water can range from 2.8 pounds to 4 pounds. For my 30-gallon blunger, I start with 300 pounds of dry ingredients.
While I know that 120 pounds of water for 300 pounds of the raw materials I use will get me very close to my numbers for specific gravity and viscosity, I always start with 10 pounds less and add additional water gradually to achieve my ideal casting state. Remembering that a gallon of water weighs about 8 pounds at standard temperature and pressure (STP), and 40 pounds of water is about 5 gallons.
In contrast, a clay body built for throwing can have up to 90 parts of plastic material (clays) and 10 percent non-plastics (filler, such as silica, feldspar, etc). Other successful ratios are 80/20, 70/30, 60/40, and I know of just two throwing bodies that are 50/50. Less water is required for a throwing/handbuilding body than for a casting body. Again, this depends on the materials chosen to build the clay body as well as how soft or stiff you prefer your clay.
Raw Material Selection
Building a successful casting body depends on testing various combinations of materials. I have designed a simple Casting Body Test Sheet (available in the Appendix of my book, The Mold-Making Manual) that is specific to materials that are domestically available in the US. Such a framework can easily be used to develop and test any number of clay bodies at a specific temperature. What I would recommend is that once you find a combination of non-plastic materials that provide a dense and vitrified fired body, that you keep these the same for all the tests. The plastics side of the formula should be those ingredients that your supplier stocks or can get for you, and as long as you adhere to the 50/50 rule, you can achieve a viable and well-balanced casting body.
Follow the 50/50 rule (50 clays or plastic materials, 50 non-clay materials or non-plastics). The 50 parts contain clays consists of kaolins and ball clays that are specifically blended for casting. Other clays can, of course, be used that are specifically designed and blended for slip casting. I designed a whiteware cone 5–6 casting body that casts easily, has low absorption and shrinkage, is fully vitrified, and fires white. Note that this is not a porcelain body; I am not concerned with translucency. My clay bodies always have a diversity of kaolins and a small amount of ball clay. Too much ball clay slows down the casting rate, and the iron content of most ball clays in a porcelain body will diminish the translucency. However, their smaller particle size will increase green strength. Old Hickory’s Ti-21 Ball Clay is an extremely low-iron ball clay but has a percentage of titanium oxide, which also may blur translucency.
My preferences for plastic materials (clays) includes: Opticast kaolin, FC340 ball clay, EPK, Ti21 ball clay, and Wilco.
Jonathan Kaplan Cone 5–6 Whiteware Slip Casting Body Formulation Worksheet
Non-Plastic Materials (Fluxes and Fillers)
I must confess that for many years when I started making my own casting slips, I was plagued by casting bodies that took a very long time to produce a sufficient casting thickness. What changed my paradigm were lengthy conversations with Russ Fish, who was then working in the lab at Old Hickory. Fish suggested FC 340, and to use only a percentage that could provide the necessary green strength and still not affect the permeability. For me, it was 5 percent and I still use this same amount today. Of course, many other clays and many other formulas will work. It is important to find the one formula that satisfies all your requirements.
My preferences for non-plastic materials (fluxes and fillers) includes: nepheline syenite A240, potash feldspar (depends on firing temperature), silica, and pyrophyllite.
Jonathan Kaplan Cone 5–6 Whiteware Casting Body 06/2018
Deflocculant Choices
The most common and popular deflocculant is sodium silicate. The chemical formula for sodium silicate is Na2SiO3. It is common practice to use sodium silicate in combination with soda ash, as soda ash helps dissolve any lignite present in the ceramic materials and helps to make the sodium silicate more workable. The combination of these materials produces a very strong and effective deflocculant. Never add undiluted sodium silicate to your mix, and never add all of it at one time.
The downside to using a sodium silicate/soda ash deflocculation system is that there is very little room for error and it is possible to quickly over-deflocculate the slip, rendering it useless. Another significant negative attribute is that it attacks the plaster mold surface, causing premature mold wear as well as a white efflorescence on the exterior of the molds. Making molds is time consuming and there is no need to cause any early mold deterioration by using sodium silicate and soda ash. Fortunately, there are other products that do not have any of these detrimental effects.
There are two Darvan products of interest to ceramic artists: Darvan 7-N (often referred to as Darvan 7) and Darvan 811.
Darvan 7-N
Darvan 7-N is recommended for use in the preparation of casting slips and refractory compositions. It is also used in glazes for rheological stability.
Darvan 7-N produces slips with a wide casting range. Furthermore, the casting rate is not decreased to a marked degree, even though the slip is near the minimum viscosity point.
Darvan 7-N produces slips that show little tendency to thicken on standing or become thixotropic. Ware cast from these slips is plastic and easy to scrap or remix.
During the casting process, very little Darvan 7-N is absorbed by the molds giving plaster molds a longer life.
Most whiteware bodies are readily dispersed by the addition of 0.2–1% of Darvan 7-N based on the dry body weight.
Darvan 811
Darvan 811 is used in vitreous and semi-vitreous bodies and in glazes.
Darvan 811 produces slips with a longer casting range.
Darvan 811 produces slips with a higher proportion of dry material
Darvan 811 does not block the pores of the mold and increases mold longevity.
Darvan 811 reduces hard spots, yellow spots, or soda spots on greenware, and it might affect the glaze application.
Darvan 811 does not produce calcium silicate scum.
Slip Preparation in the Studio
I have a 30-gallon fiberglass tank, purchased from Lehman Manufacturing many years ago, which holds 300 pounds of dry materials. I also have an older Mity Mite, a small ten-gallon mixer, also from Lehman Manufacturing, that I use for testing ten-pound casting-slip samples. I mix outside in a covered area next to my gas kiln, where I set up a table that has two scales on it, a small digital scale for measuring deflocculant, and a larger analog scale to weigh dry ingredients. My mixing procedure is fairly straightforward:
Always wear a properly fitted respirator when handling dry materials.
Make sure the mixing container is clean. Weigh out the necessary amount of water.
Weigh out the amount of deflocculant required and mix with the water.
Weigh out the plastic ingredients first, and then add them to the mixing container and allow them mix.
Weigh out the non-plastic ingredients and add them to the mix. If you are mixing slip with a handheld drill, this will take some time to properly disperse and wet the dry ingredients.
After thoroughly mixed, measure the specific gravity. If you have hit it correctly, congratulations! If not, make the necessary adjustments, and let the slip mix on a slow speed, preferably overnight to ensure complete dispersion and wetting of each clay particle.
In the morning, measure the specific gravity and viscosity. If your metrics are within the parameters, kudos to you!
Measurement: Specific Gravity and Viscosity
Specific gravity and viscosity are two very important metrics to understand, measure, and correct if necessary. After mixing slip, keeping a daily record of these numbers will provide a footprint of the inner workings, or the rheology of your casting slip. These numbers will change on a frequent basis, even daily, as the weather changes, barometric pressure differs, and depending on the conditions in your studio. Numbers don’t lie, and present conditions do not guarantee the same conditions over the week or month. Once your numbers are dialed in so that you know the best casting condition for your work, reliable, consistent, and predictable casting behavior are in your favor.
In my practice, I always test for specific gravity first, adjust if necessary, and then test the viscosity.
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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Like most things in ceramics, there are many ways to source casting slip. If making a slip yourself is of interest, the following discussion covers the basics, materials theory, and more.
Defining the Terms
Casting Rate: The time required by a casting slip to build up a sufficient clay wall thickness in the mold.
Deflocculant: Electrolytes that supply sodium ions. Chemically, electrolytes are substances that become ions in solution and acquire the capacity to conduct electricity.
Loss On Ignition (LOI): Refers to the burning out of carbonaceous materials and the driving off of chemically bound water present in all clays during the early stages of firing. LOI that does not burn off successfully can be responsible for many annoying glaze defects.
Newtonian Fluids: A fluid that has a constant viscosity, or thickness, even when force is applied.
Rheology: The study of how fluids flow, in this case, the array of characteristics that ceramic slurries exhibit (viscosity, thixotropy, etc.) over an entire range of applied shear conditions.
Shear: A force that makes one surface of a substance move over another parallel surface.
Specific Gravity: The weight of a volume of liquid divided by the weight of the same volume of water. This is sometimes referred to as relative density.
Thixotropy: The property of a slurry’s viscosity to decrease when subjected to shear, followed by a gradual increase in the viscosity when the shear is stopped. It is time dependent.
Viscosity: Resistance to flow of any fluid, caused by internal friction. High-viscosity fluids are thick (think cold honey) and low-viscosity fluids are thin (think water).
The Framework Behind Building a Slip-Casting Body
Building a successful slip-casting body requires the proper selection of materials and an understanding of the ratio between the plastic ingredients (clays) and non-plastic materials (silica, feldspar, talc, pyrophyllite, and sometimes frits). This ratio should always be 50/50, but I have seen formulas that vary slightly. Selection of the correct materials could be determined by availability, cost, and the temperature of your firing. Successful casting bodies use the correct amount of water and deflocculant, which will give a proper specific gravity and viscosity. Proper mixing and then testing your slip for the correct rheology, adjusting them if necessary, will provide you with a casting body that works for you.
Casting slips are counterintuitive to throwing bodies. To understand slip-casting bodies, we need to understand that they require more water than throwing bodies and a deflocculant. Here are some major things to know:
The 50/50 Ratio: Plastics/Non-Plastics
The goal in building a casting body is to use enough water to allow the dry materials to enter into an aqueous suspension, and then by adding a deflocculant, achieve the required rheological properties. The way to successfully achieve this is to have half the slip formula contain plastic ingredients (clays) and the remaining half, non-plastics, silica, feldspar, talc, pyrophyllite. Non-plastic ingredients can also be referred to as filler. This 50/50 ratio is the gold standard. There are, of course, exceptions to the rule, and the ones I have encountered stay pretty close to the 50/50, deviating by no more than five parts on either side to always add up to 100 parts. The one true exception I have worked with is a low-fire white casting body (cone 06–04), which is 60 parts plastic materials and 40 parts non-plastic materials (60 parts ball clay : 40 parts talc).
Water
Depending on the plastic ingredients and their metrics that yield an ideal casting body for you, the amount of water can vary from 28% to 40%. So, in a 100-pound dry mix that yields 10 gallons of liquid slip, the water can range from 2.8 pounds to 4 pounds. For my 30-gallon blunger, I start with 300 pounds of dry ingredients.
While I know that 120 pounds of water for 300 pounds of the raw materials I use will get me very close to my numbers for specific gravity and viscosity, I always start with 10 pounds less and add additional water gradually to achieve my ideal casting state. Remembering that a gallon of water weighs about 8 pounds at standard temperature and pressure (STP), and 40 pounds of water is about 5 gallons.
In contrast, a clay body built for throwing can have up to 90 parts of plastic material (clays) and 10 percent non-plastics (filler, such as silica, feldspar, etc). Other successful ratios are 80/20, 70/30, 60/40, and I know of just two throwing bodies that are 50/50. Less water is required for a throwing/handbuilding body than for a casting body. Again, this depends on the materials chosen to build the clay body as well as how soft or stiff you prefer your clay.
Raw Material Selection
Building a successful casting body depends on testing various combinations of materials. I have designed a simple Casting Body Test Sheet (available in the Appendix of my book, The Mold-Making Manual) that is specific to materials that are domestically available in the US. Such a framework can easily be used to develop and test any number of clay bodies at a specific temperature. What I would recommend is that once you find a combination of non-plastic materials that provide a dense and vitrified fired body, that you keep these the same for all the tests. The plastics side of the formula should be those ingredients that your supplier stocks or can get for you, and as long as you adhere to the 50/50 rule, you can achieve a viable and well-balanced casting body.
Follow the 50/50 rule (50 clays or plastic materials, 50 non-clay materials or non-plastics). The 50 parts contain clays consists of kaolins and ball clays that are specifically blended for casting. Other clays can, of course, be used that are specifically designed and blended for slip casting. I designed a whiteware cone 5–6 casting body that casts easily, has low absorption and shrinkage, is fully vitrified, and fires white. Note that this is not a porcelain body; I am not concerned with translucency. My clay bodies always have a diversity of kaolins and a small amount of ball clay. Too much ball clay slows down the casting rate, and the iron content of most ball clays in a porcelain body will diminish the translucency. However, their smaller particle size will increase green strength. Old Hickory’s Ti-21 Ball Clay is an extremely low-iron ball clay but has a percentage of titanium oxide, which also may blur translucency.
My preferences for plastic materials (clays) includes: Opticast kaolin, FC340 ball clay, EPK, Ti21 ball clay, and Wilco.
Non-Plastic Materials (Fluxes and Fillers)
I must confess that for many years when I started making my own casting slips, I was plagued by casting bodies that took a very long time to produce a sufficient casting thickness. What changed my paradigm were lengthy conversations with Russ Fish, who was then working in the lab at Old Hickory. Fish suggested FC 340, and to use only a percentage that could provide the necessary green strength and still not affect the permeability. For me, it was 5 percent and I still use this same amount today. Of course, many other clays and many other formulas will work. It is important to find the one formula that satisfies all your requirements.
My preferences for non-plastic materials (fluxes and fillers) includes: nepheline syenite A240, potash feldspar (depends on firing temperature), silica, and pyrophyllite.
Deflocculant Choices
The most common and popular deflocculant is sodium silicate. The chemical formula for sodium silicate is Na2SiO3. It is common practice to use sodium silicate in combination with soda ash, as soda ash helps dissolve any lignite present in the ceramic materials and helps to make the sodium silicate more workable. The combination of these materials produces a very strong and effective deflocculant. Never add undiluted sodium silicate to your mix, and never add all of it at one time.
The downside to using a sodium silicate/soda ash deflocculation system is that there is very little room for error and it is possible to quickly over-deflocculate the slip, rendering it useless. Another significant negative attribute is that it attacks the plaster mold surface, causing premature mold wear as well as a white efflorescence on the exterior of the molds. Making molds is time consuming and there is no need to cause any early mold deterioration by using sodium silicate and soda ash. Fortunately, there are other products that do not have any of these detrimental effects.
There are two Darvan products of interest to ceramic artists: Darvan 7-N (often referred to as Darvan 7) and Darvan 811.
Darvan 7-N
Darvan 811
Slip Preparation in the Studio
I have a 30-gallon fiberglass tank, purchased from Lehman Manufacturing many years ago, which holds 300 pounds of dry materials. I also have an older Mity Mite, a small ten-gallon mixer, also from Lehman Manufacturing, that I use for testing ten-pound casting-slip samples. I mix outside in a covered area next to my gas kiln, where I set up a table that has two scales on it, a small digital scale for measuring deflocculant, and a larger analog scale to weigh dry ingredients. My mixing procedure is fairly straightforward:
Measurement: Specific Gravity and Viscosity
Specific gravity and viscosity are two very important metrics to understand, measure, and correct if necessary. After mixing slip, keeping a daily record of these numbers will provide a footprint of the inner workings, or the rheology of your casting slip. These numbers will change on a frequent basis, even daily, as the weather changes, barometric pressure differs, and depending on the conditions in your studio. Numbers don’t lie, and present conditions do not guarantee the same conditions over the week or month. Once your numbers are dialed in so that you know the best casting condition for your work, reliable, consistent, and predictable casting behavior are in your favor.
In my practice, I always test for specific gravity first, adjust if necessary, and then test the viscosity.
Unfamiliar with any terms in this article? Browse our glossary of pottery terms!
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