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P300 is a mix of 45% kaolin, 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.
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).
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.
We specifically recommend liner and base glazes at our Liner and Base Glazes page. (e.g. M370 Transparent, Matte Base). We manufacture these but also provide the recipes if you want to mix your own. The M370 Transparent and Matte Base work well on P300.
As an alternative glossy transparent base, try G1216L.
For slip decoration, be careful to match drying and fired shrinkage and thermal expansion of the slip with the body.
Drying Shrinkage: 4.5-5.5% Dry Strength: n/a Water Content: 21.5-22.5% Drying Factor: c120+ Dry Density: n/a
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
Cone 4: 7.5-8.5% Cone 5: 8.0-9.0 Cone 6: 8.0-9.0
Cone 4: 1.0-2.0% Cone 5: 0.5-1.5 Cone 6: 0.0-0.5
CaO 0.3 K2O 2.1 KNaO 0.1 MgO 1.0 Na2O 0.6 TiO2 0.8 Al2O3 25.1 SiO2 63.1 Fe2O3 0.3 FeO 0.0 LOI 6.6%