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P580

Description

High temperature, smooth, medium plastic, grey-white firing, refined body for reduction and oxidation porcelainous stoneware.

P580 is a variation on the classic '25 Porcelain' mix (25% ball clay, kaolin, feldspar, silica). It uses a blend of 36% ball clays and 12% kaolin to produce a body of greater plasticity. It also has additional bentonite to increase plasticity further. P580 maintains excellent drying properties and is well suited to machine production since it has high leather hard and dry strength compared to porcelains.

While P580 is not quite as white as fine porcelains, it still maintains near zero absorption for functional ware. Thus it is a material-of-choice compared to porcelains if the fired color is acceptable for your application. However, if you can tolerate a slightly darker fired color, we recommend H555.

Process Properties

P580 is a smooth and slick fine grained body that throws and dries quite well compared to all-kaolin porcelains. Its ball clay complement gives it a robust nature that makes it a good choice for machine forming and even hand building. It generates very little slip during throwing and does not work as well when too soft as do more plastic bodies.

However, P580 is fine grained and thus it cannot be expected to dry and handle as well as a coarser stoneware. If you need to join sections, be sure to follow good practice (i.e. use slip containing an aggregate like molochite, dry ware evenly, use as much pressure and lateral movement as possible when joining, make ware with an even cross section).

Firing

P580 fired bars. Cone 10R top. Cone 8 to 11 oxidation (from bottom up).

P580 fires darker than P600 or P570. However, its color is stable across the cone 8-10 range in oxidation and its reduction color is quite consistent from firing to firing.

P580 is quite mature (but not vitrified to zero porosity), it contains 24% each of feldspar and silica. It thus produces a very strong product. Thus you must take precautions to make shapes that have inherent strength to resist warping (i.e. avoid goblets with flared bases, flared or overhung bowls). Kiln shelves must also be be flat. You may need to use a little kiln wash to separate lids and vessels.

Glazing

P580 has a fairly high flint content and is thus easier to fit glazes to than P600.

P580's high strength can be severely compromised if a glaze is under excessive compression or tension. We recommend that you stress-test a piece of ware using a boiling water:ice water test. Ware should be able to survive several two-minute cycles before trouble appears. If you need assistance to adjust the thermal expansion of your glazes, please contact Plainsman.

If you wish to use slip on your ware, make it from a base of P580 itself if possible.

Glaze Recipes

Commercial brush-on glazes offer many colors and surfaces. For functional ware check for glaze fit (vital for quality functional ware). Do not assume food safety of brightly colored glazes in your kiln and with layering without a leach test (e.g. GLLE test). Consider using transparent or white liner glazes for food surfaces.

Mixing your own glazes is practical (with our clear guidelines even beginners can make dipping glazes that go on silky smooth and evenly and dry in seconds). If you already do this using recipes from the web, be careful. High-feldspar glazes (having more than about 35%) often craze. Some recipes rely on high melt fluidity to encourage crystallization and variegation (often because of inadequate SiO2 and Al2O3 or containing Gerstley Borate or Frit), view these with suspicion for leaching and cutlery marking; test them well (also test the additionless versions). Be suspicious of any glaze not having good documentation.

Consider using our G1947U glossy or G2571A matte base recipes, just add colorants, opacifiers, variegators (you will find links to much more information and pictures about these). If you have a recipe that is troublesome, consider transplanting its opacifiers, colorants and variegators to these bases instead. http://ravenscrag.com and http://albertaslip.com also have many recipes that work well on porcelains.

Crazing: Functional ware must remain craze-free (crazing is unsanitary and drastically reduces ware strength). Because ware is not crazed out of the kiln does not mean it will not do so with time. Do cycles of a boiling water:ice water immersions (BWIW test) on a piece to test glaze fit (by stressing it to bring out any crazing or shivering tendencies).

Glaze slurry consistency and quality: A secret to achieving even glaze coverage is controlling the thixotropy and specific gravity of the slurry, both in freshly mixed and stored batches. A glaze of the right specific gravity and having a slightly gelled condition goes on to bisque ware evenly, does not drip and dries in seconds. Always screen glazes when first making them (80 mesh). Be alert to any particulate that may appear after storage (e.g. precipitates) and screen again if needed.

For slip decoration and engobes be careful to match the fired shrinkage of the slip with the body. Where we do not recommend a specific engobe recipe use a formula based on the porcelain itself. Add 2% VeeGum or Bentonite (the extra stickiness helps it adhere well to leather hard ware). Be careful about adding fluxes (e.g. frit), this increases fired shrinkage (the mismatch with body can cause flaking) and can compromise opacity.

If you want to develop and mix your own glazes and engobes consider getting an account at http://insight-live.com. You can organize a methodical development program and adopt better methods of testing (e.g. melt fluidity, thermal stress, slip-fit tests).

Thermal Expansion

We do not supply a thermal expansion value. The reason is that such numbers often mislead users. First, a body has different thermal expansion characteristics when fired at different temperatures, schedules and atmospheres. Dilatometers are only useful when manufacturers can measure bodies and glazes over time and in the same firing conditions. If a chart is supplied here, please view only as a way to compare one body with another.

Another significant issue is that many customers compare measured thermal expansion numbers with calculated values of glazes in efforts to fits those glazes to a body. This does not work. Calculated values are relative only and have limitations that must be understood. The best way to fit glazes to your clay bodies is by testing, evaluation, adjustment and retesting. For example, if a glaze crazes, adjust its recipe to bring the expansion down (using your account at insight-live), fire a glazed piece and thermal stress it (using an IWCT test, 300F into ice-water). If it still crazes, repeat the process.

If we recommend a base clear or glossy glaze, try calculating the expansion of that as a rough guide to know whether your glazes will fit.

Physical Properties

 Drying Shrinkage: 5.0-6.0%
 Dry Strength: n/a
 Water Content: 21.0-22.0%
 Drying Factor: C110+
 Dry Density: n/a

Sieve Analysis (Tyler mesh):

    +100: 0.0-0.2%
 100-150: 0.0-0.5
 150-200: 0.2-1.0
 200-325: 2.0-4.0

Fired Shrinkage:

   Cone 8: 6.0-7.0%
  Cone 10: 6.5-7.5
 Cone 10R: 7.0-8.0

Fired Absorption:

   Cone 8: 1.0-2.0%
  Cone 10: 0.3-0.8
 Cone 10R: 0.0-0.5

Chemical Analysis

 CaO       0.2
 K2O       2.9
 KNaO      0.0
 Li2O      0.4
 MgO       0.1
 Na2O      0.8
 TiO2      0.6
 Al2O3    18.9
 P2O5      0.0
 SiO2     70.2
 Fe2O3     0.6
 FeO       0.0
 MnO       0.0
 LOI       5.3%

Compared to Others

The decline of the ceramic industry in North America has impacted the price, availability and quality-for-ceramics of raw materials from which porcelains are made (especially ball clays and kaolins, which have seen increases in soluble salts, foreign particles and iron specks). In addition, we do not have filter-pressing, pre-mixing and stainless-steel pugging equipment (these would drastically increase prices). While you cannot make Wedgewood-quality ware it will be far whiter and cleaner than our stonewares. Notwithstanding that, be aware that transparent glazes carry a chance of isolated specks using our standard porcelains, it is better to use white and colored glazes on these. If you absolutely need a clean, white, translucent porcelain consider using our premium products, Polar Ice and P700 (they cost more but you get much more). We are also developing engobes for low, middle and high temperature ranges, using these you can apply a porcelain-like surface of almost any color at leather hard stage and completely hide the underlying stoneware. These engobes are so opaque that a white one can completely mask a black body underneath using only one coat. So, if you can master their use (there is lots of documentation here) many new design opportunities will offer themselves.

Gallery

P580 mugs fired at cone 10R. Pure Alberta Slip as a glaze outside. G1947U clear glaze inside. Glaze employs Grolleg Kaolin (left two mugs) and EPK (right mug). By Tony Hansen.

Large plate by Peter Flanagan. P580 body. The dark slip is colored P600.

Safety Data Sheet

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Logo Plainsman Clays Ltd.
702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508
Email: plainsman@telus.net