Understanding the Terra Cotta Slip Casting Recipes In North America

Section: Clay Bodies, Subsection: Formulation

Description

This article helps you understand a good recipe for a red casting body so that you will have control and adjustability.

Article

In North America there is a raw material that makes the creation of a low fire red casting body very easy. It is called Redart. This clay is air floated, high in iron, low in soluble salts and matures at a low temperature (around cone 1). In addition Redart deflocculates very well. Adding iron oxide to a clay causes it to gel badly on dispersion however the natural form of iron in Redart does not do this at all. To use the casting process efficiently you must understand how deflocculation works. Believe me, if you are new to casting and have never mixed a heavy casting slip with low water content you will never go back to a simple clay water mix again after you learn the deflocculation process (see the link to an article on this).

Although it is possible, there are a number reasons why you would not normally use a 100% Redart formula.

Redart plasticity and dry strength are quite low so added ball clay is needed
If you need to fit commercial glazes you need to increase the thermal expansion of Redart, this is done by adding talc
Using a body of one ingredient only is asking for trouble. Materials do vary and it is best to have a body that is a mix so that variation in any one material is diluted by the others.
There are many advantages to having some sort of non-plastic filler in the body, especially for casting since it speeds up the casting rate and improves drying performance.

Red Casting Powder Starting Recipe

Redart     50
Ball Clay  25
Talc       25

Recipe Notes

IMCO makes a red low fire clay named Banta that is much more plastic, you could restore some color by using it in place of the ball clay.
This is one time where you actually want to use a dirty talc and ball clay, that is, ones that contain some iron.
Since the Redart has been cut to 50% of the recipe it stands to reason that the body will only fire half as red. However keep in mind that the iron color does not really begin to develop into a bright red until cone 03-02, so if you are firing lower than this the color is not as much lighter as you might think it would be.
The talc does improve the casting performance and working properties of the body so it is a desirable ingredient.
You can adapt this body to higher temperatures by changing the talc to feldspar and reducing the amount of Redart in favor of kaolin and silica.
If you can find a non-plastic finely ground red burning filler (i.e. a shale, argilitic rock, etc), this could be the secret to the reddest possible color. Note that there are red burning ball clays on the market also.
If you find that a white burning scum deposits on the surface after drying you may need to add about 0.3% barium carbonate to the dry mix, it will precipitate the soluble salts inside the body so they do not come to the surface during drying.

Terra Cotta Process Considerations

At cone 06 almost any mix not containing expensive fluxes is going to be non-vitreous and highly porous. However by 04 this figure drops dramatically. By cone 03-02 it is possible to have stoneware properties in most terra cottas. This is because natural red clays contain so much flux. Consider some of the considerations you must think about when formulating a terra cotta casting body. Body color is usually important, people want the deepest possible shade of red in their terra cotta. If you are using commercial glazes fit is a big issue and the body needs to have significant talc, up to 50%. This is not a problem for white bodies since you can couple it with white burning plastic ball clay. But for red bodies it is a challenge because there is not enough room in the recipe for flux given the amount of clay needed to achieve workability (the lower plasticity red plus the ball clay). This means you have a tug of war between color, fit of commercial glazes, maturity and workability. An obvious way to ease things is to formulate your own glazes, this enables removing the talc completely. As mentioned, you cannot just add iron to a white burning clay to get terra cotta color since iron causes slip to severely gel. In my experience you need to use at least 50% dark red clay and deal with the opposing issues of maturity and working properties in the remainder. In North America the primary red clay used in terra cotta bodies is Redart. It casts very well but it is just not plastic enough so you may need to augment it with ball clay to create a slip that has adequate dry strength and shrinkage to pull away from the mold. However to get a dark enough color you normally want a high proportion of red clay. If you can fit a glaze to the talcless version of the body, you may be able to work with a recipe of up to 80% red clay and 20% ball clay.

However it is not typical to fire earthenware this high since color and resistance to warp change dramatically with only slight over firing and the red color is lost under transparent glazes. The beautiful red colors depend on stopping well short of vitrification. Many commercial wares are 6-8% porosity and yet they are very strong, so do not worry about higher-than-stoneware porosity that is common with earthenware. Just make sure the glaze fits (can survive a hot water:ice water test) and strengthens the ware.

In terra cotta bodies the glaze is not nearly as well interfaced and adhered to the body as with stonewares. Glazes are dramatically more prone to crazing, shivering and chipping off. To make good quality earthenware requires much more technical glaze knowledge than for making stoneware.

Combining Casting a Thrown or Hand-build Elements

This is a very practical aspect of terra cotta. Some commercial terra cotta throwing bodies will deflocculate. However you will find that elements cast from the recipe given here can be joined onto leather hard objects using the casting slip itself as a glue. You may need to adjust the recipe to match color it ware is not to be glazed.

Deflocculation

Deflocculation refers to the magic process of creating a slurry with only half the amount of water that would normally be necessary (see the link to an article on this topic). Generally you measure the water and put it into a container, start your propeller mixer and add most but not all of the deflocculant. Then you add the powder body mix. The slurry becomes very heavy so you need a powerful mixer than can run for hours. Getting the last bit of powder to mix in can take some time so patience is required.

Powder Mix	100
Water	36.0% of dry amt
Soda Ash	0.05-.1% of dry amt
Sodium Silicate	about 0.2 to 0.3% of dry amt
or
Water	36% of dry amt
Darvan	0.5% of dry amt

Note that water and deflocculant amounts are examples, you need to know how to measure specific gravity and viscosity to adapt their amounts to your materials and water (see the article on deflocculation for more information).

Being Realistic

Terra cotta ware is never going to be anywhere near as strong as stoneware. Terra cotta is a matter of tradition and getting a nice warm red colored body. Terra cotta is much stronger than white burning earthenware and aesthetically it goes well with brightly decorated light colored opacified glazes. If you can make fine quality earthenware then you can be proud, it is not an easy job.

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Authors

Tony Hansen (Owner)

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