Modification Date: 2016-09-30 22:47:55
Member of Group: RV6
Plainsman Cone 6 Ravenscrag Slip based version of the popular floating blue recipe. It can be found among others at http://ravenscrag.com.
David Shaner's cone 6 floating blue has been used for many years by thousands of potters. However the base is built on a the 50:30:20 Nepheline Syenite:Gerstley Borate:Silica clear base. This base has some troublesome issues which include slurry gelling because of the partially soluble Gerstley Borate and Nepheline Syenite, consistency and supply issues relating to Gerstley Borate, susceptibility to blistering and higher thermal expansion because of the high feldspar content. It was a love-hate relationship that many people had with this glaze, the pull of the appearance was enough to make many endure all of its problems.
In this recipe we have transplanted the iron, cobalt and rutile into a the 80:20 Ravenscrag:Frit base (and added more frit to get more action in the crystal development and variegation). This affords the great workability features of this base producing a much more pleasant-to-use slurry. It also eliminates the solubility and consistency issues and produces a glaze of lower thermal expansion.
Although more expensive to make that the Alberta Slip version of this recipe (because of the cobalt), this one does not require slower cooling.
You may find that the slurry shrinks too much on drying, cracking on the bisque. If that happens consider calcining some of the powder (as described at ravenscrag.com). Tune the mix of calcine to raw to get the exact slurry properties you want.
Cone 6 oxidation. GR6M Ravenscrag version is on the left. The Alberta Slip version (GA6C) is more fluid, but that also means it will run more during firing and blister more if too thick or on re-firing. Generally, the Alberta Slip version appears better on dark bodies and the Ravenscrag one on lighter burning clays. The Alberta Slip version gets its color only from Rutile (and thus requires a special drop-and-hold firing scheduel), the Ravenscrag one produces blue in any firing schedule (although the color will be better in the drop-and-hold schedule).
GR6-M Ravenscrag cone 6 Floating Blue (center) on Plainsman M340, a buff burning body. On the left is a version having 80:20 Ravenscrag:Frit 3134 (no extra 10% Frit 3124). On the right is GR6-M on porcelain (where the floating effect has been largely lost). It appears the effect benefits from the iron it finds in the stoneware body.
GR6-M Ravenscrag Cone 6 Floating Blue on Plainsman M340 buff stoneware. This glaze also has this variegated visual character on porcelain. Because it has the GR6 base recipe (which is publicly available at ravenscrag.com), the slurry has very good working properties in the studio, it is a pleasure to use. This is an excellent showcase for the variegating mechanism of rutile.
Here it is fired to cone 8 where the melt obviously has much more fluidity! The photo does not do justice to the variegation and crystallization happening on this surface. Of course it is running alot more, so caution will be needed.
These are from the same firing, glazed at the same time and are the same thickness. The floating blue effect is a fragile mechanism and affected even by the small color difference in these bodies. The small amount of extra iron in the M370 affects the glaze character more than expected.
The insides are GA6-A Alberta Slip cone 6 base. Outsides are Ravenscrag Floating Blue GR6-M. The firing was soaked at cone 6, dropped 100F, soaked again for half and hour then cooled at 108F/hr until 1400F. The speckles on the porcelain blue glaze are due to agglomerated cobalt oxide (done by mixing cobalt with a little bentonite, drying and pulverizing it into approx 20 mesh size and then adding that to the glaze slurry).
Out Bound Links
In Bound Links
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By Tony Hansen+