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This material is not typical of any other glaze material currently out there so this is likely part of the reason. Its claim-to-fame is mainly 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 will be exerting too much 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.
Out Bound Links
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In Bound Links
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<?xml version="1.0" encoding="UTF-8"?> <material name="Ravenscrag Slip" descrip="Silty fusible low iron clay" searchkey="" loi="0.00" casnumber="70694-09-6"> <oxides> <oxide symbol="CaO" name="Calcium Oxide, Calcia" status="U" percent="10.100" tolerance=""/> <oxide symbol="MgO" name="Magnesium Oxide, Magnesia" status="U" percent="1.700" tolerance=""/> <oxide symbol="K2O" name="Potassium Oxide" status="U" percent="3.000" tolerance=""/> <oxide symbol="Na2O" name="Sodium Oxide, Soda" status="U" percent="0.500" tolerance=""/> <oxide symbol="TiO2" name="Titanium Dioxide, Titania" status="" percent="0.400" tolerance=""/> <oxide symbol="Al2O3" name="Aluminum Oxide, Alumina" status="" percent="12.700" tolerance=""/> <oxide symbol="SiO2" name="Silicon Dioxide, Silica" status="" percent="62.100" tolerance=""/> <oxide symbol="Fe2O3" name="Iron Oxide, Ferric Oxide" status="" percent="0.800" tolerance=""/> </oxides> <volatiles> <volatile symbol="LOI" name="Loss on Ignition" percent="8.700" tolerance=""/> </volatiles> </material>
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