Techno File: Clay Restoration

Many potters make a practice of reclaiming clay, but how do you restore the all of the properties that originally attracted you to the clay body?

Defining the Terms
Ball Clay: A 2:1 aluminosilicate clay particle used to impart plasticity in a
clay body.
BentoneMA: Highly refined montmorillonite clay that imparts high plasticity and is used primarily in
porcelain bodies.
Clay Body: A formulation of one or more types of clay, with feldspar and silica additions to produce specific firing properties at specific cone values.
Feldspar: Natural alumino silicate minerals that supply sodium or potassium that act as a flux in a clay body.
Plasticity: Mechanical property that allows a clay body to hold its shape after pressure from various forming techniques has been applied.
Plasticizer: A clay mineral or synthetic additive that imparts malleability or flexibility to a clay body.
Silica: Silicon dioxide (SiO2), also referred to as flint in older recipes, and as quartz in others. Primary milled from quartz, it is the primary glass former in most glaze and clay recipes.

Potters refer to it as clay fatigue when describing the loss of plasticity. Every time you throw, fine particles of clay, silica, and feldspars are pulled from the clay body and deposited as slip on your hands. When clay is reclaimed after throwing, a percentage of plasticizer, silica, and feldspars are lost, pending the amount of water and pressure applied. In order to correct this loss, you must determine the level of plasticity your fresh clay had. Although it is not scientific, the first step is to conduct the “slip test.” Every potter is aware of the amount of slip that is deposited on their hands when they throw fresh clay. The amount of slip is an indication of the type and levels of plastic materials (clays) used in formulation. Use this simple guide:

  1. Heavy slip buildup that passes between your fingers and requires constant removal indicates higher levels of plasticizers.
  2. Medium slip buildup with some slip passing between your fingers requiring occasional removal indicates medium levels of plastic materials.
  3. Light slip that requires occasional removal, but typically does not ooze, indicates lower levels of plasticizers. See sidebar for slip-test protocols.

Of course every potter applies different levels of pressure and the amount of water they use differs. The trick then becomes to take the slip test after you center and are ready to start coning. Clean your hands before you start. Make four passes at coning, pulling up, and compressing, wetting one hand between each of the four passes. At the end of the fourth pass, assess the amount of slip deposited on your hands. Heavy, medium, or light slip? With this information, you now have an insight to how much plasticizer is in your favorite clay.

1 Heavy slip leaves dense build up on the hands with pieces of clay if pressure is used, and oozes through the fingers, requiring frequent removal.

2 Medium slip coats the hand with some oozing through the fingers.

If your slip test produced heavy slip (1), then your clay has higher additions of fine particle ball clay. Medium slip (2) indicates either medium levels of plasticizer additions or a lower plasticity ball clay was used to make the clay body. Light slip (3) levels indicate lower levels of an added plasticizer. All throwing bodies require the addition of plasticizers in order to form properly on the wheel. These additions range from high plasticity ball clay such as Foundry Hill Creme (FCH) ball clay, to higher levels of medium plasticity ball clay such as OM 4. Premium white porcelains often use BentoneMA as the plasticizer in order to maintain higher levels of translucency.

3 Light slip barely coats the hand, with some bare skin visible.

Rejuvenation

Each time you reclaim a batch of clay, the loss of fine particles of clays, plasticizer, silica, and feldspar increases until it begins affecting the original working properties. The loss of plasticizers affects malleability; while the loss of feldspar and silica will lower the vitrification levels and increase absorption. Using excessive water when throwing can accelerate the loss of fine particles as well. The easy fix is to keep the dry blend of your favorite clay on hand to add to your reclaim, if it is available. Most often it is not, so some simple clay chemistry is required to create a fix-it mix blend that will work with your clay body.

Most stoneware bodies have some amount of ball clay in their composition. Mix 80% OM 4 ball clay, 10% silica, and 10% nepheline syenite or 10% G200 feldspar until it is well blended. Add 1 cup of dry mix per gallon of slurry if you had heavy slip, ½ cup for medium, and ¼ cup for light slip results from your original slip test. This one-size-fits-all blend will help maintain plasticity and ensure absorption rates do not increase due to silica and feldspar loss. Check your reclaim as you go along and adjust additions to suit your taste. Before you begin to test for plasticity, remember it will take several days before full plasticity develops. You can check it by doing a simple coil test: roll the clay to a ¼-inch-thick coil and wrap it around your middle finger. If no stress cracks develop, then it is ready for use. However, if it acts rubbery, then reduce your fix-it mix additions by 13.

Making a dry blend of fix-it mix to add to porcelain requires a bit more chemistry, starting with analyzing the color of the clay. White or off-white color indicates that premium plasticizers or a whitish ball clay such as C&C have been used for plasticity. Does your porcelain body produce translucent properties after glaze firing, or does it fire to a bright white finish? Premium white porcelains use BentoneMA as the plasticizer. Most white porcelains typically create the same slip buildup because BentoneMA is the primary plasticizer. Ball clay additions would minimize translucency due to titanium content. After answering these questions, you need to decide how much of the necessary ingredients to buy, which is directly related to how much you reclaim. Most suppliers sell BentoneMA in ¼-pound, 1-pound, and 5-pound bags. This material is pricey, but the good news is that a little goes a long way. Most porcelain bodies use nepheline syenite as a body flux, so limit the feldspar replacement in your mix to nepheline syenite.

Test Protocols for Clay Restoration

Start with 1000 grams (dry weight) of 80% clay(s), 10% silica, and 10% potash feldspar—a typical stoneware formula—and mix with water to a usable consistency. Cone the clay on the wheel with heavy pressure and water. Dry and weigh the collected slip, with an added 10% approximation for slip lost in the water. For porcelain, two 1000-gram recipes were used: Porcelain A (50% Grolleg, 20% silica, 30% potash, and 2.5% BentoneMA) and Porcelain B (35% Grolleg, 15% Foundry Hill Creme ball clay, 20% silica, and 30% potash feldspar (cone 6)).

If your clay is bright white and/or translucent, dry blend a fix-it mix of 50% Grolleg kaolin, 25% nepheline syenite, and 25% silica together and set aside. Add 2% BentoneMA to this white porcelain dry mix and thoroughly blend. Add ½ cup of dry mix per gallon of slurry or ¼ cup if you prefer slightly less plasticity. Spend extra time blending this slurry addition to ensure even distribution of this specialty plasticizer.

If your fresh porcelain clay body is off white, gray, tan, or fires off white, it probably includes ball clay. Porcelain tile bodies incorporate larger particle-sized kaolin such as #6 Tile clay, and have a light tan color. Most often ball clays are used as plasticizers in order to keep their price points lower. Other porcelain bodies may be light gray in color, which is the result of another type of ball clay. White porcelain follows the pure standard of 50% kaolin, 25% silica, and 25% feldspar. Bodies that use ball clay as the plasticizer follow a standard of 35% kaolin, 15% ball clay, 25% silica, and 25% feldspar, and are less white, with no translucency. Some recipes follow a standard of 25% kaolin and 25% ball clay, 25% silica, and 25% feldspar, which is known as 50/50 porcelain.

With porcelain clay bodies made with ball clay, the slip test becomes relevant to determine the ratio of ingredients when mixing a dry maintenance blend. Heavy slip indicates the porcelain is closer to equal parts of kaolin and ball clay. Medium slip indicates a mixture toward the 35% kaolin and 15% ball clay range. In this case, leave light slip buildup off the list because of the numerous variables involved. You must also make a decision of how precisely you want to duplicate the original blend. Gray-body porcelains may incorporate FHC or an equal ball clay, which has a higher plasticity index than OM 4. FHC is medium gray in color. Tan colored porcelains use OM 4 or equivalent types of ball clay for plasticity, which give the porcelain body its color.

The fix-it mix for ball-clay porcelains that produce heavy slip buildup is 25% EPK kaolin, 25% OM 4 ball clay, 25% silica, and 25% nepheline syenite. For medium slip buildup the fix-it mix is: 35% EPK, 15% OM 4, 25% silica, and 25% nepheline syenite. Add 1 cup per gallon of slurry. If the body color is gray or if you prefer high plasticity, swap out the 15% OM 4 ball clay for FHC. Add ½ cup of dry mix per 1 gallon of slurry.

If you are certain bentonite has been used as a plasticizer, then only add up to 18 cup per 1 gallon of slurry. Bentonite should be dispersed slowly in a cup of warm water first, then add this solution slowly to your clay slurry, mixing as you add it.

Put It to the Test

These are one-size-fits-all remedial fixes that do not require exact clay chemistry and incorporate pocket-book friendly ingredients. Once you throw the reclaimed clay using these fixes, make a determination if the plasticity level is too high or too low for your personal use. The true test is to compare your reclaim to the original clay body, putting both to the coil test as mentioned above. You should also perform an absorption test if you are making functional ware. If absorption has risen above 2%: increase the fix-it mix by 10% over the initial addition to help lower absorption back into range. If you own a pugmill, estimate how many gallons of slurry the chamber holds and multiply the fix-it mix accordingly.

the author Thomas Anderson has spent the last decade researching technical papers from various universities on clay formulation. He is currently writing formula limits for clay chemistry.

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