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H441G

High temperature, grogged but fairly smooth, plastic, non-vitreous, white buff burning, native body for reduction fired handbuilding and sculpture.

This is our most refractory body. It is made from 50% of a high coal very white burning plastic fireclay along with 18% grog and a mix of stoneware and ball clays. H441G is ideal for sculpture and other types of work where forms are subject to warping in the kiln (i.e. overhung, appendages sticking out, shapes lacking strength). It is able to retain shape in the kiln because it does not vitrify.

H441G fires much whiter than typical sculpture clays. It does have some soluble salts discoloration on the surface and these enhance textures and contours on unglazed pieces. If you need a darker firing material, try H440G. Although H441G does throw quite well, we do not recommend it for functional ware because of its refractory nature.

Process Properties

H441G vents water better than our non-grogged materials, it has a lower drying shrinkage, and it does not propagate cracks as much. The grog also assists the healing process during joining. When attaching pieces we recommend that you use slip of low water content, apply as much pressure as possible, and move surfaces latterally against each other so that the slip can fix the two together firmly.

H441G has a base which masks the feel of the grog somewhat so it is possible to throw it with much less discomfort than our other grogged materials.

Thicker ware or pieces of uneven cross section should be given adequate time to dry. Thus it is often necessary to cover larger pieces with cloth and plastic to slow drying down enough to make sure it occurs evenly throughout. Final drying should be done under heated conditions to assure that all water is expelled before firing. We recommend that you preheat your kiln overnight on low to give plenty of time for remaining water to be removed. This is especially true in electric kilns where there is no draft to take water vapor away.

Our grog clays are somewhat susceptible to water splitting because the grog exists in a matrix that contains sand and coarse clay particles. While this open structure has advantages, one of the disadvantages is that splits in the plastic matrix can start and propagate more easily.

Firing

H441G's light fired color is due to its refractory nature, little glass is formed to amplify the color of iron in the body. The fired color is very stable across the cone 8 to 11 oxidation range. At cone 10R the color is quite similar to the oxidation equivalent except the surface will usually exhibit some grey patching as microscopic areas begin to vitrify, turn a light grey, and radiate outward. H441G does have some iron speckle that displays in a scattered pattern in reduction.

It is best to fire larger sculptural forms on a thick slab made from the same clay. This slab will absorb the stresses associated with shrinkage and help prevent cracking and warping.

Since H441G is quite porous and non-vitreous, it is stable in the kiln. It is the best choice if you need to create pieces that have overhung or exaggerated shapes that might otherwise warp during firing. If ware is to be put outdoors, you are advised to seal the pore structure against water absorption and associated damage due to freeze-thaw. Building supply stores have sealant products used for concrete and brick.

Glazing

Since H441G is light firing in reduction, it will not significantly affect glazes and colors. However H441G can contain significant amounts of iron stone concretion particles which melt vigorously in high temperature reduction and which bleed up through glazes to create what are called 'iron blossoms'. However you should be aware that the amount of iron speckle can vary a certain amount from run to run.

Most sculptors find that slips and oxides are effective with this type of material, especially if the surface has been altered or treated in any way. However, be careful to match the firing and drying shrinkage of the slip to the body.

Physical Properties

 Drying Shrinkage: 5.0-6.0%
 Dry Strength: n/a
 Water Content: 21.5-22.5%
 Drying Factor: C120
 Dry Density: n/a

Sieve Analysis (Tyler mesh):

     +35: 4.0-6.0%
   35-48: 2.5-4.0
   48-65: 2.5-4.0
  65-100: 3.0-5.0
 100-150: 3.0-6.0
 150-200: 4.0-8.0
 200-325: 6.0-10.0

Fired Shrinkage:

   Cone 8: 4.5-5.5%
  Cone 10: 5.0-6.0
 Cone 10R: 5.0-6.0

Fired Absorption:

   Cone 8: 7.0-9.0%
  Cone 10: 5.0-7.0
 Cone 10R: 5.0-7.0

Chemical Analysis

 BaO       0.3
 CaO       0.5
 K2O       1.6
 MgO       0.5
 Na2O      0.1
 TiO2      0.9
 Al2O3    21.0
 P2O5      0.1
 SiO2     63.3
 Fe2O3     1.3
 MnO       0.0
 LOI      10.5%

Plainsman Clays Ltd.
Box 1266, 702 Wood Street, Medicine Hat, Alberta T1A 7M9
Phone: 403-527-8535 FAX: 403-527-7508

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