Ravenscrag Slip

Description

Fine grained low plasticity cone 10 melting silty clay. Ravenscrag slip was originally developed as an almost-complete base cone 10 glaze material with superior application and multi-layering properties. In educational and beginning pottery settings it is an ideal starting-point for material-blending style glaze development and experimentation with colors, opacifiers, variegators and matting agents.

Ravenscrag Slip can be viewed as a 'plastic feldspar' that provides good melting and excellent working properties in a light colored material. It is feasible to use Ravenscrag Slip 'as is' at cone 10, on either green or bisque fired ware, to achieve a light colored silky matte surface. We anticipate that this material will make it easier for people to become more independent and learn to understand and control their own glaze mixes.

The principal ingredient in Ravenscrag Slip is mined from a huge deposit near Ravenscrag, Saskatchewan in Canada. We are confident that potters can count on its supply for many years into the future. Because this material is conditioned with mineral additives we can adjust the mix to maintain control when the mined ingredient changes. In addition every step in the mining and processing is focussed at delivering a very consistent product.

We refer to Ravenscrag as a 'silty' material because, although it contains enough clay to give it some plasticity and dry hardness, it also contains a variety of other very fine mineral particles that do not interact with water the way clay particles do. They particles give it its melting and application properties.

All of these mugs are made from glazes whose major component is either Alberta Slip or Ravenscrag Slip. They are all fired at cone 10R.

Process Properties

A fundamental advantage of basing your slips and glazes on this material is its application properties. Ravenscrag Slip contains more than 70% native silty clay. A slurry glides onto the ware like silk and you can apply multiple layers onto bisque or dry ware without drying cracks and crawling problems. Even pure Ravenscrag Slip will resist crawling on sharp interior corners and edges.

Like Albany Slip, Ravenscrag Slip is excellent for underglaze use to achieve variegation and pooling effects (i.e. a dark glossy glaze under a more stable white). However you will need to test to make sure that the overglaze does not 'pull' on the Ravenscrag Slip underlayer and compromise its dry-bond with the body. Use this material as the base for each layer if possible.

Since Ravenscrag Slip is a siltly materials it does not work well for burnishing.

Firing

Ravenscrag Slip is the opposite of Alberta or Albany Slip: The later are low melting, plastic, and dark burning; this is higher melting, non-plastic, and light burning. It melts to a semi-matte surface at cone 10. Adding small amounts of feldspar (i.e. 5-10%) will make it flow better and add gloss. We have not had good success with boron frits for cone 10 oxidation use, these tend to produce an uneven blistered surface (although some users have found that gerstley borate is ok as a flux).

Adding about 10% Ferro 3134 frit for cone 6 will produce a similar melt. Increasing the frit to 20% will improve flow and add gloss further. You can add colorants (i.e. stains, metal oxides), variegators (i.e. 5% rutile), and opacifiers (i.e. 10% Zircopax) to create almost any effect.

Since it is basically a clay material a significant amount of gas is generated during decomposition during melting. If not fired high enough and not fluxed adequately very thick layers could tend to form unbroken bubbles in the glaze layer.

Unless well melted this material does not smooth out as well on coarser stoneware bodies as on bodies made from refined kaolins, ball clays, feldspar, etc. Some stonewares seem to generate gases at the 'wrong time' during the firing cycle and Ravencrag bubbles up badly if the firing schedule is not right.

It is important to remember: Ravenscrag is a base material on which to build slips and glazes. By itself it is simply a milky glaze, it needs opacifiers, colors, and variegators to produce visually interesting surfaces.

Glaze Recipes

Ravenscrag Slip is an ideal base for almost any middle to high temperature glaze or slip since it already has most of the chemistry and physical requirements built in.

To get better melting add 10% feldspar (or whiting for a less glossy appearance). To make a high fire white, just add some opacifier or fire as is on light bodies. For a tenmoku, celadon, brown crystal, or kaki and the appropriate amount of iron (2-12%). For an oatmeal, add a little manganese dioxide (2-5%). For a blue add a little cobalt.

At cone 6 use about 15-20% Ferro frit 3134 to get a good gloss. Then add colorants and opacifiers. 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 underlayer will be exposed on edges and will often variegate with the overlayer to produce very interesting effects. When applying multilayers by dipping be sure to control the overall thickness to prevent running.

Ravenscrag based glazes should have minimal problems with crazing since it contains little sodium and plenty of magnesia. If glazes do craze and are melting well, add a little silica. Otherwise you will need to calculate the chemistry of the mix and add boron or shift to fluxes with lower thermal expansion.

Another potentially valuable use for Ravenscrag Slip is as a glaze underlayer. Glazes that normally must be thick to achieve the desired effect (i.e. rutile blues) often tend to run, but if you apply a layer of Ravenscrag Slip on the bisque first you can apply a thinner layer of your glaze over it and it will fire out as if it were thick. Even though Ravenscrag slip melts well at high fire, we have found it to be a surprisingly stable base; overglazes have not run unless applied thickly. However, check to make sure that RSlip is compatible with your body, some stonewares containing coarser materials (e.g. fireclays) tend to create bubbles in the overlying slip.

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.

If you need to duplicate Albany slip with this material try starting with 4-6% iron and about 15-25% frit 3134. Adjust for color with the iron and increase or decrease the frit to adjust melt fluidity.

Remember that this material does contain some iron so less potent coloring oxides may not produce brightly colored effects as easily as normal fritted or feldspar based glazes.

Note that all the glaze variations are not guaranteed-to-work glaze. We focus on the value of this material to make a base glaze and provides some examples of variations, but everyones circumstances are different, you must do testing and likely alteration for your situation. Visit this page for information on variegating, opacifying and coloring glazes: Variegating Glazes.

For more detailed information go to http://ravenscrag.com

Physical Properties

 Drying Shrinkage: 3.5-4.5%

Sieve Analysis (Tyler mesh):

     +100: 0.0-0.1%
 100-150: 0.5-1.5
 150-200: 2.5-3.5
 200-325: 5.5-8.0

Chemical Analysis

The analysis of this material has changed in 2013, not because the material changed, but because we have switched to an actual assay for a calcuated analysis.

 CaO      8.5
 K2O       4.1
 MgO       2.5
 Na2O      0.5
 TiO2      0.4
 Al2O3    15.4
 SiO2     56.7
 Fe2O3     1.1
 SO3       0.2
 LOI       8.7%

Downloads

  • Brochure for double-sided printing (uncheck the Fit to Page checkbox when printing or it will be too small)

Gallery

Alberta Slip/Ravenscrag celadon cone 6 on P300 and M340. By Tony Hansen.

H450 mugs fired at cone 10R with pure Alberta Slip on the outsides, G1947U transparent (left) and pure Ravenscrag Slip on the insides. By Tony Hansen.

Safety Data Sheet

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Logo Plainsman Clays Ltd.
702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508
Email: plainsman@telus.net