Monthly Tech-Tip from Tony Hansen SignUp

No tracking! No ads!

1-9 | A | B | C | D | E | F | Frits | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

Ravenscrag Slip

Description: Silty fusible low iron clay

Oxide Analysis Formula
BaO 0.30% 0.01
MgO 1.70% 0.16
CaO 10.10% 0.68
K2O 3.00% 0.12
Na2O 0.50% 0.03
TiO2 0.40% 0.02
Al2O3 12.60% 0.47
SiO2 61.80% 3.89
Fe2O3 0.80% 0.02
LOI 8.80%n/a
Oxide Weight 345.34
Formula Weight 378.66

Notes

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.

Ravenscrag Slip makes it 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).

Ravenscrag Slip 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 exerts 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).

Related Information

G2917 Ravenscrag floating blue as a dipping and brushing glaze at cone 6

Tap picture for full size and resolution
Dipping and brushed glaze on two mugs

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.

Ravencrag Slip vs Alberta Slip floating blues at cone 6 oxidation

Tap picture for full size and resolution
Two floating blue mugs

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.

A better cone 6 oatmeal glaze using Ravenscrag Slip

Tap picture for full size and resolution
Ravenscrag oatmeal glazed mug

Left: G3933EF oatmeal based on Ravenscrag Slip.
Right: G3933 oatmeal based on a mix of G2934 matte and G2926B glossy base glazes.
Both have the same added colorants. The Ravenscrag version features several advantages. Most importantly much less tendency to crawl. It has better application properties, the slurry needs less water and it is naturally thixotropic. It has an extra option for adjusting properties: Changing the ratio of roast-to-raw Ravenscrag clay. It is responsive to cooling differences - more matte on slow cool versions of the C6DHSC schedule (e.g. 150F/hr), more glossy on faster cools (e.g. 250F/hr). And, its recipe is adjustable (e.g. raising the MgO if a more persistent matte is needed). And, it looks and feels way better, interacting with dark bodies for richer color and varying in tone more for thinner and thicker sections.

Ravenscrag Cone 6 white glaze with 10% Mason chrome tin stain

Tap picture for full size and resolution

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).

A breaking glaze highlights incised decoration by its variation in thickness

Tap picture for full size and resolution

This is the Ravenscrag slip cone 6 base (GR6-A which is 80 Ravenscrag, 20 Frit 3134) with 10% Mason 6006 stain (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.

Laguna B-Mix, B-Mix+Fireclay with Ravenscrag GR10-A, GR10-C glazes

Tap picture for full size and resolution

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.

Ravenscrag Slip + 10% talc = fantastic cone 10R silky matte glaze

Tap picture for full size and resolution
Two mugs with silky matte glaze

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.

Ravenscrag Slip based dolomite matte

Tap picture for full size and resolution

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).

Ravenscrag Alberta Slip Celadon mug by Tony Hansen

Tap picture for full size and resolution

The outside glaze on this cone 10R mug (made of Plainsman H550) is G2881B, simply an Alberta Slip:Ravenscrag Slip 50:50 mix (the Ravenscrag Slip portion was roasted). 5% Ferro Frit 3134 was added to get a little better melting (to produce a high gloss). This produces a good celadon with great working and application properties. Inside glaze: Pure Ravenscrag Slip (mixed 50:50 roast and raw).

A cone 10R iron crystal glaze using only Ravenscrag Slip and Iron

Tap picture for full size and resolution
Two mugs with iron crystal glazes

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.

Ravenscrag Slip pure: Oxidation vs. Reduction

Tap picture for full size and resolution

Pure Ravenscrag Slip on a porcelain at cone 10 oxidation (left) and cone 10 reduction (right). The reduction fired sample is a very smooth pleasant semi-matte, the other is glossier but dimpled.

How to matte Ravenscrag Slip at cone 10 by adding talc

Tap picture for full size and resolution

2,5,10,15% talc added to Ravenscrag Slip on a buff stoneware fired at cone 10R. Matting begins at 10%. By Kat Valenzuela.

The surprising results of titanium/talc additions to Ravenscrag slip at cone 10R

Tap picture for full size and resolution

Talc additions to RCS produce variegated silky matte surfaces (in 10-15% amounts) at cone 10R. What if titanium is also added? This triaxial blend shows what works. Top tile #1: Pure Ravenscrag Slip (RCS) Bottom left tile #7: 85% RCS, 15% Talc. Bottom right tile #10: 95% RCS, 5% Titanium Dioxide. The addition of a little titanium further improves the effect as shown on #8 (88% RCS, 10% Talc, 2% Titanium) and #6 (90% RCS, 10% Talc, 3.5% Titanium). But too much titanium loses it (tile #9) and adding only titanium goes glossy brown. This shows that some TiO2 is good, too much and the effect is lost.

Sometimes it is better to replace the base in a production glaze recipe

Tap picture for full size and resolution

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 problems have emerged, especially a 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 accrue. 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). This 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 mix of calcine and raw Ravenscrag Slip also enable control over the slurry and application properties. I later tuned this recipe a little more and settled on G3933EF.

Roast or calcine your Ravenscrag Slip (or other clays) for much better results

Tap picture for full size and resolution
A bisque fired container of roasted Ravenscrap Slip powder

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.

Roasting Alberta and Ravenscrag Slips at 1000F: Essential for good glazes

Tap picture for full size and resolution

Roasted Alberta Slip (right) and raw powder (left). These are thin-walled 5 inch cast bowls, each holds about 1 kg. I hold the kiln at 1000F for 30 minutes. Why do this? Because Alberta Slip is a clay, it shrinks on drying (if used raw the GA6-B and similar recipes will crack as they dry and then crawl during firing). Roasting eliminates that. Calcining to 1850F sinters some particles together (creating a gritty material) while roasting to 1000F produces a smooth, fluffy powder. Technically, Alberta Slip losses 3% of its weight on roasting so I should use 3% less than a recipe calls for. But I often just swap them gram-for-gram.

Links

Articles 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.
Articles 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.
Materials Barnard Slip
Materials Alberta Slip
Albany Slip successor - a plastic clay that melts to dark brown glossy at cone 10R, with a frit addition it can also host a wide range of glazes at cone 6.
Materials Albany Slip
A pure low plasticity clay that, by itself, melted to a glossy dark brown glaze at cone 10R. It was a popular glaze ingredient for many decades.
Materials Ravenscrag Slip 1900F Calcined
Materials Ravenscrag Slip 1000F Roast
URLs http://ravenscrag.com
Ravenscrag web site
URLs https://plainsmanclays.com/data/index.php?product=12911
Ravenscrag Data Sheet at Plainsman Clays
Typecodes Ravenscrag Slip Recipes
Recipes based on Ravenscrag Slip from Plainsman Clays.
Typecodes Clay Other
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.
Recipes GR10-J - Ravenscrag Cone 10R Dolomite Matte
Plainsman Cone 10R Ravenscrag Slip based glaze. It can be found among others at http://ravenscrag.com.
Troubles Crawling
Ask yourself the right questions to figure out the real cause of a glaze crawling issue. Deal with the problem, not the symptoms.
By Tony Hansen
Follow me on

Got a Question?

Buy me a coffee and we can talk



https://digitalfire.com, All Rights Reserved
Privacy Policy