Modification Date: 2017-08-08 11:24:18
|M23 Ball Clay||54.0||30.8%|
|Mason 6336 Stain||0.2||0.1%|
More information and pictures on this recipe can be found in the Products section of http://plainsmanclays.com.
This recipe has been tuned to have about the same drying and fired shrinkage as the target clay body (Plainsman M390). It has more Zircopax, more stain and less flux than previous ones. The objective has been to match fired shrinkage (see bi-clay strips below). Thus, this is not vitreous like a porcelain.
To prepare the slurry mix equal parts water and powder and fine tune thixotropy using Epsom Salts and Darvan (until it is gelled the right amount to stay on ware without dripping after dipping).
The color is whiter than previous trials (because of the zircopax), but it would benefit from more blue stain.
2000 clay, 2000 water = 1.44 specific gravity. Had to use Darvan to thin it after over flocculating.
Left is Plainsman M340. Right is M390. Each mug has been white engobed inside and half-way down the outside. The insides have been glazed using G2926B clear. The inside surface has more depth and has a richer appearance than you could achieve using a white glaze (especially over the dark burning body). The outside of the left one is Alberta Slip base GA6A using Frit 3249 (it produces a more stable glass of lower thermal expansion). The outside glaze on the right is the clear plus 4% iron oxide. This technique of using the engobe enables porcelain-like functional surfaces on the insides and striking visual contrast and character on the outside of the dark body mug.
This is a white engobe (L3954B) drying on two dark burning cone 6 stoneware leather-hard mugs (Plainsman M390). Those lumps are on the left cannot be screened out, they are agglomerates. That slip has excessive flocculant (Epsom salts are added to gel the slurry so that it stays put on the piece after dipping). About 4 drops of Darvan were added to one gallon of the slurry, this immediately transformed the slurry, making it smooth and a perfect consistency for application. It remains stable on ware (without runs). Engobes require tight control to have the right specific gravity, viscosity and thixotropy. When they are right they are a joy to use, when they are not your ware is ruined.
These two mugs are made from a dark red burning stoneware and fired in a cool-and-soak firing schedule. A white engobe (L3954A) has been applied on the inside and half way down the outside. Both are glazed inside with G2926B whiteware transparent glaze. The outside glaze on the left is the same transparent with 4% added iron oxide. It has been sieved to 80 mesh. Notice the iron agglomerates and still produces specking (an effect that may be desired, but difficult to keep consistent). Interestingly, that iron is producing a clear amber-colored glass about equal in color to the Alberta Slip GA6A base glaze (80% Alberta Slip, 20% Frit 3195) on the mug on the right.
The body is buff burning Plainsman M340 (cone 6). The amber colored glaze is 80% Alberta Slip (raw:calcine mix) with 20% of each frit. The white engobe on the inside of mug 1 is L3954A (also glazed inside using transparent G2926B). These frits are producing an amber gloss glaze of high quality. On the outside of mug 1 we see the 3195 version on the white slip until midway down, then on the bare buff clay (the other has the 3249 version). These mugs are fired using a drop-and-soak firing schedule. There is a caution: Frit 3249 has a very low thermal expansion, use it on bodies that craze other glazes (like Plainsman P300), it could shiver on stonewares like this.
The foot ring on these hard mugs has already been trimmed. At the stiff-leather-hard stage an engobe was applied to the inside. This rewet the bodies of the mugs, almost to the same point as freshly-thrown. But the handles did not get rewetted. To re-dry these mugs to the point of being able to turn them over will take 4-6 more hours. But by that time the handles will be bone dry. To prevent that I waxed them after trimming. That slows their drying down enough to keep them even with the body of the mug. To dry ware successfully the key is to keep all parts of a piece of the same water content throughout the process.
On the right is what it will look like when fired with a clear glaze on the inside and amber-clear on the outside. On the left it has dried and is ready for a little fix-up before bisque firing.
This is the standard Plainsman L3954D white engobe recipe with the 10% Zircopax switched for Raw Umber. The result is a very dark, rich, ultra-gloss brown (almost black). The engobe is applied inside and half-way down the outside. The mug on the left is glazed inside and out with the base GA6A Alberta Slip cone 6 recipe (but uses Ferro Frit 3195 instead of 3134). The one on the right has the same glaze on the outside but the G2926B clear transparent on the inside (it is micro-bubbling).
Black burning bodies are popular with many potters. They are normally manufactured by adding around 10% burnt or raw umber to an existing buff-burning cone 6 stoneware. Umbers are powerful colorants, they have high iron and also contain manganese (the latter being the primary source of the color). But these clays can be troublesome. First, good kiln venting is needed to avoid breathing the dangerous manganese metal vapors. Micro-bubble clouding/gloss-loss in the glazes and blistering/bloating of the bodies are common. But this mug fired perfectly. Why? The umber was added to a cone 10 stoneware instead (and it has fluxed the body to mature at cone 6). The mug has been white engobed on the inside and partway down the outside during leather hard stage. After bisque it was clear glazed on the inside giving a flawless surface (using G2926B) and dipped in GA6-A Alberta Slip base amber-clear. The GA6-A over the black clay produces a very deep, rich, almost black ultra-gloss surface.
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By Tony Hansen+