Attractive high temperature, sandy textured, plastic, semi-vitreous, dark red speckled burning, native body for reduction fired iron stoneware. H440 displays an attractive mottled surface with a range of deep reds and browns when fired in cone 10 reduction. H440 is not a vitreous body, it stops short of complete maturity to retain interesting color. The body is best suited for decorative pieces large or small (vases, bowls, planters, etc.). H440's lower fired strength means that it is not nearly as good as H550 or H450 for use in functional ware. However, its compelling color, robust drying properties and excellent green strength have meant that significant amounts of functional ware are made from this material.
H440 is dark brown in pugged form. It has good plasticity but has significant kaolinized sand in its recipe. Thus it has a more sandy texture during throwing (more than 443). It has a wide distribution of particle sizes in the plus 200 mesh range producing a high dry density and strength.
H440 has a fairly high drying shrinkage and thus care and attention during drying are necessary for larger pieces, especially flat plates, shallow bowls, and sculptural ware. Cover pieces to make sure they dry evenly, take extra time drying if needed, and start the kiln slowly (candle overnight) for final water expulsion.
These test bars (left to to right) compare H440 and H443 at cone 10R (top) and cone 10, 8 and 8 oxidation.
H440 is a deep red firing iron reduction body. The significant Firered native clay complement in its recipe is the reason for the attractive color. Like other bodies of this type, H440 has been formulated so that cone 10R is at its transition point from a light brown to a dark red brown. When fired just right the surface is a patchy network of darker vitrified areas beginning to invade the lighter colored matrix. If over-fired or over-reduced the surface will burn a dark solid red brown. Good color is possible as low as cone 8 reduction (where it redder).
H440 contains significant amounts of iron stone concretion particles which melt vigorously in high temperature reduction and which blossom on the base surface and bleed up through glazes. The degree to which these speckles melt and develop is dependent on the degree of reduction.
H440 can be fired in oxidation, but its color is much lighter; a leather greenish tan with a very fine speckle. It will tend to blister and bloat sooner in oxidation (i.e. cone 11) because the coarser particles in the clay are much more active in producing gaseous by-products during decomposition.
Since H440 is high in iron and fires to a dark color, the finely divided iron in the clay will bleed into glazes. For example bright glazes will not be nearly as vivid as they would be if used on buff stonewares. On the other hand, this effect will enhance the appearance of earthtone and variegated glazes.
H440 has a reasonably high porosity so the fired body will tend to absorb water. Thus it is important that glazes not be crazed (strength can be significantly impacted if glazes do not fit). Use an ice water:boiling water immersion test to make sure.
H440 bisque ware is quite dense with channels formed by larger particles. This means that glazes can pinhole during drying as water-displaced air escapes from larger-than-normal surface holes that channel air out from inside.
Consider using our standard G2571A matte and G2947U glossy base glazes as starting points. Information is given on adding colorants, opacifiers and variegators to make any effect you want. You will also find excellent recipes made from Ravenscrag Slip and Alberta Slip (on their respective websites, Ravenscrag.com and AlbertaSlip.com). For slip decoration, be careful to match drying and fired shrinkage of the slip with the body.
Drying Shrinkage: 6.0-7.0% Dry Strength: n/a Water Content: 20.0-21.0% Drying Factor: C120 Dry Density: n/a
Sieve Analysis (Tyler mesh):
+48: 0.0-0.5% 48-65: 1.0-3.0 65-100: 4.0-6.0 100-150: 2.5-5.0 150-200: 4.0-7.0 200-325: 6.0-10.0
Cone 8: 4.5-5.5% Cone 10: 5.0-6.0 Cone 10R: 5.0-6.0
Cone 8: 4.0-6.0% Cone 10: 3.0-4.0 Cone 10R: 2.5-3.5
BaO 0.5 CaO 0.4 K2O 1.7 MgO 0.5 Na2O 0.1 TiO2 0.7 Al2O3 16.4 P2O5 0.1 SiO2 68.0 Fe2O3 3.1 MnO 0.0 LOI 8.4%
H440 planter fired at cone 10R.
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|Plainsman Clays Ltd.|
702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508