P300

Mid-temperature, medium plastic, vitreous, off-white burning, refined material porcelain. P300 is a mix of 45% of two plastic kaolins, 35% of two feldspars, and 12% flint. It also has a small amount of added talc for better vitrification and 2-3% bentonite for extra plasticity. However, even though it does not contain any ball clay like the darker burning P380, P300 is not a true porcelain at cone 6 since it is not translucent and it does not have a totally clean and pure white fired surface. However, it does provide a better working and more tolerant firing alternative to true porcelains.

Process Properties

P300 is a smooth and slick fine grained body. It relies almost entirely on plastic kaolin for workability and dry strength (although there is a small amount of added bentonite). The result is a body which is not as plastic and does not throw as well, especially when soft, as more traditional white stonewares containing plastic ball clay.

P300 also demands more than the usual attention during drying, especially on difficult shapes like large flat plates. Time is required to remove all the water since the bentonite acts as a barrier to its passage. If you need to join sections, be sure to follow good practice (i.e. use slip containing an aggregate like molochite, use pressure and lateral movement when joining, dry pieces evenly, avoid making ware with an uneven cross section, put the focus on 'even' drying).

Firing

P300 is intended to vitrify to near zero porosity just as it reaches cone 6 and to be fairly stable through cone 7. At cone 8 it is not reliable and will begin to bloat and warp badly. Below cone 6 its porosity rises rapidly, however, for most functional applications it has adequate strength at cone 4-5.

Like all bodies of this type, P300 has a fairly high fired shrinkage so ware must be able to slide against the shelf or it will warp. It is also wise to make sure kiln shelves are covered with a layer of kiln wash to prevent ware from sticking and that kiln wash or alumina is used to separate lids fired on the ware. Also, be sure that your ware has a cross section with inherent structural strength that makes it resistant to warping during firing.

P300 fires almost as white as our M370 and quite a bit whiter than our P380. However it is not a 'bone china' in that it is not translucent, smooth surfaced, and paper-white. P300 does provide much better working properties than true porcelains, but it is not as good as P380 or M370.

Glazing

P300 has a fairly low thermal expansion compared to our stoneware bodies, thus glazes which work fine on the latter often craze on P300. Even if your fired glaze appears to fit we recommend that you do an ice water:boiling water immersion test to make sure. Feel free to call us if you need help in adjusting the expansion of a glaze to make it fit this body better.

Since P300 is so low in iron you will find that colored glazes will produce much brighter and cleaner colors.

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/cone6.htm. This page mentions a variation named G1214W glossy that works well. Use G1214V for a matte and add as much silica as it will take. 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: 4.5-5.5%
 Dry Strength: n/a
 Water Content: 21.5-22.5%
 Drying Factor: c120+
 Dry Density: n/a

Sieve Analysis (Tyler mesh):

     48-65: 0.0-0.1%
  65-100: 0.0-.1
 100-150: 0.0-0.1
 150-200: 0.3-0.8
 200-325: 3.0-5.0

Fired Shrinkage:

 Cone 4: 7.5-8.5%
 Cone 5: 8.0-9.0
 Cone 6: 8.0-9.0

Fired Absorption:

 Cone 4: 1.0-2.0%
 Cone 5: 0.5-1.5
 Cone 6: 0.0-0.5

Chemical Analysis

 CaO       0.3
 K2O       2.3
 KNaO      0.2
 MgO       2.1
 Na2O      2.1
 TiO2      0.6
 Al2O3    23.5
 SiO2     62.2
 Fe2O3     0.5
 FeO       0.0
 LOI       6.3%