The story of how Ravenscrag Slip was discovered and developed might help you to recognize the potential in clays that you have access to.
The village of Ravenscrag in southern Saskatchewan, Canada is on a lonely country road at the bottom of a large valley that’s drops away from the flat prairies. Not much happens there now but in the time of the dinosaur it was an exciting place. Their bones are easy to find and there is lots of archaeological activity.
To Luke Lindoe, a potter and geologist/prospector, this valley was a most interesting place. In the 1940s he worked in the local heavy stoneware and brick industries and had a dream to start a pottery clay manufacturing company using the amazing clays found here. That idea was the birth of Plainsman Clays in the 1960s. In 1972 I joined Luke and John Porter (a British trained master potter) at the new company and have been working there since. While Luke was trouncing around the countryside with pick and shovel and John was into the business of clay, I was interested in working with the materials Luke brought in, learning their properties. Luke taught me to see each material as a living entity to be studied and understood in a physical way, not just as a bunch of numbers on a data sheet.
When I drive along the valley bottom looking up at the hills on both with outcroppings of pure white clay I am still amazed there is a place like this. There is no rock, just hundreds of different layers of clay. In ancient times the area was at the bottom of an inland sea. The weathering of the mountains far to the west created vast quantities of micro fine particulates that washed out into the sea and settled over long periods. Each layer is unique depending on the conditions of the geologic period. While they often have obvious boundaries some layers blend into the next. To potters the most interesting of the layer-groups is called the ‘Whitemud Formation’. These secondary materials (transported and settled) are amazingly homogeneous and pure mechanically, many can be used as-is for pottery by just slaking in water. Chemically the layers are diverse and are stained by iron to some degree. Mineralogically the whitemuds have a nice range of properties. There are refractory ball clays, buff burning high and moderately plastic stonewares, a kaolinized sand and a bentonite. The one of interest is a very fine sand-clay-feldspar blend known internally as ‘3D’. Plainsman Clays has used it as a body ingredient since the early 1970s to help vitrify medium fire clay bodies and improve drying properties (it has low plasticity).
The whitemuds are perhaps 200 feet below the surface. However the Ravenscrag is in a valley carved out by the Frenchman river so it is just a matter of finding a hill at the bottom whose top is a little above the whitemud layers. They can then be peeled off one-by-one and stock piled.
One day I plugged the percentage analysis of 3D clay into the Digitalfire desktop INSIGHT (which I wrote). Notice the unity formula (I have shown the ranges for a typical cone 10 glaze on the right).
BaO Barium 0.05* 0-0.3 CaO Calcium 0.07* 0.35-0.7 MgO Magnesium 0.26* 0-0.35 K2O Pottasium 0.58* 0.2-0.45 KNaO Na2O Sodium 0.03* TiO2 Titanium 0.16 Al2O3 Alumina 3.34 0.3-0.55 SiO2 Silica 24.89 3.0-5.0 Fe3O3 Iron 0.14
At first it does not look much like the chemistry of a typical high-fire glaze because the alumina and silica are 10 times recommended and there is little CaO (the basic flux of such glazes). Thus it struck me that adding whiting to source only CaO could drastically reduce the alumina and silica (you can learn more about unity formula at the Digitalfire Reference Database). Amazingly, as you can see below, it takes only 10% whiting to turn the original chemistry (left) into a balanced glaze (right).
BaO Barium 0.05* 0.01 CaO Calcium 0.07* 0.88 MgO Magnesium 0.26* 0.03 K2O Pottasium 0.58* 0.08 Na2O Sodium 0.03* tr TiO2 Titanium 0.16 0.02 Al2O3 Alumina 3.34 0.45 SiO2 Silica 24.89 3.32 Fe3O3 Iron 0.14 0.02
The non-chemistry approach would have been to blend feldspar with this clay to get it to melt more. But we can see from the chemistry that this is exactly the wrong thing to do. However the chemistry was not a perfect predictor either. 10% whiting is not really enough to get the needed melting and the CaO is actually too high. So I did some juggling and employed a couple of other flux sourcing materials and ended up with a recipe that does melt well at cone 10, namely Ravenscrag Slip. You might think it would be better just to sell the pure natural material, but conditioning it with other minerals buffers mining variations and gives Plainsman the power to adjust the chemistry if needed. It also produces the right balance of clay, alumina and silica to be a complete cone 10 glaze that melts well to a clear eggshell non-crazing semi-gloss.
Adding about 10-20% Ferro 3134 to Ravenscrag Slip frit will produce a good to reactive melt at cone 6. Ravenscrag is thus an ideal starting point for material-blending style glaze development and experimentation with colorants (although it has a little iron that can impede bright colors), opacifiers (eg. 10% Zircopax), variegators (eg. 5% rutile), and glossing/matting/reacting agents. You can make tenmoku, celadon, brown crystal, or kaki at cone 10R with the appropriate amount of iron (2-12%).
However this is only part of the story. This material has a unique physical presence also. I was originally attracted most by its amazing slurry properties (it is surprising how many potters tolerate difficult-to-use glazes). Yet even at high temperatures where it is easy to formulate glazes with good slurry properties it has often not been done. Why? Many formulators start with the raw material that melts the best (feldspar) and then add things until the melt is stabilized. The problem is that clay is not a good addition. Why? The best answer is a chemistry one: feldspar has plenty of alumina and thus little is needed from other sources. Unfortunately the major other source is clay, it is needed to create a good suspension. The result has been a bunch of high-feldspar low-clay glazes having rotten slurry properties that craze because of the high sodium content!
Ravenscrag is an excellent solution to this. It imparts beautiful working properties to the glaze slurry: suspending and improving evenness of application, drying speed, reducing shrinkage enabling multi-layering without cracking (many of the most beautiful art glaze effects can be achieved by layering). Ravenscrag glides onto your ware like silk and hangs on without dripping.
To learn more visit www.ravenscrag.com. The site illustrates interesting cone 6 and 10R Ravenscrag-based glazes that Kat Valenzuela developed. They have more reactive and interesting surfaces and produce good multi-layer effects. Ravencrag is excellent for underglaze use to achieve variegation and pooling effects. For double layering try applying a fluid dark colored version (more frit) and overlay it with lighter colored less fluid version. Or try three layers (i.e. light fluid, dark stable, light fluid). During firing the layers separate to expose others and rivulets and variegation occur (be sure to control the overall thickness to prevent running).
Another potentially valuable use for Ravenscrag Slip is as a stable glaze underlayer. Glazes that normally must be thick to achieve the desired effect (i.e. rutile blues) may run. If you apply a layer of Ravenscrag Slip on the bisque first with a thinner layer of your glaze over it will fire as if it were thick. A very interesting variation of this is to employ Ravenscrag as an underlayer for metal sulfate decoration. Cobalt sulfate, for example, can be painted over a Ravenscrag Slip layer to produce watercolor-like visual effects.
However, remember that layering glazes can be a tricky business. Test to make sure that overlayers do not ‘pull’ on underlying ones and compromise their bond with the body. Remember also that Ravenscrag is a raw material and thus produces gases during firing, so bubbles and blisters can occur in thick layers.
Ravenscrag Cone 10R Sample Board from Plainsman Clays
Ravenscrag Cone 6 Glazes Sampleboard
Layers of the Whitemud Formation are being mined. The layer being extracted is a silty stoneware they referred to as the "D member" (equivalent to Plainsman 3D which is mined several miles to the east). Below the D they continued to mine a much whiter kaolinized sand of equal or more thickness. Above the D is a ball clay (equivalent to Plainsman A2). Above that is a light burning stoneware (the combined layers that Plainsman extracts separately as A3 and 3B). A foot-thick layer of much harder volcanic ash is visible in the green over burden at the top. From these stoneware clays they made brick of exceptional quality, firing it as high as cone 10. Twenty years later the company reclaimed this land and today you would be unable to find where the quarry was located.
Out Bound Links
How Alberta Slip was created by analysing and duplicating the physical and chemical properties of Albany Slip
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.
By Tony Hansen