Raku Throwing

Low expansion plastic throwing and handbuilding body with fine grog.

Raku-Throwing tries to achieve a balance between to opposing challenges of the raku process: The ability to withstand rapid changes in temperature without cracking and being smooth enough to throw comfortably on the potter's wheel.

The secret to the unique fired properties of Raku-Throwing are four-fold:

1) It employs 15% calcined kaolin grog rather than traditional silica sand, brick or fireclay grogs. The grog particles contain almost no quartz, thus they are much more dimensionally stable during cooling through quartz inversion.

2) It contains a significant amount of fine phyrophyllite powder, a mineral with exceptional stability in radical temperature-change environments.

3) The body contains minimal quartz sand particles; it is made purely from our finest high quality stoneware materials.

4) It contains a significant addition Kyanite, a material well known to stabilize raku bodies against thermal shock failure.

Process Properties

The grog constituent greatly improves the drying performance, although there is still need for normal care since the grog is of a fairly fine particle size. The grog helps to terminate micro-cracks before they grow and assists in the healing process during joining.

Thrown forms should dry without problem unless they have a very uneven cross section. It may be necessary to cover larger pieces with cloth and plastic to slow drying down enough to make sure it occurs evenly throughout. Drying time and technique should be whatever is necessary to achieve evenness, whether that be a day or a month.

Raku-Throwing is relatively smooth compared to other raku bodies containing coarse angular silica sand grog (some contain 20%+ plus-35 mesh). It throws very well. The fine stoneware base coupled with the small size and flat particle shape of the grog suppresses the normal abrasive nature of added grog.

Since Raku-Throwing has a fine clay base it burnishes well for a grogged body. You might find it helpful to make a slip of the material and screen out the grog, then paint this on and burnish.

Firing

The Raku-Throwing base clay vents water very well during early stages of firing. However if you bisque in an electric kiln, consider preheating overnight on low to give plenty of time for all water to be removed.

Raku-Throwing is a significant departure from our previous raku body which had no anti-thermal shock additives and was very refractory. Like any other raku material, it is porous and non-vitreous in a typical raku firing. However, you can fire it from cone 6-10 to produce very strong and vitreous ware. However be careful with sculpture pieces and overhung shapes as these will warp during firing at higher temperatures.

One caution about firing: The raku community tends to employ some fairly hazardous materials and these are often used in quite large amounts and are applied in manners involving somewhat less care than is warranted. Be careful about breathing kiln vapors, using leaded and heavy metal materials. Be cautious about eating and drinking from raku vessels.

Glazing

Since this body has a low thermal expansion, glazes will tend to craze. While this is often desirable in raku, keep in mind that a crazing glaze also severely compromises the already weak ware. To minimize crazing, avoid large amounts high expansion materials like nepheline syenite and potash feldspar in the glaze recipes.

Designing a raku glaze is somewhat like making one for low fire ceramics. A viable approach is to create a good clear base glaze and then add colorants, opacifiers, and modifiers. The metallic effects, for example, are simply made by the addition of significant amounts of metal oxides to a clear base; you can learn these by studying existing recipes. Lead materials are not really necessary for raku, there are plenty of boron frits that will melt in low fire (i.e. Ferro 3110, 3124, 3134, 3195). A good base glaze can be as simple as 90% frit and 10% kaolin. Many raku potters also employ gerstley borate as a glaze base. A raku glaze can be made from 80% of this material and 20% kaolin. Others use a 50:50 mix of nepheline syenite and gerstley borate, although this is likely to produce severe crazing. Note however that gerstley borate is not a consistent material and it imposes application problems with the glaze slurry.

Physical Properties

 Drying Shrinkage: 5.5-6.5%
 Dry Strength: n/a
 Water Content:  21.5-22.5%
 Drying Factor: C110+
 Dry Density: n/a

Sieve Analysis (Tyler mesh):

     +35: 5.0-7.0%
   35-48: 3.0-5.0
   48-65: 3.0-5.0
  65-100: 4.0-7.0
 100-150: 3.0-7.0
 150-200: 6.0-10.0
 200-325: 8.0-12.0

Fired Shrinkage:

 Not tested in raku
  Cone 6: 4.5-5.5
  Cone 8: 5.0-6.0
 Cone 10: 5.0-6.0

Fired Absorption:

 Not tested in raku
  Cone 6: 2.0-3.0
  Cone 8: 0.5-1.5
 Cone 10: 0.0-0.5

Chemical Analysis

 CaO       0.2
 K2O       1.8
 MgO       0.5
 Na2O      0.1
 TiO2      0.8
 Al2O3    22.6
 P2O5      0.0
 SiO2     66.7
 Fe2O3     1.3
 MnO       0.0
 LOI       6.0%