|Monthly Tech-Tip |
Description: Silty fusible low iron clay
At cone 10R it fires as a natural silky transparent. With 20% frit it melts at cone 6. It is a natural base for the addition of pigments, opacifiers and variegators.
This material is not typical of any other glaze material currently available. It is a conditioned clay. The base material, which comprises the bulk of the recipe, is a silty clay that couples very good suspension and drying properties with a low firing temperature. This clay is blended with a mix of minerals to produce the chemistry of a middle-of-the road silky cone 10 glaze.
The principle claims-to-fame of Ravenscrag Slip is twofold:
-It is possible to use it 100% to create a silky to glossy glaze at cone 10 because it contains all the feldspar, silica and clay needed (supplied in majority from the natural clay).
-It imparts very good working properties to the glaze slurry: it suspends it and improves evenness of application, drying speed, reduces shrinkage.
Thermal Expansion of Ravenscrag Slip
COEs are not available for Ravenscrag. Since the material is natural, it contains particles of dozens of different minerals, each particle type has it's own behavior during heat-up, and they interact in complex ways that relate to their chemistry, mineralogy and particle size. This is completely unlike a frit where all the particles are non-mineral, they are all a glass of the same chemistry.
The calculated thermal expansion of Ravenscrag slip is not reliable, it is not a glass; the mineralogy of the particles exert influence. Consider, for example, the SiO2. Unlike a frit where this oxide simply exists as a percentage in the glass, in Ravenscrag it exists in the crystal structure of pretty well all the mineral particles, each of these particles exhibits varying resistance to melting that is related to not just the chemistry, but the mineralogy. Thus, while it might calculate to a very low thermal expansion, you likely need not worry about shivering, just do a boiling water:ice water test if you are concerned.
The analysis of this material was changed here in Sept 2013, not because the material changed, but because they switched to an actual assay instead of a calculation of the chemistry (the chemical analysis of one of the ingredients was off).
Usable, reliable, non-crazing floating blue glazes are difficult to achieve at cone 6. Not these, they pass all the tests yet fire like the original classic G2826R floating blue from David Shaner. Both have been applied at moderate thickness on Plainsman M325 (using a slurry of about 1.43-1.45 specific gravity, higher values end up getting them on too thick). The Ravenscrag version (left) highlights contours better (the edges are black because of the black engobe underneath). It also produces the blue color whether or not the kiln is slow-cooled (although drop-and-hold PLC6DS schedule usually fires more blue). The Alberta Slip version has zero cobalt so it is less expensive to make (but it does require the C6DHSC slow-cool firing schedule). It produces a deeper color over the L3954F black engobe on these pieces. Both of these produce a wide range of effects with different thicknesses, bodies and firing schedules.
The dipping glaze version of G2917 was used on the left, we make that by the bucket and it drains and dries in seconds after dipping bisque ware. We also make a brushing glaze version in our studio (it is not sold). That was used on the outside of the mug on the right. Of course, it is much slower to apply but there are some advantages. First, it was easy to control the thickness to maximize the variegation this glaze gives with thin and thick, revealing the throwing lines better. And where thicker application was needed (e.g. at the rim) it was easier to achieve that. Near the foot ring, it has been applied in a thinner layer. And the brush strokes do impart a more handmade look that is also nice.
Left: G3933E oatmeal based on Ravenscrag Slip.
Right: G3933 oatmeal based on a mix of G2934 matte and G2926B glossy base glazes.
The Ravenscrag one features a number of advantages. Most important: Left tendency to crawl. It is also more responsive to cooling differences (more matte on slow cool, more glossy on fast cool). And its recipe is adjustable, it is easy to raise the MgO if a more persistent matte is needed.
The body is Plainsman M340 and these two glazes are based on the GR6-A recipe (Ravenscrag Slip + 20% frit). The GR6-C creamy white glaze adds 10% Zircopax to opacify it. The pink version, our code number GR6-L, adds Mason 6006 stain instead. The GR6-A base is zinc-free and just hits the 10% minimum CaO recommended to get color development with a chrome tin stain. This recipe also couples a low MgO level (MgO can kill the color in chrome tin stains).
This is the Ravenscrag slip cone 6 base (GR6-A which is 80 Ravenscrag, 20 Frit 3134) with 10% Mason 6006 stain (producing our code GR6-L). Notice how the color is white where it thins on contours, this is called "breaking". Thus we say that this glaze "breaks to white". The development of this color needs the right chemistry in the host glaze and it needs depth to work (on the edges the glaze is too thin so there is no color). The breaking phenomenon has many mechanisms, this is just one. Interestingly, the GR6-A transparent base has more entrained micro-bubbles than a frit-based glaze, however these enhance the color effect in this case.
Left two mugs are glazed with pure Ravenscrag Slip (roast:raw combo), far right one is RavenTalc silky matte (GR10-C). The speckled mugs have 10% of a Plainsman Fire-Red (a blend of a red fireclay, M2 and a heavily speckled ball clay). Ravenscrag Slip is an ideal base for cone 10R glazes, so many glazes can be made by adding pigments, opacifiers, variegators and matting agents.
The clays are Plainsman H450 and H550. Firing is cone 10 reduction. A 50:50 mix of roasted and raw Ravenscrag slip was used. L3954N black engobe was applied at leather hard stage (on the insides and partway down the outsides). We call this recipe GR10-C Ravenscrag Talc Matte, it is on the insides of both and on the outside of the one on the left. The outside of the other is G2571A Bamboo, it is also an excellent matte base. The silky matte surfaces produced by these two are both functional (they are very durable and do not stain or cutlery mark). And they are very pleasant to the touch.
GR10-J Ravenscrag dolomite matte base glaze at cone 10R on Plainsman H443 iron speckled clay (actually, the MgO is being sourced from talc instead of dolomite). This recipe was created by starting with the popular G2571 base recipe (googleable) and calculating a mix of materials having the maximum possible Ravenscrag Slip percentage. The resultant glaze has the same excellent surface properties (resistance to staining and cutlery marking) but has even better application and working properties. It is a little more tan in color because of the iron content of Ravenscrag Slip (see ravenscrag.com).
The inside glaze is pure Ravenscrag Slip and the outside glaze is a 50:50 mix of Ravenscrag and Alberta Slips. Each of the glazes employs an appropriate mix of calcined and raw clay to achieve a balance of good slurry properties, hardening and minimal drying shrinkage. Ravenscrag needs less calcined since it is less plastic than Alberta Slip. The clay is Plainsman H550.
Ravenscrag Slip, by itself, produces a silky transparent glaze at cone 10R. It is an excellent base to which to add colorants and modifiers. This is a simple addition of 10% iron oxide (Ravenscrag Slip already contains 2% iron, making about 12% total Fe2O3). This GR10-L recipe produces a stunning crystalline fired surface on these two porcelains. This "beyond-tenmoku" effect happens because of the extra iron and a slow cooling rate. The 12% iron dissolves in the glaze melt during heatup in the firing, but during cooling, the extra 2% precipitates out to produce these surfaces. The iron also acts as a flux in reduction firing, greatly increasing melt fluidity. Take that last statement seriously: The iron is a flux and the glaze will melt much more (it can wreck your kiln shelves if it runs). That being said, Ravenscrag Slip is more melt stable than other bases, making it a more stable host for the iron addition.
Triaxial blend showing talc additions (left) and titanium additions (right).
Top tile: Pure Ravenscrag Slip (RCS)
Bottom left tile: 85% RCS, 15% Talc.
Bottom right tile: 95% RCS, 5% Titanium Dioxide.
Clearly, talc additions to RCS produce striking variegated silky matte surfaces (in 10-15% amounts). But the addition of a little titanium further improves the effect (especially on the second title in the bottom row: 88% RCS, 10% Talc, 2% Titanium. And the center one: 90% RCS, 10% Talc, 3.5% Titanium). But surprising are the two tiles on the bottom right: 90% RCS,10% Talc, 3.5% Titanium and 96.5 RCS, 3.5 % Titanium - They are glossy brown. This suggests that the MgO in the talc impedes the development of brown and the iron in the RCS is all the TiO2 needs to make a dark brown.
Almost all ceramic glazes are a base recipe with additions of colors, opacifiers, variegators, etc. Our traditional G3933 oatmeal glaze is a good example (recipe on the left). It can produce rich brown silky matte surfaces, especially on dark burning bodies. But one problem has emerged: The tendency to crawl. Much testing has yet to reveal the reason. Would it be possible to base the recipe on Ravenscrag Slip and achieve the same chemistry? Yes. And some unexpected benefits accrued. In the recipe on the right I sourced MgO (the key to the matte surface) from dolomite and Ferro Frit frit 3249 (earlier tests sourcing from talc were unsuccessful, off-gassing from the talc was puffing up the glaze with micro-bubbles). The all-new G3933E recipe has the same chemistry (I derived it in my account at insight-live.com). It is not likely to be without problems, but it looks identical (with richer color from a little more iron oxide), it does not crawl and it's recipe and chemistry are flexible. It is glossy when cooled fast and silky matte when cooled slowly. The MgO can be increased easily to get matteness with quick cool also. The mix of calcine and raw Ravenscrag Slip also enable control over the slurry and application properties.
Calcined or roasted clays are indispensable in making many types of glazes, they reduce drying shrinkage (and thus cracking and crawling) compared to those made using raw clay. In a glaze, you can fine-tune a mix of raw and roast clay to achieve a compromise between dry hardness and low shrinkage.
This is Ravenscrag Slip, we roast it to 1000F (roasting is adequate to destroy plasticity and produces a smoother powder than calcining at higher temperatures). To make sure the heat penetrates for this size vessel I hold it for 2 hours at 1000F. Calcined koalin is getting harder to find, this same process can be used to make your own from a raw kaolin powder. One thing is worth noting: Weight lost on firing actually means that less of the roasted powder is needed to yield the same amount of material to the glaze melt, it can be anywhere from 5-12% less.
Creating a Non-Glaze Ceramic Slip or Engobe
It can be difficult to find an engobe that is drying and firing compatible with your body. It is better to understand, formulate and tune your own slip to your own body, glaze and process.
Ravenscrag Slip is Born
The story of how Ravenscrag Slip was discovered and developed might help you to recognize the potential in clays that you have access to.
Ravenscrag Slip 1000F Roast
Ravenscrag Slip 1900F Calcined
Ask yourself the right questions to figure out the real cause of a glaze crawling issue. Deal with the problem, not the symptoms.
GR10-J - Ravenscrag Cone 10R Dolomite Matte
Plainsman Cone 10R Ravenscrag Slip based glaze. It can be found among others at http://ravenscrag.com.
Ravenscrag Data Sheet at Plainsman Clays
Ravenscrag web site
Clays that are not kaolins, ball clays or bentonites. For example, stoneware clays are mixtures of all of the above plus quartz, feldspar, mica and other minerals. There are also many clays that have high plasticity like bentonite but are much different mineralogically.
Ravenscrag Slip Recipes
Recipes based on Ravenscrag Slip from Plainsman Clays.
|By Tony Hansen|
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