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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 two fold:
-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 and these are supplied in majority from the natural clay.
-It imparts beautiful working properties to the glaze slurry: it suspends it and improves evenness of application, drying speed, reduces shrinkage and enables multi-layering (many of the most beautiful art glaze effects can be achieved by layering one glaze over another).
You can visit www.ravenscrag.com to learn more. The site shows a lot of very interesting glaze formulation work and recipes and comments are recorded on the site. In addition to some of the nice multi-layer effects, you will find one glaze in particular, a blue cone 6, to be quite stunning (it is a worthy successor to floating blue because it lacks its persnickety nature). If you want to dabble in developing your own glazes, Ravenscrag Slip is a great place to start.
One exciting thing that has emerged is that Ravenscrag is at home in high, medium and low fire glazes. At high temperature it can be used as-is with additions of color, opacifier, etc to create glossy and matte glazes. At middle temperature, 20-30% frit seems to work well. At low temperature 50-60% is needed. Frit 3195 at low fire produces a very attractive silky matte surface having a nature that is much more lively than the typical toilet-bowl type low fire glaze we are used to. Ravenscrag also has potential as a tile glaze since it applies well on dry ware.
Plainsman Clays made a change in the Ravenscrag recipe starting with mix number 4288 from May 2002. It now has 10% added calcium carbonate to make it melt better. If you have a material with a code number whose first four digits are lower than this, add 10% whiting to get the same results as on the web site.
You can order a sample of this material at https://digitalfire.com/plainsman/store/store.php
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. Understandably, it is practical to measure frit COE with some degree of confidence that as long as the user melts it completely they will get the published COE. But it is not practical with Ravenscrag Slip, it does not melt completely (every particle) unless it has at least some other flux.
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).
At cone 5R pure Alberta Slip (left) is beginning to melt and flow down the runway of this tester. It is producing a matte gunmetal surface. Pure Ravenscrag Slip (right) is just starting (it needs frit to develop melt fluidity at this temperature). The iron in the Alberta Slip is melting it because of the reduction atmosphere in the kiln (it does not move like this in oxidation).
Cone 5 GR6-A glaze at cone 5R on Plainsman M340 (left) and pure Ravenscrag Slip at cone 10R on H550 (right).
This iron crystal glaze is Ravenscrag slip plus 10% iron oxide fired to cone 10R on a buff stoneware (Plainsman H550). Since Ravenscrag slip is a glaze-by-itself at cone 10, it is an ideal base from which to make a wide range of glazes. It has its own website at http://ravenscrag.com. It was originally formulated using Digitalfire Insight software. The project built on the merits of a specific silty clay that was noted to couple very good suspension and drying properties with a low firing temperature. The process involved calculating what minerals needed to be added to it to produce the chemistry of a middle-of-the road silky cone 10 glaze; the product was Ravenscrag Slip.
The mug is the buff stoneware Plainsman M340. Firing is cone 6. On the inside is the GR6-A Ravenscrag transparent base glaze. The outside glaze is GA6-C Alberta Slip rutile blue on the outside. The transparent, although slightly amber in color compared to a frit-based transparent, does look better on buff burning stoneware bodies this.
This is the Ravenscrag Slip I used to calcine at 1850F (about 10lbs in a bisque vessel). I am now roasting it to 1000F instead, this produces a smoother powder, less gritty. I hold it for 2 hours at 1000F to make sure the heat penetrates. It is not actually calcining, since not all crystal water is expelled, so we call it "roasting". Why do this? Ravenscrag Slip is a clay, it shrinks. If the percentage is high enough the glaze can crack on drying (especially when applied thickly). The roast does not shrink. The idea is to tune a mix of raw and roast Ravenscrag to achieve a compromise between dry hardness and low shrinkage. Technically, Ravenscrag losses 3% of its weight on roasting so I should use 3% less. But I often swap them gram-for-gram.
Plainsman M340 with Ravenscrag clear (inside) and pink glazes at cone 6. These were fired in a larger electric kiln at a slower firing rate, so the glaze has no imperfections. This pink glaze has proven to be very reliable and attractive on many clay bodies, it employs a stain rather than raw chrome and tin oxides.
Crawling of a cone 10R Ravenscrag iron crystal glaze. The added iron oxide flocculates the slurry raising the water content, increasing the drying shrinkage. To solve this problem you can calcine part of the Ravenscrag Slip, that reduces the shrinkage. Ravenscrag.com has information on how to do this.
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.
This liner glaze is 10% calcium carbonate added to Ravenscrag slip. Ravenscrag Slip does not craze when used by itself as a glaze at cone 10R on this body, so why would adding a relatively low expansion flux like CaO make it craze? It does not craze when adding 10% talc. This is an excellent example of the value to looking at the chemistry (the three are shown side-by-side in my account at Insight-live.com). The added CaO pushes the very-low-expansion Al2O3 and SiO2 down by 30% (in the unity formula), so the much higher expansion of all the others drives the expansion of the whole way up. And talc? It contains SiO2, so the SiO2 is not driven down nearly as much. In addition, MgO has a much lower expansion than CaO does.
This high-Alberta-Slip glaze is shrinking too much on drying. Thus it is going to crawl during firing. This common issue happens because there is too much plastic clay in the glaze recipe (common with slip glazes). Clay is needed to suspend the other particles, but too much causes the excessive shrinkage. The easiest way to fix this is to use a mix of raw and calcined Alberta Slip (explained at albertaslip.com). The calcined Alberta Slip has no plasticity and thus much less shrinkage (but it still has the same chemistry). Many matte glazes have high kaolin contents and recipes will often contain both raw and calcined kaolin for the same reason.
If your drying glaze is doing what you see on the left, do not smooth it with your finger and hope for the best. It is going to crawl during firing. Wash it off, dry the ware and change your glaze or process. This is Ravenscrag Slip being used pure as a glaze, it is shrinking too much so I simply add some calcined material to the bucket. That reduces the shrinkage and therefore the cracking (trade some of the kaolin in your glaze for calcined kaolin to do the same thing). Glazes need clay to suspend and harden them, but if your glaze has 20%+ kaolin and also bentonite, drop the bentonite (not needed). Other causes: Double-layering. Putting it on too thick. May be flocculating (high water content). Slow drying (try bisquing lower, heating before dipping; or glaze inside, dry it, then glaze outside).
Out Bound Links
An amber-colored glaze that produces a clean, micro bubble free transparent glass on brown and red burning stonewares.
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In Bound Links
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.
The story of how Ravenscrag Slip was discovered and developed might help you to recognize the potential in clays that you have access to.
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Ask yourself the right questions to figure out the real cause of a glaze crawling issue. Deal with the problem, not the symptoms.