Terrastone

Low temperature, fine ground, plastic, red-burning, native terra cotta body intended for use in schools. Terrastone is a blend of two iron bearing native Plainsman materials. Unlike most of our other bodies, it does not contain any barium carbonate. Terrastone is pleasant to work with and burns to the classic terra cotta color. It is suitable for the production of functional and decorative ware in low temperature electric oxidation firing. We recommend the use of commercial bottled glazes. If you are using this material in an educational setting, on our Internet web site there is an article on making the children's first experience with clay a memorable one. We also have a number of videos available on clay basics.

Process Properties

Terrastone is dark brown-red in pugged color and has medium plasticity. Terrastone feels smooth but yet it still has a wide range of particle sizes immediately below the tactile threshhold, producing good dry strength and density combined with a pleasant texture. However it is still considered a fine-grained body and is not suitable for large sculptural forms. Big bowls, plates and other shapes that present difficulty in drying, should be made with caution. If you are using this body in educational settings with children, we have information that will help you make their first experience with clay a memorable one.

To assure successful drying without cracks, use low water content slip to join pieces, apply as much pressure and lateral movement as possible when joining, and dry the ware evenly (cover with cloth and plastic if necessary).

Firing

Cone 04

Because it does not contain barium carbonate, Terrastone has one limitation that you must be aware of. The natural soluble salts in the clay come to the surface during drying and are left in a layer whose distribution across the surface is determined by the nature of the drying. After firing this surface film appears as a whitish scum on the normally red body. This is the same effect as the efflorescing that occurs on red terra cotta brick walls. However, this behavior is not normally an issue if the ware is being glazed. If you are doing larger unglazed pieces you can wash this scum off using an acid product intended for this purpose (available at brick or building supply stores).

Terrastone fires to a light red at cone 04-06. As temperature is gradually increased, the red color intensifies to a brick red at cone 02. Further increase shifts toward brown, until it reaches its maximum strength around cone 1. The body becomes volatile above this range and small changes in chemistry, mineralogy and particle size can mean that at cone 4 one run may hold together while another may fail with sudden expansion, bloating, or warping. Furthermore, the color evolution of this material is not linear. Around cone 01 the shift is much more rapid. Thus, ware fired in this range will be subject to wide variations in color with only a slight changes in firing temperature.

Glazing

Although it is relatively easy to adapt our recommended low-fire glaze recipes on this body, most people use commercial glazes. You may have to try different brands to find one that fires craze-free in your circumstances. However in many educational settings, crazing will not matter.

If you intend to use a white zircon opacified boron glaze on this body, we recommend our Majolica clay instead. Its recipe is very similar to Terrastone. Remember that making functional ware at low temperature requires good command of every aspect of the process to produce serviceable ware.

Glaze Recipes

You can develop a compatible glossy or matte base for this body from our suggested starting point base recipes available on our Internet web site at http://digitalfire.com/education/glaze/cone06.htm. Information is given on how to fit the glaze to your body and how to customize it it for colors, opacity, speck, variegation, etc. For slip decoration, be careful to match drying and fired shrinkage of the slip with the body since low temperatures generate little glass to adhere the slip.

Physical Properties

 Drying Shrinkage: 5.5-6.5%
 Dry Strength: n/a
 Water Content: 20.0-21.5%
 Drying Factor: C120
 Dry Density: n/a

Sieve Analysis (Tyler mesh):

     +48: 0.5-1.5%
   48-65: 3.0-5.0
  65-100: 4.0-6.0
 100-150: 4.0-6.0
 150-200: 5.5-8.5
 200-325: 6.0-9.0

Fired Shrinkage:

 Cone 04: 2.0-3.5%
 Cone 02: 4.0-5.0
  Cone 2: 6.5-7.5

Fired Absorption:

 Cone 04: 8.0-11.0%
 Cone 02: 5.0-7.0
  Cone 2: 1.0-2.5

Chemical Analysis

 BaO       0.0
 CaO       0.4
 K2O       3.1
 MgO       1.1
 Na2O      0.1
 TiO2      0.6
 Al2O3    14.6
 P2O5      0.2
 SiO2     69.6
 Cr2O3     0.0
 Fe2O3     5.1
 MnO       0.0
 LOI       5.1%