|Monthly Tech-Tip |
Modified: 2023-10-23 18:06:17
Drying and firing shrinkage fitted to Plainsman M390, M340
|OM #4 Ball Clay||54.00||62.1|
This is white because of the 10% Zircopax. Increase that 20% for maximum white. The black version of this is L3954F.
This recipe has been tuned to have about the same fired shrinkage as Plainsman M390, Coffee Clay and M340 at cone 6 (we measure this at 6.7% dry-to-fired shrinkage using the SHAB test). We also confirmed fired-shrinkage-match using the EBCT test). If the body you use has between 6.5 and 7.0 fired shrinkage, this should work. Do not ignore this detail. If you apply this as an engobe (e.g. by dipping leather-hard functional pieces), and it is not firing-shrinkage-compatible with the body, it could eventually flake off during use.
The color is whiter than previous trials (because of the zircopax), but some might prefer a little blue stain to brighten the color. We used 0.2% Mason 6336 with good success, other stains we tried agglomerated and caused specking). For a super white see the L3954R recipe.
As will other engobes we recommend mixing in a kitchen blender, it is an ideal way to get the lumps out and thoroughly wet all particle surfaces.
A thixotropic dipping slurry is needed and we have found it is possible to create a deflocculated one using 1000 powder, 780g water, ~3g Darvan. This produces a specific gravity of ~1.55. If it is too viscous add more Darvan, too fluid add a tiny amount of Epsom salts to bring it back to a creamy texture.
A brushing version can also be made, see the notes for the L3954F black engobe.
Left is Plainsman M340. Right is M390. Each mug has been white-engobed inside, using L3954B, 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 could be achieved using a white glaze (especially over the dark burning body). The outside of the left one is Alberta Slip base GA6-B. 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 the L3954B engobe. 15% Mason 6600 black body stain has been added (instead of the normal 10% Zircopax for white). Of course, a cover glaze is needed for a functional surface, thickly applied engobes must have the same fired maturity as the body for good fit. Lots of information is available on using L3954B (including mixing and adjustment instructions). Engobes are tricky to use, follow the links below to learn more. L3954B is designed to work on regular Plainsman M340 (this piece), M390 and Coffee Clay (it is not a porcelain but has adjustable maturity to be fitted to them if needed). These bodies dry better than porcelains and are much less expensive, so coating them with an engobe to get a surface like this makes a lot of sense.
M340 stoneware fired to cone 6 (drop-and-hold schedule). The L3954B engobe fires deep black (it has 10% Mason 6600 black stain instead of the normal 10% Zircopax). It was applied inside and partway down the outside (a much less messy process than using a black clay body). They were bisque fired and glazed inside using the base GA6A Alberta Slip amber clear (using Frit 3195). The outside glaze is Alberta Slip Rutile Blue (you are seeing it on the bare buff body near the bottoms and over the black clay surface on the uppers). To learn more about how to make the engobe and start making black pots click "Product Data Sheets" at PlainsmanClays.com and go to the section on Medium Temperature.
Black burning bodies are popular with many potters. This one is stained by adding 10% raw umber to a buff-burning stoneware. Umbers are powerful natural clay colorants, they have high iron and also contain manganese oxide. This mug fired perfectly. For both aesthetic and safety reasons. A white engobe, L3954B, was applied during leather hard stage, on the inside and partway down the outside. After bisque, transparent G2926B glaze was applied inside and GA6-B outside (over the engobe it fires amber but over the black clay it produces a deep brown). What about safety? Notice the clay has not bloated, the glaze has no bubble clouds or blisters, that means no manganese fumes were being generated. This happened because the top temperature on the firing was closely guarded to not exceed cone 6. And the body is a cone 10 stoneware, not cone 6, there is insufficient feldspar to trigger release of the MnO from the umber (and its subsequent decomposition to metallic state).
This is the standard Plainsman L3954D white engobe recipe with the 10% Zircopax switched for Burnt Umber. The result is a 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). This engobe works even better with a black stain.
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.
The foot ring on these leather-hard mugs has already been trimmed. At the stiff-leather-hard stage an engobe was applied to the inside. This rewets 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 too dry. To prevent that I wax them after trimming (leaving a just the inside handle-curves bare). That slows their drying down enough to keep them even with the body of the mug. This method works well enough that none of my mugs need covering during drying, even in our desert climate. Keeping all parts of a piece at the same water content throughout the process, that is a key to successful drying.
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. A couple of caveats: 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. Both of these frits prevent the formation of bloating blues (with additions of rutile or titanium).
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. Neither has the micro bubbles that mar a typical clear glaze on bodies like this.
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 (powdered Epsom salts are added to gel it 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 made it smooth and a perfect consistency for application. It remains stable on ware (without runs). Engobes require tight control to have the right viscosity and thixotropy (which can be achieved over a range of specific gravities (about 1.45-1.6). When they are right they are a joy to use, when they are not ware is ruined.
White Cone 6 Engobe, New Alberta Slip glazes, Firing Schedule
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|By Tony Hansen|
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